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Sci.Electronics.Basics -> Help needed. Zero crossing with RC snubber problem
There are 33 messages in this thread.
You are currently looking at messages 1 to 20.
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Author: michael nikolaouDate: 04:00 26-02-08
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Hi
I have a 12 v relay driving an large 220 volt AC relay . Across the contact
of the driver relay i placed one RC snubber circut (27NF with 100 R
resisitor in series) to help with some spikes that were influencing the low
voltage driver circuits.
The driver circuit is able to detect mains zero crossing and fire the
driver relay at an angle i choose .
From what i read the best point to switch off the power relay is at zero
crossing . I did that and i show a large spike up to 1 KV at the relay
contact followed by a decaying 500hz waveform to 0 volts . After some
experimentation the best point came exactly when switching off at the peak
of the mains voltage .At this point there is smooth decaying waveform to 0
volt after 5 periods of 500 HZ but no overshoot. The relay presents no
arcing. If i remove the snubber and make the experiment the best place to
switch is zero crossing but i also see large SHARP spikes up to 500 Volts
Peak.
My question is
The switching with snubber must be made at zero crossing or at the peak of
an ac voltage waveform ?
What is the behaviour of the circuit ?.
As i understand any large spikes can harm the X2 capacitor i'm using so
what is the best operating practise ?.
Any help will be appreciated
Michael
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Author: petrus bitbyterDate: 06:34 26-02-08
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"michael nikolaou" <michaelnikolaou_remove_me_@yahoo.com> schreef in
bericht
news:fq0kg4$1n3d$1@ulysses.noc.ntua.gr...
> Hi
>
> I have a 12 v relay driving an large 220 volt AC relay . Across the
> contact of the driver relay i placed one RC snubber circut (27NF with 100
> R resisitor in series) to help with some spikes that were influencing the
> low voltage driver circuits.
> The driver circuit is able to detect mains zero crossing and fire the
> driver relay at an angle i choose .
> From what i read the best point to switch off the power relay is at zero
> crossing . I did that and i show a large spike up to 1 KV at the relay
> contact followed by a decaying 500hz waveform to 0 volts . After some
> experimentation the best point came exactly when switching off at the peak
> of the mains voltage .At this point there is smooth decaying waveform to
> 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents
> no arcing. If i remove the snubber and make the experiment the best place
> to switch is zero crossing but i also see large SHARP spikes up to 500
> Volts Peak.
> My question is
> The switching with snubber must be made at zero crossing or at the peak of
> an ac voltage waveform ?
> What is the behaviour of the circuit ?.
> As i understand any large spikes can harm the X2 capacitor i'm using so
> what is the best operating practise ?.
>
> Any help will be appreciated
>
> Michael
>
>
The best moment for switching off, highly depends on the load. As long as
the load is resistive, the zero crossing point of the voltage is best as
switching is done at minimum voltage and current. As soon as the load has a
reactive component, zero crossing of the voltage differs from zero crossing
of the current. It is the breaking of the current that causes the voltage
spikes.
petrus bitbyter
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Author: ChrisDate: 07:43 26-02-08
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On Feb 26, 3:00=A0am, "michael nikolaou"
<michaelnikolaou_remove_...@yahoo.com> wrote:
> Hi
>
> I have a 12 v relay driving an large 220 volt AC relay . Across the contac=
t
> of the driver relay i placed one RC snubber circut (27NF with 100 R
> resisitor in series) to help with some spikes that were influencing the lo=
w
> voltage driver circuits.
> The driver circuit is able to detect =A0mains zero crossing and fire the
> driver relay at an angle i choose .
> From what i read the best point =A0to switch off the power relay is at zer=
o
> crossing . I did that and i show a large spike up to 1 KV =A0at the relay
> contact followed by a decaying 500hz waveform to 0 volts . After some
> experimentation the best point came exactly when switching off at the peak=
> of the mains voltage .At this point there is smooth decaying waveform =A0t=
o 0
> volt after 5 periods of =A0 =A0500 HZ =A0but no overshoot. The relay prese=
nts no
> arcing. =A0If i remove the snubber and make the experiment the best place =
to
> switch is zero crossing but i also see large SHARP spikes up to 500 Volts
> Peak.
> My question is
> =A0The switching with snubber must be made at zero crossing or at the peak=
of
> an ac voltage waveform ?
> What is the behaviour of the circuit ?.
> As i understand any large =A0spikes can harm the X2 capacitor i'm using so=
> what is the best operating practise ?.
>
> Any help will be appreciated
>
> =A0Michael
Hi, Michael. First off, you should spec a 12V relay that's made to
handle inductive loads (you can see a HP rating). This type of relay
has contacts which open more quickly, and are farther apart when
open. A small standard relay might not even open up far enough to
stop an inductive arc at line voltage.
Next, when using a relay to drive a relay, you have to be aware of the
delay-on-make, which can be several milliseconds, especially for
larger relays. That might help explain some of the curious results
you're getting. Turn-on delay can be affected by wear and aging. It
also varies from unit to unit, even in relays with the same part and
lot number. Trying to get this kind of timing accuracy might be the
wrong way to go.
It might be better to take a look at suppressing the arc, which you've
already started to do. Here's an intuitive way to start. First off,
remember your basic goal: you want the voltage across the contacts to
rise just slowly enough so the contacts can pull away without
sustaining an arc. That's all.
Remove the cover from the driving relay so you can see the contacts.
Next, find the current rating of those contacts, and use Ohm's law to
find a resistor which will result in about half that current.
For example, if you have a 220VAC load, and your relay can handle
10A,:
R =3D 220V / 5A =3D 44 ohms
Choose a 47 ohm, 1 watt resistor here (carbon comp is best). Now get
a selection of self-healing AC line-rated capacitors, and switch the
inductive load with the 47 ohm and C snubber directly across the load,
repeating and increasing the cap value until the contact arcing
disappears, or at least is minimized. Without knowing anything about
your 220VAC relay, it sounds like your 0.027uF cap might be on the
small side.
Note that ITW Paktron makes a series of Quencharc snubbers that you
can just plug in, which makes selection a snap. They're one-piece,
epoxy-encapsulated units, and are very easy to use.
http://www.paktron.com/pdf/Quencharc_QRL.pdf
If all else fails, remember that physical distance is also helpful.
Do what you can to place the arcing contact as far away as possible
from sensitive circuitry.
Good luck
Chris
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Author: leggDate: 07:52 26-02-08
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On Tue, 26 Feb 2008 11:00:51 +0200, "michael nikolaou"
<michaelnikolaou_remove_me_@yahoo.com> wrote:
>Hi
>
>I have a 12 v relay driving an large 220 volt AC relay . Across the contact
>of the driver relay i placed one RC snubber circut (27NF with 100 R
>resisitor in series) to help with some spikes that were influencing the low
>voltage driver circuits.
>The driver circuit is able to detect mains zero crossing and fire the
>driver relay at an angle i choose .
>From what i read the best point to switch off the power relay is at zero
>crossing . I did that and i show a large spike up to 1 KV at the relay
>contact followed by a decaying 500hz waveform to 0 volts . After some
>experimentation the best point came exactly when switching off at the peak
>of the mains voltage .At this point there is smooth decaying waveform to 0
>volt after 5 periods of 500 HZ but no overshoot. The relay presents no
>arcing. If i remove the snubber and make the experiment the best place to
>switch is zero crossing but i also see large SHARP spikes up to 500 Volts
>Peak.
>My question is
> The switching with snubber must be made at zero crossing or at the peak of
>an ac voltage waveform ?
>What is the behaviour of the circuit ?.
>As i understand any large spikes can harm the X2 capacitor i'm using so
>what is the best operating practise ?.
>
>Any help will be appreciated
>
> Michael
>
If your load is inductive, the current lags the voltage by 90 degrees.
Generally, to avoid switch stress, you would try to switch inductive
loads at zero current, capacitive loads at zero voltage.
RL
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Author: sycochknDate: 08:14 26-02-08
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"michael nikolaou" <michaelnikolaou_remove_me_@yahoo.com> wrote in
message
news:fq0kg4$1n3d$1@ulysses.noc.ntua.gr...
> Hi
>
> I have a 12 v relay driving an large 220 volt AC relay . Across the
> contact of the driver relay i placed one RC snubber circut (27NF with 100
> R resisitor in series) to help with some spikes that were influencing the
> low voltage driver circuits.
> The driver circuit is able to detect mains zero crossing and fire the
> driver relay at an angle i choose .
> From what i read the best point to switch off the power relay is at zero
> crossing . I did that and i show a large spike up to 1 KV at the relay
> contact followed by a decaying 500hz waveform to 0 volts . After some
> experimentation the best point came exactly when switching off at the peak
> of the mains voltage .At this point there is smooth decaying waveform to
> 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents
> no arcing. If i remove the snubber and make the experiment the best place
> to switch is zero crossing but i also see large SHARP spikes up to 500
> Volts Peak.
> My question is
> The switching with snubber must be made at zero crossing or at the peak of
> an ac voltage waveform ?
> What is the behaviour of the circuit ?.
> As i understand any large spikes can harm the X2 capacitor i'm using so
> what is the best operating practise ?.
>
> Any help will be appreciated
>
> Michael
>
>
Use a solid state releay and dont worry about it.
Bob
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Author: PieterDate: 08:46 26-02-08
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What chris writes really makes sense. I am adding my extra's here too.
On Tue, 26 Feb 2008 04:43:44 -0800 (PST), Chris <cfoley1064@yahoo.com>
wrote:
>On Feb 26, 3:00 am, "michael nikolaou"
><michaelnikolaou_remove_...@yahoo.com> wrote:
>> Hi
>>
>> I have a 12 v relay driving an large 220 volt AC relay . Across the contact
>> of the driver relay i placed one RC snubber circut (27NF with 100 R
>> resisitor in series) to help with some spikes that were influencing the low
>> voltage driver circuits.
>> The driver circuit is able to detect mains zero crossing and fire the
>> driver relay at an angle i choose .
>> From what i read the best point to switch off the power relay is at zero
>> crossing . I did that and i show a large spike up to 1 KV at the relay
>> contact followed by a decaying 500hz waveform to 0 volts . After some
>> experimentation the best point came exactly when switching off at the peak
>> of the mains voltage .At this point there is smooth decaying waveform to 0
>> volt after 5 periods of 500 HZ but no overshoot. The relay presents no
>> arcing. If i remove the snubber and make the experiment the best place to
>> switch is zero crossing but i also see large SHARP spikes up to 500 Volts
>> Peak.
>> My question is
>> The switching with snubber must be made at zero crossing or at the peak of
>> an ac voltage waveform ?
>> What is the behaviour of the circuit ?.
>> As i understand any large spikes can harm the X2 capacitor i'm using so
>> what is the best operating practise ?.
>>
>> Any help will be appreciated
>>
>> Michael
>
>Hi, Michael. First off, you should spec a 12V relay that's made to
>handle inductive loads (you can see a HP rating). This type of relay
>has contacts which open more quickly, and are farther apart when
>open. A small standard relay might not even open up far enough to
>stop an inductive arc at line voltage.
Very true!
>Next, when using a relay to drive a relay, you have to be aware of the
>delay-on-make, which can be several milliseconds, especially for
>larger relays. That might help explain some of the curious results
>you're getting. Turn-on delay can be affected by wear and aging. It
>also varies from unit to unit, even in relays with the same part and
>lot number. Trying to get this kind of timing accuracy might be the
>wrong way to go.
There also is a relay-off delay. And the RC snubber makes the turn-off
delay even worse as the current will run a little longer. Expect
something here between 5 to 20 milliseconds.
>It might be better to take a look at suppressing the arc, which you've
>already started to do. Here's an intuitive way to start. First off,
>remember your basic goal: you want the voltage across the contacts to
>rise just slowly enough so the contacts can pull away without
>sustaining an arc. That's all.
What you need to know is the current running through the contacts, and
the voltage spike you want to allow. What a snubber does is store the
energy of the coil in the capacitor. The resistors "eats" this up as
the capacitor discharges over the load. The voltagespike at turnoff
(aasuming an empty capacitor at that moment) is the load current *
resistor. So if you have R= 100, C= 47n, I=2A, you get a voltage spike
of 100*2 = 200 volt even before your capacitor charges. The load
inductance with the current give the stored energy: Q=L*I=C*U. So the
capacitor size should match the load inductance, otherwise you get a
high voltage there too. The capacitor will for example be loaded to
100 volts with a Q = 27n*100V = 2,7 uCoulomb. An inductance that
contains this would be L=Q/I= 2,7uC/2A = 1,35 uH. But the resistor
already "eats" up a lot, so the voltage will be lower. When you turn
off larger motors, transformers, inductors (the coil of a large
relay!), the capacitor must match the load to keep voltages limited.
The prevent oscillations, resistance values must not be too low,
ususally between 30 to 100 Ohm is ok. For large inductive loads I
would not take 100 Ohm but 47 Ohm as normal value (see below what
Chris wrote), you contact must ve able to handle that. The capacitance
could be anything you want, for larger devices 220n, 470n etc.
>Remove the cover from the driving relay so you can see the contacts.
>Next, find the current rating of those contacts, and use Ohm's law to
>find a resistor which will result in about half that current.
>
>For example, if you have a 220VAC load, and your relay can handle
>10A,:
>
>R = 220V / 5A = 44 ohms
>
>Choose a 47 ohm, 1 watt resistor here (carbon comp is best). Now get
>a selection of self-healing AC line-rated capacitors, and switch the
>inductive load with the 47 ohm and C snubber directly across the load,
>repeating and increasing the cap value until the contact arcing
>disappears, or at least is minimized. Without knowing anything about
>your 220VAC relay, it sounds like your 0.027uF cap might be on the
>small side.
Also very true: the RC snubber also gives a current peak when you turn
ON the relay. Nice thing about inductive loads is that the current
does not rise fast, so the load and RC current do not occure exactly
at the same time (also depending on RC, where a smaller R gives a
higher rush-in current but also a better timing).
The peak where the snubber works is at turning the relay OFF again.
What I often use is 47 Ohm to 100 Ohm with 100 nF.
>Note that ITW Paktron makes a series of Quencharc snubbers that you
>can just plug in, which makes selection a snap. They're one-piece,
>epoxy-encapsulated units, and are very easy to use.
>
>http://www.paktron.com/pdf/Quencharc_QRL.pdf
>
>If all else fails, remember that physical distance is also helpful.
>Do what you can to place the arcing contact as far away as possible
>from sensitive circuitry.
>
>Good luck
>Chris
You need TWO RC networks: one across the driving relay, one across the
large relay, and do not forget the diode (or also RC) across the coil
of the driving relay. So you have 3 things that need attention.
Do not forget that long cables (also with resistive loads like lamps)
act as inductance. 10 meters or more may also need a RC!
Regards,
Pieter
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Author: Fred BloggsDate: 09:22 26-02-08
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>
> I have a 12 v relay driving an large 220 volt AC relay . Across the contact
> of the driver relay i placed one RC snubber circut (27NF with 100 R
> resisitor in series) to help with some spikes that were influencing the low
> voltage driver circuits.
> The driver circuit is able to detect mains zero crossing and fire the
> driver relay at an angle i choose .
> From what i read the best point to switch off the power relay is at zero
> crossing . I did that and i show a large spike up to 1 KV at the relay
> contact followed by a decaying 500hz waveform to 0 volts . After some
> experimentation the best point came exactly when switching off at the peak
> of the mains voltage .At this point there is smooth decaying waveform to 0
> volt after 5 periods of 500 HZ but no overshoot. The relay presents no
> arcing. If i remove the snubber and make the experiment the best place to
> switch is zero crossing but i also see large SHARP spikes up to 500 Volts
> Peak.
WTF are you talking about, the arc across the driver relay contacts or
the arc across the "large 220 volt AC relay?" It gets reall aggrvating
when you don't make yourself clear...
> My question is
> The switching with snubber must be made at zero crossing or at the peak of
> an ac voltage waveform ?
> What is the behaviour of the circuit ?.
> As i understand any large spikes can harm the X2 capacitor i'm using so
> what is the best operating practise ?.
>
Your whole idea is dumb anyway, you're working with relays and pull-in
times that are a substantial fraction of line voltage period and vary
with age and operating conditions. Snubbers are spedc'd around worst
case and not timing. They are only an approximation, the inductance of
the "large 220 volt AC relay" is non-linear and may differ by 10:1
between pulled in and out. And some relays require an arc for contact
longevity. You let the relay manufacturer take care of the switching and
just work out a better 12V drive circuit immune to the dV/dt current
from that HV ringing, usually means lower impedance drive.
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Author: Sjouke BurryDate: 12:33 26-02-08
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michael nikolaou wrote:
> Hi
>
> I have a 12 v relay driving an large 220 volt AC relay . Across the contact
> of the driver relay i placed one RC snubber circut (27NF with 100 R
> resisitor in series) to help with some spikes that were influencing the low
> voltage driver circuits.
> The driver circuit is able to detect mains zero crossing and fire the
> driver relay at an angle i choose .
> From what i read the best point to switch off the power relay is at zero
> crossing . I did that and i show a large spike up to 1 KV at the relay
> contact followed by a decaying 500hz waveform to 0 volts . After some
> experimentation the best point came exactly when switching off at the peak
> of the mains voltage .At this point there is smooth decaying waveform to 0
> volt after 5 periods of 500 HZ but no overshoot. The relay presents no
> arcing. If i remove the snubber and make the experiment the best place to
> switch is zero crossing but i also see large SHARP spikes up to 500 Volts
> Peak.
> My question is
> The switching with snubber must be made at zero crossing or at the peak of
> an ac voltage waveform ?
> What is the behaviour of the circuit ?.
> As i understand any large spikes can harm the X2 capacitor i'm using so
> what is the best operating practise ?.
>
> Any help will be appreciated
>
> Michael
>
>
Dont switch off at zero voltage, but at zero current.
So monitor across a small sense resistor, and wait for
zero across that.
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Author: michael nikolaouDate: 12:39 26-02-08
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Thank guys for your replies .Some of them i have to study first
Let me make some things clear about the circuit and values chosen
1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All
results are
after calculating this delay .So what is see on the scope is at the moment
i'm explaining
2. the arc is across the driver relay .The power board is inside a control
unit box so
i have to leave with small distances and cpu disturbances.Its actually a
microcontroller
having the problem .Driver relay contact current rating is 5A at 220V
.Power relay coil is rated is 6
watts consumption at 220V.
3. Using large value capacitors over 33 nf was causing sometimes latch of
the power relay so i have
value limitation here
4. The capacitors i've chosen are X2 self healing 275VAC. With no ZC control
they are blown
to 0 nf value (some of them) after 10-12 months of operation.
5. I don't have the space or budget to use large sized capacitors rated at
higher voltages or SSR .
The idea was to use ZC to avoid using expensive and large size protection
snubber
So the question is .
Does the relay On/OFF time differs with time .If it's 10% it's not a
problem since again
the arc will not be so high .Since its the current break that causes the
arc i must switch off at Peak of the
ac voltage .This is what my reading confirmed .In this case switching a
resistive load must i change the driver
algorithm ???
Any helpful comments will be apreciated
"Fred Bloggs" <nospam@nospam.com> wrote in message
news:47C42089.3040701@nospam.com...
>
>>
>> I have a 12 v relay driving an large 220 volt AC relay . Across the
>> contact of the driver relay i placed one RC snubber circut (27NF with 100
>> R resisitor in series) to help with some spikes that were influencing the
>> low voltage driver circuits.
>> The driver circuit is able to detect mains zero crossing and fire the
>> driver relay at an angle i choose .
>> From what i read the best point to switch off the power relay is at zero
>> crossing . I did that and i show a large spike up to 1 KV at the relay
>> contact followed by a decaying 500hz waveform to 0 volts . After some
>> experimentation the best point came exactly when switching off at the
>> peak of the mains voltage .At this point there is smooth decaying
>> waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The
>> relay presents no arcing. If i remove the snubber and make the
>> experiment the best place to switch is zero crossing but i also see large
>> SHARP spikes up to 500 Volts Peak.
>
> WTF are you talking about, the arc across the driver relay contacts or the
> arc across the "large 220 volt AC relay?" It gets reall aggrvating when
> you don't make yourself clear...
>
>> My question is
>> The switching with snubber must be made at zero crossing or at the peak
>> of an ac voltage waveform ?
>> What is the behaviour of the circuit ?.
>> As i understand any large spikes can harm the X2 capacitor i'm using so
>> what is the best operating practise ?.
>>
>
> Your whole idea is dumb anyway, you're working with relays and pull-in
> times that are a substantial fraction of line voltage period and vary with
> age and operating conditions. Snubbers are spedc'd around worst case and
> not timing. They are only an approximation, the inductance of the "large
> 220 volt AC relay" is non-linear and may differ by 10:1 between pulled in
> and out. And some relays require an arc for contact longevity. You let the
> relay manufacturer take care of the switching and just work out a better
> 12V drive circuit immune to the dV/dt current from that HV ringing,
> usually means lower impedance drive.
>
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Author: michael nikolaouDate: 13:18 26-02-08
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can you be more specific about the sense resistor . My circuit creates 500
micosecond pulse at every 50 hz zero voltage crossing.
At this pulse edge i activate relay considering turn-off / off delay to
calculate the peak voltage so 0 current for my inductive load .
Do you have something different in mind?
"Sjouke Burry" <burrynulnulfour@ppllaanneett.nnlll> wrote in message
news:47c44d7e$0$25500$ba620dc5@text.nova.planet.nl...
> michael nikolaou wrote:
>> Hi
>>
>> I have a 12 v relay driving an large 220 volt AC relay . Across the
>> contact of the driver relay i placed one RC snubber circut (27NF with 100
>> R resisitor in series) to help with some spikes that were influencing the
>> low voltage driver circuits.
>> The driver circuit is able to detect mains zero crossing and fire the
>> driver relay at an angle i choose .
>> From what i read the best point to switch off the power relay is at zero
>> crossing . I did that and i show a large spike up to 1 KV at the relay
>> contact followed by a decaying 500hz waveform to 0 volts . After some
>> experimentation the best point came exactly when switching off at the
>> peak of the mains voltage .At this point there is smooth decaying
>> waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The
>> relay presents no arcing. If i remove the snubber and make the
>> experiment the best place to switch is zero crossing but i also see large
>> SHARP spikes up to 500 Volts Peak.
>> My question is
>> The switching with snubber must be made at zero crossing or at the peak
>> of an ac voltage waveform ?
>> What is the behaviour of the circuit ?.
>> As i understand any large spikes can harm the X2 capacitor i'm using so
>> what is the best operating practise ?.
>>
>> Any help will be appreciated
>>
>> Michael
>>
>>
> Dont switch off at zero voltage, but at zero current.
> So monitor across a small sense resistor, and wait for
> zero across that.
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Author: Sjouke BurryDate: 14:18 26-02-08
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michael nikolaou wrote:
> can you be more specific about the sense resistor . My circuit creates 500
> micosecond pulse at every 50 hz zero voltage crossing.
> At this pulse edge i activate relay considering turn-off / off delay to
> calculate the peak voltage so 0 current for my inductive load .
> Do you have something different in mind?
> "Sjouke Burry" <burrynulnulfour@ppllaanneett.nnlll> wrote in
message
> news:47c44d7e$0$25500$ba620dc5@text.nova.planet.nl...
>> michael nikolaou wrote:
>>> Hi
>>>
>>> I have a 12 v relay driving an large 220 volt AC relay . Across the
>>> contact of the driver relay i placed one RC snubber circut (27NF with 100
>>> R resisitor in series) to help with some spikes that were influencing the
>>> low voltage driver circuits.
>>> The driver circuit is able to detect mains zero crossing and fire the
>>> driver relay at an angle i choose .
>>> From what i read the best point to switch off the power relay is at zero
>>> crossing . I did that and i show a large spike up to 1 KV at the relay
>>> contact followed by a decaying 500hz waveform to 0 volts . After some
>>> experimentation the best point came exactly when switching off at the
>>> peak of the mains voltage .At this point there is smooth decaying
>>> waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The
>>> relay presents no arcing. If i remove the snubber and make the
>>> experiment the best place to switch is zero crossing but i also see large
>>> SHARP spikes up to 500 Volts Peak.
>>> My question is
>>> The switching with snubber must be made at zero crossing or at the peak
>>> of an ac voltage waveform ?
>>> What is the behaviour of the circuit ?.
>>> As i understand any large spikes can harm the X2 capacitor i'm using so
>>> what is the best operating practise ?.
>>>
>>> Any help will be appreciated
>>>
>>> Michael
>>>
>>>
>> Dont switch off at zero voltage, but at zero current.
>> So monitor across a small sense resistor, and wait for
>> zero across that.
>
>
Just use a comparator IC to test the current, how this fits into
your design, is hard to see from this distance.
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Author: leggDate: 17:37 26-02-08
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On Tue, 26 Feb 2008 19:39:06 +0200, "michael nikolaou"
<michaelnikolaou_remove_me_@yahoo.com> wrote:
>Thank guys for your replies .Some of them i have to study first
>
>Let me make some things clear about the circuit and values chosen
>
>1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All
>results are
>after calculating this delay .So what is see on the scope is at the moment
>i'm explaining
>2. the arc is across the driver relay .The power board is inside a control
>unit box so
> i have to leave with small distances and cpu disturbances.Its actually a
>microcontroller
> having the problem .Driver relay contact current rating is 5A at 220V
>.Power relay coil is rated is 6
>watts consumption at 220V.
>3. Using large value capacitors over 33 nf was causing sometimes latch of
>the power relay so i have
>value limitation here
>4. The capacitors i've chosen are X2 self healing 275VAC. With no ZC control
>they are blown
> to 0 nf value (some of them) after 10-12 months of operation.
>5. I don't have the space or budget to use large sized capacitors rated at
>higher voltages or SSR .
>The idea was to use ZC to avoid using expensive and large size protection
>snubber
>So the question is .
>
>Does the relay On/OFF time differs with time .If it's 10% it's not a
>problem since again
>the arc will not be so high .Since its the current break that causes the
>arc i must switch off at Peak of the
>ac voltage .This is what my reading confirmed .In this case switching a
>resistive load must i change the driver
>algorithm ???
However the timing is controlled, yes, that's probably what you need
to do.
The larger relay coil will be rated for power consumption in the
continuous active condition - with the armature closed and inductance
fairly high. The inductance limits the current flow that generates
power loss in the coil, if your phase angle observations are correct.
It's therefor possible that current in the coil is higher than a
guestimate (using the 6W label) might produce. What's the DC
resistance of the bigger coil? What's the actual coil current with the
voltage applied?
Larger currents could account for the control contact arcing and large
voltages that you see with 100R snubber....unless the resistor is open
circuit or intermittent.
RL
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Author: cadmanDate: 17:58 26-02-08
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I have used a triac assisted relay before.
The triac shorts out the relay contacts just before the relay is
switched on and off.
Because the relay is no longer swicthing current they pretty much last
forever.
You can get away with a small triac as its not on for long.
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Author: john jardineDate: 18:27 26-02-08
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"michael nikolaou" <michaelnikolaou_remove_me_@yahoo.com> wrote in
message
news:fq1is0$2i0f$1@ulysses.noc.ntua.gr...
> Thank guys for your replies .Some of them i have to study first
>
> Let me make some things clear about the circuit and values chosen
>
> 1. I've measured turn-on , turn-off delay at 3.3 ms for the driver
relay.All
> results are
> after calculating this delay .So what is see on the scope is at the moment
> i'm explaining
> 2. the arc is across the driver relay .The power board is inside a control
> unit box so
> i have to leave with small distances and cpu disturbances.Its actually
a
> microcontroller
> having the problem .Driver relay contact current rating is 5A at 220V
> .Power relay coil is rated is 6
> watts consumption at 220V.
[ ... ]
From your info', my guesstimate says the big relay has a 8kohm, 4H coil (I'm
working from a UK 50Hz). This means the current running through it will be
nearly in phase with the voltage across it.
Let's say the current is running about 1mS late, then add on the 3.3mS delay
for the relay to pull in and it seems not unreasonable that you need to be
switching ON about 4-5 mS from the sensed 0V Xover to give minimum
transients. This -coincidentally- just happens to be at the peak AC voltage
point.
A different relay may well have bigger inductance and lower resistance,
these would need measuring and the Arrctan thing doing to get a new phase
angle/delay for the current.
If you put say a 10k ohm 2Watt resistor in // with the power relay coil this
will be effective in quickly quenching the relay coil energy. Or better
still, fitting a Varistor will simply clip the transient to whatever it's
rated voltage is.
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Author: JamieDate: 20:20 26-02-08
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michael nikolaou wrote:
> Hi
>
> I have a 12 v relay driving an large 220 volt AC relay . Across the contact
> of the driver relay i placed one RC snubber circut (27NF with 100 R
> resisitor in series) to help with some spikes that were influencing the low
> voltage driver circuits.
> The driver circuit is able to detect mains zero crossing and fire the
> driver relay at an angle i choose .
> From what i read the best point to switch off the power relay is at zero
> crossing . I did that and i show a large spike up to 1 KV at the relay
> contact followed by a decaying 500hz waveform to 0 volts . After some
> experimentation the best point came exactly when switching off at the peak
> of the mains voltage .At this point there is smooth decaying waveform to 0
> volt after 5 periods of 500 HZ but no overshoot. The relay presents no
> arcing. If i remove the snubber and make the experiment the best place to
> switch is zero crossing but i also see large SHARP spikes up to 500 Volts
> Peak.
> My question is
> The switching with snubber must be made at zero crossing or at the peak of
> an ac voltage waveform ?
> What is the behaviour of the circuit ?.
> As i understand any large spikes can harm the X2 capacitor i'm using so
> what is the best operating practise ?.
>
> Any help will be appreciated
>
> Michael
>
WHen you say relay, I assume you mean a mechanical contact?
if so, It takes time for the contacts the release. If you turn it
off at what you detect as the zero crossing point, the contacts most
likely will not actually release until some where in some mid point .
Many contactors are fast but not fast enough to open before current
can get a charge going.
That's just my evaluation of what you're doing.
By you signaling to turn off the relay at a peak, the contactor will
most likely not open until it gets near the zero crossing point.
--
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Author: ChrisDate: 22:49 26-02-08
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On Feb 26, 7:14=A0am, "sycochkn" <sycoc...@earthlink.net> wrote:
> "michael nikolaou" <michaelnikolaou_remove_...@yahoo.com> wrote in
message=
>
> news:fq0kg4$1n3d$1@ulysses.noc.ntua.gr...
>
>
>
>
>
> > Hi
>
> > I have a 12 v relay driving an large 220 volt AC relay . Across the
> > contact of the driver relay i placed one RC snubber circut (27NF with 10=
0
> > R resisitor in series) to help with some spikes that were influencing th=
e
> > low voltage driver circuits.
> > The driver circuit is able to detect =A0mains zero crossing and fire the=
> > driver relay at an angle i choose .
> > From what i read the best point =A0to switch off the power relay is at z=
ero
> > crossing . I did that and i show a large spike up to 1 KV =A0at the rela=
y
> > contact followed by a decaying 500hz waveform to 0 volts . After some
> > experimentation the best point came exactly when switching off at the pe=
ak
> > of the mains voltage .At this point there is smooth decaying waveform =
=A0to
> > 0 volt after 5 periods of =A0 =A0500 HZ =A0but no overshoot. The relay p=
resents
> > no arcing. =A0If i remove the snubber and make the experiment the best p=
lace
> > to switch is zero crossing but i also see large SHARP spikes up to 500
> > Volts Peak.
> > My question is
> > The switching with snubber must be made at zero crossing or at the peak =
of
> > an ac voltage waveform ?
> > What is the behaviour of the circuit ?.
> > As i understand any large =A0spikes can harm the X2 capacitor i'm using =
so
> > what is the best operating practise ?.
>
> > Any help will be appreciated
>
> > Michael
>
> Use a solid state releay and dont worry about it.
>
> Bob- Hide quoted text -
>
> - Show quoted text -
Right. If you have a resistive load for the final 220VAC load, a
plain old solid state relay will work fine (be sure to heat-sink the
SSR at about 1.5 watts per amp load). If you have an inductive load,
you may want to spec a SSR made to switch these loads, which have back-
to-back SCRs to eliminate the possibility of not being able to turn
off the SSR).
If this is a class project, you won't lose any points by going for the
simple solution, as long as it also happens to be the best one.
Good luck
Chris
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On 27 =D6=E5=E2, 03:24, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
> michael nikolaou wrote:
> > Hi
>
> > I have a 12 v relay driving an large 220 volt AC relay . Across the cont=
act
> > of the driver relay i placed one RC snubber circut (27NF with 100 R
> > resisitor in series) to help with some spikes that were influencing the =
low
> > voltage driver circuits.
> > The driver circuit is able to detect =A0mains zero crossing and fire the=
> > driver relay at an angle i choose .
> > From what i read the best point =A0to switch off the power relay is at z=
ero
> > crossing . I did that and i show a large spike up to 1 KV =A0at the rela=
y
> > contact followed by a decaying 500hz waveform to 0 volts . After some
> > experimentation the best point came exactly when switching off at the pe=
ak
> > of the mains voltage .At this point there is smooth decaying waveform =
=A0to 0
> > volt after 5 periods of =A0 =A0500 HZ =A0but no overshoot. The relay pre=
sents no
> > arcing. =A0If i remove the snubber and make the experiment the best plac=
e to
> > switch is zero crossing but i also see large SHARP spikes up to 500 Volt=
s
> > Peak.
> > My question is
> > =A0The switching with snubber must be made at zero crossing or at the pe=
ak of
> > an ac voltage waveform ?
> > What is the behaviour of the circuit ?.
> > As i understand any large =A0spikes can harm the X2 capacitor i'm using =
so
> > what is the best operating practise ?.
>
> > Any help will be appreciated
>
> > =A0Michael
>
> WHen you say relay, I assume you mean a mechanical contact?
>
> =A0 if so, It takes time for the contacts the release. If you turn it
> =A0 off at what you detect as the zero crossing point, the contacts most
> likely will not actually release until some where in some mid point .
> =A0 =A0Many contactors are fast but not fast enough to open before current=
> can get a charge going.
> =A0 =A0 That's just my evaluation of what you're doing.
>
> =A0 =A0By you signaling to turn off the relay at a peak, the contactor wil=
l
> most likely not open until it gets near the zero crossing point.
>
> --- =C1=F0=FC=EA=F1=F5=F8=E7 =EA=E5=E9=EC=DD=ED=EF=F5 =F3=E5 =F0=E1=F1=DC=
=E8=E5=F3=E7 -
>
> - =C5=EC=F6=DC=ED=E9=F3=E7 =EA=E5=E9=EC=DD=ED=EF=F5 =F3=E5 =F0=E1=F1=DC=E8=
=E5=F3=E7 -
I observe all signals with a scope . I see actually the "driver relay"
signal .So all my observations
about timing are correct . My notion was different though .I expected
to turn off at zero crossing
voltage and have no arc .Actualy when the transition happents at the
peak of voltage then i see only a
decaying waveform ,no overshoot, for which i suspect since i know the
capacitor and measure the
frequency one could calculate the total inductance value (cables+relay
coil). Since its clear that V(emf)=3D-L*di/Dt
as pointed out by your emails also then the correct point to switch
off the driver relay is at peak voltage
since current lags voltage by 90 deg in any inductor.
Now i only have to find out how much can a relay type deviate from the
measurements i have made.
Thank's for all you help guys
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Author: OppieDate: 11:19 27-02-08
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I have had some occasions using a solid state relay to switch a large
contactor. We found that a random turn-on relay was as good as a zero
crossing turn-on (and cost a bit less too). Turn-off with a solid state
relay will be at the next zero *current* crossing regardless of the type of
SSR used. As to snubbing, I found that a snubber across the contactor's coil
was more significant than one across the SSR. A secondary snubber can be
applied across the SSR if still needed.
The main purpose of snubbing with a SSR is to control the slew rate and peak
voltage across the SSR's switching element (traic or scr) to keep it below
the point of causing a spurrious trigger/conduction cycle. You should also
be mindful that triacs are more sensitive to triggering in certain quadrants
and may lead to an asymetrical output conduction (aka a net DC component)
which can cause inductive loads to saturate.
Oppie
"michael nikolaou" <michaelnikolaou_remove_me_@yahoo.com> wrote in
message
news:fq0kg4$1n3d$1@ulysses.noc.ntua.gr...
> Hi
>
> I have a 12 v relay driving an large 220 volt AC relay . Across the
> contact of the driver relay i placed one RC snubber circut (27NF with 100
> R resisitor in series) to help with some spikes that were influencing the
> low voltage driver circuits.
> The driver circuit is able to detect mains zero crossing and fire the
> driver relay at an angle i choose .
> From what i read the best point to switch off the power relay is at zero
> crossing . I did that and i show a large spike up to 1 KV at the relay
> contact followed by a decaying 500hz waveform to 0 volts . After some
> experimentation the best point came exactly when switching off at the peak
> of the mains voltage .At this point there is smooth decaying waveform to
> 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents
> no arcing. If i remove the snubber and make the experiment the best place
> to switch is zero crossing but i also see large SHARP spikes up to 500
> Volts Peak.
> My question is
> The switching with snubber must be made at zero crossing or at the peak of
> an ac voltage waveform ?
> What is the behaviour of the circuit ?.
> As i understand any large spikes can harm the X2 capacitor i'm using so
> what is the best operating practise ?.
>
> Any help will be appreciated
>
> Michael
>
>
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Author: Fred BloggsDate: 16:00 27-02-08
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> Thank guys for your replies .Some of them i have to study first
>
> Let me make some things clear about the circuit and values chosen
>
> 1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All
> results are
> after calculating this delay .So what is see on the scope is at the moment
> i'm explaining
> 2. the arc is across the driver relay .The power board is inside a control
> unit box so
> i have to leave with small distances and cpu disturbances.Its actually a
> microcontroller
> having the problem .Driver relay contact current rating is 5A at 220V
> .Power relay coil is rated is 6
> watts consumption at 220V.
> 3. Using large value capacitors over 33 nf was causing sometimes latch of
> the power relay so i have
> value limitation here
> 4. The capacitors i've chosen are X2 self healing 275VAC. With no ZC control
> they are blown
> to 0 nf value (some of them) after 10-12 months of operation.
> 5. I don't have the space or budget to use large sized capacitors rated at
> higher voltages or SSR .
> The idea was to use ZC to avoid using expensive and large size protection
> snubber
> So the question is .
>
> Does the relay On/OFF time differs with time .If it's 10% it's not a
> problem since again
> the arc will not be so high .Since its the current break that causes the
> arc i must switch off at Peak of the
> ac voltage .This is what my reading confirmed .In this case switching a
> resistive load must i change the driver
> algorithm ???
>
> Any helpful comments will be apreciated
>
See section 15.2.12.5 and in particular Circuit B of figure 15.6 in the
link below. You will want 450WVDC electrolytics and 600PIV rated diodes
along with possibly a small inrush limiting resistor in series with the
whole lot. The idea is to dynamically form a peak clamp circuit and this
has been used in many high end line surge protection devices for many
years with good success. The zero crossing helps a bit but is largely a
waste of time.
http://www.leachintl2.com/english/english2/vol6/properties/00053.html
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Author: PieterDate: 18:08 27-02-08
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On Tue, 26 Feb 2008 18:33:50 +0100, Sjouke Burry
<burrynulnulfour@ppllaanneett.nnlll> wrote:
>michael nikolaou wrote:
>> Hi
>>
>> I have a 12 v relay driving an large 220 volt AC relay . Across the contact
>> of the driver relay i placed one RC snubber circut (27NF with 100 R
>> resisitor in series) to help with some spikes that were influencing the low
>> voltage driver circuits.
>> The driver circuit is able to detect mains zero crossing and fire the
>> driver relay at an angle i choose .
>> From what i read the best point to switch off the power relay is at zero
>> crossing . I did that and i show a large spike up to 1 KV at the relay
>> contact followed by a decaying 500hz waveform to 0 volts . After some
>> experimentation the best point came exactly when switching off at the peak
>> of the mains voltage .At this point there is smooth decaying waveform to 0
>> volt after 5 periods of 500 HZ but no overshoot. The relay presents no
>> arcing. If i remove the snubber and make the experiment the best place to
>> switch is zero crossing but i also see large SHARP spikes up to 500 Volts
>> Peak.
>> My question is
>> The switching with snubber must be made at zero crossing or at the peak of
>> an ac voltage waveform ?
>> What is the behaviour of the circuit ?.
>> As i understand any large spikes can harm the X2 capacitor i'm using so
>> what is the best operating practise ?.
>>
>> Any help will be appreciated
>>
>> Michael
>>
>>
>Dont switch off at zero voltage, but at zero current.
>So monitor across a small sense resistor, and wait for
>zero across that.
The relay is too slow. So you need a delay to switch off at the NEXT
crossing.
Pieter.
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