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Sci.Electronics.Basics -> 240V relay contact suppression

There are 12 messages in this thread.
You are currently looking at messages 1 to 12.


Author: MarkMc
Date: 14:02 21-08-06

Hi

I'm using a relay with 240VAC coil and 240VAC contacts. From what I've
been advised before with relays (not 240VAC), I understand it's a good
idea to protect the contacts with a capacitor and possibly a MOV like
below;

Is this required for AC and is it a good idea to have a MOV?

Also, I assume I need do nothing on the coil side, seeing as it's AC?
Normally, I'd expect a reverse biased diode in there somewhere.

Regards,
Mark




240VAC(in)
o o
| |
|L N|
| |
| |
| |
)| o |
RY )| \ |
_)| o |
| ___ || |
|-|___|--||-|
| || |
| LCR |
.-. |
| |MOV |
| | |
'-' |
| |
o o

240VAC(out)
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)
Author: Stanislaw Flatto
Date: 15:21 21-08-06


MarkMc wrote:
> Hi
>
> I'm using a relay with 240VAC coil and 240VAC contacts. From what I've
> been advised before with relays (not 240VAC), I understand it's a good
> idea to protect the contacts with a capacitor and possibly a MOV like
> below;
>
> Is this required for AC and is it a good idea to have a MOV?

If the relay is just replacement of manual switch for equipment designed
for line supply then don't improve on it. If the load is sensitive then
add whatever is needed.
>
> Also, I assume I need do nothing on the coil side, seeing as it's AC?

Right.

> Normally, I'd expect a reverse biased diode in there somewhere.

What is "normally"? On DC the diode allows (shorts) the magnetic field
for faster collapse and faster opening of the contacts to eliminate
"welder" spark between them. On AC the spark can at max be 10msec.(50Hz)
till next zero voltage crossing of the line.
>
> Regards,
> Mark

Cheers

Stanislaw
Author: John Fields
Date: 18:09 21-08-06

On Mon, 21 Aug 2006 19:21:25 GMT, Stanislaw Flatto
<compaid@shoalhaven.net.au> wrote:

>MarkMc wrote:
>> Hi
>>
>> I'm using a relay with 240VAC coil and 240VAC contacts. From what I've
>> been advised before with relays (not 240VAC), I understand it's a good
>> idea to protect the contacts with a capacitor and possibly a MOV like
>> below;
>>
>> Is this required for AC and is it a good idea to have a MOV?
>
>If the relay is just replacement of manual switch for equipment designed
>for line supply then don't improve on it. If the load is sensitive then
>add whatever is needed.
>>
>> Also, I assume I need do nothing on the coil side, seeing as it's AC?
>
>Right.
>
>> Normally, I'd expect a reverse biased diode in there somewhere.
>
>What is "normally"?

---
Normally is almost always.
---

>On DC the diode allows (shorts) the magnetic field
>for faster collapse and faster opening of the contacts to eliminate
>"welder" spark between them.

---
Actually, no.

The diode provides a path to Vcc for the high-voltage spike
generated when the relay driver turns off, in order to protect the
driver. The price, though, is that the current won't only flow
through the diode, it'll also flow through the coil (since that's
where it comes from) until it's dissipated to the point where it can
no longer hold the armature closed. That means that the relay will
stay closed longer than if the diode wasn't there, but the driver
won't be destroyed.


--
John Fields
Professional Circuit Designer

Author: Stanislaw Flatto
Date: 18:44 21-08-06

John Fields wrote:

> Actually, no.
>
> The diode provides a path to Vcc for the high-voltage spike
> generated when the relay driver turns off, in order to protect the
> driver. The price, though, is that the current won't only flow
> through the diode, it'll also flow through the coil (since that's
> where it comes from) until it's dissipated to the point where it can
> no longer hold the armature closed. That means that the relay will
> stay closed longer than if the diode wasn't there, but the driver
> won't be destroyed.
>
>

Too modern for me. What "driver"?
~1960 AD working on telephone exchanges, relay technology, diodes(?),
were called rectifiers then, selenium plates, and were used (rarely!)
to improve responses by allowing faster collapse of the magnetic field.
Anything changed since?

Have fun

Stanislaw
Author: John Popelish
Date: 18:55 21-08-06

MarkMc wrote:
> Hi
>
> I'm using a relay with 240VAC coil and 240VAC contacts. From what I've
> been advised before with relays (not 240VAC), I understand it's a good
> idea to protect the contacts with a capacitor and possibly a MOV like
> below;
>
> Is this required for AC and is it a good idea to have a MOV?
>
> Also, I assume I need do nothing on the coil side, seeing as it's AC?
> Normally, I'd expect a reverse biased diode in there somewhere.
>
> Regards,
> Mark
>
>
>
>
> 240VAC(in)
> o o
> | |
> |L N|
> | |
> | |
> | |
> )| o |
> RY )| \ |
> _)| o |
> | ___ || |
> |-|___|--||-|
> | || |
> | LCR |
> .-. |
> | |MOV |
> | | |
> '-' |
> | |
> o o
>
> 240VAC(out)
> (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)
>
To protect AC contacts, it is common to add an RC across wither the
load or the contact. You get slightly better protection if it is
across the contact, but more off state leakage current.

The resistor should limit the peak instantaneous current at contact
closure to something like half of the contacts current rating. The
size of the capacitor depends on the inductive energy the load will
dump when its current is interrupted by the contact opening. If you
under size it, it might be damaged by over voltage, unless it and the
resistor are paralleled by an MOV, as a voltage clamp. However, if
the MOV fails shorted by line over voltage (and the network is across
the load), it will probably destroy the contacts. However, if the
network is across the contacts, not only is it much harder to blow the
MOV (because the load impedance limits peak current during a line
voltage spike) but failure shorted just turns the load on.

You cannot put an MOV in series with the load, as you have shown,
because it has a high resistance, except during over voltage
situations. So you would get very little voltage across the load.

I have often used an MOV across AC relay coils, to limit the rate of
rise of voltage (by virtue of the MOV's inherent capacitance) and a
voltage limit by virtue of its voltage dependent resistance. This
will make contacts driving the coil last longer and make less
electrical noise.
Author: Phil Allison
Date: 21:28 21-08-06


"Stanislaw Flatto"
>>John Fields The diode provides a path to Vcc for the high-voltage spike
>> generated when the relay driver turns off, in order to protect the
>> driver. The price, though, is that the current won't only flow
>> through the diode, it'll also flow through the coil (since that's
>> where it comes from) until it's dissipated to the point where it can
>> no longer hold the armature closed. That means that the relay will
>> stay closed longer than if the diode wasn't there, but the driver
>> won't be destroyed.
>>
>>
>
> Too modern for me. What "driver"?


** General term for whatever solid state device switches the coil current on
and off.


> ~1960 AD working on telephone exchanges, relay technology, diodes(?),
> were called rectifiers then, selenium plates, and were used (rarely!)
> to improve responses by allowing faster collapse of the magnetic field.
> Anything changed since?


** The laws of magnetics must have been suspended in that clunky old
exchange.





..... Phil
Author: Jamie
Date: 00:20 22-08-06

MarkMc wrote:
> Hi
>
> I'm using a relay with 240VAC coil and 240VAC contacts. From what I've
> been advised before with relays (not 240VAC), I understand it's a good
> idea to protect the contacts with a capacitor and possibly a MOV like
> below;
>
> Is this required for AC and is it a good idea to have a MOV?
>
> Also, I assume I need do nothing on the coil side, seeing as it's AC?
> Normally, I'd expect a reverse biased diode in there somewhere.
>
> Regards,
> Mark
>
>
>
>
> 240VAC(in)
> o o
> | |
> |L N|
> | |
> | |
> | |
> )| o |
> RY )| \ |
> _)| o |
> | ___ || |
> |-|___|--||-|
> | || |
> | LCR |
> .-. |
> | |MOV |
> | | |
> '-' |
> | |
> o o
>
> 240VAC(out)
> (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)
>
put a snubber across the contacts./ it will suppress some of the
inductive arcing.
normally a .1 cap with a 100 ohm R in series.


--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Author: MarkMc
Date: 04:58 22-08-06

John Popelish wrote:
> You cannot put an MOV in series with the load, as you have shown,
> because it has a high resistance, except during over voltage
> situations. So you would get very little voltage across the load.

Thanks for the advice. I've obviously misunderstood the use of MOV's.

I'll put one (I'll have to check at home to see what rating it is) in
parallel across my RC (LCR - combined 100R and 0.1 Cap)

Regards,
Mark
Author: MarkMc
Date: 05:43 22-08-06

So I'd end up with this?







240VAC(in)
L o o N
| |
--------- |
| | | |
| .-. | |
| | | | |
)| o | | .-. |
RY )| \ '-' | | MOV |
_)| o | | | |
| | '-' |
| --- | |
| --- | |
| | | |
|-------- |
| |
| |
| |
| |
o o

240VAC(out)
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

Regards,
Mark

MarkMc wrote:
> John Popelish wrote:
> > You cannot put an MOV in series with the load, as you have shown,
> > because it has a high resistance, except during over voltage
> > situations. So you would get very little voltage across the load.
>
> Thanks for the advice. I've obviously misunderstood the use of MOV's.
>
> I'll put one (I'll have to check at home to see what rating it is) in
> parallel across my RC (LCR - combined 100R and 0.1 Cap)
>
> Regards,
> Mark
Author: default
Date: 06:50 22-08-06

On Mon, 21 Aug 2006 22:44:44 GMT, Stanislaw Flatto
<compaid@shoalhaven.net.au> wrote:

>John Fields wrote:
>
>> Actually, no.
>>
>> The diode provides a path to Vcc for the high-voltage spike
>> generated when the relay driver turns off, in order to protect the
>> driver. The price, though, is that the current won't only flow
>> through the diode, it'll also flow through the coil (since that's
>> where it comes from) until it's dissipated to the point where it can
>> no longer hold the armature closed. That means that the relay will
>> stay closed longer than if the diode wasn't there, but the driver
>> won't be destroyed.
>>
>>
>
>Too modern for me. What "driver"?
>~1960 AD working on telephone exchanges, relay technology, diodes(?),
>were called rectifiers then, selenium plates, and were used (rarely!)
>to improve responses by allowing faster collapse of the magnetic field.
>Anything changed since?
>
>Have fun
>
>Stanislaw

Solid state transistor drivers are/can be destroyed by the voltage
spike caused by the collapsing magnetic field. Back in the selenium
rectifier era, they used toobs which were unaffected by the spikes.

The diode acts to short the reverse emf developed by the coil and this
supports the magnetic field for a short period of time, slowing the
action of the contacts opening. - similar to a the shading pole they
use on AC relays to prevent them from chattering.

Slower collapse - the diode causes a current to flow in the coil, that
current produces a magnetic field.

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Author: John Popelish
Date: 08:37 22-08-06

MarkMc wrote:
> So I'd end up with this?
>
>
>
>
>
>
>
> 240VAC(in)
> L o o N
> | |
> --------- |
> | | | |
> | .-. | |
> | | | | |
> )| o | | .-. |
> RY )| \ '-' | | MOV |
> _)| o | | | |
> | | '-' |
> | --- | |
> | --- | |
> | | | |
> |-------- |
> | |
> | |
> | |
> | |
> o o
>
> 240VAC(out)
> (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)
>
> Regards,
> Mark

That should work if both the capacitor and MOV is rated for 240 volts
AC operation. If the capacitor is not AC rated, it should have
something like a 1000 volt DC rating (or at least as high as the MOV
clamping voltage specification).
Author: MarkMc
Date: 10:38 22-08-06

Yes, the LCR (combined cap and Resistor) and MOV are rated for about
400VAC (I don't have the items here with me at work to confirm).

Thanks
Mark

John Popelish wrote:

> That should work if both the capacitor and MOV is rated for 240 volts
> AC operation. If the capacitor is not AC rated, it should have
> something like a 1000 volt DC rating (or at least as high as the MOV
> clamping voltage specification).
1


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