Surge Pulse Clamping with Ceramic Capacitors

Hi

For an application I need to protect the electronics of an RS485
interface from surges (8/20us 1kV pulse)

The normal way be to use a transzorb, in a SMA housing or even a SOT23
device.

That is not possible since we have to be able to withstand up to 30V
DC on the bus also (that is handled by a special RS485 IC)

The problem is that the selected breakdown voltage of the transzorb
therefore is high (>30V) and a lot of energy is dissipated into the
transzorb

In another product I have used a diode from the affected node to a
ceramic capacitor with a bleeder resistor in parallel to clamp the
energy and dissipate the energy into the bleeder and that worked fine.
The diode sees very little energy and the capacitor is just charged
during the pulse

I never saw any problems doing that, but I would like to know if
anyone here has tried the same and has any inputs into failure cases
or even a better way to clamp the pulse?

One "feature" of the diode-capacitor clamp is that closely spaced
pulses will eventually destroy the capacitor, but anyhow closely
spaced pulses in a tranzorb will also destroy that one....

Thanks

Klaus

Klaus Kragelund wrote:
> Hi
> 
> For an application I need to protect the electronics of an RS485
> interface from surges (8/20us 1kV pulse)
> 

What, you also? I am just looking at RS485 protection for an industrial
system.


> The normal way be to use a transzorb, in a SMA housing or even a SOT23
> device.
> 

Normally you need special ones with low capacitance or TVS in series
with regular diodes. Ordinary transzorbs with their huge capacitance can
muffle the data signal because of the lowpass effect. They'd "kill it
dead", as John Wayne would have put it.


> That is not possible since we have to be able to withstand up to 30V
> DC on the bus also (that is handled by a special RS485 IC)
> 
> The problem is that the selected breakdown voltage of the transzorb
> therefore is high (>30V) and a lot of energy is dissipated into the
> transzorb
> 
> In another product I have used a diode from the affected node to a
> ceramic capacitor with a bleeder resistor in parallel to clamp the
> energy and dissipate the energy into the bleeder and that worked fine.
> The diode sees very little energy and the capacitor is just charged
> during the pulse
> 
> I never saw any problems doing that, but I would like to know if
> anyone here has tried the same and has any inputs into failure cases
> or even a better way to clamp the pulse?
> 

It works but can blunt the edges of your signal in times when the cap is
not yet charged at all. You can also limit the charge on the cap via a
big TVS in parallel to it.


> One "feature" of the diode-capacitor clamp is that closely spaced
> pulses will eventually destroy the capacitor, but anyhow closely
> spaced pulses in a tranzorb will also destroy that one....
> 

Why not clamp directly to the 30V DC rail? You just have to make sure
that this rail won't start to float up when lots of pulses appear and
the load on the DC rail is too light. A TL431 can help with that
although it'll be marginal in this case with its 36V limit. Maybe
another homemade shunt regulator.

-- 
Regards, Joerg

http://www.analogconsultants.com/

On May 30, 12:08=A0am, Joerg <inva...@invalid.invalid> wrote:
> Klaus Kragelund wrote:
> > Hi
>
> > For an application I need to protect the electronics of an RS485
> > interface from surges (8/20us 1kV pulse)
>
> What, you also? I am just looking at RS485 protection for an industrial
> system.
>
> > The normal way be to use a transzorb, in a SMA housing or even a SOT23
> > device.
>
> Normally you need special ones with low capacitance or TVS in series
> with regular diodes. Ordinary transzorbs with their huge capacitance can
> muffle the data signal because of the lowpass effect. They'd "kill it
> dead", as John Wayne would have put it.
>

Yes, we are using a low capacitance type (in other case put a diode in
series to quinch the tranzorb capacitance)
>
>
>
> > That is not possible since we have to be able to withstand up to 30V
> > DC on the bus also (that is handled by a special RS485 IC)
>
> > The problem is that the selected breakdown voltage of the transzorb
> > therefore is high (>30V) and a lot of energy is dissipated into the
> > transzorb
>
> > In another product I have used a diode from the affected node to a
> > ceramic capacitor with a bleeder resistor in parallel to clamp the
> > energy and dissipate the energy into the bleeder and that worked fine.
> > The diode sees very little energy and the capacitor is just charged
> > during the pulse
>
> > I never saw any problems doing that, but I would like to know if
> > anyone here has tried the same and has any inputs into failure cases
> > or even a better way to clamp the pulse?
>
> It works but can blunt the edges of your signal in times when the cap is
> not yet charged at all. You can also limit the charge on the cap via a
> big TVS in parallel to it.
>

I have no room for it, don't even have room for 3 SMAs


> > One "feature" of the diode-capacitor clamp is that closely spaced
> > pulses will eventually destroy the capacitor, but anyhow closely
> > spaced pulses in a tranzorb will also destroy that one....
>
> Why not clamp directly to the 30V DC rail? You just have to make sure
> that this rail won't start to float up when lots of pulses appear and
> the load on the DC rail is too light. A TL431 can help with that
> although it'll be marginal in this case with its 36V limit. Maybe
> another homemade shunt regulator.
>

The 30V is only available since some idiot would connect 30V power to
the A/B line of the RS485 interface

One way to do it to add a set of biasing resistors, so one cap is
charged to +12V and another to -7V (the RS485 CM range) and connect
that surge capacitor to the bus via the diode.

Another idea is to use the Bourns CDSOT23-SM712, specifically designet
for surge protection for RS485 devices, but add ceramic caps in series
with each line connection so it can tolerate 30V without creating
wonderful smoke, but will be able to clamp surge pulses without
affecting the high speed bus.

Regards

Klaus

On May 30, 12:20=A0am, Klaus Kragelund <klausk...@hotmail.com> wrote:
> On May 30, 12:08=A0am, Joerg <inva...@invalid.invalid> wrote:
>
>
>
>
>
>
>
>
>
> > Klaus Kragelund wrote:
> > > Hi
>
> > > For an application I need to protect the electronics of an RS485
> > > interface from surges (8/20us 1kV pulse)
>
> > What, you also? I am just looking at RS485 protection for an industrial
> > system.
>
> > > The normal way be to use a transzorb, in a SMA housing or even a SOT2=
3
> > > device.
>
> > Normally you need special ones with low capacitance or TVS in series
> > with regular diodes. Ordinary transzorbs with their huge capacitance ca=
n
> > muffle the data signal because of the lowpass effect. They'd "kill it
> > dead", as John Wayne would have put it.
>
> Yes, we are using a low capacitance type (in other case put a diode in
> series to quinch the tranzorb capacitance)
>
>
>
>
>
>
>
>
>
>
>
> > > That is not possible since we have to be able to withstand up to 30V
> > > DC on the bus also (that is handled by a special RS485 IC)
>
> > > The problem is that the selected breakdown voltage of the transzorb
> > > therefore is high (>30V) and a lot of energy is dissipated into the
> > > transzorb
>
> > > In another product I have used a diode from the affected node to a
> > > ceramic capacitor with a bleeder resistor in parallel to clamp the
> > > energy and dissipate the energy into the bleeder and that worked fine=
.
> > > The diode sees very little energy and the capacitor is just charged
> > > during the pulse
>
> > > I never saw any problems doing that, but I would like to know if
> > > anyone here has tried the same and has any inputs into failure cases
> > > or even a better way to clamp the pulse?
>
> > It works but can blunt the edges of your signal in times when the cap i=
s
> > not yet charged at all. You can also limit the charge on the cap via a
> > big TVS in parallel to it.
>
> I have no room for it, don't even have room for 3 SMAs
>
> > > One "feature" of the diode-capacitor clamp is that closely spaced
> > > pulses will eventually destroy the capacitor, but anyhow closely
> > > spaced pulses in a tranzorb will also destroy that one....
>
> > Why not clamp directly to the 30V DC rail? You just have to make sure
> > that this rail won't start to float up when lots of pulses appear and
> > the load on the DC rail is too light. A TL431 can help with that
> > although it'll be marginal in this case with its 36V limit. Maybe
> > another homemade shunt regulator.
>
> The 30V is only available since some idiot would connect 30V power to
> the A/B line of the RS485 interface
>
> One way to do it to add a set of biasing resistors, so one cap is
> charged to +12V and another to -7V (the RS485 CM range) and connect
> that surge capacitor to the bus via the diode.
>
> Another idea is to use the Bourns CDSOT23-SM712, specifically designet
> for surge protection for RS485 devices, but add ceramic caps in series
> with each line connection so it can tolerate 30V without creating
> wonderful smoke, but will be able to clamp surge pulses without
> affecting the high speed bus.
>
> Regards
>
> Klaus

Like this:

www.electronicsdesign.dk/tmp/RS485_cap_protection.pdf

Klaus Kragelund wrote:
> On May 30, 12:08 am, Joerg <inva...@invalid.invalid> wrote:
>> Klaus Kragelund wrote:

[...]


>>> That is not possible since we have to be able to withstand up to 30V
>>> DC on the bus also (that is handled by a special RS485 IC)
>>> The problem is that the selected breakdown voltage of the transzorb
>>> therefore is high (>30V) and a lot of energy is dissipated into the
>>> transzorb
>>> In another product I have used a diode from the affected node to a
>>> ceramic capacitor with a bleeder resistor in parallel to clamp the
>>> energy and dissipate the energy into the bleeder and that worked fine.
>>> The diode sees very little energy and the capacitor is just charged
>>> during the pulse
>>> I never saw any problems doing that, but I would like to know if
>>> anyone here has tried the same and has any inputs into failure cases
>>> or even a better way to clamp the pulse?
>> It works but can blunt the edges of your signal in times when the cap is
>> not yet charged at all. You can also limit the charge on the cap via a
>> big TVS in parallel to it.
>>
> 
> I have no room for it, don't even have room for 3 SMAs
> 

Almost the same here :-)

> 
>>> One "feature" of the diode-capacitor clamp is that closely spaced
>>> pulses will eventually destroy the capacitor, but anyhow closely
>>> spaced pulses in a tranzorb will also destroy that one....
>> Why not clamp directly to the 30V DC rail? You just have to make sure
>> that this rail won't start to float up when lots of pulses appear and
>> the load on the DC rail is too light. A TL431 can help with that
>> although it'll be marginal in this case with its 36V limit. Maybe
>> another homemade shunt regulator.
>>
> 
> The 30V is only available since some idiot would connect 30V power to
> the A/B line of the RS485 interface
> 

One way to handle that would be to sense A and B. The instance either
one goes beyond 12V cut the data line electronically or shunt to ground
with Polyfuses in line. I don't like the Polyfuse approach because they
become hot, if possible I'd try to electronically switch open. Two
back-to-back P-channels or something like that, maybe use an array if
space is really tight. That avoids dissipation issues.


> One way to do it to add a set of biasing resistors, so one cap is
> charged to +12V and another to -7V (the RS485 CM range) and connect
> that surge capacitor to the bus via the diode.
> 

That would be a very nice way to do it. In my case I don't have any
negative supply, and also very little space. But you'd have to make sure
it can't run away. So either the dividers have to contain low enough
resistor values or there have to be clamps. With the chance of some dude
connecting a hard 30VDC that's next to impossible. Better to use an
electronic protection like above.


> Another idea is to use the Bourns CDSOT23-SM712, specifically designet
> for surge protection for RS485 devices, but add ceramic caps in series
> with each line connection so it can tolerate 30V without creating
> wonderful smoke, but will be able to clamp surge pulses without
> affecting the high speed bus.
> 

Sure, but then you'd have DC drift on the bus. Might be ok but it can
get iffy.

-- 
Regards, Joerg

http://www.analogconsultants.com/

On May 29, 2:15=A0pm, Klaus Kragelund <klausk...@hotmail.com> wrote:
> Hi
>
> For an application I need to protect the electronics of an RS485
> interface from surges (8/20us 1kV pulse)
>
> The normal way be to use a transzorb, in a SMA housing or even a SOT23
> device.
>
> That is not possible since we have to be able to withstand up to 30V
> DC on the bus also (that is handled by a special RS485 IC)
>
> The problem is that the selected breakdown voltage of the transzorb
> therefore is high (>30V) and a lot of energy is dissipated into the
> transzorb
>
> In another product I have used a diode from the affected node to a
> ceramic capacitor with a bleeder resistor in parallel to clamp the
> energy and dissipate the energy into the bleeder and that worked fine.
> The diode sees very little energy and the capacitor is just charged
> during the pulse
>
> I never saw any problems doing that, but I would like to know if
> anyone here has tried the same and has any inputs into failure cases
> or even a better way to clamp the pulse?
>
> One "feature" of the diode-capacitor clamp is that closely spaced
> pulses will eventually destroy the capacitor, but anyhow closely
> spaced pulses in a tranzorb will also destroy that one....
>
> Thanks
>
> Klaus

what's the impedance of the pulse? if low, wont work.

or, rather how many joules do you expect in the pulse?

I once had to protect telco equipment whose pulse went to around 500V,
BUT could supply around 500A  With protection like you describe you
didn't lose a tranzorb, you'd lose the PCB!

Klaus Kragelund wrote:
> On May 30, 12:20 am, Klaus Kragelund <klausk...@hotmail.com> wrote:

[...]


>> Another idea is to use the Bourns CDSOT23-SM712, specifically designet
>> for surge protection for RS485 devices, but add ceramic caps in series
>> with each line connection so it can tolerate 30V without creating
>> wonderful smoke, but will be able to clamp surge pulses without
>> affecting the high speed bus.
>>
>> Regards
>>
>> Klaus
> 
> Like this:
> 
> www.electronicsdesign.dk/tmp/RS485_cap_protection.pdf


But that would not protect against a hard 30VDC applied because an
installer miswired something. It could cause your RS485 chip to go PHUT
unless it has internal protection against this.

-- 
Regards, Joerg

http://www.analogconsultants.com/

Can give latches a try.  There are SIDACs made for protection duty. 
That'll short the line down to a few volts within a microsecond or so (if 
your bitrate isn't too high, you could filter out the remaining fractional 
microseconds of overshoot).  Should still work if the line remains 
active -- RS485 levels of 200mV won't keep a SIDAC on, so you could use 
two (one per line to ground) or three (full delta).

MOVs handle gobs of energy, but of course, they are rather high 
capacitance, no good for a terminated line.

There are low-capacitance TVSs, made from some sort of plastic I think, 
that'd do this sort of job, but I don't know that any will handle the kind 
of energy a 1kV spike will most likely deliver.  Here's an example:
http://www.littelfuse.com/products/pulseguard-esd-suppressors/~/media/Files/Littelfuse/Technical%20Resources/Documents/Data%20Sheets/Littelfuse_PulseGuard%20ESD_PGB2.pdf
Yeah, no mention of avalanche energy... hmm...

Other than that, diodes into a rail or conventional TVS -- as suggested --  
is your best alternative.

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

"Klaus Kragelund" <klauskvik@hotmail.com> wrote in message 
news:9269edc5-9fae-4f7e-bd35-ed7b02c4fa02@k4g2000vba.googlegroups.com...
> Hi
>
> For an application I need to protect the electronics of an RS485
> interface from surges (8/20us 1kV pulse)
>
> The normal way be to use a transzorb, in a SMA housing or even a SOT23
> device.
>
> That is not possible since we have to be able to withstand up to 30V
> DC on the bus also (that is handled by a special RS485 IC)
>
> The problem is that the selected breakdown voltage of the transzorb
> therefore is high (>30V) and a lot of energy is dissipated into the
> transzorb
>
> In another product I have used a diode from the affected node to a
> ceramic capacitor with a bleeder resistor in parallel to clamp the
> energy and dissipate the energy into the bleeder and that worked fine.
> The diode sees very little energy and the capacitor is just charged
> during the pulse
>
> I never saw any problems doing that, but I would like to know if
> anyone here has tried the same and has any inputs into failure cases
> or even a better way to clamp the pulse?
>
> One "feature" of the diode-capacitor clamp is that closely spaced
> pulses will eventually destroy the capacitor, but anyhow closely
> spaced pulses in a tranzorb will also destroy that one....
>
> Thanks
>
> Klaus 

On May 30, 12:31=A0am, Joerg <inva...@invalid.invalid> wrote:
> Klaus Kragelund wrote:
> > On May 30, 12:08 am, Joerg <inva...@invalid.invalid> wrote:
> >> Klaus Kragelund wrote:
>
> [...]
>
>

[Snip]

> > One way to do it to add a set of biasing resistors, so one cap is
> > charged to +12V and another to -7V (the RS485 CM range) and connect
> > that surge capacitor to the bus via the diode.
>
> That would be a very nice way to do it. In my case I don't have any
> negative supply, and also very little space. But you'd have to make sure
> it can't run away. So either the dividers have to contain low enough
> resistor values or there have to be clamps. With the chance of some dude
> connecting a hard 30VDC that's next to impossible. Better to use an
> electronic protection like above.
>

The electronic protection would probably be sensitive to ESD and
burst, and is a pain to design with a possible 30V voltage applied

> > Another idea is to use the Bourns CDSOT23-SM712, specifically designet
> > for surge protection for RS485 devices, but add ceramic caps in series
> > with each line connection so it can tolerate 30V without creating
> > wonderful smoke, but will be able to clamp surge pulses without
> > affecting the high speed bus.
>
> Sure, but then you'd have DC drift on the bus. Might be ok but it can
> get iffy.
>

The Bourns part has 7V/12V transzorbs back-to-back, so it will handle
the CM range of the bus. A resistor should be added in the schematics
in parallel with each capacitor

Cheers

Klaus

On May 30, 12:32=A0am, Robert Macy <robert.a.m...@gmail.com> wrote:
> On May 29, 2:15=A0pm, Klaus Kragelund <klausk...@hotmail.com> wrote:
>
>
>
>
>
>
>
>
>
> > Hi
>
> > For an application I need to protect the electronics of an RS485
> > interface from surges (8/20us 1kV pulse)
>
> > The normal way be to use a transzorb, in a SMA housing or even a SOT23
> > device.
>
> > That is not possible since we have to be able to withstand up to 30V
> > DC on the bus also (that is handled by a special RS485 IC)
>
> > The problem is that the selected breakdown voltage of the transzorb
> > therefore is high (>30V) and a lot of energy is dissipated into the
> > transzorb
>
> > In another product I have used a diode from the affected node to a
> > ceramic capacitor with a bleeder resistor in parallel to clamp the
> > energy and dissipate the energy into the bleeder and that worked fine.
> > The diode sees very little energy and the capacitor is just charged
> > during the pulse
>
> > I never saw any problems doing that, but I would like to know if
> > anyone here has tried the same and has any inputs into failure cases
> > or even a better way to clamp the pulse?
>
> > One "feature" of the diode-capacitor clamp is that closely spaced
> > pulses will eventually destroy the capacitor, but anyhow closely
> > spaced pulses in a tranzorb will also destroy that one....
>
> > Thanks
>
> > Klaus
>
> what's the impedance of the pulse? if low, wont work.
>

The impedance is 40ohms, so the peak current as test reveals is about
20A

> or, rather how many joules do you expect in the pulse?
>
> I once had to protect telco equipment whose pulse went to around 500V,
> BUT could supply around 500A =A0With protection like you describe you
> didn't lose a tranzorb, you'd lose the PCB!

I have a surge test earlier passed using the CDSOT device, without the
requirement for 30V applied

Regards

Klaus