Surge Pulse Clamping with Ceramic Capacitors

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

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.

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.

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.

Yes, we are using a low capacitance type (in other case put a diode in series to quinch the tranzorb capacitance)

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

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:

Almost the same here :-)

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.

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.

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

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!

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.

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:

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

Tim

-- Deep Friar: a very philosophical monk. Website:

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

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

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

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

Regards

Klaus

The RS485 IC has +/-60V protection rating, so its ok

Regards

Klaus

... resistor added to avoid stair casing of the voltage on the capacitor from multiple surges (discharge time of less than a second, the standard defines wait times between pulses of maximum 60 seconds, which of course does not mimic real life pulses)

Which one do you use? The ones on mine (clients's choice) have abs max ratings of -8V to +12V, and no internal circuitry given.

Also, make sure for how long it can take that. Sometimes there is only a poly-resistor that eventually hisses out.

Looks like a nice part, had not heard of them before. Seems it will be a little too big, AFAICS the same size as SMA tranzorbs.

MOVs derate over time, every pulse applied reduces the breakdown voltage, so after some use the product self-destructs

A ceramic TVS, like the CT0805K14 is better, so they say, but it has very soft knee and large capacitance

Regards

Klaus

You'd need a 40V or so TVS upfront but I don't see the 30V presenting a problem. I was thinking about something like this:

Yes, it'll handle it alright but the bus will meander around a bit depending on the data transmitted. But how would it prevent 30VDC from being sent down a data line?

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LTC2862, can take 60V indefinitely, but you need to add clamping circuit to the VDD node since it will dump current into that node if it is in transmit mode and is subjected to back fed voltage.

Cheers

Klaus

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Our device will survive, but other devices on the same bus from other suppliers will probably be destroyed.

A lot of the device I have reverse engineered has no protection at all and will be destroyed from this

Cheers

Klaus

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It would need levelshifting and another supply rail to control the FETs at all input voltages. be immune to bursts and allow to be turned on from regular signals from the RS485 device

I tried to draw it up, became a nightmare

Cheers

Klaus

Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding price tag :-)

But isn't that bleed-through only an issue if the ground of the LTC2862 has come off? Otherwise this would really be a problem because you'd just have moved the dissipation from one place to another. I think the only real protection for 30V continuously is some sort of cut-off, whether inside or outside a chip. At least a partial one where the current becomes very small.

Yeah, you do need a negative supply voltage. Sources and gates tied together, zener between them, resistor from gates to negative supply. The OVP trigger could then shunt gate-source via an optocoupler, or you could have a transistor at the base (in that case there needs to be also a gate-source bleed resistor).

There may even be a ready-to-go OVP protector chip with all this in there, but probably expensive.

Then there are always the Supertex high voltage mux chips but they will probably be too big. I wish they made something like this for RS485 level trip points but haven't seen it yet: