Surge Pulse Clamping with Ceramic Capacitors

On May 30, 12:34=A0am, Joerg <inva...@invalid.invalid> wrote:
> 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.

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

Regards

Klaus

On May 30, 12:44=A0am, Klaus Kragelund <klausk...@hotmail.com> wrote:
> 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 sur=
e
> > 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 dud=
e
> > 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 designe=
t
> > > for surge protection for RS485 devices, but add ceramic caps in serie=
s
> > > 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

... 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)

Klaus Kragelund wrote:
> On May 30, 12:34 am, Joerg <inva...@invalid.invalid> wrote:
>> 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.
> 
> The RS485 IC has +/-60V protection rating, so its ok
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com/

On May 30, 12:43=A0am, "Tim Williams" <tmoran...@charter.net> wrote:
> Can give latches a try. =A0There 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 fractiona=
l
> microseconds of overshoot). =A0Should 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).

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

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

Klaus Kragelund wrote:
> On May 30, 12:31 am, 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
> 

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:

http://www.vishay.com/docs/71433/71433.pdf


>>> 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
> 

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?

-- 
Regards, Joerg

http://www.analogconsultants.com/

On May 30, 12:59=A0am, Joerg <inva...@invalid.invalid> wrote:
> Klaus Kragelund wrote:
> > On May 30, 12:34 am, Joerg <inva...@invalid.invalid> wrote:
> >> 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 design=
et
> >>>> for surge protection for RS485 devices, but add ceramic caps in seri=
es
> >>>> 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 PHU=
T
> >> unless it has internal protection against this.
>
> > The RS485 IC has +/-60V protection rating, so its ok
>
> 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.
>

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

On May 30, 1:06=A0am, Joerg <inva...@invalid.invalid> wrote:
> Klaus Kragelund wrote:
> > On May 30, 12:31 am, 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 su=
re
> >> 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 du=
de
> >> 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
>
> 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:
>
> http://www.vishay.com/docs/71433/71433.pdf
>
> >>> Another idea is to use the Bourns CDSOT23-SM712, specifically designe=
t
> >>> for surge protection for RS485 devices, but add ceramic caps in serie=
s
> >>> 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
>
> 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?
>

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

On May 30, 1:06=A0am, Joerg <inva...@invalid.invalid> wrote:
> Klaus Kragelund wrote:
> > On May 30, 12:31 am, 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 su=
re
> >> 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 du=
de
> >> 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
>
> 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:
>
> http://www.vishay.com/docs/71433/71433.pdf
>

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

Klaus Kragelund wrote:
> On May 30, 12:59 am, Joerg <inva...@invalid.invalid> wrote:
>> Klaus Kragelund wrote:
>>> On May 30, 12:34 am, Joerg <inva...@invalid.invalid> wrote:
>>>> 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.
>>> The RS485 IC has +/-60V protection rating, so its ok
>> 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.
>>
> 
> 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.
> 

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.

-- 
Regards, Joerg

http://www.analogconsultants.com/

Klaus Kragelund wrote:
> On May 30, 1:06 am, Joerg <inva...@invalid.invalid> wrote:
>> Klaus Kragelund wrote:
>>> On May 30, 12:31 am, 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
>> 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:
>>
>> http://www.vishay.com/docs/71433/71433.pdf
>>
> 
> 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
> 

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:

http://www.supertex.com/pdf/datasheets/MD0100.pdf

-- 
Regards, Joerg

http://www.analogconsultants.com/