On Wed, 29 May 2013 14:15:58 -0700 (PDT), Klaus Kragelund <klauskvik@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 > >KlausWhat's the source impedance of that 1KV pulse? Maybe a transzorb can handle the energy. If you use a diode, be careful about the diode's forward recovery time. Sometimes it works to use a series resistor to limit the current, instead of sinking all that impulse energy. What's your data rate? A big series resistor or two will limit downstream current but slow down the data, too. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Surge Pulse Clamping with Ceramic Capacitors
Started by ●May 29, 2013
Reply by ●May 29, 20132013-05-29
Reply by ●May 30, 20132013-05-30
On Thursday, May 30, 2013 1:32:10 AM UTC+2, Joerg wrote:> Klaus Kragelund wrote: >=20 > > On May 30, 12:59 am, Joerg <inva...@invalid.invalid> wrote: >=20 > >> Klaus Kragelund wrote: >=20 > >>> On May 30, 12:34 am, Joerg <inva...@invalid.invalid> wrote: >=20 > >>>> Klaus Kragelund wrote: >=20 > >>>>> On May 30, 12:20 am, Klaus Kragelund <klausk...@hotmail.com> wrote: >=20 > >>>> [...] >=20 > >>>>>> Another idea is to use the Bourns CDSOT23-SM712, specifically desi=gnet>=20 > >>>>>> for surge protection for RS485 devices, but add ceramic caps in se=ries>=20 > >>>>>> with each line connection so it can tolerate 30V without creating >=20 > >>>>>> wonderful smoke, but will be able to clamp surge pulses without >=20 > >>>>>> affecting the high speed bus. >=20 > >>>>>> Regards >=20 > >>>>>> Klaus >=20 > >>>>> Like this: >=20 > >>>>> www.electronicsdesign.dk/tmp/RS485_cap_protection.pdf >=20 > >>>> But that would not protect against a hard 30VDC applied because an >=20 > >>>> installer miswired something. It could cause your RS485 chip to go P=HUT>=20 > >>>> unless it has internal protection against this. >=20 > >>> The RS485 IC has +/-60V protection rating, so its ok >=20 > >> Which one do you use? The ones on mine (clients's choice) have abs max >=20 > >> ratings of -8V to +12V, and no internal circuitry given. >=20 > >> >=20 > >=20 >=20 > > LTC2862, can take 60V indefinitely, but you need to add clamping >=20 > > circuit to the VDD node since it will dump current into that node if >=20 > > it is in transmit mode and is subjected to back fed voltage. >=20 > >=20 >=20 >=20 >=20 > Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding >=20 > price tag :-) >=20 >=20 >=20 > But isn't that bleed-through only an issue if the ground of the LTC2862 >=20 > has come off? Otherwise this would really be a problem because you'd >=20 > just have moved the dissipation from one place to another. I think the >=20 > only real protection for 30V continuously is some sort of cut-off, >=20 > whether inside or outside a chip. At least a partial one where the >=20 > current becomes very small. >=20When the device is in transmit mode, it has no way of knowing if an applied= voltage is just a load or a fault. So it has current limit on the outputs = and if sourcing into the VDD is detected, the driver is shut off. If 5V is = supplied to the device externally, only the current limit is active and the= current is steered to the GND or VDD rail. Regards Klaus
Reply by ●May 30, 20132013-05-30
On Thursday, May 30, 2013 1:51:03 AM UTC+2, John Larkin wrote:> On Wed, 29 May 2013 14:15:58 -0700 (PDT), Klaus Kragelund >=20 > <klauskvik@hotmail.com> wrote: >=20 >=20 >=20 > >Hi >=20 > > >=20 > >For an application I need to protect the electronics of an RS485 >=20 > >interface from surges (8/20us 1kV pulse) >=20 > > >=20 > >The normal way be to use a transzorb, in a SMA housing or even a SOT23 >=20 > >device. >=20 > > >=20 > >That is not possible since we have to be able to withstand up to 30V >=20 > >DC on the bus also (that is handled by a special RS485 IC) >=20 > > >=20 > >The problem is that the selected breakdown voltage of the transzorb >=20 > >therefore is high (>30V) and a lot of energy is dissipated into the >=20 > >transzorb >=20 > > >=20 > >In another product I have used a diode from the affected node to a >=20 > >ceramic capacitor with a bleeder resistor in parallel to clamp the >=20 > >energy and dissipate the energy into the bleeder and that worked fine. >=20 > >The diode sees very little energy and the capacitor is just charged >=20 > >during the pulse >=20 > > >=20 > >I never saw any problems doing that, but I would like to know if >=20 > >anyone here has tried the same and has any inputs into failure cases >=20 > >or even a better way to clamp the pulse? >=20 > > >=20 > >One "feature" of the diode-capacitor clamp is that closely spaced >=20 > >pulses will eventually destroy the capacitor, but anyhow closely >=20 > >spaced pulses in a tranzorb will also destroy that one.... >=20 > > >=20 > >Thanks >=20 > > >=20 > >Klaus >=20 >=20 >=20 > What's the source impedance of that 1KV pulse? Maybe a transzorb can >=20 > handle the energy. >=20 >=20Its 40ohms, 1kV, 8/20us, so approx 20A pulse.>=20 > If you use a diode, be careful about the diode's forward recovery >=20 > time. >=20 >=20 >=20 > Sometimes it works to use a series resistor to limit the current, >=20 > instead of sinking all that impulse energy.=20 >=20 >=20 >=20We have no room for it, 0402 and 0603 resistors is only option a=B4nd they = really cannot handle much Cheers Klaus
Reply by ●May 30, 20132013-05-30
Joerg <invalid@invalid.invalid> writes:> 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.TI do some fault protected ones, probably going to use some in a redesign, where they keep blowing up the tranceiver. <http://www.ti.com/lsds/ti/interface/rs-485-products.page> <http://www.ti.com/product/sn65hvd1781> -- John Devereux
Reply by ●May 30, 20132013-05-30
Klaus Kragelund wrote:> On Thursday, May 30, 2013 1:32:10 AM UTC+2, Joerg wrote:[...]>> 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. >> > > When the device is in transmit mode, it has no way of knowing if an > applied voltage is just a load or a fault. So it has current limit on > the outputs and if sourcing into the VDD is detected, the driver is > shut off. If 5V is supplied to the device externally, only the > current limit is active and the current is steered to the GND or VDD > rail. >They probably should have built in an internal VDD run-up trigger that then limits the current to zero. If it doesn't let go of the bus at that point that could present a problem. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●May 30, 20132013-05-30
John Devereux wrote:> Joerg <invalid@invalid.invalid> writes: > >> Klaus Kragelund wrote:[...]>>> 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. > > TI do some fault protected ones, probably going to use some in a > redesign, where they keep blowing up the tranceiver. > > <http://www.ti.com/lsds/ti/interface/rs-485-products.page> > > <http://www.ti.com/product/sn65hvd1781> >Those are nice but one should read the disclaimers on page 12. Doesn't sound like 100% useful protection to me. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●May 30, 20132013-05-30
Klaus Kragelund wrote:> On Thursday, May 30, 2013 1:51:03 AM UTC+2, John Larkin wrote: >> On Wed, 29 May 2013 14:15:58 -0700 (PDT), Klaus Kragelund >> >> <klauskvik@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 source impedance of that 1KV pulse? Maybe a transzorb can >> >> handle the energy. >> >> > Its 40ohms, 1kV, 8/20us, so approx 20A pulse. > >> If you use a diode, be careful about the diode's forward recovery >> >> time. >> >> >> >> Sometimes it works to use a series resistor to limit the current, >> >> instead of sinking all that impulse energy. >> >> >> > We have no room for it, 0402 and 0603 resistors is only option a�nd they really cannot handle much >I really do not like PTC fuses but they might be the only option here. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●May 30, 20132013-05-30
On Thu, 30 May 2013 00:17:43 -0700 (PDT), Klaus Kragelund <klauskvik@hotmail.com> wrote:>On Thursday, May 30, 2013 1:51:03 AM UTC+2, John Larkin wrote: >> On Wed, 29 May 2013 14:15:58 -0700 (PDT), Klaus Kragelund >> >> <klauskvik@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 source impedance of that 1KV pulse? Maybe a transzorb can >> >> handle the energy. >> >> >Its 40ohms, 1kV, 8/20us, so approx 20A pulse. > >> >> If you use a diode, be careful about the diode's forward recovery >> >> time. >> >> >> >> Sometimes it works to use a series resistor to limit the current, >> >> instead of sinking all that impulse energy. >> >> >> >We have no room for it, 0402 and 0603 resistors is only option a�nd they really cannot handle much > >Cheers > >KlausYou have room for power diodes and capacitors but not for resistors? Anybody know how much voltage an 0603 resistor can stand? I'd guess 500 at least, assuming not a lot of joules were absorbed. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●May 30, 20132013-05-30
Joerg <invalid@invalid.invalid> writes:> John Devereux wrote: >> Joerg <invalid@invalid.invalid> writes: >> >>> Klaus Kragelund wrote: > > [...] > > >>>> 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. >> >> TI do some fault protected ones, probably going to use some in a >> redesign, where they keep blowing up the tranceiver. >> >> <http://www.ti.com/lsds/ti/interface/rs-485-products.page> >> >> <http://www.ti.com/product/sn65hvd1781> >> > > Those are nice but one should read the disclaimers on page 12. Doesn't > sound like 100% useful protection to me.True, but in fact 30V is enough for my application. Basically they keep connecting/shorting the data lines to a nominal 24VDC. -- John Devereux
Reply by ●May 30, 20132013-05-30
John Larkin wrote:> On Thu, 30 May 2013 00:17:43 -0700 (PDT), Klaus Kragelund > <klauskvik@hotmail.com> wrote: > >> On Thursday, May 30, 2013 1:51:03 AM UTC+2, John Larkin wrote: >>> On Wed, 29 May 2013 14:15:58 -0700 (PDT), Klaus Kragelund >>> >>> <klauskvik@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 source impedance of that 1KV pulse? Maybe a transzorb can >>> >>> handle the energy. >>> >>> >> Its 40ohms, 1kV, 8/20us, so approx 20A pulse. >> >>> If you use a diode, be careful about the diode's forward recovery >>> >>> time. >>> >>> >>> >>> Sometimes it works to use a series resistor to limit the current, >>> >>> instead of sinking all that impulse energy. >>> >>> >>> >> We have no room for it, 0402 and 0603 resistors is only option a�nd they really cannot handle much >> >> Cheers >> >> Klaus > > You have room for power diodes and capacitors but not for resistors? > > Anybody know how much voltage an 0603 resistor can stand? I'd guess 500 at > least, assuming not a lot of joules were absorbed. >I think legally (datasheet) many are restricted to 50V. What they can really take depends on things such as the laser cut method for trimming. -- Regards, Joerg http://www.analogconsultants.com/
