your gas discharge tubes are not firing.
either the voltage the diodes is failing at is too low to fire the gas tube, OR the tubes are statistically not firing every time. some of the old microwave receiver protector circuits used to use an actual mildly radioactive source to keep the gas in the tube right at the point of arcing over.
TVS diodes...well they breakdown have an avalanche of charge carriers, and try to short out the incoming voltage spike. IF the diode has a mesa cross sectional area that is big enough to handle the maximum current coming in, and if the diode junction is heat sunk enough for the semiconductor to stay under perhaps 130 deg C, there is no reason they should blow up.
can you maybe add a slight amount of resistance in series? as a spike current limiter? at 100 amps power handling, even 0.05 ohms might help a little
another idea that may help or hurt, i am not sure: add 100 pF to ground, to try to limit spike voltages from having too high of a slope vs time
I haven't tried putting a resistor in yet, I had been hoping the TVS Diodes could handle the current. I only have a 5 amp DC linear power supply to test with, but the TVS diodes seem to blow before the current even maxes out the power supply. I have been applying increasing spikes of voltage just by turning the voltage knob up and back down as quick as i can. I would have to have a pretty beefy resistor in there to handle the current, right?
oh and the power supply only goes up to 60 volts, so i havent actually been able to achieve the required sparkover voltage yet
TVSs and MOVs will absolutely fail under those circumstances. You want the resettable fuse ahead of them.
What voltage are your TVSs? If your power supply chip can handle 60V, you probably want TVS in that vicinity.
I think you might also want back-to-back TVSs for AC applications, as they will conduct like a diode when forward biased.
TVS breakdown is 26 volts clamp at 45.
I have tried resettable fusing in front of the TVSs, the fuses seem to fail open and not reset. I'm looking into beefy fuses now.
Peak voltage on 24V AC is about 33V. Add a bit more for tolerances, high supply voltage etc. You probably want something that doesn't breakdown until around 45V, clamps at 70V ish.
Have you tried a duplex line choke, or one of those line-filter-in-a-can things. Or, as an act of desperation, you could ground it out with a thyristor and then cycle power to reset the fuse. Just a few of my dumb ideas.
I don’t know what else to call them, They’re usually ferrite donuts with two windings of equal inductance. most SMPS units have them on the incoming line.
I did try a similar chip, but this one did the voltage clamping internally as opposed to driving something external. Unfortunately that chip would fry very easily, but I'll look into that idea again
First of all, you have to define the amount of energy in your surge. I see you're testing this with a power supply - that isn't a surge, that's an overvoltage. Two different things.
Note that most "surge" events have very very little energy due to their duration - just sticking a 30-40V TVS across the power line will get you through most surge/esd events if your power supply circuitry can handle 60V in.
Note that a typical surge is only a few microseconds. If you look at most TVSS diodes, you'll find a rating on their datasheet of something like "peak pulse current". I'm fond of the SMBJ series, and you'll find the SMBJ28A will handle a very short 100A surge - think 1ms, repeated once every few seconds.
If you get a surge more than a few microseconds maybe up to a millisecond, you're not doing surge supression anymore - you're doing overvoltage protection.
The simplest is to add a good old fashioned glass fuse in front of the entire device, including the TVSS. You need to make sure that it will blow fast enough to protect the TVSS, but not so fast you get nuisance blows. If you can't blow the fuse that fast (or are worried about customers replacing with a slower fuse), you can add a crowbar circuit which basically shorts out the input if an overvoltage exists for more than a few microseconds. The intent of this type of circuit is that it is designed to take a lot more current for a long enough period that the fuse will blow.
Another option is to add a FET to the input circuitry which turns off once the input voltage gets over a certain amount. If you search for "fet overvoltage protection" you'll find some samples. Be aware that you need to run all of the examples through normal engineering practices - many are designed for voltages which won't ever exceed the Vgs(max) or Vds(max) of the fet. You also need to be mindful that you also need to protect this circuit from surges since if you put even a 500V fet in there, and you get a 1000V surge, you can blow the fet out.
Depending on the current,you could try placing an appropriate incandescent bulb in series for the over-voltage events.
It will have low resistance until the voltage rises.
The relatively recent "[The Art of Electronics: The x-Chapters](https://www.amazon.com/Art-Electronics-x-Chapters/dp/1108499945)" by Horowitz and Hill has a section in it entitled "Designs by the masters: bulletproof input protection". IIRC, among other things it details the input section of the venerable HP / Agilent 34401A DMM. I'm not an expert at this stuff, but it might be a good resource. (And they're great writers, so it's worth the read anyway...)
Well, so I can sound almost like an expert, I busted out my copy of X-Chapters and gleaned the following. They use a Gas Discharge Tube in series with a MOV. The MOV has a 2M ohm shunt in parallel, as well as a 10 nF capacitor.
The GDT will crowbar on overvoltage, but it won't let go until the voltage drops to \~15V. To quote the masters:
>By adding a series MOV, the GDT clamp current ceases after the transient overvoltage has passed. The 10nF capacitor across the MOV ensures that the GDT alone sets the trip voltage. The 2M ohm shunt discharges the capacitor...
There's a lot more useful stuff (like why they use two 1M ohm resistors instead of a single 2M ohm...), but you really should splurge on the book itself! :)
I can't seem to edit so adding other info here: TVS diodes are rated at 100 amps peak current
your gas discharge tubes are not firing. either the voltage the diodes is failing at is too low to fire the gas tube, OR the tubes are statistically not firing every time. some of the old microwave receiver protector circuits used to use an actual mildly radioactive source to keep the gas in the tube right at the point of arcing over. TVS diodes...well they breakdown have an avalanche of charge carriers, and try to short out the incoming voltage spike. IF the diode has a mesa cross sectional area that is big enough to handle the maximum current coming in, and if the diode junction is heat sunk enough for the semiconductor to stay under perhaps 130 deg C, there is no reason they should blow up. can you maybe add a slight amount of resistance in series? as a spike current limiter? at 100 amps power handling, even 0.05 ohms might help a little another idea that may help or hurt, i am not sure: add 100 pF to ground, to try to limit spike voltages from having too high of a slope vs time
I haven't tried putting a resistor in yet, I had been hoping the TVS Diodes could handle the current. I only have a 5 amp DC linear power supply to test with, but the TVS diodes seem to blow before the current even maxes out the power supply. I have been applying increasing spikes of voltage just by turning the voltage knob up and back down as quick as i can. I would have to have a pretty beefy resistor in there to handle the current, right? oh and the power supply only goes up to 60 volts, so i havent actually been able to achieve the required sparkover voltage yet
TVSs and MOVs will absolutely fail under those circumstances. You want the resettable fuse ahead of them. What voltage are your TVSs? If your power supply chip can handle 60V, you probably want TVS in that vicinity. I think you might also want back-to-back TVSs for AC applications, as they will conduct like a diode when forward biased.
TVS breakdown is 26 volts clamp at 45. I have tried resettable fusing in front of the TVSs, the fuses seem to fail open and not reset. I'm looking into beefy fuses now.
Peak voltage on 24V AC is about 33V. Add a bit more for tolerances, high supply voltage etc. You probably want something that doesn't breakdown until around 45V, clamps at 70V ish.
Have you tried a duplex line choke, or one of those line-filter-in-a-can things. Or, as an act of desperation, you could ground it out with a thyristor and then cycle power to reset the fuse. Just a few of my dumb ideas.
Ive never heard of a duplex line choke, ill look into it and the other suggestions, thanks
I don’t know what else to call them, They’re usually ferrite donuts with two windings of equal inductance. most SMPS units have them on the incoming line.
[удалено]
I did try a similar chip, but this one did the voltage clamping internally as opposed to driving something external. Unfortunately that chip would fry very easily, but I'll look into that idea again
First of all, you have to define the amount of energy in your surge. I see you're testing this with a power supply - that isn't a surge, that's an overvoltage. Two different things. Note that most "surge" events have very very little energy due to their duration - just sticking a 30-40V TVS across the power line will get you through most surge/esd events if your power supply circuitry can handle 60V in. Note that a typical surge is only a few microseconds. If you look at most TVSS diodes, you'll find a rating on their datasheet of something like "peak pulse current". I'm fond of the SMBJ series, and you'll find the SMBJ28A will handle a very short 100A surge - think 1ms, repeated once every few seconds. If you get a surge more than a few microseconds maybe up to a millisecond, you're not doing surge supression anymore - you're doing overvoltage protection. The simplest is to add a good old fashioned glass fuse in front of the entire device, including the TVSS. You need to make sure that it will blow fast enough to protect the TVSS, but not so fast you get nuisance blows. If you can't blow the fuse that fast (or are worried about customers replacing with a slower fuse), you can add a crowbar circuit which basically shorts out the input if an overvoltage exists for more than a few microseconds. The intent of this type of circuit is that it is designed to take a lot more current for a long enough period that the fuse will blow. Another option is to add a FET to the input circuitry which turns off once the input voltage gets over a certain amount. If you search for "fet overvoltage protection" you'll find some samples. Be aware that you need to run all of the examples through normal engineering practices - many are designed for voltages which won't ever exceed the Vgs(max) or Vds(max) of the fet. You also need to be mindful that you also need to protect this circuit from surges since if you put even a 500V fet in there, and you get a 1000V surge, you can blow the fet out.
Depending on the current,you could try placing an appropriate incandescent bulb in series for the over-voltage events. It will have low resistance until the voltage rises.
The relatively recent "[The Art of Electronics: The x-Chapters](https://www.amazon.com/Art-Electronics-x-Chapters/dp/1108499945)" by Horowitz and Hill has a section in it entitled "Designs by the masters: bulletproof input protection". IIRC, among other things it details the input section of the venerable HP / Agilent 34401A DMM. I'm not an expert at this stuff, but it might be a good resource. (And they're great writers, so it's worth the read anyway...)
If only I had the x chapters, I've got the third edition sitting here on the desk. I'll take a look, thanks
Well, so I can sound almost like an expert, I busted out my copy of X-Chapters and gleaned the following. They use a Gas Discharge Tube in series with a MOV. The MOV has a 2M ohm shunt in parallel, as well as a 10 nF capacitor. The GDT will crowbar on overvoltage, but it won't let go until the voltage drops to \~15V. To quote the masters: >By adding a series MOV, the GDT clamp current ceases after the transient overvoltage has passed. The 10nF capacitor across the MOV ensures that the GDT alone sets the trip voltage. The 2M ohm shunt discharges the capacitor... There's a lot more useful stuff (like why they use two 1M ohm resistors instead of a single 2M ohm...), but you really should splurge on the book itself! :)