Help with Zener identification - Page 1
Forum section: electronics / repair

author: Fixalot
author_rank: Newbie
timestamp: August 23, 2019, 10:52:01 am
content: Hi,Because I can't measure a shorted Zener I need to identify it by the marking.A guess would be that it is a 24V zener but I want to be sure.The marking says "24 0K" as best as I can tell. The lead is about 1.2 mm in diameter, so probably a high Watt version?Vintage of PCB it came from is around 1990.Used with a LT1070HVCT (https://www.analog.com/media/en/technical-documentation/data-sheets/10701fe.pdf)The "Totally Isolated Converter" in the datasheet on page 12 is close to what is used. On page 11 in the "Flyback Converter" the zener position is shown right above the LT1070.I don't quite understand its function. Is it part of a snubber circuit to limit the voltage seen on Vsw?

author: Renate
author_rank: Super Contributor
timestamp: August 23, 2019, 07:42:26 pm
content: How about this:https://www.digikey.com/products/en?keywords=BZT03C24-TRThe snubber is there to keep a limit on the flyback.The zener is there to keep a limit on the snubber.What's your input voltage?Add 24 to it.Is that less than 75V (the absolute max on the switch in the LT1070HVCT)?

author: Fixalot
author_rank: Newbie
timestamp: August 23, 2019, 08:21:33 pm
content: The maximum input voltage is around 36 V DC. It comes from a full bridge rectifier and the AC side har around 28 VAC if I remember correctly.I know that the markings can be deceiving, like for example this could also have been 2.4V also but that seems quite low in this application.Marking is probably "24 0K" but it could also be "0K 24". Tried Google but did not end up with any examples of that as a marking on Zener diodes.I guess 24V should be OK, it's there to protect the film cap and resistor from overvoltage as you say.Just wanted to get a second opinion before I make stupid guessesThanks!

author: fzabkar
author_rank: Super Contributor
timestamp: August 23, 2019, 08:32:15 pm
content: I would think that the circuit should still function if the Zener is shorted?!Were any other components damaged? Did you check the resistor and capacitor? AISI, the R and C should take most of the current. Perhaps the Zener failed because either R or C wasn't doing its job?

author: Renate
author_rank: Super Contributor
timestamp: August 23, 2019, 08:50:40 pm
content: Diode says, "I'm the one that knows what's going on, I can see that flyback spike coming and I direct it."Capacitor says, "I'm in the firing line, when that spike comes, I absorb it."Resistor says, "I help out, without me Capacitor would just keep charging."Zener says, "A fat lot you all care. If you aren't up to the job you might have 100 volts across you.I know that you don't care personally, but I'm responsible for seeing that things don't get so out of whack that Switching Transistor explodes!"

author: Fixalot
author_rank: Newbie
timestamp: August 23, 2019, 08:52:02 pm
content: Yes, the circuit should work. I will investigate it further tomorrow and I suspect that there are something on the other side of the transformer that is shorting the supply causing problems on the feeding side.I'm not sure why the Zener decided to give up. I'm not sure yet if the switching controller is working as expected.The resistor is a 1k in a big package, probably a 1W. I just calculated that if the designers wanted a 2x security margin they would need to clamp at just above 22.3V (500mW through the resistor). The film cap (big package) also measures good capacitance and ESR so should not be a problem.The other components damaged are on the other side. I replaced and fixed the visible ones but there may hide something not visible. Tried to measure out where it could be but did not get any meaningful results.The PCB is a Power supply and IGBT driver card in a CNC Mill and the damage is caused by high voltage DC to the gate of the IGBT. They shorted from the emitter to the gate injecting probably well over 200V DC to the driver circuit that was trying to keep the gate low. Some drivers let the magic smoke out before the main circuit breaker tripped. Copper traces on the PCB disintegrated from the high current caused by the short.I would really love some fuse and TVS combo on each gate. There are a total of 18 drivers on the board (6 IGBT per motor).Lots to test tomorrow.Thanks!

author: Fixalot
author_rank: Newbie
timestamp: August 23, 2019, 08:56:24 pm
content: Quote from: Renate on August 23, 2019, 08:50:40 pmThat's a good one!I mostly understood the circuit but always good to get someone else to verify