| Electronics > Repair |
| Help Identifying Vintage Component |
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| Drjaymz:
I was testing an over voltage relay and it was not working (its from 1970s home built aircraft). I reverse engineered it and its reasonably straight forward, potential divider, NPN transistor activating the relay when the supply > 16V. The potential divider is actually 3 resistors, with the middle one being a trimmer, the transistor base tap comes from the top of the first resistor that is grounded. This was a dead short to ground (hence why it wouldn't activate) with a component in parallel which I assumed is a capacitor to prevent it triggering on noise etc. When I remove that component the relay works correctly switching at just over 16V. Mystery component attached. What made me question my assumption was that I wondered if it was a zener or something else and I wondered if anyone else could identify it. It looks like a polystyrene cap and the positive lead is offset which is consistent with some capacitor designs. 15V75 sounds a bit like a zener voltage but we have to remember that its also from around 1975 and may just be a date code of something like week 15 1975 maybe. The 4070 possibly the value 40n *10^7 or something buy thats a weird value. 4.7 something would be more normal. Its worth checking over voltage relays because when they don't work the first you know if when your avionics explode or your battery literally boils. PS I already had an account on here but it says I didn't exist so just had to create account again. |
| floobydust:
It looks like a tantalum capacitor, probably wet 75uF 15V. |
| factory:
Looks like an axial wet tantalum capacitor to me as well, they do seem to use odd values a lot. David |
| fzabkar:
I used to see these tantalum capacitors in old Control Data disc drives. When they failed, they would burn a hole in the PCB. They are definitely not suitable for aerospace applications. |
| Drjaymz:
There are actually a couple of them in there. I know that tants like to short when they fail. The relay module is a Prestolite X17621 and they are certified but obviously do fail. Being certified doesn't necessarily mean it uses military or special grade equipment and in fact a lot of what I have seen is of a standard you wouldn't allow in a car. I'd expected there to be some form of freewheeling diode or snubber but there is nothing at all, to be fair it shouldn't usually activate and it has done 50 years. The reason I took it apart was that the contacts had a fair bit of resistance, which with the ropey design of aircraft alternator systems results in strong oscillation in the charge current. Are we saying its 75uF 15V? Sounds high for a cap that isn't much larger than a 1/2W resistor and from the 1970's. I imagine that the capacitance has probably escaped over the years. I'd like to put another cap in, we don't want it spuriously activating when the radio transmits. I cleaned the contacts carefully and got the 2 ohms down to the same reading as just the test leads so thats good and there's a rubber seal which used to seal it all up but that gave up sealing probably 25 years ago and let corrosion in, so I will deal with that too. |
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