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| HP 3465A DMM repair |
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| bdunham7:
--- Quote from: bd139 on May 24, 2022, 09:07:25 pm ---This is an HP 1901-0586 which is completely undocumented and expensive to get hold of. The service manual specifies it as a general purpose diode 30V 25mA, so pretty much anything will do here apparently. Well that's not good enough for this as it's very high impedance IMHO and it was clearly affected by leakage of some sort. I'm not paying for an original diode so after doing some research and simulations of the clamp in LTspice, the B-E diode in a 2N3904 is sufficient for the job so I will snip the collector off one and solder it in, then carefully clean it and re-test. --- End quote --- The original HP part appears to be a "low-leakage PIN diode" according to some descriptions, but it also might be a JFET in disguise. I believe someone else has already pointed out an appropriate replacement from InterFET. In this case, I think it is just the low reverse leakage that matters, not much else. However, looking at the circuit you posted, I think a reverse-biased 2N3904 BE junction won't quite cut it as the circuit as shown could have as much as 8 volts reverse bias across one diode before the other one clamps. Having the reverse-biased BE junction go avalanche might actually sort of work in an unintended way, but I'm not sure it is a good long-term plan. The CB junction might be a better bet, otherwise the PN4393 JFET from Central Semiconductor is something I've used in this sort of application with excellent results. At 4 volts they would likely leak <1pA. And you should check to make sure that the resistors and zeners in the biasing circuit for these clamps are all working properly as well. |
| bd139:
Good points there. Actually I just noticed there was an error in the LTspice simulation I did of the clamp. Just running my test cases through it again now... :palm: |
| Kleinstein:
There are also afforable low leakage diodes BAV199 available in a SMD case. Not tested for super low current, but typical leakage current < 10 pA. |
| Caliaxy:
--- Quote from: bdunham7 on May 25, 2022, 03:07:56 pm ---In this case, I think it is just the low reverse leakage that matters, not much else. However, looking at the circuit you posted, I think a reverse-biased 2N3904 BE junction won't quite cut it as the circuit as shown could have as much as 8 volts reverse bias across one diode before the other one clamps. Having the reverse-biased BE junction go avalanche might actually sort of work in an unintended way, but I'm not sure it is a good long-term plan. --- End quote --- Totally agree, the B-C junction would be the appropriate one here (for 2N3904), as both diodes are can get reverse biased at a voltage higher than the reverse breakdown voltage of the B-E junction... |
| TimFox:
After the introduction of silicon planar transistors, you should always assume that the B-E reverse breakdown voltage is much lower than the B-C breakdown voltage, even for high-voltage VCEO devices. |
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