A14U diode replacement

[FlyingHacker]

Hi, (in the famous Dave voice)

I am repairing an old multimeter that had a dead current measurement. I found a blown trace, and discovered a couple of protection diodes (in parallel in reverse directions to ground) were fried. I am trying to find suitable replacements, but I am not sure the right way to do so. The old diodes are marked A14U (data sheet here http://www.digitroncorp.com/Documents/Datasheets/A14A,-A14F,-A14U.aspx?ext=.pdf )

In this type of application what are the key things I need to look for in a replacement? I assume the average forward current of 2.5A is important, and I am guessing the non-repetitive peak surge current comes into play. I just wondered if someone more experienced could walk me through the thought process for choosing a replacement. Of course just a part number would be helpful, but I would like to learn why I should use a particular part.

The parts list for the meter just lists them as low leakage rectifiers (and a Data Precision part number). I tried searching for the part number and cross reference, etc., and did not get very far.

In a circuit as I describe above the idea is that if there is that in the case of overload the diode breaks down and becomes reversed biased, thus shunting the current to ground (which is what lead to the trace on the board blowing,out, but the ADC, etc of the meter being preserved). Right?

I believe the meter works fine, as I removed those diodes from the circuit in a controlled environment, and the current measurements worked again. One of the diode broke (very easily) when I removed it. The other I got out intact, but I think the one that broke may have been the one that blew.

Thanks for any input.

[SeanB]

Going to be hard to get them in that glass package, but you can see if you can shoehorn a 1N5404 in place of  them, or a 3A bridge rectifier with the + and - terminals shorted.

[FlyingHacker]

Going to be hard to get them in that glass package, but you can see if you can shoehorn a 1N5404 in place of  them, or a 3A bridge rectifier with the + and - terminals shorted.

Thanks for the reply. I can probably make a larger diode fit if I have to (though this little Data Precision 245 is small), but I am also trying to learn why I would pick a particular replacement.

Does the higher 200A peak forward surge current of the 1N5404 ( vs 30A in the original) cause a problem in this application ( two opposite polarity diodes in parallel to ground) ?

Edit: I should add these diodes are right after the mode (volts, ohms, amps) selector, but before the shunt resistors used when measuring current. It looked like somebody pumped 110v AC into it when the meter was set to milliamperes. I got the meter for $9 delivered off eBay, which was a steal.


Thanks.