Measuring diodes in circuit

[dwinstonwood]

Hello all,
I was testing the four 1N4007 diodes in a rectifier I soldered up in an amp, and got some weird readings. I am using the Diode test on my MM.

With forward biased testing everything looks right: 0.533V to 0.535V for all four diodes.

But, when reverse biased testing, the meter quickly (1 to 2 seconds) rises to 2.9xx volts and then switches to "OL."

Could the reservoir cap be holding the voltage that the MM is feeding through the diodes?

Or, do the diodes only stop passing reverse voltage when they see more than 3V? Maybe my meter takes a second or two to send the voltage?

I also did a resistance test and all of the diodes measure about +/- 12.25M ohms when measured forward or reverse, which I understand means they're good.

Thanks for any insight on this!

[shooter]

scrounge up a 1N400X, bench test, That gives you a "base-line" from which all future diodes will be held to.
now gator-clip a cap + to cathode, meter to anode and cap ground, read. discharge the cap, reverse leads, read, test complete, head will begin to itch 


I prefer one leg hanging "in circuit".  a scope will "see" if there's 1/2 waves or sine waves on the down-stream side of the diode.

[sluckey]

I also did a resistance test and all of the diodes measure about +/- 12.25M ohms when measured forward or reverse, which I understand means they're good.

A good diode will have a low resistance one way and a high resistance the other way.

If you want a 'for sure' check on a diode always disconnect one end.

[dwinstonwood]

Thanks shooter and shooter.

I will lift one side and see what I get.


That was supposed to be shooter and sluckey! 

[dwinstonwood]

I have four 1N5408's on hand, but only two more 1N4007's left. If the ones in the circuit still act weird with a lead lifted, I'm just going to put the 3A parts in. Overkill, but peace of mind.

[PRR]

...Could the reservoir cap be holding the voltage that the MM is feeding through the diodes?...

Absolutely.

Don't try to measure "Ohms" when there is ANY other source of energy in the circuit. Even the small stored cap-charge from a previous reading. (And you could verify this by bleeding the cap between reads, or using your second meter to watch the Volt on the cap.)