Checking for ripple voltage - 04D2

[JFD]

AC in your DC ?

With engine running at idle speed, negative lead of digital multimeter on chassis negative, postive lead of dmm on alternator positive and dmm on the AC reading, how much are you suppose to read ?

 

[rovercanus]

Don't have an answer but, why?

 

[p m]

to check for burnt-out diodes, for instance.

 

[Cozy41EF]

Theoreticaly zero, but in any case less than .7vac Which I believe is the breakdown voltage for a diode. I'm making a guess, but anything more than .7vac would make me think a diode went.

 

[p m]

.7vac cannot be a measure of the breakdown voltage for a diode.

.7V DC while applied to a diode in conducting direction may or may not burn it, and it won't harm it if applied in reverse polarity.

In any case, it is rather irrelevant - a burnt-out diode out of a 6-pack means one phase of three-phase AC current will not be rectified. It would be difficult to detect it with a DMM with the battery connected, and I would not test the alternator with the battery disconnected. A handheld scope would give a better idea.

 

[Jake1996D1]

.7vac cannot be a measure of the breakdown voltage for a diode.

.7V DC while applied to a diode in conducting direction may or may not burn it, and it won't harm it if applied in reverse polarity.

In any case, it is rather irrelevant - a burnt-out diode out of a 6-pack means one phase of three-phase AC current will not be rectified. It would be difficult to detect it with a DMM with the battery connected, and I would not test the alternator with the battery disconnected. A handheld scope would give a better idea.

Yea what he said

 

[JFD]

to check for burnt-out diodes, for instance.

My point exactly.

 

[JFD]

.7vac cannot be a measure of the breakdown voltage for a diode.

.7V DC while applied to a diode in conducting direction may or may not burn it, and it won't harm it if applied in reverse polarity.

In any case, it is rather irrelevant - a burnt-out diode out of a 6-pack means one phase of three-phase AC current will not be rectified. It would be difficult to detect it with a DMM with the battery connected, and I would not test the alternator with the battery disconnected. A handheld scope would give a better idea.

Just put a new diode plate in my Valeo and changed one negative diode and I'm getting 30V

 

[p m]

30V of what - AC ripple?

 

[discostew]

this is more relavalent than you may think. if you had a a/c voltage as high as .7 v then you would be seeing its results in some real messed up ways. If ever I check everything on my list and still can't come up with a solid answer for why some kind of fucked up shit is happening, this is the next measurement I take. But keep in mind that the alt. is not the only source of this voltage bleeding into the system. I would like to see less than .3 of a volt of a/c current. and even that is a sign of something going on. This is one of those VERY good questions.

 

[JFD]

30V of what - AC ripple?

DMM on the AC side, pos. lead on pos. alt. and neg. lead on chassis neg.

 

[p m]

30V of AC ripple does not sound right. I'd get on it and try to do whatever I could to ensure that, for instance, my DMM is not picking up some high-frequency spikes (a true RMS meter should be better), and that the DMM is AC-coupled (through a capacitor) so it does not count DC voltage offset as AC.

State-side, I'd take the alternator to the shop to be tested. I'll try to grab a couple of different DMMs this afternoon and see what my truck has, just for the kicks.

 

[JFD]

Let me know what you get.

 

[p m]

Just checked with a cheap DMM - set to 200VAC, ground lead to battery "-", signal lead to battery "+," engine at idle - read 29.9V. Same with leads reversed - 0 V.

With a .047uF capacitor in series (A/C coupled) - 0V.

Do not use cheap DMMs to measure voltage ripple.

 

[JFD]

There is another method, I'm going to look for it.