reliably testing diodes in circuit ?

while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in one direction then swap probes around and expect (0 V) but i get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does not indicate a circuit error or fault with diode?

thanks, robb

Reply to
robb
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Certainly. Your meter can't tell the electrons to *only* to through the diode. They will go through whatever else is in the vicinity.

A diode by itself is not .567 one way and 0 the other. It is .567 (or so) one way and infinity ("overload") the other way, indicating that there is no path for the electrons.

In this case the electrons are finding a path through other components.

Reply to
mc

Why would you expect 0V ? That would indicate a short !

0.567V is about the right forward voltage for a diode check.

That's the non-conducting direction for the diode. The 1.897 V will come from other components in parallel with it. It's hardly unexpected !

I think you need to practice measuring some diodes before drawing incorrect conclusions as at present.

Graham

Reply to
Eeyore

Yes.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

LOL !

That's an unusually terse response from you there Spehro.

Graham

Reply to
Eeyore

DMM

V) in

i

through the

.567 (or so)

that there is no

components.

Thanks for help and explanation.

regards, robb

Reply to
robb

DMM

V) in

apology, i mistyped. i meant to type 0L, at least that is what the meter displays when i use diode test on the DMM

Thanks for help.

Reply to
robb

(1.897 V) the other .

thanks Spehro.

Reply to
robb

I ran into a situation like that when trying to figure out what a little four-leaded device was. Turns out it's a diode ring - four diodes connected anode-to-cathode - intended for use in a balance mixer/modulator. For any two adjacent leads, it would read about .5 volts with the leads in one orientation and three times that voltage in the other direction... I was actually reading the forward voltage drop of either one or three diodes.

In your case, I expect that you're probably reading the forward voltage drop of the diode in one direction, and reading the effect of other components (diodes with the opposite orientation, shunt resistors, etc.) in the other. Since you're reading in-circuit, you cannot be certain that either reading is due solely to the diode you think you're measuring, and the actual number displayed is going to depend somewhat on the measurement technique used by the meter.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Reply to
Dave Platt

Hi, Robb. When on diode check, the DMM typically puts out a current of a couple of mA, and then measures the voltage across the probes.

As mentioned elsewhere, doing a diode check on a diode in-circuit also measures everything else in parallel with the diode. The readings may have little or no meaning.

One exception -- if you're measuring a diode's forward voltage drop in- circuit, the reading has to be less than or equal to the diode drop out of circuit. If the reading is greater, that reliably indicates a problem with the diode. Since diodes do sometimes fail open, that data point can be of some use.

Cheers Chris

Reply to
Chris

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