The return of PACO

[saturated]

He's back 

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Time to do some exercises

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[saturated]

A baffling experience was recorded today 

Using a multimeter where you have to select the range the resistance of a forward biased diode was about 3.3 Mohm and infinite reverse biased
Then using the diode function the typical forward voltage drop was obtained without incident.

So what's the beef  well the auto ranging multimeter showed infinite resistance when forward biased and a few megohm when reverse biased  (and it has no diode test function)

 

[saturated]

I went back and rechecked all my results and even added a couple other multimeters to the mix

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Gonna have to think about this one for a while 

[g1]

Set the Ideal or the Centech to measure DC volts.
Check the polarity (and voltage) of the DC being put out by the Amprobe's when they are set to ohms or diode check functions.

[saturated]

I now see that this perceived "problem" is mere child's play for the Mighty G1 

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[Kaz Kylheku]

The diode testing mode is showing 601 mV.  It's a good diode with a reasonable forward drop.

These meters work by putting out some known, fixed current into the device and measuring the voltage. That current is going to be tiny! Remember that silicon diodces have a nominal forward drop of 0.7V; but the detailed model is that they start conducting at significantly below that, at small currents. That's why you can get readings below 600 mV for diodes.

Measuring ohms is a crapshoot. Multimeters measure resistance also by putting out a fixed current and measuring the voltage. The current used varies with the range: like millamp-level currents for the smallest ranges, and down into hundreds of nanonamperes for the biggest range.

Let's say that your multimeter is using a 100 nA test current for the multi-megohm range. The voltage drop would be 0.1V across a 1M resistor.   The voltage across a diode conducting 100 nA could also be 0.1V, so that it reads as a 1M resistor. With a forward bias of 0.1V, it's possible for a diode to be conducting 100 nA.

Try this diode current calculator: https://www.everythingpe.com/calculators/diode-current-calculator

You can't pick a diode from a list here, needing to know the saturation current. This is like 2 to 4 nA for a 1N4148, so let's use, oh, 2.5 nA.  Then we put in a voltage of 0.1 and hit calculate. The current is 1.1E-07A, which is 110 nA. So that checks out.

But actually, 3.3 MΩ was observed on saturated's auto-ranging meter.  Can we use that to guess at approximately what current it is using for probing the component?

Yes! The meter is using around 20 nA.   At that value, a 3.3M resistor drops a voltage of 0.06V.  And 20 nA is in the ballpark of the forward current through a diode at 0.06V.