How do you test a diode?

Set up the multimeter to measure AC or DC voltage as needed. Turn the dial to the resistance mode (Ω). It may share a space on the dial with another function. Connect the test leads to the diode after it has been removed from the circuit.

Being able to determine the polarity (cathode against anode) and the basic functionality of a diode is a very important skill for the hobbyist or the electronics technician.

Since we know that a diode is essentially nothing more than a one-way valve for electricity, it is logical that we should be able to verify its uni-directional nature using a DC ohmmeter (battery powered) as in the following figure. Connected in one way through the diode, the meter should show very low resistance in (a). Connected to the other side of the diode, it should show a very high resistance in (b) (“OL” on some models of digital meters).

Of course, to determine which end of the diode is the cathode and which is the anode, it is necessary to know with certainty which meter test lead is positive (+) and which is negative (-) when set to “resistance” or Ω ” With most digital multimeters I’ve seen, the red lead becomes positive and the black negative is set to measure resistance in accordance with the standard electronic color coding convention, but this is not guaranteed for all meters.

Analog multimeters, for example, make their negative position positive (+) and their red negative signals (-) when they switch to the “resistance” function because it is easier to manufacture in this way!

A problem with using an ohmmeter to check a diode is that the readings have only a qualitative, not a quantitative value. In other words, an ohmmeter tells you exactly how the diode is driving; the indication of the low value obtained during driving is not necessary.

If an ohmmeter shows a “1.73 ohm” value while a forward polarizing diode, this value of 1.73 Ω is not a real world quantity useful to us as circuit technicians or designers. It does not represent any direct voltage drop or mass resistance in the diode material of the diode itself, but is rather a figure dependent on both quantities and will substantially vary with a particular ohmmeter used for reading.

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