Electronic – How to interpret diode-mode readings from a multimeter


I'm using this multimeter to examine a heat gun fan. The fan has four contacts placed in a circle (well in a square inside a circle). Two opposite each other are labeled + and -; the other two are unlabeled. When I extracted the fan from the heat gun, the AC inputs were connected to the unlabeled contacts.

Visible on the other side of this circular piece of PCB are four black cylinders with silver stripes at one end. Each end of a cylinder connects to a contact from a different pair (i.e. they go around the circumference of the squared circle, forming the "edges" of the square).

My primitive understanding of electronics leads me to conclude that these cylinders are diodes arranged so as to create a full-wave rectifier, which I suppose makes sense since they are regulating the powering of a fan motor.

When I measure across these contact points using the multimeter in diode mode, I get values of 500-700 for adjacent (non-paired) contacts and 1200-1400 for the AC contacts (consistently varying by about 100 depending on polarity). Conversely, measuring the resistance across the DC contacts gives about 6 ohms.

Are these diode-mode readings normal? What conclusions can be drawn from them? Is it possible to determine from them whether or not there is a continuity problem in the circuitry that would prevent the fan from running on AC power? I have tested it with success by hooking a 9V battery up to both pairs of contacts.

Best Answer

Your description sounds exactly like a full wave bridge rectifier.

A multimeter in diode mode usually displays the voltage drop measured across the diode under test, as a small current is passed through it.

A single silicon diode usually has a forward bias voltage drop around 0.7V, so your reading of 500 to 700 probably corresponds to 500 to 700 milivolts.

When you measure across the AC input, you'll be measuring 2 diodes in series, plus the series resistance of the motor winding. Your measurement of 1200 to 1400 mV still makes perfect sense.