2018 February 16     Tech,     Electronics


Diodes are very common in radios, power supplies, and other electronics.  They go bad sometimes.

This is just a quick article on testing diodes in-circuit. 

(Caution: electronics repair can be dangerous.  Disclaimer.)

In This Article

Safety First

How To Test Diodes

Other Stuff In Parallel


Safety First

If you don't know how to work safely with electronics, please practice on battery-powered circuits until you have the skills and knowledge.  It's up to you to learn the techniques to be safe. 

Many diode circuits use low voltage, but power circuits can easily have 50 to 200 volts, sometimes more.  These are dangerous;  test the capacitors even when the power is off.


How To Test Diodes

As I said, first unplug the radio and check capacitor voltages carefully with your multimeter.  (This assumes you know the basic safety precautions; disclaimer, again.)

A digital multimeter (DMM) in Ohms mode may not supply enough voltage to bias the semiconductor junction.  At least that's how it used to be, though now I've seen at least one multimeter company that talks about measuring ohmic resistance of diodes with their DMM's.  But cheap ones may not be able to do that;  they tend to be a lot more limited and also less accurate.  This is why it really helps to buy a good one.  The multimeter is your core piece of test equipment. 

However, even the cheap DMM's usually have a "diode test" mode that works well enough.  We're going to assume your meter is set to "diode test", not ohms.

So, the diode tests:

Good Diode:  Put the positive probe of your digital multimeter at the unmarked end of a diode. Put the negative at the silver-banded end.  The meter should read less than 800, or 0.800 depending on your DMM.  High 500's to low 800's is generally OK; a cheap multimeter like mine could be off a little bit.  But anything well outside this range is probably a bad diode. 

Now, reverse the probe polarity and it should read open circuit ("OL").  Some meters say "1" or "I" for infinite.  Conductivity one way, but not the other, means diode OK. 

Shorted Diode
If the diode test shows a voltage drop in both directions, that's a shorted diode.  The number is typically around 400 (0.400 volts) each direction, but I've found shorted diodes where the number was higher.

Open Diode:  Open means no electricity can flow. When they fail, power diodes tend to allow current to flow in either direction, as opposed to neither.  But some of them do go open-line. If the meter reads "1" or "OL" in both directions, that's what happened.

So... "OL" or "1" in both directions = bad diode.  Numeric reading in both directions = bad diode.


Other Stuff

Will other components throw off the measurement?  How much?

Trying to answer this by testing possibly-bad components in a non-functioning radio is only going to cause confusion.  Here's a better way to learn this.  Get a solderless breadboard (try this link), a couple 1N4001 diodes, and a ceramic capacitor.  It's possible to get a no-frills breadboard for a dollar or two, but the better ones have features that are a lot more convenient. For example, non-skid feet, external power-supply connectors, etc.  (Also try this link and this one for some good ones.)

For learning what various components do, this is one of the most effective ways.

Suppose we've got a circuit where there are a couple of diodes in parallel.  Maybe there are other things in parallel, too.  Can you still accurately test the diodes in-circuit?  Time for the breadboard:

How is the reading affected?

Two Or More Diodes in Parallel:  No real difference.  However, if a diode is bad here, you might not know whether it's both or just one.  If you get conductivity in both directions, desolder them to see which one is bad.

Diodes in Parallel with Capacitor:  No real difference.  You can still test the diodes, because DC voltage cannot cross the caps. (In "diode test" mode, the meter applies enough DC voltage to make a diode conduct.)

Here's why you might find caps in parallel with diodes.  High frequencies can pass more easily through a capacitor than low frequencies can.  The right caps can block 60 Hz completely and let RF through.  Together with diodes, you get something that will pass DC and RF, but not 60 Hz AC.  It makes sense that this might be used in radio circuits.

Diodes in Parallel with Resistor:  Low-ohm resistors should not be in parallel with diodes for any reason I can think of;  this would defeat the purpose of diodes.  If you did find this in a real circuit, it would test like a shorted diode.  Forward and reverse voltage tend to look a bit different, though;  a real shorted diode usually has forward and reverse voltage drops pretty much the same.

From what I tested, higher-ohm resistors have less and less effect.  Once you're up around 2200 ohms, the forward and reverse directions test much differently.  Reversed, it's almost "out of limit" on the diode tester.  In other words, it's like not having anything in parallel with the diode, so it's not even a concern.

I still don't know why there'd be low- or medium-ohm resistors in parallel with a diode.  But let's say that were a possibility.  Often you can figure out portions of a schematic if you start at the corners of a circuit board (where power often goes to the board through diodes).  So, you might be able to rule out stuff like that.  (Another thing to look out for is some other component that's shorted, making a low- or no-ohm connection where it shouldn't be.)



Diodes are often OK to test in-circuit.  Most of the time, if you get continuity or open-circuit in both directions, that's probably a bad diode.  And if you're not sure, just desolder one lead and test it again.

Diodes go bad fairly often.  Semiconductors really don't like overvoltage.  Static electricity or undetected power spikes can kill your electronics on a sunny day, no thunderstorms necessary. This is another reason why I keep recommending these for AC-powered electronics. 

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