[Elecraft] How to check a diode?

Fri Feb 28 15:05:00 2003

First be sure it is a "multimeter" designed for solid state gear. Older
"multimeters" use a voltage source for resistance checks that often far
exceeds the allowable limits on solid state devices, including some
diodes. 

I had an old "VOM" that had a 22.5 volt battery for resistance
measurements! Fine for vacuum tubes, but a disaster around anything more
sensitive than a power rectifier diode! 

As for checking a diode "in circuit" you can learn a lot by taking
resistance and voltage measurements. First, recall that a diode conducts
current in the "forward" direction  - positive lead to the anode (end
with the arrowhead on the schematic) - and will not conduct current in
the "reverse" direction. 

Of course, resistance is done with the power off.  If the resistance is
much lower in the "forward" direction than it shows when you reverse the
leads, the diode is probably good. I say "probably" because there might
be ANOTHER diode somewhere in the circuit that is showing up, provided
the diode being checked is open, not shorted. 

Reverse the leads and see if the resistance is much, much higher,
indicating that the diode is "reverse biased" now and not conducting.
It's very likely good if you see those readings. If the resistance is
the same in either direction, it may well be shorted out. In that case
I'd lift one end and test it "isolated". 

If the circuit has a d-c current flowing through the diode, you can
check it with the power on by measuring the voltage to ground on both
sides of the diode. Silicon diodes show, typically, a 0.6 volt drop
across them. So the voltage on the anode side should be 0.6 volts higher
than the other side. If the diode has a d-c reverse bias on it, the
cathode end will show a positive voltage that is higher  - should be
much higher - than the cathode side. 

In either case, it is very likely good if you get those readings.

Of course there are no "moronic" questions...

Ron AC7AC
K2 # 1289

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of [email protected]

If your only equipment is a multimeter, what is the proper way to check
if a diode is good as an isolated component? What is the proper way to
check if its shorted out when installed? I know, moron questions. Thanks
guys. 73/Tim NZ7C

_