[Elecraft] How to check a diode?

[Ed Tanton]

Hi Tim... OK... (I'm going to start at ZERO-so don't get your feelings hurt 
if it's redundant) for starters: there are regular VOMs (meter types, 
generally older-stands for Volt-Ohms-Meter)) and there are 'modern' digital 
DVMs (Digital-Volts-Meter). Many DVMs actually have a diode-checking 
position. If your doesn't don't worry about it.
1) A diode is nothing more than a P-N junction. It conducts in one 
direction when enough voltage is applied to it: generally 0.5 V for 
silicon; 0.3 V for (older) germanium types; and sometimes as low as 0.3 
volts for the latest Schottky diodes. The reverse voltage-e.g. how many 
volts it can stand before breaking down, depends on the diode. 1N4000 
series diodes for example, use the last digit of the ID to tell you that 
voltage: 1N4007s are 1000 V diodes capable of conducting 1A in the real 
world-often rated to 2.5A-but I sure wouldn't try it.
2) To test an ordinary rectifier, signal (silicon such as the 1N914/1N4148 
series), or even Germanium (such as the 1N34 or the 1N270s) all you have to 
do is apply a positive voltage to the ANODE of the diode (from your DVM, 
set to OHMS) and see if you get a reading. The actual reading isn't 
indicative of much-except that the DUT (Device-Under-Test... e.g. your 
diode) will pass current. IF it is not in a circuit, and it reads BOTH 
ways, it is shorted. If it won't read either way, it is probably open. I 
say 'probably' because one meter I own, a Fluke 8020A has an OHMS position 
that will NOT forward-bias a silicon junction (e.g. makes OHMS measurements 
using something less than 0.5 VDC.) That's very handy when you want to 
measure something that has a diode connected (besides the diode, that is.) 
Unless you meter SPECIFICALLY mentions something like that, don't worry 
about it.

One thing to be careful of: every once in a while, a manufacturer used to 
(carelessly) make the meter's OHMS measurements w/o regard for the polarity 
of the voltage at the leads. THE standard for such things is that the RED 
lead has a positive voltage, and of course, the BLACK lead will have the 
negative return. Again, probably not a problem for you. IF your 
out-of-circuit diode conducts when the RED lead is on the anode, and the 
BLACK lead is on the cathode (the banded end)... then your meter is fine. 
If the reverse is true for more than just one diode (manufacturers DO screw 
up once in a great while and put the K band on the wrong end!!!) then your 
meter likely has the reversed polarity. In truth, I haven't seen one like 
that in a LONG time-but it bears mentioning.

So, to repeat: for out of circuit tests, an ordinary diode should conduct 
with RED lead to the anode, and the BLACK lead to the cathode end. It 
should then NOT conduct when you reverse the leads.

Now, about in-circuit tests... everything I just said is still true... BUT 
now, everything around it is going to affect the readings. Diodes are 
frequently connected to transformers... which look a lot like a short if 
you just measure across the leads. The same can hold true when detector 
diodes are around toroids. There are also effects from the electrolytic 
capacitors that typically follow a diode. It all depends on the circuitry 
involved. All this means, is that when testing things IN-CIRCUIT don't go 
flying off the handle removing parts. Easiest thing to do is lift one end 
of the DUT and then you have isolated it, and can measure to your heart's 
content.

One final comment is about ZENERs. Consider that 1000V that the 1N4007 may 
break-down at... what if you put 1000V on it, then limited the current 
through it (using a resistor) so that the actual power rating of the P-N 
junction would not be exceeded. You would be 'trimming' anything over 1000V 
because the zener would breakdown. That's how it works... except zeners are 
designed to have very specific breakdown voltages. So that means, for, say, 
a 12V zener, anything over 12V applied to the cathode (remember it is an 
actual designed-in breakdown) would be 'bled' away in what is meant to be a 
controlled fashion. Again, the series resistor limits the current, and so 
keeps it from exploding (literally). To check a zener, in-circuit would be 
nice. If you know what the voltage-rating is, just check the voltage.

More often than not, zeners short (rather than pop open.) So you CAN do a 
partial test in-or-out of circuit by testing it just like a regular diode: 
RED lead to anode, BLACK lead to cathode. If you've been observant testing 
other diodes, you will have noticed that they do not go all the way to ZERO 
ohms, but stop somewhere that is fairly consistent for the diode material. 
A zener measured like that is just a forward-biased diode. Ought to look a 
lot like any other silicon diode. You can test the actual voltage by 
setting your DVM to measure DC VOLTS. Now hook the CATHODE of the zener to 
the POSITIVE source of a power supply; and your meter's negative (BLACK) 
lead to the MINUS connector. Use the POSITIVE (RED) lead of the DVM to 
measure the DC voltage present at the DUT's anode. The actual ZENER voltage 
is then calculated by subtracting the measured voltage from the power 
supply voltage. E.g. a 28VDC supply and a 12V zener, would result in ~ 16V 
as measured at the zener's anode. Sort-of like a constant-voltage dropping 
resistor.

I think that should cover it. If I left anything out, or unanswered, just 
let me know.



73 Ed Tanton N4XY <n4xy@earthlink.net>

Ed Tanton N4XY
189 Pioneer Trail
Marietta, GA 30068-3466

website: http://www.n4xy.com

All emails <IN> & <OUT> checked by
Norton AntiVirus with AutoProtect

LM: ARRL QCWA AMSAT & INDEXA;
SEDXC NCDXA GACW QRP-ARCI
OK-QRP QRP-L #758 K2 (FT) #00057

--------------------------------------------------
"Suppose you were an idiot ...
And suppose you were a member of
Congress...  but I repeat myself."
--Mark Twain
--------------------------------------------------