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From: ornitz@kodak.UUCP
Newsgroups: sci.electronics
Subject: Re: Infra-Red Receiver?
Message-ID: <770@kodak.UUCP>
Date: Tue, 31-Mar-87 08:46:25 EST
Article-I.D.: kodak.770
Posted: Tue Mar 31 08:46:25 1987
Date-Received: Thu, 2-Apr-87 02:39:14 EST
References: <1922@ihlpl.ATT.COM> <562@puff.WISC.EDU> <3972@fritz.UUCP> <801@cooper.UUCP> <1640@ttrdc.UUCP>
Reply-To: ornitz@kodak.UUCP (barry ornitz)
Organization: Eastman Kodak Co, Rochester, NY
Lines: 24
Keywords: LED, ohmmeter, DVM, light emitting diodes
Summary: Ohmmeter tests often do not work with LEDs.

In article <1640@ttrdc.UUCP> levy@ttrdc.UUCP (Daniel R. Levy) writes:
>
>Why not just test with an ohmmeter (compare with the way the ohmmeter reads
>on a known-good garden-variety diode)?

Light emitting diodes have a much higher forward drop than silicon or germanium
diodes.  Many ohmmeters have too low a voltage to indicate forward conduction
with LED's.  This is particularly true with digital multimeters which often
have a special diode conduction setting to test silicon devices.  The higher
voltage found on some older VOM's can damage some LED's by exceeding their
reverse breakdown ratings.  I have found that on many meters set on a 10K 
scale, LED's show a slight conduction in one direction and virtually none in
the other.  This has usually been adequate to determine their polarity.  Often
this conduction is enhanced by exposing the LED to a bright light.

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|  ___  ________  |
| |  / /        | |  Barry L. Ornitz       UUCP:...!rochester!kodak!ornitz
| | / /         | |  Eastman Kodak Company
| |< < K O D A K| |  Eastman Chemicals Division Research Laboratories
| | \ \         | |  P. O. Box 1972
| |__\ \________| |  Kingsport, TN  37662       615/229-4904
|                 |
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