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mutimeter problem reading LEDs

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scoutuk

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I have the above multimeter and when i try to use the diode setting to read an LED i get no reading stays on 0L, Im no expert on this stuff but was following instructions from a robotic book, i tried a few LEDs to make sure and also connected them to a battery and they all lit up, any ideas were im going wrong please, According the the book i should be getting a voltage drop of about 1.6 V thanks for any help.
 
Perhaps the multimeter doesn't measure that high a diode voltage. Try measuring two or three normal diodes in series and see what that indicates.
 
Not really a problem with your multimeter. Nowhere on the spec page you reference does it specify how your multi-meter "tests" a diode.

In the diode measure mode, my Fluke meters apply a constant current of about 1mA, and then display the resulting forward voltage (in volts). But what happens when the forward drop (at ~1mA) of the diode under test exceeds the compliance of the current source? What is the compliance of the current source? I measured the open circuit voltage across the probes with another meter; my Fluke 75 only puts out about 2.5V, while the Fluke 87 puts out about 3V. Some LEDs have forward drops this high, so my conclusion is that these meters are not capable of measuring the forward drop using the built-in constant-current source.

You can always forward bias the LED using a constant-voltage power supply and a current limiting resistor (or by using a constant-current power supply set to the desired current) while using your DMM (in volts mode) to measure the forward voltage drop.
 
Thanks Guys,
I guess your right about the DMM, I dont really need to be be testing the LED in diode mode, was just following the book example, I will try a normal diode crutschow thanks, Yes Mike funnily enough i had the LED connected to a power supply and resistor and did read the drop across the LED, this was also in the book lol, Gues the DMM the author of the book uses is capable of reading the drop across LEDs in diode mode, once again thanks
 
Put the LED in a circuit with a dropper resistor so that it takes about 10mA. Read the voltage across the LED. I would never try to read anything about a LED on the diode scale of ANY MULTIMETER. The voltage of a LED will vary slightly from one manufacturer to another and will depend heavily on the colour. In addition, it will change slightly according to the current and also from one lED to another in the same batch. For instance I have superbright white LEDs and they vary from 3.4v to 3.6v within the same batch of 1,000 LEDs.
Even the voltage across a power diode should only be measured when the diode is taking the current you are expecting it to take as the voltage across a simple 1N4004 rises to over 1.1v when the current is 1amp. These diodes are really only suitable for 700mA.
 
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