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  1. #1
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    multimeter questions

    So when trying to check my led"s I was unable to check them on diode mode and have the led light up and am trying to figure out why that might be. I checked the led on my "decent" $60 unit and couldnt get it to work. So I went and got my brothers $10 unit and it worked like a charm. So the question is why wouldnt it work on mine. I messed up 2 MCE and was able to test them that voltage was showing up meaning they were bad/shorted out so I know that it is "working". Are there some that dont put voltage through the diode/led to make it light up?
    This is the meter I have:
    http://compare.ebay.com/like/1904245...=263602_263632
    trying to figure this out before I screw any more led's up...

  2. #2
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    I have the same, one meter lights LEDs the others don't and it's the cheap one that does.
    They all run off a nine volt battery too.

    Don't know why though.

  3. #3
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    It's because normal (non-led) diodes typically have a Vf between 0.3 and 0.7 volts. So meters have to use a voltage slightly higher than 0.7 volts in that mode. Meter diode test modes are really intended to measure normal diodes, not LEDs. LEDs will typically start to light up at little over 1 volt. So I just depends on if the meter is using something like 0.9V to make the test in that mode or 1.1 volts.

  4. #4
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    If the diode check mode on your meter does not output a voltage near the Vf of your LED it will not light. I suspect that some meter designs were intended to only test "regular" diodes. The Vf of a "regular" diode varies, but is typically quite a bit lower than that of a high power LED. .7Vf is typical for a common silicon diode.

  5. #5
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    The resistors in the scaling network (for diode checking) generally limit the voltage to around .7 volts. This is the conduction voltage for a silicon diode or transistor. The led's we use have a conduction voltage around 3 to 3.5 volts. Use a 9 volt battery and a 100 ohm 1/2 watt resistor to check your led's. DO NOT attempt this without using the resistor. The led will fail instantly!!!!!!!!!!!!. The resistor limits the current to around 60mA, this won't damage the led even if it isn't heatsinked. The cheaper multimeter outputs more current. This might puke small leds like T1's and such. I have left a long trail of broken, burnt, crushed and shorted parts. I try to do better; (I'm not sure George at Taskled believes that however). Don't worry about the bodies left behind you; press on. James

  6. #6
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    got it. thanks! that explains alot...

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