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DMM diode test voltage & current query

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seb1982:
Hi!

I'm trying to find a DMM that performs well in Dave's infamous "white LED test" - can it light and take a reading from a decent LED?  I note from his $100 multimeter shootout video that the Fluke 87-V has no problem in this respect.

So, I had a look at its user manual (.pdf here http://docs-europe.electrocomponents.com/webdocs/0adc/0900766b80adcb1d.pdf) which says on p.51 that in diode mode, it has an open circuit test voltage of 3.9vDC and a current of 0.6mA.

In the same video, the BK Precision 2709B can't cope with this test - it lights up the LED but can't display a reading. Looking at the datasheet for this DMM (.pdf here http://www.bkprecision.com/downloads/datasheets/2709B_datasheet.pdf), it says on p.1 that it has an open circuit voltage of 3.5vDC at a current of 1.2mA.

So, on paper, I'd expect the BK to perform pretty much as well as the Fluke in this regard, although there's clearly something I'm not understanding properly, as I'd expect an LED to need more like 15-20mA to light up like that rather than less than 1mA. 

If anyone can explain this to me, or let me know what I should be looking for in spec sheets to make sure DMMs can perform this task, I'd be very grateful!  Or is it one of those things you can only test when you've actually got your hands on the meter?

Thanks ever so much.

Ghydda:
Fortunately LEDs light up at near zero current. Now whether or not your eye can detect that is another matter.

All modern signal indicator LEDs light up fine with 1 mA of current for your eye to see.
And most higher power LEDs also light up enough to to make you able to determine if the diode is ok and what the colour is.

Now to the DMM. The open circuit voltage of the BK 2709B as you mention is adequately high to put a light into most if not all single chip LEDs.
But that does not translate to the DMM will read out the diode voltage all the way up to its open circuit voltage level.

So if you need your DMM to read out the voltage for all single chip LEDs then I suspect you must verify this yourself by doing a hands on test at you local store on every DMM you consider buying.

seb1982:

--- Quote ---All modern signal indicator LEDs light up fine with 1 mA of current for your eye to see
--- End quote ---
Ah, I did not know this - thank you very much!  My old electronics teacher (years ago) taught me to use a rule of thumb of 15-20mA for an LED to light up, and I've never had cause to question it until now!  :palm:

That'll teach me to tinker more!  :-+

codeboy2k:

--- Quote from: seb1982 on April 11, 2013, 01:22:45 pm ---
--- Quote ---All modern signal indicator LEDs light up fine with 1 mA of current for your eye to see
--- End quote ---
Ah, I did not know this - thank you very much!  My old electronics teacher (years ago) taught me to use a rule of thumb of 15-20mA for an LED to light up, and I've never had cause to question it until now!  :palm:

That'll teach me to tinker more!  :-+

--- End quote ---

I used to design for 20ma, then I switched to 15ma... now I use 10ma as my design figure. As seb1982 said, all modern LEDs do not require much current, not like they used to in the 80s and 90s.  The efficiency has become so good now, that 20ma is almost always too bright.  Sometimes even 5ma is good enough to see, and I would use it if I was trying to save battery energy, for example.


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