Multimeters rarely make LEDs emit light.
With GW121 the maximum current delivered seems to be (15V - 3.2V) / 2200Ω ≈ 5mA, based on specs.
As a blanket statement, that’s complete nonsense. Not every LED will illuminate on every multimeter, but a great many will.
As cortex_m0 already said, many modern LEDs are insanely efficient. On my current project, I have various diagnostic LEDs, and to keep power consumption low, I am using white LEDs (which are the most efficient ones I have) and run them at around 50 μA IIRC. More than bright enough to be seen indoors.
Probably more correctly, InGaN LEDs are usually easily lit by multimeters with an output voltage above 2.5V, but AlGaInP, GaInP LEDs are still widely used for indicators which will not always be illuminated in diode test mode.
1. I never said “always”, I was very clear on this.
2. The old LED chemistries tend to be lower voltage, so have little trouble illuminating on even rudimentary diode test modes. The newer, higher voltage chemistries may require a higher test voltage, but many multimeters are perfectly capable of supplying this, including many of the most popular models (like the Fluke 87 V).
I own a number of different multimeters, some cheap, some expensive, which I think are very representative of the world of digital multimeters. I just tested numerous LEDs (some vintage, some modern) with six different meters:
- Fluke 87 V ($$$)
- Keysight U1252B ($$$)
- Keithley 2015 ($$$$)
- Aneng AN8008 ($)
- Mastech MY64 ($)
- No-name VC830L (¢)
All of them could light and measure the least efficient of all the LEDs, a very old red LED that lit very dimly on all the meters. (Even at 20mA it’s a very dim LED.)
All of them could light and measure a typical vintage red LED.
All of the meters except the Mastech could light the most challenging of all the LEDs, a modern UV LED (those have a Vf approaching 4V at 20mA), enough to see it in a lit room. Only the Fluke, Keithley, and Aneng could measure it, but the no-name and the Keysight could light it without having a reading.
A modern blue LED, two different modern green LEDs, and a white LED were lit by all meters, but again, only the Fluke, Keithley, and Aneng could measure them.
A modern red LED, a modern yellow LED, a vintage yellow LED, and a vintage green LED lit on all meters, and could be measured on all except the no-name.
Remember that the statement I was refuting is this one:
Multimeters rarely make LEDs emit light.
These quick tests not only disprove that nonsense claim, but disprove it
decisively, with literally only a single LED, the challenging UV LED, refusing to illuminate visibly on one single meter out of six. I tested 11 different LEDs, meaning a total of 66 tests. Of those 66 combinations of meters and LEDs,
only one single one failed to light.
So I stand by my statement that
most LEDs will light on
most multimeters.
Had the question been “which meters can
measure high-Vf LEDs?”, the discussion would be rather different. But the question was whether they
emit light.
[Edited repeatedly to fix the mathml, which isn’t parsed in the preview, misconstruing the dollar signs.]