I need someone to educate me on what I’m doing wrong. While tinkering with some LED Christmas lights I started checking various lights with my multimeter in the diode/continuity test mode. On a LED that I know was good, the continuity tester didn’t indicate an open circuit. I tried the leads both ways, but no luck. I’m sure I’m doing something stupid wrong. YouTube even showed some people able to light up their LED with the multimeter.
Does the DMM have a “diode check” function? If so, try that.
In the “diode check” function, the DMM sources a constant current (typically around 1 mA) and measures the voltage drop across the diode. For a good silicon diode, the meter will show a voltage of around 0.6 V when forward-biased and an open circuit when reverse-biased.
Depending on the meter, the “diode check” function may or may not work with an LED. The reason is that the forward voltage drop across an LED is around 1.6 V to 4 V depending on the color, and the meter may (or may not) be able to produce this voltage when using the “diode check” function.
Honestly, the best way to check an LED is to simply light it up using a 9 V battery and series resistor. (The value of the resistor will depend on the nominal forward voltage drop across the LED, which is determined by the color it emits.) An even better approach is to use 9 V battery and LM317 configured as a constant current source.
I have a Mastech MS8268. The diode setting is the same as the continuity, then you press ‘select’ to put it in the right mode. I tried both sides by the way. Both show an open connection. I appreciate the top on using a transistor battery. Mostly I was just trying to learn how to use my dmm little better.(I’m a failed electronics hobbyist) From what you’re saying I need a better Meter.
I don’t think there’s anything wrong with your meter. It’s just not designed to test LEDs with the diode test function. Like Crafter_Man said, LEDs have a significantly larger forward voltage drop and I think (but couldn’t find official specs) not many DMMs will test them correctly.
I just grabbed two random LEDs; one was red, and one was green. Both had a forward voltage of 2V as indicated by a multi-tester. I tried a couple of digital multimeters on them, without success, while a normal diode tested fine. So, yes, it seems that you need a LED tester, or go for the multi-tester (mine handles transistors, diodes inc. LEDs and Zener, resistors, inductors, capacitors, etc.)
The LED should have a forward voltage drop similar to the number of electron volts of energy of the photons emitted, so something like 2 V for red and 4 V for blue. At lower voltages they should draw very very little current and look like an open circuit. Around their typical forward voltage drop they will appear to conduct and, incrementally, conduct vastly more current for a small voltage increase, and blow out if connected to a strong voltage source somewhat above their rated forward voltage drop. The difference in current should grow exponentially with increased voltage. There’s a small temperature dependence too.
My DMM also has ‘multi function socket’ which I think is used for testing transistors. Would that work? If so, any tips?
I tried sticking the LED’s anode into “C” and the cathode into “E” on the NPN tester socket, and it worked (lit up). It seems you can get around 2.8V there, as opposed to 1.5V max in diode-test mode.
Don’t forget Crafter_Man’s suggestion to use a 9V battery, resistor, and/or LM317…
ETA On the multimeter I used, the transistor-test socket is a permanent part of the meter; it does not look like your linked image.