Multimeter Diode Test Maximum Forward Voltage  - Page 1
Forum section: electronics / beginners

author: cjs
author_rank: Contributor
timestamp: July 10, 2017, 03:30:59 am
content: I've been trying out a few different multimeters and have noticed that the cheaper ones don't seem to give forward voltage beyond much over a volt. They'll show around 1.2 V forward on a series pair of signal diodes, but, though they'll light even blue and white LEDs, they show "OL" as the forward voltage on even a red LED with around 1.7 V forward voltage. More expensive meters will show forward voltages even up around 2.5-2.8 V (IIRC) for the white and blue LEDs (as well as lighting them up, of course).Is being unable to show >= 1.5 V (or whatever) forward voltages on diodes likely to be a problem, or even an annoyance for people doing electronics? If so, for what kinds of activities (beyond, e.g., sorting large piles of diodes) is it likely to be annoying?And how does the forward voltage test work, exactly, anyway?

author: alm
author_rank: Super Contributor
timestamp: July 10, 2017, 03:46:52 am
content: My guess is that it is not so much the downsides, but more the lack of upsides. Needing a forward voltage beyond 1 V is mostly limited to LEDs (and zener diodes, but they will likely be much higher). Unless you are doing lighting, you likely do not spend that much time measuring LEDs.The forward voltage works by trying to force a constant current of e.g. 1 mA through the device under test (diode). Forcing the current through the DUT will drop voltage (I * R for a resistor, and the forward voltage for a forward-biased diode). The maximum voltage is limited to something like 2 V. It is similar to how resistance is measured, but instead of calculating the resistance from the voltage and the current using Ohms law, it displays the forward voltage directly. So if the 1 mA results in a drop of 0.5 V, the resistance range would display 500 Ohm (0.5 V / 1 mA), while the diode range would display 0.5 V.For measuring resistance you usually want a fairly low voltage limit because you do not want to forward bias any diodes in the circuit. If the voltage becomes to high, you would rather switch to a higher resistance range with a lower current (and hence a lower voltage drop). So the diode range would need to measure a larger voltage range than the resistance range.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2017, 04:03:42 am
content: Ideally there's 3 types of diode tests you'd want to accomplish.Standard silicon and Shockley diodesLED'sHV diodes with Vf in the 6-12V range.Very few DMM's can do all these, but IIRC Dave's new 121GW DMM has high Vf capability.

author: alm
author_rank: Super Contributor
timestamp: July 10, 2017, 04:17:20 am
content: Shockley should probably be Schottky, unless you still have somevery ancient diodes.The Keithley 2000 DMM has 3 V and 10 V diode test ranges and a selectable test current of 10 µA, 100 µA and 1 mA. Granted, it is kind of overkill to measure an LED.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: July 10, 2017, 12:48:36 pm
content: Yes, Shockley is a psychologist.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 10, 2017, 02:52:13 pm
content: And also one of the three Nobel-prize recipients, for their work on the transistor at Bell Labs:https://en.wikipedia.org/wiki/TransistorBut back to the OP's question; one of the reasons that cheap multimeters will show "OL" while measuring white or blue LEDs, is that the LED's Vf exceeds the 2000 count capability of their displays.

author: Neilm
author_rank: Super Contributor
timestamp: July 10, 2017, 06:44:12 pm
content: Quote from: tautech on July 10, 2017, 04:03:42 amI have seen HV dioded with Vf of over 30V. Can't remember the part number but do remember the problems the test gear had when probing the boards

author: alm
author_rank: Super Contributor
timestamp: July 10, 2017, 07:22:28 pm
content: But at some point it is outside the realm of a DMM and you should just use a source measure unit or current source + volt meter instead.

author: bson
author_rank: Unknown
timestamp: July 10, 2017, 09:38:36 pm
content: It's not a problem at all. Use a power supply and a series resistor or pot, then measure the voltage directly across the LED. If you actually need better than a ballpark measurement, then this will let you measure it at the brightness level (= forward current) you will be using it at...

author: KL27x
author_rank: Super Contributor
timestamp: July 12, 2017, 09:04:57 am
content: Before i had a current limited bench supply, i just used a little 12V key fob alarm battery and a 10k resistor. I put a socket in my dmm where i could plug it in, when needed. Set the dmm to dc volts and plug the battery in. Dmm shows 12v. Touch the leads across a diode and the voltage will drop to Vfd. (And/or reverse breakdown of a sub 12V zener.)Oftentimes, i use diode test to figure out transistors. If they are pnp, pfet, npn, or nfet, the diode test will show it. I have a little board with 1 of each soldered down for reference. Cuz i cant remember how it goes.

author: 2N3055
author_rank: Super Contributor
timestamp: July 12, 2017, 02:45:23 pm
content: If you need to test diodes a bit more into detail maybe something like this ?