Current Limit for Diode Testing - DMM

[ultrasmurf]

Anyone has a schematic for Uni T 33B DMM ? I have this cheap DMM and would like to increase the maximum current supply when doing the diode test. I have bunch of optoisolator that I would like to do a quick test, and it seems the DMM hits the max current limit before the led side of the opto turns on. I have test few opto with a power supply/freq gen/oscilloscope and they are confirmed good, just that when I use the DMM to do a quick test, it is not able to turn on the led side..

Thank you for your help.

[ultrasmurf]

Thanks for the suggestion blueskull. I have that a jig done up in the lab setting with freq generator and oscilloscope that can test the optoisolator. I wanted to mod my dmm to be able to do the simple led side test because most of the time if I need to do a quick test in the field, I only have my dmm with me. So far, this test has helped me to troubleshoot optoisolator/photointerruptor issue almost 90% of the time. i.e. quick confirmation that an opto/photointerruptor is not working. The rest of 10% is where I need to dismantle the pcb, come back to lab, test the opto, and find out they are ok.... Trying to cut these down, if it cannot be done, then yes I will build another small jig just to test the opto.

[digsys]

Generally it's not just a c/current, but a MAX voltage as well. This is why there are many DMMs that can't turn on LEDS, and others that go to 3V
I'd guess that there'd be a voltage clamp or CC/CV limiter, so it may NOT be able to be done, without triggering a fault. You are better off finding a DMM that
can go to 3V, then you can do all sorts of LEDs.

[ultrasmurf]

multimeter_ut33d_sch.pdf_2 by ultrasmurf, on Flickr

Am I reading this schematic correctly ? The resistor R18 is the one that will be limiting the current when the DMM is set as a diode/continuity testing ? Seems it will be limitting the current to about 16mA for 1.5-1.6V diode.

BTW, my DMM is UT33D not B as initially described, didnt have the DMM with me when I post the question.

[Wytnucls]

Max diode test current on the 33D is unlikely to be above 1.5mA.
The best meter for diode testing is the Chauvin Arnoux MTX3293, which has a Zener mode (26V 10mA), but it is expensive.