multimeter pcb optocoupler test? - Page 1
Forum section: electronics / repair

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2022, 04:24:30 am
content: So I have a 4N32 optocoupler on a goo boardIts a darlington. When I power it from a 1.3V (like 50mA) power source with clips going to PCB and connect a multimeter across the output in ohms it measures like alot of ohms in the Kohms+ rangeIf the current transfer ratio is 500% for a darlington, the resistance should read much lower right? It should be like in the low ohms range?The part has a discoloration/polish on it that makes me suspicious of it like the die over heated. I dont want to desolder more goo shit unless I have to

author: abdulbadii
author_rank: Frequent Contributor
timestamp: February 18, 2022, 03:48:08 am
content: is it true 6/5 to 4 polarity ?

author: aqibi2000
author_rank: Regular Contributor
timestamp: February 18, 2022, 12:14:49 pm
content: Why the hell are you measuring the resistance? It’s not a POTENTIOMETER.

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2022, 04:07:25 pm
content: cuz I recall measuring another one some where and it was like 10 ohmswhatever, i ordered a replacement. all the effort to remove that 1$ part from circuit will be replaced

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 18, 2022, 11:15:46 pm
content: I don't know optocouplers or Darlingtons that well, but since this one brings out the base connection couldn't you just do a diode check B to E and B to C like any other BJT? Then diode check the LED on the other side.

author: coppercone2
author_rank: Super Contributor
timestamp: February 19, 2022, 12:28:06 am
content: lol i totally forgot about diode test... lets seeSo 6(P) - 5(N) is 0.7V, ohm reading 100Kohm,if I try to measure 6-4 with ohms (diode shows nothing), I get 2 megaohms.Not able to get any diode measurements measuring anything else. I think it should give me two readings. Base emitter should read something I thinkBut I am pretty sure I measured a turned on opto before (non darlington) and it has a pretty low resistance, around 20 ohms.. which is why I did the test in the first placeI think the 34401A has a 1V range on diode.. so a darlington is a problem I guess, since it would be 1.4V.Let us connect to the second base rather then leaving a stupid NC pin! but that would be an isolation problem for 4kV

author: coppercone2
author_rank: Super Contributor
timestamp: February 19, 2022, 10:06:28 pm
content: Well I decided to try other multimeters to test the darlington, and a fluke 289 shows a correct voltage drop of 1.4V. At least I think thats correct, if the transistors have similar drops. The test currents are different on Ohms mode, they don't agree.Now it does not line up with what I have seen other turned on opto couplers read on Ohms mode (but that was a different part and a different meter), but its different by orders of magnitude (~20 ohms vs 200Kohms), and it does not explain the discoloration on top of the chip which I would naturally attribute to die heating... but its something to think about in this wild goose chase.It's assosiated with the signal chain though, so it might be worth replacing for 90 cents and the power transistor nearby.