Must zener diodes be desoldered for test reading via multimeter? - Page 1
Forum section: electronics / beginners

author: niemand
author_rank: Contributor
timestamp: September 14, 2022, 03:51:11 am
content: I have a zener diode (1N4735A, supposedly) that I tested using a DMM while the diode was still soldered on the circuit board. Must a zener diode be removed from the circuit before being tested with a DMM?The readings I got were:forward: 0.50 vreverse: 0.55 vresistance: 508Ω" alt="" class="bbc_img" />

author: ledtester
author_rank: Super Contributor
timestamp: September 14, 2022, 04:21:35 am
content: Generally, yes. But you don't need to completely remove it - disconnecting one end is sufficient.

author: EPAIII
author_rank: Super Contributor
timestamp: September 14, 2022, 10:41:59 pm
content: Since you often (usually?) don't know exactly what the rest of the circuit will do to your meter reading, yes removing ANY component from the circuit is usually the only way to go.The general rule is that if a component has N leads, then N-1 of those leads should be disconnected. So for resistors, diodes, inductors, all components with 2 leads, one lead should be disconnected. For a transistor with 3 leads, two of them should be disconnected. Etc.That being said, sometimes you get lucky because the rest of the circuit has a negligible effect on the readings. This can tell you that a component is OK if the readings are what is expected for a good one. While this can work, in my experience it is a rare occurrence; perhaps less than 1 in 100 or more.But in circuit checks do not work to tell you if it is bad because a low reading can be due to another part of the circuit.Another good idea is as long as you need to ask, just remove the component first. I have been at troubleshooting for a long time and still use that rule.

author: Nusa
author_rank: Super Contributor
timestamp: September 14, 2022, 11:51:45 pm
content: In-circuit checks work better if you have a known-good unit to compare to or a schematic that includes expected readings at test points (once upon a time, this was a common practice on things like TV sets). Without that, there are a lot fewer assumptions you can make.

author: niemand
author_rank: Contributor
timestamp: September 15, 2022, 01:42:52 am
content: Here are the readings with the anode disconnected from the circuit:forward voltage: 0.70 vreverse voltage: OLresistance (relevant?): 2.90 MΩHere are the specs for the 1N4735A zener diode:https://components101.com/diodes/1n4735a-zener-diode-pinout-datasheetThe readings look to be within spec, I'm guessing.

author: ledtester
author_rank: Super Contributor
timestamp: September 15, 2022, 01:59:35 am
content: That is a 6.2V Zener and the OL indicates that your multimeter wasn't able to determine the diode drop voltage. Most multimeters can't measure a diode drop of more than 3 or 4 volts.Try a circuit like this:Code:+9 ---/\/\/--A---|<|---GND1Kand measure the voltage between point A and GND. That should read around 6.2V.

author: theHWcave
author_rank: Regular Contributor
timestamp: September 15, 2022, 07:33:44 pm
content: Unless the circuit is completely dead, the fastest way how I usually confirm Z-diodes, is in-circuit while the circuit is running. Your 6.5V Z-diode should have a voltage drop of about 6.5V across it. If it doesn't, and the power-rail itself is ok, the next thing is checking the voltage drop across series resistor feeding the Z-diode. With that voltage and the resistor value, you can calculate the current going through it. Depending on the circuit some part of the current will be going through the z-diode and some (usually much smaller) part going elsewhere to use the stabilised voltage. Check the data sheet and see what the minimum current for that diode is. If the current you measured is too small, the Z-diode can't produce the correct voltage drop and the fault probably somewhere in the circuit that supplies the Z-diode (including the series resistor). If the current you measured is above the minimum or much higher, I would suspect the circuit that takes the output of the Z-diode of s drawing too much current and you should look closely at those components. Only if either these checks don't seem to get anywhere would I desolder the Z-diode and use a circuit like this:Quote from: ledtester on September 15, 2022, 01:59:35 am

author: james_s
author_rank: Super Contributor
timestamp: September 15, 2022, 08:39:08 pm
content: It depends. I often can find shorted diodes in circuit, and if it shows completely open circuit you can tell it's bad, but there is a lot of gray area where you can't easily tell. It helps if you have a schematic and can see what else is connected to it in the circuit.

author: strawberry
author_rank: Super Contributor
timestamp: September 15, 2022, 11:47:13 pm
content: check film capacitorcapacitor dropper power supply might e bad news510ohm resistor might be coincidence