How to find the lowest resistance components?  - Page 1
Forum section: electronics / beginners

author: mopplayer
author_rank: Contributor
timestamp: December 15, 2024, 11:44:19 am
content: Hi all,I have stuck finding the lowest resistance components on a PCB for a long time.I have to find about 560 ohms components to repair on a 3.3V rail of a PCB, because a good PCB is about 8.4K ohms.Typically, the resistance of 3.3V rail would auto-calcauted to a nearby value (both 560 and 8.4K) from top to bottom or bottom to top by Brymen 869s digital meter, so I think it should be a damaged capacitor, if I was wrong, please remind me.However, most methods are trying to find a short component on that PCB, i.e. ~ 0 ohm. I have replaced the shorted components, but I still get a low resistance on 3.3V rail (560 vs 8.4K).Is there a way to find the damaged components? I don't want to desolder about a hundred of capacitors.

author: kkontak
author_rank: Contributor
timestamp: December 15, 2024, 11:58:02 am
content: If nothing gets hot enough to be detectable by a thermal camera or any of the cheap tricks with vaporized IPA or smoke, I would try to isolate any 3V3 rail branches if possible, to minimize the search area.

author: mopplayer
author_rank: Contributor
timestamp: December 15, 2024, 05:52:36 pm
content: Quote from: kkontak on December 15, 2024, 11:58:02 amThermal threshold trickies have found the short 0 ohm components, but 560 ohms components are not enough to identify that, the thermal is near by normal.Any help would be appreciated.

author: inse
author_rank: Super Contributor
timestamp: December 15, 2024, 08:27:01 pm
content: It may as well be a damaged semiconductor - a shorted zener diode that is causing higher current consumption, a damaged OPAMP with output at supply rail level or a shorted transistor for example.Or a broken capacitor that is shorting a signal, not the power supply.Defining the deviation via the calculated resistance is an unusual approach, normally one would say “the circuit draws 6mA instead of 0.4mA on the 3.3V supply…

author: mopplayer
author_rank: Contributor
timestamp: December 21, 2024, 01:02:54 pm
content: Quote from: inse on December 15, 2024, 08:27:01 pmThanks,I have checked and desoldered suspected diodes and other 3.3V-rail chips, but don't find any wrong components. I don't have the power supply equipment to check consumed current, so I only check the resistance.DT4282 open-circuit 2.07 V get test on 3.3V-rail:reverse resistance: 419.40 ohmsforward resistance: 557.42 ohmsBrymen 869s open-circuit 1.15V get test on 3.3V-rail:reverse resistance: 0.5640K ohmsforward resistance: 0.5666K ohmsMight the test voltage on different reverse resistance values have some ideas, was there really damaged capacitors?Any help would be appreciated.

author: CaptDon
author_rank: Super Contributor
timestamp: December 21, 2024, 02:21:08 pm
content: If you find one damaged integrated circuit holding down the 3.3 rail then the entire board is suspect for repeated failure. If you are going to do this level of troubleshooting then you should have a CV / CC power supply available. Set it at 3.3v and start increasing the current limit until you find a component that seems unusually warm. We have gone clear up to 10 amps with a voltage limit set at 1vdc and simply blown shorted tantalums right off the board. Often you will find an opamp so hot it will burn your fingers and also if there are buffer I.C.'s for signals entering or leaving the board they are likely to have suffered damage from external static or EMI so look at them as a possible failure point.

author: inse
author_rank: Super Contributor
timestamp: December 21, 2024, 07:18:14 pm
content: But you do have a multimeter, put it in line with the supply and tell us how much current a good vs a bad unit draws.Are we also allowed to see a photo of the PCB or schematics?