Circuit board has a dead short across power leads - Page 1
Forum section: electronics / repair

author: EasyGoing1
author_rank: Regular Contributor
timestamp: February 06, 2025, 10:03:39 am
content: This circuit controls an electrostatic water atomizer (creates a fog used to humidity a small terrarium). It stopped working so I decided to check it out and when I put the bench power supply to it, with the power switch in the off position (the potentiometer doubles as the power switch) it clearly is a dead short and the MOSFET began heating up rather quickly. I checked the power leads with the meter and it reads 1 ohm from the positive pin to the ground pin. Not sure where to start looking. Also not sure what that dome-looking thing is ...I removed C21 cause it looked like it might have had some burn damage on one side but I checked it with the meter and it read 22µF which is whats printed on it.Any assistance or guidance would be much appreciated.

author: benj38
author_rank: Frequent Contributor
timestamp: February 06, 2025, 01:15:49 pm
content: If the MOSFET started heating up then you have your main suspect right there!why don't you remove it and do the resistance test again?

author: kripton2035
author_rank: Super Contributor
timestamp: February 06, 2025, 01:31:16 pm
content: build yourself a shorty-with-display (link in my signature)it is an incredibly useful tool to always have on hand for this kind of short finding...

author: EasyGoing1
author_rank: Regular Contributor
timestamp: February 06, 2025, 02:14:13 pm
content: Quote from: benj38 on February 06, 2025, 01:15:49 pmWell what bothers me is that the mosfet is getting power at all with the power switch in the off position... But I have no problem removing it and doing the test again.Quote from: kripton2035 on February 06, 2025, 01:31:16 pmI watched your videos ... so its basically an insanely accurate resistance meter?

author: benj38
author_rank: Frequent Contributor
timestamp: February 06, 2025, 03:04:03 pm
content: Quote from: EasyGoing1 on February 06, 2025, 02:14:13 pmIt is not so uncommon to have the switch only control the MOSFET drive circuit, and not cut the power off to everything. If this is the case here should not be too hard to figure out by tracing and writing down a schematic of the part of the circuit involving the switch. If it looks like some of these traces go into internal planes than this would be hard to do unless you get rid of the short first, since you will then have to rely on continuity measurements to figure out the circuit.Since it would take seconds to remove the MOSFET, I would start with that.

author: kripton2035
author_rank: Super Contributor
timestamp: February 06, 2025, 03:39:36 pm
content: Quote from: EasyGoing1 on February 06, 2025, 02:14:13 pmit's a relative (i.e. not absolute) accurate resistance meter yes. 0.1mΩ resolution. you can pinpoint a short within a few mm of pcb.

author: ledtester
author_rank: Super Contributor
timestamp: February 07, 2025, 02:59:12 am
content: If you have a LM317 lying around this shorty implementation might do the job:Convert Your MULTIMETER into an accurate SHORT CIRCUIT TRACER Finder for less than $1 Build This DIY -- Learn Electronicshttps://youtu.be/P_2GGNr4q1sIt doesn't attempt to measure resistance in an absolute sense but just relative to other points in the circuit.

author: EasyGoing1
author_rank: Regular Contributor
timestamp: February 08, 2025, 07:52:50 pm
content: Quote from: benj38 on February 06, 2025, 03:04:03 pmI went and did that right after replying to you and it did prove to be the problem.However, I only had a SIMILAR mosfet (same package type - slightly different specs) - rated for higher current, lower voltage... long story short, I put one in that I had and the short was gone so I connected power but then saw a transistor light up really nice and bright and the it gave up the smoke ...Since I have some delicate young plants relying on that thing, I was kind of in a hurry but after I smoked the board, I just ordered a completely different (and much higher quality) humidifier which was delivered the next day by Amazon ... it's installed and running and everything is happy.I threw the broken one in the trash where it belongs.

author: EasyGoing1
author_rank: Regular Contributor
timestamp: February 08, 2025, 07:58:26 pm
content: Quote from: kripton2035 on February 06, 2025, 03:39:36 pmI'm interested in building one ... I got a grip of Nanos laying around and some of the other components in the list. I was a little surprised that you hadn't designed a PCB for it.If you're interested, I could draw one up.

author: kripton2035
author_rank: Super Contributor
timestamp: February 08, 2025, 08:40:25 pm
content: I build a pcb, just after releasing the project, but in the meantime building a version 2 of shorty... that did not work like expectedso the pcb is for a version that does not work.I left the project aside because of other life focus, but it's still in my mind to make it betterso one day there will be a new version and a pcb with it !version 2 would have a better constant current source, an extended range, and easier to build (less components)

author: ledtester
author_rank: Super Contributor
timestamp: February 08, 2025, 10:23:34 pm
content: On the subject of shorties, this video just came out today:Designing the B-Beep! The Ultimate Smart Diode Tester That Beeps!! -- Electronics Old and Newhttps://youtu.be/KteGN6v-WBgMain ideas:- displays voltage drop- audible feedback identifies type of PN junction (number of beeps)- button on handle to reverse polarity of probes

author: EasyGoing1
author_rank: Regular Contributor
timestamp: February 09, 2025, 04:20:10 am
content: Quote from: kripton2035 on February 08, 2025, 08:40:25 pmWell I drew one up last night ... I made a screen video of it with a couple of images you can see them herehttps://simtechdata.com/shorty/

author: kripton2035
author_rank: Super Contributor
timestamp: February 09, 2025, 08:51:23 am
content: nice. I would just have set the arduino more to the right so that if you make a box the usb plug is accessible ?anyway it's open source do what you want with it.others on this forum have made a similar version using a stm instead of an arduino.https://www.eevblog.com/forum/testgear/finding-short-on-motherboards-with-a-shorty-(with-display)/

author: EasyGoing1
author_rank: Regular Contributor
timestamp: February 11, 2025, 09:46:04 pm
content: Quote from: kripton2035 on February 09, 2025, 08:51:23 amI'm a fan of the Waveshare RP2040-Zero myself ...https://www.waveshare.com/media/catalog/product/r/p/rp2040-zero-2.jpgDual core lightening fast processor puts the Nano to shame... built several projects with it over the last few months.The only real core design change I would make in your schematic would be to use a Drok mini-buck converter.https://www.amazon.com/gp/product/B08JZ5FVLCThey're really nice and extremely efficient. You can cut a trace on the board then solder a jumper to select a voltage (1.8, 2.5, 3.3, 5, 9 and 12) or use the pot to dial it in but I always do the jumper for a stable voltage. I would power the nano from the 9 volt then power everything else from the buck converter ... where the power switch of course cuts power to all of it.