What happened to this Resistor ? - Page 1
Forum section: electronics / repair

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 04:48:33 pm
content: It tests good at 9.9k ohmsBut visually, it looks like it blew something out onto the PCB.I accidentally reverse connected a car battery to this Power Supply.Going by the tests of the resistor (All OK) is it possible something could still be wrong with this resistor?In one photo you can see what appears to be a burn on the resistor and in the other photo you can see a small smudge on the PCB under the resistor.Under magnification there are no cracks or holes but there is a microscopic silver speck as if a tiny piece of metal might be protruding but only by maybe 1000th of an inch or less.I don't know what to make of this or whether I should replace this resistor?Could a resistor pass all tests and still be bad?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 21, 2021, 05:17:28 pm
content: seems the resistor has spitted some stuff, burnt maks seen on the pcbOverheating, over current etc .... i would not hesitate to change it even if the value seems fine

author: TimFox
author_rank: Super Contributor
timestamp: July 21, 2021, 05:19:46 pm
content: There should be no way to smoke a 10 k resistor, apparently a “flame-proof” cermet, with 12 V (only 15 mW power), so the junk you see must have come from another part (maybe of the semiconductor persuasion, unhappy with reverse or over voltage), or from over temperature caused by another component. Therefore, I would look for the other culprit, as well as replacing this inexpensive component.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 21, 2021, 05:24:54 pm
content: Could it be smoke stained from whatever took the main fault current? D21 seems to be missing !

author: Gyro
author_rank: Super Contributor
timestamp: July 21, 2021, 05:35:51 pm
content: Quote from: TimFox on July 21, 2021, 05:19:46 pmNot very evident from the OP, but he says that he reverse connected the car battery to a power supply (bench?). From the bit of yellow tape visible, it looks like a mains SMPS. Unfortunately the photo is too close in to see whether the resistor is on the primary or secondary side (it would have to be primary to burn), but it's probably a symptom of a rather bigger problem - eg blown secondary rectifier causing primary side damage.Edit: A better title might be 'What happened to this Power Supply'.@OP: Can we have a photo of the whole board please, so we can see the position of the resistor in context.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 06:47:38 pm
content: Quote from: Gyro on July 21, 2021, 05:35:51 pmThe location of the Resistor in question is located midway between E and F at 1.5 under the edge of the toroidal filter near the read wire

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 06:54:23 pm
content: And here is a photo of that location without components in the way

author: Gyro
author_rank: Super Contributor
timestamp: July 21, 2021, 07:11:31 pm
content: Ha, those output rectifiers and heatsinks appeared quickly!Ok, so definitely secondary side. Reading between the lines, it looks as if you've already removed the output rectifiers and maybe found a problem with them?Reverse 12V at car battery sort of current will have reverse biassed all of the secondary side regulation and feedback circuitry (the output electrolytics too at probably a significant current).R47 looks as if it might be part of the feedback from the output (you'll need to reverse engineer that part of the circuit) but it must be going to a higher voltage node to justify its wattage (?). There's unfortunately quite a lot of stuff on the secondary side of the isolation barrier.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 07:15:07 pm
content: Quote from: Gyro on July 21, 2021, 07:11:31 pmThanks.Yeah, I've replaced some components and repaired a blown out section of track and it works now but this resistor looks suspicious and I just noticed it today.In this post, I'm really only trying to absorb info on that one resistor and why it appears like there is a burned spot on it and the adjacent board with no other components nearby that could have caused it, yet the resistor still tests OK?

author: bdunham7
author_rank: Super Contributor
timestamp: July 21, 2021, 07:27:09 pm
content: Quote from: ThermallyFrigid on July 21, 2021, 07:15:07 pmThe likely explanation is that it has been running hot, but hasn't failed yet. That is unlikely to have anything to do with the reverse polarity incident, unless perhaps the PSU partially fires up but the secondary side is still shorted or damaged in a way that grossly overloads the primary in a way that overheats the resistor. If I were fixing it, I would move on to other issues and then recheck it if and when the PSU is nominally functioning.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 10:53:59 pm
content: IT LIVES ! (WITH the questionable resistor)So, I just installed the ES2G I received from Mouser, put everything back together and powered a 12v, .01Amp fan with it....no problems.Hoo Rah!Yeah, I know. It could fail on higher current / voltage. But...the fact that it worked at this low amperage probably means I'm going in the right direction.So I'll be spot checking the temps of all the power components as I incrementally increase the load up to the full 5 amps.Next I'll find a 1amp load and try it.Ended up replacing the P75N75 Mosfet, both IRF840's, an ES2G diode and repairing a blown out track on the PCB at the output side (figures since I connected a battery backwards there) I also ripped out a few pads and had to repair those.Going to hook it up to the O-Scope and see what the output looks like.who knows....maybe I fixed it.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 11:51:43 pm
content: Up to 2 amps no problem. Checked the board with thermal imaging...no hot spots.SFSGWhat worries me now is will it blow again if I connect it (properly) to a 12v battery ?Just because it will power a load such as a motor or fan, does not mean it will not malfunction if connected to a battery.The difference being connecting to a battery imposes a reverse load into the power supply when connected and the power supply voltage is below the voltage of the battery IF.....the board does not have protection (which I think should be diodes)

author: helius
author_rank: Super Contributor
timestamp: July 22, 2021, 04:19:41 am
content: Large lead-acid batteries have low internal impedance, so can supply high current. Power supplies, generally, are not designed to absorb power from their output terminals. The consequence of this is, if you wish to use a power supply as a lead-acid charger, you must set its output voltage, turn output on, and THEN connect it to the battery to be charged (yes, there is a small spark). When charging is complete, disconnect the leads before turning the supply off.There is a further issue with many power supplies made for sensitive transistor circuits, using OVP (over voltage protection) devices. These cannot be used as battery chargers, as they intentionally short their outputs together when a voltage is supplied there.