Why is the rectifier in my psu smoking? - Page 1
Forum section: electronics / beginners

author: rusty_uk
author_rank: Newbie
timestamp: August 30, 2023, 03:14:01 pm
content: Hi allJust getting back into electronics after a 25 year break (i studied in college) and have a faulty game console steering wheel to fix.The psu comprises of a triac (activated by 5v usb), a transformer and a bridge rectifier (i think). When i plug in the psu it does nothing and has no output which is expected. When i plug in the usb input it cooks the diodes in the rectifier. This has happened to 2 psu's now so I assume its something else thats perhaps drawing too much current???Any ideas on how i could start to fault find this?ThanksR

author: Eraldo
author_rank: Regular Contributor
timestamp: August 30, 2023, 04:33:36 pm
content: Is it a linear power suplly (with just a big transformer and the bridge rectifier) and could you post a photo of it?Also does the power supply not work at all or are just the diodes a bit smoky (do you get an output from the psu).As said above a big help would be to post some good quality photos from different angles and see the problem closely.Most likely the bridg rectifier has been shorted (dead diodes) and need replacement. Hopefully the transformer is protected with a thermal fuse or something.Does the transformer get hot?Also something i don't understand is about the triac. Do you need another external 5 volt source to turn on the power supply?

author: rusty_uk
author_rank: Newbie
timestamp: August 31, 2023, 08:41:18 am
content: HiApologies for the lack of info. Answers to your questions are as follows:Im not sure if the supply works or not. There is a fair bit of smoke so i don't really want to run it. The diodes on the rectifier are cooked so I suspect that it does not work. I must admit that I have not tested while its not plugged into the gaming unit so do not know if it works outside of this configuration. Perhaps i can replace the diodes on the rectifier and see as a first test.Just testes the rectifier and all are showing 4v in both direction which leads me to assume that they are dead!About the triac...It has an external usb (from the gaming console) that poweres it up. When it is not powered up the transformer should deliver no power. With 5v across the triac, the transformer should deliver 24vImages are attached

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2023, 09:51:02 am
content: Post a picture of the track side of that Triac board. I suspect its a dangerous non-isolated circuit that is totally unsuitable for use with external USB power. That sh!t may fly in 3rd world hell-holes where life is dirt cheap, but in any 1st world country could put you in jail for negligent homicide! It may also be a contributory factor to smoking the diodes!

author: rusty_uk
author_rank: Newbie
timestamp: August 31, 2023, 10:26:10 am
content: Underside of the triac board attached.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2023, 11:40:38 am
content: Quote from: rusty_uk on August 31, 2023, 10:26:10 amYour attachment is AWOL!

author: Eraldo
author_rank: Regular Contributor
timestamp: August 31, 2023, 12:10:16 pm
content: Honestly if possible i would just replace the entire thing with another switch mode power supply you might have laying around that you don't use (maybe from a laptop). 24v is a common voltage so a replacement is not that hard to find.As for the comment from Ian, it seems to have a triac driver optoisolator so it's not that dangerous from that point of view and i get the idea of turning on the steering wheel only when connectet to the console (since the transformer might get heated) but it's a stupid and unnecessary idea nonetheless.Now if you want to repair it and not replace it you just need to find some new diodes and replace them, preferably with higher rated current capability (but make sure they fit).ALSO if you have a multimeter with diode testing, test each diode individually (try it in both polarities if you don't know how) and every diode with a voltage higher than 0.8 v or lower than 0.3 is propably bad.But also check the traces if they are damaged and clean them. The capacitor looks fine and doesn't really need replacement. Hopefully the transformer is fine.But yeah, a better idea to replace the entire thing with another switch mode psu of the same voltage rating (you can measure the transformer rms ac voltage after you have removed the bad diodes and multiply that voltage with square root of 2 ≈ 1.41 to find the peak voltage. Most likely there is another post voltage regulation inside the device.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: August 31, 2023, 12:28:08 pm
content: From what I can see, the diode rectifier is powering the opto isolator which turns on the triac. So how does the triac turn on in the first place if there is no power flowing to the bridge rectifier? I think there is some nasty voltage feedback going on, but without a circuit diagram... draw one and things may become obvious.Seriously, take a lump hammer to this antique and find a safer alternative.

author: rusty_uk
author_rank: Newbie
timestamp: August 31, 2023, 12:45:11 pm
content: Forgot to attach the underside of the triac board

author: rusty_uk
author_rank: Newbie
timestamp: August 31, 2023, 12:50:24 pm
content: So just to add some clarification to this post since im not sure that I was as clear as I could be....The psu is the actual one supplied with the steering wheel and not some 3rd party junk (although im not saying that it isn't still junk).I have a second identical supply which I was told tested fine when removed from a different wheel. But once plugged into this wheel, started to smoke as well. This leads to to believe that something else is trashing these psu's. Perhaps by drawing too much current?My question would be around how to fault find what it might be. Are there any specific components that tend to be culprits for this? Is there some methodology that you guys usually use?

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2023, 01:16:42 pm
content: Originally I wasn't sure if it had an optoisolator - the topside photo was too blurry to be certain. However even if its got one, if there's less than 4mm creepage/clearance under it I wouldn't trust it, hence my desire to see the track side. The new underside photo shows adequate creepage/clearance distance and is characteristic of an optocoupler (probably an OptoTRIAC) soI withdraw my objections to its safety.I'd bet it draws 'wakeup' power to light the opto's LED from the console's USB Vbus.I *think* this Rube Goldberg abomination is due to theEU standby powerregulations, hence the need to cut power to the transformer primary, as they couldn't be arsed to redesign it with a SMPSU that meets the standby requirements in no load 'burst' mode.Time to tear the wheel apart, look for any evidence of overheating or blown parts and if nothing found, try powering it up on a current limited bench supply, measure its supply current and figure out if its excessive for the transformer and diodes used in the PSU.To work on the PSUs, start by cold-checking all the diodes with the output and one transformer secondary wire disconnected, replace any bad ones, then power them up with another approx. 5V source to drive the optocoupler (e.g. a lab PSU, or 3x fresh AA cells or a USB 'charger') and check they can supply the rated load current (if known) without excessive voltage drop or overheating.If you don't have access to a lab PSU to test the wheel and a DC load to test the PSUs it gets a bit harder to diagnose properly. . .N.B. I have seen a misfiring TRIAC burn out a transformer load by creating a DC imbalance. If the TRIAC firing remains symmetrical but is severely delayed the increased crest factor could theoretically overheat the secondary side bridge rectifier, but that's *far* less likely.

author: Eraldo
author_rank: Regular Contributor
timestamp: August 31, 2023, 01:22:35 pm
content: Quote from: AndyBeez on August 31, 2023, 12:28:08 pmIt is powered from the data usb that connects to the console. So it turns on and off when the console does and this way it doesn't overheat the transformer. It doesn't have anything to do with the bridge rectifier.But on another note seeing that there were two psu burnt the same way it lead to believe there is a possible short or a faulty component inside.I'm not familiar with these devices so better to wait for someone else to reply. But in the end you have to take it apart and see the problem. Maybe a capacitor in parallel went bad or the internal voltage regulator is not working as intended. Either way you will have to open it to find the fault.Now if we were to test it without opening it we would need to know the input voltage and feed that voltage and measure the current with a multimeter. Then compare the values with the typical power consumption of the device.But before even that, i would just try and measure the imput impedance (resistance) and see if it is indeed a dead short or not. And then open it if something looks amiss.

author: golden_labels
author_rank: Super Contributor
timestamp: August 31, 2023, 01:26:48 pm
content: Part numbers are not clearly visible in the photos, but I will try making some guesses from the layout and what I can see.It seems like there is a triac (ST BTA06?), controlled by a phototriac on the first board. Any misbehavior (edit: but please see Ian M’s post above regarding misfiring TRIAC) there would not end in diodes overheating. A transformer could fail in a manner destructive to diodes only if the primary winding shorted in a very peculiar and improbable way, or if there was a short between windings. The latter would likely kill more than just the diodes. So IMO only suspects remain is on the second board and beyond.Perhaps there is a short inside the wheel?

author: AndyBeez
author_rank: Frequent Contributor
timestamp: August 31, 2023, 02:02:20 pm
content: In the photograph, where is the USB connection with a clean 5 volts? I'm looking at an opto isolator driven by a rectified and smoothed voltage. Thisunregulatedvoltage then passes through a tordial with possibly a high reactance - then into the opto's LED. No USB involved in the test rig?