Industrial 24 VDC PS troubleshooting

[RD_Atlanta]

New to the forum...
The Power Supplies I'm having trouble with is branded under different brands (Rhino PSC-24-090 or Traco TBL 090-124) but I think the actual mfg is Convertec 090PSL182/E out of a factory in China. We have hundreds of these installed, and have operated flawlessly for many years. But suddenly there is no 24 VDC when the AC power is cycled. I was hoping that someone in the forum may have access to a schematic or is so knowledgeable about power supplies, could help me to better understand this power supply. Please refer to the [PSC-24-090 Top View Photo.JPG] and [PSC-24-090.PNG] attachment.
Though it's not much, here is what I know...
1.) Tracing back from the 24v output terminals, (I think) the earliest point where voltage can be measured on the circuit is at the point of the rectified and filtered DC coming off the mains.
2.) At this earliest point of measuring voltage, please refer to [PSC-24-090.PNG] and not the differences that I measure between a working unit and one of the failed units.
3.) All of these fail when AC power is applied.

All of my searches has not yet netted a schematic.

Thank you for any help that anyone can offer.

[R_J]

Re: Industrial 24 VDC PS troubleshooting

I would try replacing C16 and the smaller electrolytic beside it, these are likely the startup vcc capacitors and are likely drying out over time and loosing capacity.

[CapLeaker]

Re: Industrial 24 VDC PS troubleshooting

Welcome to the forum! A few more pics from the sides would be nice, as some parts are covered. I’d start on the secondary, making sure the rectifier diodes aren’t dead shorted. A 0.2V drop isn’t a short. Then measure the DC voltage on the big main filter capacitor on the primary. Do not get your grounds mixed up and be careful poking around in the primary. There can be quite a few reasons why this PSU isn’t working.

[harp]

Re: Industrial 24 VDC PS troubleshooting

All Good PS voltages are different, all Bad PS voltage are same... have you check for short on caps/diode?

[RD_Atlanta]

Re: Industrial 24 VDC PS troubleshooting

Thank you for your responses !!
I attached an 8 page PDF with additional photos & measurement points.

When I compare measuring components and voltages between one of the good & bad power supplies...I don't think that I see a single discrete component that measures bad (in comparison).

I realize that I could be off base but please let me share my (mostly anecdotal) thoughts...I'm somewhat suspicious of the TL3843b PWM controller is failing on these power supplies. Part of my logic is these hundred (literally) power supplies have been running 24/7 for 10 years. About 70 of them have failed while performing AC power cycle tests. I don't 'think' that this number of capacitors would fail simultaneously. All of the diodes measure a healthy looking fwd & reverse voltage drop & the caps all seem to charge & discharge (with an ohm meter), the transformer windings measure the same with an ohm meter (between the good & bad PS). As is noted on the last slide (8), it appears that the TL3843b PWM controller is somehow taking fdbk from a section of the transformer (I'm assuming the the transformer is purely used for switching DC purposes, but I really don't yet understand this section). There are no voltages present on the bad PS units beyond what I show on slide one.

I've tried to replace the TL3843b PWM controller on one of the bad PS units. I did surface mount soldering for a year and achieved a pretty good skill level. My method has always been to apply a small amount of upward force while blowing a stream of hot air on the SM pins & never had an issue with this method. That is until I ran into these PS units. On Saturday I applied the hot air stream (as I always did in the past) with a slight amount of upward force. But the TL3843b never would lift even though I could see the solder flowing. I sacrificed one of the PS units and cut the legs (with a dremel). Still the TL3843b would not lift (even though the legs were severed). Turns out the China factory used some serious adhesive to hold the TL3843b in place. So, I will need to come up with a way to overcome this (surprising) obstacle. The factory that I worked in, we never glued the SMT components. I had to pry it up with a screwdriver (after the legs were cut), it took some serious force. In the meantime, I thought I would reach out to see if I could gain confidence that the TL3843b could be the likely failure component. Does anyone have a good idea of how the TL3843b works in this type of circuit? I attached the datasheet for the TL3843b.

Thank you for your help!!

[stj]

Re: Industrial 24 VDC PS troubleshooting

if you have switching psu's the first thing to know is 95% of them have a capacitor and sometimes a couple of resistors that are responsible for starting them up - that often fail.

[Per Hansson]

Re: Industrial 24 VDC PS troubleshooting

On Saturday I applied the hot air stream (as I always did in the past) with a slight amount of upward force. But the TL3843b never would lift

Instead of trying to remove the control IC use that heat gun to heat the small capacitors and see if it comes to life. (Higher heat lowers the ESR of the capacitor).
Small caps from bad manufacturers are very prone to failure.
But even small caps from good manufacturers are prone to drying out when running hot, like they are here when next to a diode & heatsink.

[RD_Atlanta]

Re: Industrial 24 VDC PS troubleshooting

STJ & Per Hansson, thank you for your advice. I'm grateful to learn caps & sometimes a couple of resistors actually start the switcher (I did not know this, thanks). Also, good idea on increasing the temp of some of the capacitors, the SMT heat gun that I have has nozzles to where I can focus the heat stream. I'll follow with the results. Much appreciated.

[RD_Atlanta]

Re: Industrial 24 VDC PS troubleshooting

Raising the temperature of C9 & C16 allows the switcher to start on the bad power supplies.

Many thanks to you guys !!

[redwire]

Re: Industrial 24 VDC PS troubleshooting

I think the lone TO-220 covered in heatshrink provides startup power to C9 or C16.

[CapLeaker]

Re: Industrial 24 VDC PS troubleshooting

A lot of the times you have to look at the small caps, specifically those on the primary and whenever they are located near a heat sink. I did replace dead and weird acting PWMs (works for 40 seconds and then quits) like the TL3843 before, but that’s more of a rare case. One of your main tools for troubleshooting is your hot air solder station, a can of air upside down for freezing, a bottle of 99% IPA and a DMM. That’s to 99% all I use to fix a PSU.

A give away on these start up caps is: PSU ran a long time and doesn’t turn on after a power outage. If you are really in a pinch just warm up the PSU with a hair dryer and plug it it in. You did good!

[Dunerdr]

Re: Industrial 24 VDC PS troubleshooting

Raising the temperature of C9 & C16 allows the switcher to start on the bad power supplies.

Many thanks to you guys !!

Sorry to bump an old thread but i've got these things crapping out 20+ at a time during switching and power outages. Were you able to re use them?