Would a shorted pass transistor cause this failure? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 24, 2014, 11:02:45 am
content: This is hopefully my final question re restoring a big Farnell 60V at 50 amp linear power supply. I bought it with 2 blown components on the control board. A friend with a similar unit has experienced this and he found a shorted pass transistor. I therefore looked there first, and sure enough one of the pass transistors has a shorted emitter to collector, and a fairly low resistance to the base, too. I have renewed all ten. Could someone far more capable than I have a look and see if this issue was the likely cause of a blown D1 and R5 at :http://www.chriswilson.tv/H60002-1.jpgThe pass transistor schematic is at:http://www.chriswilson.tv/H60005.jpgand the interconnect schematic is at:http://www.chriswilson.tv/H60001.jpgThanks.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 24, 2014, 02:19:54 pm
content: Hi chris .You didn't say if D1 was open or short but (they usually fail shorted due excess voltage but they can fail open if you have put a large current through them )Anyway I would say yes a shorted pass transistor could cause R5 and D1 to fail (and possibly p2,p1 voltage adjust pot if your unlucky) due to excess current through those devices .Explanation :- If a pass transistor is shorted then V output would then be at max and someone would try to reduce ouput by turning V adj to minimum value ,now since the +-15V aux supply gnd rail is tied to main V+ out (so you will now have 60 V between this and -Vout) so now an excessive current will flow through D1,R5 via P1-P2 voltage adj control (which now is set to a low ohmic value when you tried to turn voltage down )back to -Vout .Note that a failed D1 on it's own ( D1 is shorted) could also cause R5 to fail because the output could again be at max V (this time the inputs of IC1 would be shorted together by D1 and so IC1 output would be stuck at one of the supply rails due to it's own input offset voltage ). Again trying to adjusting the output voltage down would cause large current through R5 ,P1-P2 to -Vout .

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 24, 2014, 03:19:03 pm
content: Wow, I am impressed, you are spot on, that explains why a pot had been replaced, this may have happened before, and it took a pot out. Someone has replaced the pot with a skeleton type. It must have suffered another pass transistor failure, after the diode and resistor and pot were previously changed. The skeleton pot survived the next failure, as a new diode and R5 fixed it up! You are spot on!!Thank you so much for the detailed and easy to follow explanation Kevin, can't thank you enough Sir.http://www.gatesgarth.com/boards/boards.htmlshows the board before the last repairD1 had indeed failed shorted.

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 27, 2014, 05:41:25 pm
content: I am afraid I am still having issues with this. I repaired the board, and replaced all 10 pass transistors (including the "pre regulator" pair. I know the repaired board, and the other 2 control boards work as I can test them in a sister power supply, that uses the same boards, albeit the supply is only a 25 Amp one. It still does 0 to 60V like the bigger one though, just fewer pass transistors and a smaller transformer.When I put the repaired board in the big supply, with all new pass transistors fitted, all rectifiers tested, and all output smoothing caps checked for value and ESR (about 0.04 Ohms for each of them). I suffered the same two components failing. R5 lit up like a light bulb. D5 has this time failed pretty much open, testing at a couple of megohms both ways. I had brought the supply up on a Variac and was monitoring output voltage both on the supplies panel meter, and on an external multimeter across the output terminals. I had the PSU front panel volts set low, as well as maximum Amp control set low. As soon as I got about 190V AC into the supply the volts on the rear terminals came up to about 4V, nothing on the front panel meter though, then R5 lit up and that was itWithout the board in at all I measured the AC volts on the secondary of the main transformer. It was weird, from switch on it started at about 14V rose very slowly over a few minutes to 60 or more V when I decided it was prudent to turn it off. The same happened across the smoothing caps, albeit DC, of course. Could I trouble someone for more ideas as to what may be causing this please? I have re checked the repaired (again...) board in the smaller supply, along with the other two boards, and the smaller supply is working fine. Sorry to be a pain, thought I had it licked. Cheers! The full schematics and operators manual are at :http://www.gatesgarth.com/supply/supply.zip

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 27, 2014, 08:03:07 pm
content: Hi again Chris . If your sure all your 4 boards are ok since you tested them in another supply then it must be something left inside the case , interconnecting wiring and plugs etc shorts/opens ? check this first .From R5 D1 blowing again it still sounds like your output is still unregulated and going to max .What I would do here is repair D1 and temporary replace R5 with a 22K (disconect a leg of C10 if your supply works but starts to oscillate) .With that larger resistor in there you may be able to probe your working voltages on the output and around IC1 to see whats going on .

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 27, 2014, 08:43:59 pm
content: On the "Control Board" PCB that was burnt up when I got the bigsupply, with R5 and D1 blown, the tracks to pin 3 and pin 5 weredestroyed. So the excess current came via those tracks. Pin 5 on theControl Board goes to the +15 0 -15V from the "Auxiliary Supplies"board, specifically to the 0V rail. Pin 3 goes to P2 and P1 which arethe front panel pots for voltage adjustment. One for fine, one forcoarse. Then on to the - output terminal via D26.What could P1 or P2 pots being shorted or open cause? If open nocurrent would flow... Can a pot go short?I am wondering if there's an issue with P1 or P2? Open circuit? Thething's in the garage as it's plain too damned heavy to get upstairssingle handed, so test gear and itself are some way apart. I willcheck the pots though, for sure.How is voltage regulation shared between the pass transistor array, andthe thyristors feeding the main transformer's primary? I was assumingthe thyristors just tweaked input voltage to allow for mains sag, andthe bulk of voltage output variation and control was via the passtransistor array. Maybe I am underestimating how much importance thethyristors have in output voltage control?Thanks Kevin! I will also try the substitution for R5.

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 27, 2014, 09:03:24 pm
content: P1 front panel fine voltage adjust pot is open at any position other than fully counter clockwise.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 27, 2014, 09:35:53 pm
content: quote :- "The thing's in the garage as it's plain too damned heavy to get upstairssingle handed, so test gear and itself are some way apart. I willcheck the pots though, for sure."How heavy is a multi meter to carry ?.Your thyristor pre reg just keeps the collectors a few volts above the output,so the pass transistors dont have to dissipate so much power. I dont think it has anything to do with your issue .P1 failed same time as D1 .After you have checked p2 and wiring is ok and no shorts (also recheck C3),do the R5 substition and note the output voltages ,plus those around IC1 , Vt2 .(relative to aux gnd)

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 27, 2014, 10:08:51 pm
content: Quote from: Kevin.D on April 27, 2014, 09:35:53 pmI will substitute a similar size fixed resistor instead of the 250 Ohm P1, for now, until I can find a new pot. I will substitute a higher value resistor as you suggest, for R5, and take notes of all voltages on the pins of IC1 and the pins of VT2. I just checked C3 again, and it's OK, 4.93uF with an ESR of 0.05 Ohms. That's with a Peak ESR70 mini ESR meter. I'll report back tomorrow, thanks again Kevin.

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 28, 2014, 12:35:43 pm
content: Morning!I replaced R5 on the "Control Board" with a 22K resistor, and temporarily replaced the open fine voltage adjustment front panel pot with a 230 Ohm resistor.The supply seemed to behave with full control of the voltage out at the rear terminals. I took the measurements suggested at the pins of IC1 and VT2. I also found a couple of other issues. The front panel volt meter is stuck, looks like a bearing issue. I think they are still available though. The coarse front panel pot for current is scratchy and intermittent. I will replace all the front panel pots I think! Current control worked against shorted rear terminals. Whether the thing seems to work is a result of removing the open front panel fine voltage adjustment pot i don't know, or as a result of the much higher 22K resistor in position R5 as opposed to the original 470 Ohm one.With 7.95 V DC showing on the rear output terminals, and 5.32V AC on the main transformer secondary, these are the pin out volts relative to the +15 0 -15V supply 0 volt rail.IC1:Pin 1 - 15.32Pin 2 0.0Pin 3 0.04Pin 4 - 15.53Pin 5 - 15.55Pin6 - 0.540Pin 7 15.39Pin 8 Not UsedVT2:Collector 8.56VBase 2.22VEmitter 1.609VThe voltage output of the transformer secondary adjusted from 0.14V AC to 25.32V AC with the front panel voltage controlThe rear panel output terminals voltage adjusted from 0.33V DC to 56.8V DC with the front panel voltage controlCould the problem have just been that the front panel fine voltage control was open circuit anywhere other than fully counter clockwise?Dare I reinstate the 470 Ohm resistor at R5?Thanks!Oh, and the rest of the parts to build a duplicate "Supplies Board" to replace the missing one arrived, the board's fully populated now, and works fine in both power suppliesPITA about the meter, I can see one being quite dear, if available at all anywhere.UPDATE!!I added a parallel resistor across the 22k R5, and monitored the voltages. I used another 22k at first, then reduced the resistance in three steps. All was still well, so I removed the 22K and refitted the 470 Ohm one. All still fine. Needs a new front panel fine voltage adjust pot, so I will renew all 4 pots. All I can assume is having the series connected 10 Ohm and 1K ohm front panel voltage control post go open at anything other than fully counter clockwise on the fine one, was the cause. Is this feasible? I am happy it lives again, would just like to understand why though

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 28, 2014, 07:40:02 pm
content: Those test Voltages around IC1 vt2 all look correct .So it looks like your regulation is currently working fine ,which is a bit of a surprise really since when you tested it previously I was pretty sure the output was unreg and max ,and thats what burned your D1 and R5 and fine cntrl P1 (If P1 was already open before this you see then it would have caused your output to max out BUT D1 and R5 would not have blown since no current path through P1) .So maybe when you where doing your latest board repairs and plugging things in and out you moved some wire/pcb connector in the case which before wasn't making good contact and so caused the previous unreg but now it seems good .Looking at those voltage above you have posted it would seem to be ok to replace R5 with original resistor if you wanted .(could use pot and wind down slowly if you want to test if gets hot .If this was my supply though I'd probably replace those cheap course+ fine controls with a nice ten turn pot and then set R5 to 10K and reduce C10 to 1nso that any future probs with unreg faults like shorted pass transistors doesnt take out D1,R5 and my expensive adjust pots with it .(reducing C10 in proportion to increasing R5 keeps the pole at 1/2 pi R5 C10 approx the same cuttoff frequency ,since this poles Fc may play a part in loop stability) (or keep same value and use fusable resistor for R5)