What happened to the diff amp in PSU  ? - Page 1
Forum section: electronics / repair

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 03, 2019, 05:15:36 pm
content: I made a PSU based on a Farnell L30 , with a 2n3055 on the output, and broke it for the 2nd time, while I was running it at 15V and a little over 1A into a MOSFET load tester I made. I think I had tried to adjust the voltage up on the PSU. I can't say if the current spiked after I noticed the output voltage going wacko and I shut it right off.Farnell - L series Bench Power Suppliesthe very last schematic on page 11http://www.introni.it/pdf/Farnell%20-%20L%20series%20Bench%20Power%20SuppliesWhen it worked, the diff-amp worked and the feedback loop worked and it would balance. And so (VT3)/Q3 Vbe was held around 0.55V and barely moved a few milli-volts, never even goes over 0.6V. I thought maybe thats too low, but IDK. I could set the voltage from 0-25V or so with 30.5K total voltage control potentiometers P1 and P2Now the output is stuck at max, which is the primary winding at 43V unloaded (it's a 36Vdc@1.5A winding).When only the control circuit is turned on, everything seems about right. (VT2)/Q2 is off all the way (I think it used to be on? I don't remember). When the whole circuit is powered, (VT1)/Q1 BE is reverse baised, so off, and Q2 is fully turns on, so Q3 Vbe is 0.7V now, and saturated so Q3 Vce is under 0.1V , and so pulling the 10k collector resistor all the way low, and so the PSU output is stuck wide open.I pulled the diff.amp BC558's, and I matched their beta and Vbe, and they both still read the same and match. The 2 1n4148 diodes seem ok in circuit, I matched them too.When it's un-powered, everything else seems to checkout in circuit with a DMM, the voltage control resistors are all still working this time.I should check the zener that fried last time, but it's at least working, last time it went short.What would break the diff-amp ??

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 05, 2019, 01:00:41 pm
content: The output 2n3055 collector-emitter went short. IDK if it was the poor design of my load tester, previous damage, or maybe this is not a real 15A 2n3055So it's working again, the Vbe of Q3 is back around 0.500mV

author: xavier60
author_rank: Super Contributor
timestamp: June 05, 2019, 11:16:26 pm
content: Although I didn't make sense of all of it, some of the voltages were not consistent with having a shorted output transistor.VT1 and VT2 are part of a constant current source for Z4 so that it will produce a stable reference voltage. The only thing that can affect this area is low rail voltage.EDIT: I was looking at the wrong schematic.

author: floobydust
author_rank: Super Contributor
timestamp: June 06, 2019, 12:45:41 am
content: It's the last schematic page, all discretes?The pass-transistor output stage is usually full on, due to R10 10k turning on VT5...VT6, VT7. VT3 (or VT4) shuts off the pass-transistor pre-driver VT5. So VT3 on, output should be zero. But if any VT7,VT6,VT5 shorted it would damage parts upstream like VT3 or worse.It's a weird circuit, using 16 as common, measure voltage on midpoint Z2 (10V) Z4 (5.1V) voltage I would check too. R8 might have cooked or the V adj potentiometer if you tried dialing down output voltage to zero when it's stuck at full output voltage.There are no current-limiting resistors on the triple EF so the driver VT6 can dump a lot of base current into the VT7.

author: xavier60
author_rank: Super Contributor
timestamp: June 06, 2019, 03:33:48 am
content: Am I seeing clearly? VT6 and VT7 appear to be two 2N3055's as a Darlington pair. That would be nuts!

author: floobydust
author_rank: Super Contributor
timestamp: June 07, 2019, 03:02:23 am
content: I checked and in 1976 2N3055 cost almost the same as a 2N3054 (TO-66):Practical Wireless 19762N3055 55p, today $4.942N3054 50p, today $4.492N3053 25p, today $2.24The British pound got trashed in 1975/76, so these figures might be off (used 1£=$1.78 USD).