Will it hurt if I do this? - Page 1
Forum section: electronics / repair

author: eas
author_rank: Frequent Contributor
timestamp: May 28, 2015, 12:27:42 am
content: I have a Power Designs TP340A triple channel bench power supply that I'm refurbishing for my own edification and eventual use (provided I don't ruin it first).First the back story:Due to some foolishness on my part, I thought I'd blowed-up the thing after replacing electrolytic caps and putting two of them in in the wrong polarity. In the process of troubleshooting it, I learned that a number of voltages in the control section of each channel are out of spec, but also, that if I actually set the current limiters correctly, the supply behaves, superficially, as expected in that it delivers the voltages and currents expected across a resistive load. Ripple and transient response is another story, and not something I've checked yet. There is one obvious issue though, which is that the tracking between channel A & B is wrong. Channel B doesn't respond until voltage gets to about 8v, and then at 16v, it starts dropping back down.The current situation:Channels 2 & 3 have voltages across zener voltage references that are out of the specified range, in some cases, they are a little bit out, but one is off by a factor of 4. They also produce a bias voltages that are out of spec.So, here is what I'm wondering:Each channel in the supply is powered by two windings off the main transformer. One provides most of the power for the channel's output. The other powers the circuitry that produces the bias voltages for the series regulators and powers the various op-amps that control the output voltage and current. Can I use another adjustable bench PSU to power just the rectified output of that winding in place at a range of voltages (up to but not exceeding the normal supply voltage) so that I can better understand how the circuit works?If the above course of action would be foolish, I could instead hook my 4-channel scope up to points of interest and record what happens when I power up the device. What's the proper way to establish the scopes ground reference? Hook the scope ground to the PSU's ground, and tie the channel's negative terminal to the PSU's ground?The schematic of two of the channels is attached.Thanks for any advice!

author: IanB
author_rank: Super Contributor
timestamp: May 28, 2015, 01:25:19 am
content: You should be able to use a floating power supply to provide external power to a part of a circuit that normally has in-circuit power. You would normally want to isolate the in-circuit supply first so it doesn't interfere. You should also watch out for the power up sequence. In some designs it may be necessary for the control circuits to be powered up first before power is applied to the regulated part.

author: eas
author_rank: Frequent Contributor
timestamp: May 28, 2015, 02:58:10 am
content: Thanks for the input, Ian.I was thinking that I'd actually leave the rest of the device un-powered. That is, I'd unplug it from the wall, and then apply power to the section of interest without disconnecting anything else.I guess the analogous situation would be if there was a problem with the other winding for the channel I was looking at. The first series regulator would have a base current, but no potential between emitter and collector. There would also be no negative feedback to the control circuit of rising voltage in the output section, but the same thing would be true if the secondary protection fuse blew. I'm guessing that the device is designed such that the blowing of the secondary protection fuse isn't going to result in damage to the unit, and so I'd reason that my plan shouldn't be risky. Am I missing important details?