Faulty power supply channel - Page 1
Forum section: electronics / beginners

author: mmilejski
author_rank: Contributor
timestamp: September 20, 2013, 07:40:00 pm
content: I got myself a triple output power supply - Protek 3303 to be precise - and I have a problem with one of the channels. As this is my first repair ever I thought I better take some advice.When I turn the unit off, voltage on channel one jumps to about 15V before completely ramping to 0V - even when dials are set to 0 volts it spikes to give or take 15V. I don't see the issue on the other channel, though. Channel numer two has a nice ramp down on power off and no glitches.I was not able to find a service manual for it and I thought I would reverse engineer it, but it is built using some custom single in-line coated circuits... And that took all the fun away. But I come up with something, I think.The transformer is tapped at about 15V and the spike at the output is associated with the relay clicking, so it is probably an relay issue, is it? I turned on the channels separately and relays sound the same on both channels.Have you seen similar issue on switched tap linear power supply? I have CRO and a multimeter at the bench, any suggestions what to check?I really would like to repair it as it has some nice LED displays for both voltage and current and also is pretty tough - it has nice metal case.

author: Jonny
author_rank: Regular Contributor
timestamp: September 20, 2013, 10:05:57 pm
content: Maybe this is normal behavior for this model? Perhaps the relay has 2 positions, when the relay coil is energized it's passing one circuit, when not it passes a different one. So when you turn the unit off, the control to the relay is lost and so the contacts pass through that rail to the output. Trace the output terminals back through the relay to prove this theory.

author: mmilejski
author_rank: Contributor
timestamp: September 21, 2013, 11:04:37 am
content: Quote from: Jonny on September 20, 2013, 10:05:57 pmI don't think it is normal behaviour as it may damage powered device, and as I said, second channel does not have this issue and it is exactly the same build. This supply is 2 x regulated 0-30V 1.5A + 5V 5A.I did some probing and it turns out that relays work okay. It is the driver transistor / SIP circuit that is causing this weird spike, it outputs voltage left on the caps/voltage regulators on to base of the transistor until base current of the driver transistor discharges decoupling caps and it passes 17V or 30V (Depending on previous transformer winding and assuming no load - it does not have discharge resistor! I'll have to add two for both regulated channels) from the bridge rectifier capacitor to the output.So it seems the problem lies within those obscure SIP packed circuits... so I will probably bodge in a relay circuit that pulls the driver transistor to ground on power off. As the spike on the output is the only issue there is, it should work fine. Do you think it is a good idea?

author: Paul Price
author_rank: Super Contributor
timestamp: September 21, 2013, 11:35:55 am
content: Would be a really better idea to connect the output terminal of the defective channel through a relay and then it would really disconnect the output upon power turnoff.

author: mmilejski
author_rank: Contributor
timestamp: September 21, 2013, 12:11:12 pm
content: Well, I connected the relay to pull the driver transistor down, and it works a treat!Now i have to assemble it back togetherQuote from: Paul Price on September 21, 2013, 11:35:55 amBanana jacks are screwed onto a PCB and to put a relay directly on the output I'd have to cut tracks and so on, so I'll stay with pulling the driver transistor down.