0-5a 0-30v psu blowup - Page 1
Forum section: electronics / repair

author: p.larner
author_rank: Frequent Contributor
timestamp: March 24, 2024, 01:49:29 am
content: I built a chinese 0-5amp 0-30v psu kit,it was fine until i hooked it up backwards to a 12v car battery,i have changed the opamps,tip41 driver +the pass transistor emiter resitors,its still totaly dead,any idea where to go next?,schematic attatched,its the same but the except the zenners,they are now 79/9 regulators.

author: tggzzz
author_rank: Super Contributor
timestamp: March 24, 2024, 01:55:22 am
content: Quote from: p.larner on March 24, 2024, 01:49:29 amWhere to go next? The rubbish tip, unless it would be collected from the kerbside.Afterwards, sit down on your hands.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 24, 2024, 02:01:50 am
content: Quote from: p.larner on March 24, 2024, 01:49:29 amAny other details are moot!QuoteStart again - from the very start - bin the ashes

author: floobydust
author_rank: Super Contributor
timestamp: March 24, 2024, 02:44:54 am
content: Uh that PSU schematic has an auxiliary (floating) +/-12V rails for the control op-amps, how does that work off a car battery?edit: The 0-30V 2mA-3A PSU boards do not work with DC input (of the correct polarity). They have a -ve rail generator that needs AC to make the -5.1V and shutoff the transistor that shorts the op-amp output during power-up/down. In other words AC yes, DC no.

author: pqass
author_rank: Super Contributor
timestamp: March 24, 2024, 03:05:35 am
content: Did you connect the battery to the transformer (L1 and L2) primary windings inputs?Or to the output terminals; OUT+ and GND?If the former, the transformers are likely destroyed (unless there was a fuse in series); replace fuse and check the primary windings resistance to confirm. If open, yer SOL.If the latter, powerup (transformer primaries with mains AC) without anything at the output terminals and verify voltages from the transformer secondaries onward. That is, on the upper L1 part, check if there is +15 and -6.2 across either zeners (center tap being neg. of your multimeter). Check if there +2.5V across the TL431. On the lower L2 parts, check across C16 to give the rectified, filtered, unreg. voltage before the series pass transistors. On the bottom LM7812 regulator section (used for L2 tap switching), check if there is +12V across output and GND.

author: p.larner
author_rank: Frequent Contributor
timestamp: March 24, 2024, 03:19:15 am
content: just to clear things the output of the psu was hooked up to the car battery with the polarity reversed.

author: floobydust
author_rank: Super Contributor
timestamp: March 24, 2024, 05:59:52 am
content: hmmm big smokum from 1N5404.

author: xavier60
author_rank: Super Contributor
timestamp: March 24, 2024, 09:34:13 am
content: That's a Harrison type design. Except for there being a diode across the output, the design can briefly tolerate applied reverse polarity until the output transistors overheat and fail short. Had the output transistors actually failed?

author: p.larner
author_rank: Frequent Contributor
timestamp: March 24, 2024, 02:36:21 pm
content: its been a while but if i remember the pass transistors shorted in every direction,also the 5w emitter resistors went open,all those components were renewed.

author: xavier60
author_rank: Super Contributor
timestamp: March 24, 2024, 02:48:33 pm
content: Confirm that the control supply ground is connected to the PSU's + output terminal. The junction of D9 and D10 is ground for the control supply rails.Then measure the voltage on all pins of U1 WRT the + output terminal.

author: factory
author_rank: Super Contributor
timestamp: March 24, 2024, 06:15:37 pm
content: why not tri the skytrronic psu? i'm sure it wass highy reckomended byem3vuvfor charging classic kar batery

author: Jwillis
author_rank: Super Contributor
timestamp: March 25, 2024, 04:18:21 pm
content: Quote from: p.larner on March 24, 2024, 01:49:29 amCheck the all polarized capacitors specifically C12, 13, 14,16, Output Capacitor and C1 and C2. Polarized electrolytic capacitors don't always explode when put into reverse polarity but can be servery damaged.