Benchtop Power supply project in an old flip clock - Page 1
Forum section: electronics / projects

author: waste
author_rank: Contributor
timestamp: March 22, 2021, 03:55:59 pm
content: Well I got these nice chinese variable power supplies, that actually need a transformer and a nice boxAnd I had a nice box from an old flip clock around and a Big Bad Audio transformer around so I thought cool, seems we have a deal.The transformer is center tap 34V RMS to center (70V end to end), so I thought I could come close to 50V which is a nice upgrade from my current 30V5A bench powersupply.After I drilled , I hacked and I fitted everything inside I start having my first problems. The inrush current was too high that it was burning the fuse. I put a 10ohm NTC inrush resistor in the input but still the same. When I put a higher value fuse, it fried the NTCSo then I went to my rectification stage which was causing the problem and found out that one of my shottky diode was dead. I was using a couple of D10SC6M common cathode double diodes. These are rated for 10A continous 65V reverse voltage and I connected the anodes together so they don't overheat. I thought 20A as rectifier diodes has some headroom to play. But I was wrong and after I changed the one dead D10SC6M another one also died, so I thought ok now I'm sure I'm doing something wrong. But I cant figure out what. So that's were I need some help.Is there something obvious Im doing wrong? My understanding is that with full wave rectification I will get 35VAC RMS = around 48-49V DC which is totally into the specs of the diodes and the capacitors

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 22, 2021, 05:17:08 pm
content: Drawing out the schematic always helps, both you and anyone helping. I'm also not sure you can just parallel diodes like that (if I'm understanding correctly, again, schematic) the curves will never be a perfect match so one will see more current than the other.

author: waste
author_rank: Contributor
timestamp: March 22, 2021, 06:22:06 pm
content: Quote from: BrokenYugo on March 22, 2021, 05:17:08 pmIt is a very simple schematic because I didn't connect anything after the capacitors, as I m still on the testing phase (and for good reasonsI used these double rectifying diodes because they are usually used together in SMTPs (and because I had them around). What amazed me is that I didn't attach any load so the only load is the charging up of the capacitors.I also tried the transformer without the rectification and capacitors and it worked as expected.

author: erikj
author_rank: Contributor
timestamp: March 22, 2021, 06:34:58 pm
content: With that type of setup the diodes will see the full transformer voltage in reverse, not just the center tap voltage.So 70V RMS or about 100V peak.The diodes died from over-voltage and not over-current would be my guess.

author: waste
author_rank: Contributor
timestamp: March 22, 2021, 06:44:02 pm
content: Quote from: erikj on March 22, 2021, 06:34:58 pmOh that would be my obvious mistake. I thought because they are rectifying only half of the wave each, they will also get half of the voltage. Is there another way to make the rectification, or I just have to find better voltage rated diodes?