Unknown mistakes. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 13, 2021, 09:26:24 pm
content: For a long time now I've been thinking about a failed power supply project. This was a 10 Amp 12 volt linear PSU, using just a L7812 and two TIP36C power transistors for the extra current. It seemed ok, but it was dropping to much voltage. I tried two TIP147 Darlington transistors, and it doubled the voltage. So I put the TIP36C's back in, after that I put it aside didn't get round to working out what went wrong. Recently I put together a bigger 20Amp 12 Volt power supply, that wouldn't get above 12 Amps, it should have done 20 Amps at 12 Volts easily. Serveral weeks passed, I took another look and realised I had the transistors emitter and collectors the wrong way round. It's such a stupid mistake, I corrected it and tried a heavy load again, it reached 25 Amps for a brief period. I didn't want to overload it, it's all working fine still. So if I had made this mistake on the earlier 10 Amp power supply, but with TIP147 transistors with the emitters and collectors reversed by mistake, would that have doubled the output voltage ? I ask as I've no idea if that's possible, with Darlington transistors. I'm a bit embarrassed, I can't believe I made such a daft mistake. As I've been using molex connectors to attached the transistors, I got the idea that the third lead of each molex connector was the collector, but it wasn't it was the emitter. Wire colour threw me as well, I'm not making excuses, it's such a stupid mistake to have made.

author: Jwillis
author_rank: Super Contributor
timestamp: September 14, 2021, 02:26:14 am
content: The TIP147 has a protection diode built in. If use in reverse then the diode would just pass voltage straight through from collector to emitter. That would bypass the BJT rendering any regulation useless . So yes depending on the source voltage of your power supply before regulation.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 14, 2021, 10:27:25 am
content: more transistors don't 'double' the voltage in series regulationyou have make a mistake in implementing that type of reg, put schematic and details for clarifybtw, 7812 is what it is, using 7812 for this i find being not the best solution. opamp V/I control is the right choice, if done by the book, for linear regulation

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 14, 2021, 07:11:08 pm
content: Yes I think it was the filtered supply voltage after rectification. I remember looking for a short, anywhere that would explain why I was getting the unregulated voltage on the output. When I swapped back to TIP36C transistors the voltage was normal at 12.60 Volts, the the collector emitter leads where to the wrong way round. But I've looked this up, and I found single bjt with pass some current if this reverse of collector and emitter are the wrong way round. That explains why the current was low as well. Because I had the transistors off the board, it never crossed my mind that these leads where the problem. The circuit is the L7812 1 Amp linear voltage regulator, with multiple PNP power transistors and current sharing ballast resistors. At least I finally found out why I couldn't find the problem on the board. I won't be making silly mistakes like that in the future. Thanks for the explanation.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 16, 2021, 12:26:34 am
content: As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don’t know we don’t know. ~There are thinks I know that I just dont want to know. I think this is for the best.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2021, 10:23:35 am
content: BJTs will work the wrong way round, but the breakdown voltage and hFEare much lower. The reverse hFEand VCEare often not specified on the datasheets, but hFEis normally a factor of 20 below the forward value and VCEis typically 5V to 7V. Running a BJT in reverse will give a lower saturation voltage, if sufficient base current is provided to make up for the low hFE.In the cause of a linear regulator, reversing the ouput transistor can either cause the output voltage to be higher, or lower, than the expected value, depending on the transistor, base drive and voltage across it.The output voltage will be lower, if hFEis very low, so the transistor doesn't have enough base drive, to pass the required collector current.The output voltage will be higher, if the transistor starts to breakdown due to the emitter-collector voltage exceeding the much lower reverse rating, or the transistor has a built-in reverse voltage protection diode, as mentioned above.