Tracing out PSU circuit - Page 1
Forum section: electronics / projects

author: steve30
author_rank: Frequent Contributor
timestamp: November 01, 2012, 10:40:18 pm
content: I have an old benchtop PSU which I would like to mod. I can probably ditch this part of the circuit all together as I'll probably replace it with an L200C or similar, but I thought it would be good to fully understand how the existing part works.It has a BD223 PNP transistor and I'm not sure of the pinout. So, could someone take a look at the diagram attached and tell me if it looks reasonable? Or could the transistor be the wrong way round?It is just a linear power supply with a fixed 24V output.Thanks.steve30.

author: Simon
author_rank: Unknown
timestamp: November 01, 2012, 10:53:47 pm
content: It is a basic regulator, the zener diode is the voltage reference, the output voltage is the zener voltage minus 2 diode drops of the transistors through the bases to the emiters (yes you have that transistor the wrong way around possibly). So the transostors are driven by the zener and the transistors will keep the emiter at the zener voltage-2 x 0.7V no matter what the load current.It's not the most accurate and reliable regulation but it's good and simple for a basic regulation.

author: steve30
author_rank: Frequent Contributor
timestamp: November 01, 2012, 11:06:53 pm
content: I thought the BD223 looked peculiar.Any idea why they would have used a PNP though instead of an NPN?The regulation isn't that good. It starts at about 24V, then creeps up to 25V after a couple of minutes. Most of the supply seems to have been made in around 1984, but the 2N2055 appears to have been made in 1978 so I suppose its doing pretty well for its age. Apparently the PSU was originally for some kind of microprocessor development system.

author: ttp
author_rank: Regular Contributor
timestamp: November 02, 2012, 12:34:05 am
content: You need to trace the circuit again, if it was wired as you show BD223 or Zener (or both) would release magick smoke long time ago. BD223 has 9V across emmiter-base junction - impossible. The rectifier is not wired as you show too. It's hard to understand how it works from wrong schematic.

author: steve30
author_rank: Frequent Contributor
timestamp: November 02, 2012, 01:50:59 am
content: Quote from: ttp on November 02, 2012, 12:34:05 amlol.I couldn't draw a bridge rectifier properly if my life depended on it. Normally I just write down the words 'bridge rectifier' and that will do, but this time I decided to draw it.

author: Simon
author_rank: Unknown
timestamp: November 02, 2012, 07:26:29 am
content: your bridge is not too bad. Get something like kicad and you can bash out that shematic in a few minutes. Something is screwy with that PNP transistor. Usually it would be an NPN and that would make a darlington pair although you can have a mixed pnp and npn darlington pair. The 2N3055 connected before the fuse is worrying. Are you sure that schematic is correct ? it's neither here not there if your taking it out but yes it's a basic and poorly regulated supply using a zener and no feedback.

author: steve30
author_rank: Frequent Contributor
timestamp: November 02, 2012, 03:45:14 pm
content: I quite like KiCAD. Been using on and off for a few years now. I still have a preference for hand drawing though.The 2N3055 does appear to be connected before the fuse, as does the 78H05 on the other part of the circuit (not shown in the first post). That would certainly explain why some voltage appears on the output with a blown fuse.I think just for the sake of it, I'll build it up in Multisim and/or LTSpice and see how well it works.

author: Simon
author_rank: Unknown
timestamp: November 02, 2012, 04:53:09 pm
content: To be honest it's a horrible circuit, it will roughly regulate but it's nothing special. You could get a better result from a few parts lashed together as a switch-mode supply

author: steve30
author_rank: Frequent Contributor
timestamp: November 02, 2012, 07:48:27 pm
content: I'm a daft sod. I just took the transistor out to check it and it is actually a BD233 (not a 223), which is NPN. The pinout is such that the BD233 and 2N3055 are a Darlington pair.

author: Simon
author_rank: Unknown
timestamp: November 02, 2012, 07:49:57 pm
content: Well i thought they would be, although you can have one of each to make a darlington but as those are connected yes it would have to be 2 npn's