I'm trying to get the above power supply working. On the outside it's a neat little thing that you can dial up any voltage from 0-36 V and limit the current via two rotary encoders connected to two LED displays, and independently switch the output on and off (link to manufacturer's site and also sold under various brand names). Someone's done an independent stripdown here of the version shown by Maplin in the UK, which reveals it all to be a bit of a tight squeeze with lots of output capacitors sandwiched between two boards and further insulated with a nest of wires.
I got it as not working with no history of what had gone wrong. It lit up, but the two displays showed 0.00 or would occasionally flicker with a hundredth of a volt, but I could still set a couple of limits.
A look inside showed effectively two power supplies on the board, one to run the internal electronics (the smaller, top, inverter in the picture attached) and the other to supply the output voltage. After following a few dead leads I worked out that the PWM chip (just below the lower heat sink) for the output wasn't working. I changed that and now I can get an output voltage, but it's neither adjustable nor stable. It starts at about 3.5 v then gradually wobbles its way upwards at about an average speed of a hundredth of a volt every three or four seconds, although it isn't smooth.
But what is more puzzling for me is the unstable voltages I'm getting on the primary side. At the bridge rectifier (top left) I'm getting a stable 237 V AC input, but on the DC side pins it's anywhere between 288 and 318 (I don't have a scope to get any more detail). I pulled the big red capacitor and measured it with my ESR meter but it seemed OK. Then across the two input capacitors (Capxon 250v 220 μF) I'm getting between 428 and 449 volts. That's only 20 volts of wobble rather than the 30 at the rectifier, so they are doing some smoothing, and again they check out OK on the ESR meter.
So does anyone have any suggestions on what could be causing the unstable voltage on the primary side? Could that yellow transformer-like thing above the smoothing capacitors have anything to do with it? What exactly is that yellow thing? It's labelled F8080000 which throws up too much noise for Google to be any help. And what in the circuit is boosting the voltage on the primary side? I would have thought that ordinary rectified mains would be enough for the Mosfets.
Thanks in advance for any ideas or hints.
I got it as not working with no history of what had gone wrong. It lit up, but the two displays showed 0.00 or would occasionally flicker with a hundredth of a volt, but I could still set a couple of limits.
A look inside showed effectively two power supplies on the board, one to run the internal electronics (the smaller, top, inverter in the picture attached) and the other to supply the output voltage. After following a few dead leads I worked out that the PWM chip (just below the lower heat sink) for the output wasn't working. I changed that and now I can get an output voltage, but it's neither adjustable nor stable. It starts at about 3.5 v then gradually wobbles its way upwards at about an average speed of a hundredth of a volt every three or four seconds, although it isn't smooth.
But what is more puzzling for me is the unstable voltages I'm getting on the primary side. At the bridge rectifier (top left) I'm getting a stable 237 V AC input, but on the DC side pins it's anywhere between 288 and 318 (I don't have a scope to get any more detail). I pulled the big red capacitor and measured it with my ESR meter but it seemed OK. Then across the two input capacitors (Capxon 250v 220 μF) I'm getting between 428 and 449 volts. That's only 20 volts of wobble rather than the 30 at the rectifier, so they are doing some smoothing, and again they check out OK on the ESR meter.
So does anyone have any suggestions on what could be causing the unstable voltage on the primary side? Could that yellow transformer-like thing above the smoothing capacitors have anything to do with it? What exactly is that yellow thing? It's labelled F8080000 which throws up too much noise for Google to be any help. And what in the circuit is boosting the voltage on the primary side? I would have thought that ordinary rectified mains would be enough for the Mosfets.
Thanks in advance for any ideas or hints.
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