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| Dual channel 0-15V 0-500mA power supply |
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| xavier60:
--- Quote from: kallek on February 28, 2020, 08:09:08 am ---Thank you xavier60, some good points! I am not very experienced in electronics yet, and not very good at english, so there are couple of things i have to clear. I somehow understand the purpose of resistor feeding pin 6, but what that diode across it do? How I should bias it? I put D8 there to prevent Q1 turning on if pin 7 can not go complitely down. But is it something to do with switching speed? Did you mean reverse biased diode across B-E junction? Because I made the board before understanding to ask help, I will now experience with it and modify it if necessery. I will use your tips to do things better in the future projects. I really appreciate you sharing your knowledge! Edit. Thank you xavier60 for your time! So you only recommend resistor or resistor and diode both? Kalle --- End quote --- The diode across the resistor to pin 6 isn't needed now. While you were posting I did another experiment. I set the unreg rail to 20V and put a 2.2K resistor from unreg to the output of the CC op-amp. The op-amp's output was only able to pull down to 1.2V with about 9mA load. Add to this the 0.7V drop of D9 increases to about 1.9V, so D8 is needed where you have put it and may not lose enough voltage to allow Q1 to turn off fully. My idea was for the op-amps to be able to pull some current out of the Base of Q1 to help it to turn off more quickly when the power supply is suddenly unloaded. A diode anti-parallel with D8 should help. Have you tried bread-boarding the circuit so that you can further investigate these issues? |
| kallek:
That confirms what I have experienced. Did you have that 2,2k pull-down resistor too? If I remember correct, I measured something like 0,7V from pin 7. I have modified circuit since last bread-boarding, but I will do it and further investigate it. Kalle |
| xavier60:
I didn't have the 2.2K to ground. And I'm using an LM358 which might be a bit different. Using a 1A schottky for D9 will also help. |
| Alti:
LM324 does not have input current bias compensation so you have to make sure input impedance of both input nodes is equal. The LM324A has 0.1R at one node and 470k at the other. As reference voltage, you use zener diode. Get a TL431. A single one will do. Do not connect voltage feedback before ammeter but after it. I'd put a fast 0.5A fuse and a varistor just after feedback tap. |
| xavier60:
Another surprise. Pulsing the output with 1A load caused a very small 50mV overshoot with 100uF output capacitance and a D44H BJT. It still would be a good idea to add the anti-parallel diode in case the BJT leakage rises too much with temperature. |
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