I was given an old Bio-rad 160/1.6 electrophoresis power supply that wasn't working. It normally puts out 0-60 VDC at 1.8A or 0-160VDC at 0.6A and I thought that it would make a great bench power supply. I contacted Bio-rad who did not have any information at all on this item as it is long obsolete. I drew out the schematic myself and replaced all of the transistors inside to get it working again, as they were all shorted except for Q5. I replaced the 4 JE3440 with NTE157 and the two SJ6916 with NTE165.
After powering it up everything seemed to work fine, although regulation under load wasn't very good. I put a scope on the output and found that, while it looked fine initially, as soon as any significant load was put on the supply it would oscillate at around 22kHz. If the output is set to 50 volts then I am seeing about 2 volts of oscillation superimposed on the output DC voltage. A higher load or output voltage setting will increase the amplitude of the oscillation, with it dropping down to nothing as the output voltage is decreased to near zero. Adjusting R14 does not appear to make much difference to anything, so I'm not sure what it's function actually would be.
I have tried capacitors and snubbers in various points on the circuit to dampen the oscillations, but nothing eliminates it, and in some cases it makes it worse. Putting any capacitance at all across the output terminals makes the problem worse. The two electrolytic caps in the unit are both working fine.
I'm starting to think that this is just a poor design, after all, the oscillation probably wouldn't effect it's function as an electrophoresis power supply, but it pretty much makes it useless for audio. So my choices now are either find some way to improve or fix the oscillations, rebuild the internal circuitry with a better design, or just toss it and get a different supply altogether.
Can anyone offer any ideas for either fixing this power supply or using as much as possible of the original circuitry to create a new one that functions correctly? Is it possible that the NTE replacement parts are somehow causing the problem? Any and all ideas and suggestions are welcome!
Take care,
Doug
After powering it up everything seemed to work fine, although regulation under load wasn't very good. I put a scope on the output and found that, while it looked fine initially, as soon as any significant load was put on the supply it would oscillate at around 22kHz. If the output is set to 50 volts then I am seeing about 2 volts of oscillation superimposed on the output DC voltage. A higher load or output voltage setting will increase the amplitude of the oscillation, with it dropping down to nothing as the output voltage is decreased to near zero. Adjusting R14 does not appear to make much difference to anything, so I'm not sure what it's function actually would be.
I have tried capacitors and snubbers in various points on the circuit to dampen the oscillations, but nothing eliminates it, and in some cases it makes it worse. Putting any capacitance at all across the output terminals makes the problem worse. The two electrolytic caps in the unit are both working fine.
I'm starting to think that this is just a poor design, after all, the oscillation probably wouldn't effect it's function as an electrophoresis power supply, but it pretty much makes it useless for audio. So my choices now are either find some way to improve or fix the oscillations, rebuild the internal circuitry with a better design, or just toss it and get a different supply altogether.
Can anyone offer any ideas for either fixing this power supply or using as much as possible of the original circuitry to create a new one that functions correctly? Is it possible that the NTE replacement parts are somehow causing the problem? Any and all ideas and suggestions are welcome!
Take care,
Doug
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OK, silly me forgot to check the zener diode, it was shorted as well. I replaced it and now the oscillations are gone most of the time, however at higher voltage and current loads it does come back, although it's not as pronounced as before. R14 appears to adjust for the voltage dropping under load, although even when set to the best position it still does drop significantly. I guess that I will experiment some more to see if i can dampen the oscillations somehow.
Take care,
Doug
Take care,
Doug
Are all the resistors ok , if so try putting a 100 ohm resistor in series with the base of Q3 as a base stopper resistor .
R14 is a current limit preset probably to limit the 60V range to 1.8A however there doesnt appear to be any circuitry to adjust the limit to 0.6A when the higher voltage range is selected .
R14 is a current limit preset probably to limit the 60V range to 1.8A however there doesnt appear to be any circuitry to adjust the limit to 0.6A when the higher voltage range is selected .
The resistors all appear to be OK. I found that adding some capacitance right at the output terminals will almost eliminate the 22kHz oscillation there, with 500 - 1000pF being the most effective. After adding a 680pF ceramic cap along with a 10uF electrolytic I am getting less than 20mV of oscillation worst case at about 150V and 1.5A output. No oscillations at all can be seen at the lower voltage range. So there may be hope for this supply yet.
You are absolutely correct that R14 is a current limit preset for the 60V range, it is working properly now that the zener diode has been replaced. For the 160V range they are apparently relying only on the 1A circuit breaker to protect from over-current. I will try the 100 ohm resistor on Q3 to see if it helps with the oscillations, but I won't get a chance to do that until Monday.
Thank you for your suggestions!
Take care,
Doug
You are absolutely correct that R14 is a current limit preset for the 60V range, it is working properly now that the zener diode has been replaced. For the 160V range they are apparently relying only on the 1A circuit breaker to protect from over-current. I will try the 100 ohm resistor on Q3 to see if it helps with the oscillations, but I won't get a chance to do that until Monday.
Thank you for your suggestions!
Take care,
Doug
I have now added the 100 ohm resistor in series with the base of Q3 and the 22kHz oscillation is now completely gone. Thank you epicyclic for your helpful advice!
Other than the poor regulation and the lack of current control, the power supply is functioning reasonably well, so I'm going to keep it as a bench supply for now and see how it works out.
Take care,
Doug
Other than the poor regulation and the lack of current control, the power supply is functioning reasonably well, so I'm going to keep it as a bench supply for now and see how it works out.
Take care,
Doug
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