I am building a adjustable lab DC power supply under the instruction of a electronic book. ("Electricity and Electronics" 2nd edition by G. R. Slone). I have encountered some serious problem.
The power supply uses two 24 volt transformers in series to step down line power with the centre of the two being the circuit common. The AC voltage is then rectified by a 6-amp 200 PIV bridge rectifier and filtered by two pairs of capacitors. (As Shown in the first picture). [The instruction requires me to use two 4400-uF 50WVDC capacitors. However, they are hard to find, so I used four 2200-uF 80V capacitors instead.]
Up to this point, I have pretty much tested the power supply under the instruction and everything turned out to be as expected. The power supply is able to give positive and negative 34V raw DC.
The last part of the power supply is a regulator circuit. (The second picture shows the positive part of the regulator circuit, the negative part is just a mirror image of the positive part). I have built the circuit, and connected the two positive and negative outputs to binding posts with fuses in between. I have also connected the circuit common to another binding post.
During the testing, I set P1 to its approximate centre position (i.e., 500Ω). And then, I used my DVM to test the voltage between positive output and circuit common. I believed I have actually measured 34 volts, and then quickly after, the fuse blowed. Same happened in the negative side. Latter, I continued the measurement without the fuse (I shouldn't have done this), and that destroyed all the transistors in the circuit.
So I am probably going to rebuild the regulator circuit, and I have no idea where I have made wrong. All the components in picture 2 expect C1 are soldered to a PCB. It's hard to test the PCB with DVM especially with the transistor broken. Since the fuse is rated 2-amp, the current limiter must not be working properly (it supposed to limited the current at roughly 1.5-amp). However, the DVM showed a 34v measurement when I set P1 to the middle, which means the voltage regulator was not working neither. And since both sides failed with the same problem, could there be something wrong with the circuit schematic? Do you guys have any suggestions on how to find the problem? Or suggestions on where I may have made wrong?
Beside, I have two more questions that I don't quite understand regarding the circuit.
1. What is C3 doing there?
2. If I set the output to be 17V and the power supply is working properly, how do I calculate the power dissipated by Q5 and Q3?
Your help is greatly appreciated.
Thanks a million.
The power supply uses two 24 volt transformers in series to step down line power with the centre of the two being the circuit common. The AC voltage is then rectified by a 6-amp 200 PIV bridge rectifier and filtered by two pairs of capacitors. (As Shown in the first picture). [The instruction requires me to use two 4400-uF 50WVDC capacitors. However, they are hard to find, so I used four 2200-uF 80V capacitors instead.]
Up to this point, I have pretty much tested the power supply under the instruction and everything turned out to be as expected. The power supply is able to give positive and negative 34V raw DC.
The last part of the power supply is a regulator circuit. (The second picture shows the positive part of the regulator circuit, the negative part is just a mirror image of the positive part). I have built the circuit, and connected the two positive and negative outputs to binding posts with fuses in between. I have also connected the circuit common to another binding post.
During the testing, I set P1 to its approximate centre position (i.e., 500Ω). And then, I used my DVM to test the voltage between positive output and circuit common. I believed I have actually measured 34 volts, and then quickly after, the fuse blowed. Same happened in the negative side. Latter, I continued the measurement without the fuse (I shouldn't have done this), and that destroyed all the transistors in the circuit.
So I am probably going to rebuild the regulator circuit, and I have no idea where I have made wrong. All the components in picture 2 expect C1 are soldered to a PCB. It's hard to test the PCB with DVM especially with the transistor broken. Since the fuse is rated 2-amp, the current limiter must not be working properly (it supposed to limited the current at roughly 1.5-amp). However, the DVM showed a 34v measurement when I set P1 to the middle, which means the voltage regulator was not working neither. And since both sides failed with the same problem, could there be something wrong with the circuit schematic? Do you guys have any suggestions on how to find the problem? Or suggestions on where I may have made wrong?
Beside, I have two more questions that I don't quite understand regarding the circuit.
1. What is C3 doing there?
2. If I set the output to be 17V and the power supply is working properly, how do I calculate the power dissipated by Q5 and Q3?
Your help is greatly appreciated.
Thanks a million.
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