Problems encountered during building a lab power supply

Discussion in 'The Projects Forum' started by piracyer, Jun 19, 2012.

  1. piracyer

    Thread Starter Member

    Jun 9, 2012
    32
    1
    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.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,150
    3,058
    I don't see anything to limit the base current on Q5 once it starts to turn on.

    C3 quiets (filters) the control signal, preventing noise from propagating.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    That's a horrible design. It starts up "wide open" and waits for feedback to throttle back the pass transistor. I have screwed around with such designs and they are losers.

    BTW: that design is not short circuit protected, it will blow up often.
     
    Last edited: Jun 20, 2012
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Q5 is the current limiter, as it turns on it sucks current away from Q1 which is the main current source to the output. Starving Q1 limits current to Q5.


    C3 looks like loop compensation on the voltage feedback path.

    Power dissipation is:

    VIN (unreg) - Vout x (ILOAD)
     
    Last edited: Jun 20, 2012
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Here is a very good general purpose linear regulator design I use for my lab supply.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,041
    3,243
    The maximum dissipation in Q3 would be equal to the maximum dissipation in R1, which occurs when half the voltage (minus the three emitter diode drops) appears across each component. For 34VDC this would be about (34V-2V)/2 = 16V which gives a maximum power of 16^{2}/1k ≈ 1/4W.

    Q5 conducts no current unless the current is being limited, so it dissipates no power under normal operation.

    Edit: I see no obvious reason the circuit should have failed so I would double-check all the wiring.
     
  7. mcasale

    Member

    Jul 18, 2011
    210
    12
    I hope you are doing this because you want to learn analog electronics - not because you need a bench supply.

    When many EEs first start out, their first job is to design a power supply. That was my case, too. I made a lot of mistakes. No one is infallible in this area - not even the Pope! :)

    Bench supplies are getting real cheap now, and they are adjustable and have current limiting.

    Good luck.
     
  8. piracyer

    Thread Starter Member

    Jun 9, 2012
    32
    1
    bountyhunter, can you explain more on this short circuit protection problem? Is it gonna be dangerous if I use the power supply for other projects?
    The circuit you gave looks much more complicate :( I will try to understand it. However, since I already build the other parts beside the circuit in an aluminum box, I kind of stuck with the circuit at this size.

    Why the maximum dissipation in R1 occurs when half the voltage? Why not when the output is at its lowest, say 2.1 volts plus 0.7 volts that drop at Q1 and 0.7 volts drop at Q2 (i.e., 3.5 volts)? Then R1 will drop (34 - 3.5) = 30.5 volts, isn't it?

    What are the effective ways to check the wiring on PCB?
    And what's the best way to connect components on a PCB?

    Thank you guys for the help :) I really appreciate it!!
     
  9. piracyer

    Thread Starter Member

    Jun 9, 2012
    32
    1
    Thanks man, it's my first project in EE. I am actually studying computer science in university, but I have strong interests in EE. So I am self-studying it through some books and website.
    Since, it's my first project, I really want to make it work.
    And I believe I can do it! :)
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    It has a sort of current limiting which is crude and non adjustable. Assuming the power transistors can handle the current and power dissipation, it should survive.
     
  11. crutschow

    Expert

    Mar 14, 2008
    13,041
    3,243
    You are correct. I misspoke in my explanation. :p It should be that the maximum power in Q3 occurs when its voltage drop is equal to the voltage drop across R1 (from the maximum power theorem).
     
  12. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Here is one good circuit for linear power supply, however in general I would not use this for more than 1A/1 MOSFET max.

    I have actually built this circuit, with some modifications, for instance lower input voltage, or to regulate upto 100V. The original design is sharply limited to 60V output, you would need to change some resistor values.

    And I use 50K pots...

    Current regulation is excellent!

    It's from joretronik.de (German language).

    But it can be built like it is, you don't need to read/understand the text on the website, even if it can be helpful.

    If you can do me a favour, don't really build this using 10X MOSFETs and large cooling system, just one MOSFET, and use it for whatever max. Amperes you'll get out from this.

    And I use 2x 2n3904, one gets a little warm but should be OK for 50V.
    I found it easy actually to get along with this circuit, and modify it.

    Edit: I have added redrawn schematic, might be easier to follow.
    MJE340/BC547 = 2n3904, OK for 40V to 50V but check heat developement.
    It should also be possible to use power PNP transistor instead of MOSFET.

    Pots for voltage/current are both 50K in the schematic.
     
    Last edited: Jun 21, 2012
  13. piracyer

    Thread Starter Member

    Jun 9, 2012
    32
    1
    Thank you guys, you are very helpful.
    I found out that I still have lots of things to learn :) in order to really understand and modify this kind of circuit. So I will devote myself, hopefully, into tons of readings on these kind of stuff, if I am not gonna play too much diablo lol. If you know any good book or website about transistors, regulator circuit or electronics in general, please let me know.

    And takao, thank you for the circuit, but I am afraid I am not able to build the circuit for now. I might give it a try in the future when I fully understand the mechanism behind it.
     
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