Wide input voltage low current PSU not working as it should...

Discussion in 'General Electronics Chat' started by seanstevens, Jul 24, 2018.

  1. seanstevens

    Thread Starter Well-Known Member

    Sep 22, 2009
    53
    2
    Hi All,

    Looking around to design a simple transistor assisted zener wide range voltage regulator capable of 10-60VDC @ 60mA for example, I started by trying the simple circuit below however, the output voltage is not exactly the same as the zener diode used due to vbe drops across 2 transistor I guess. Some may say increase the zener to compensate which is one way but just not happy with this circuit (will be last resort).

    upload_2018-7-24_12-1-6.png

    I came across a board that has a wide range voltage input so I tried to reverse engineer the circuit and I kind of think I have got the circuit right but it is not working for some reason. Also the circuit is markedly different to all the other circuits such as above as it uses an NPN & a PNP transistor, whereas all the other circuits such as above use a pair of NPN only. I have tried simulating the circuit in Multisim and it seems to work perfectly but when I make one on a vero or bread board it doesn’t work – it just outputs the same input voltage on its output!

    upload_2018-7-24_12-2-10.png
    Q3 as far as I can ID from marking looks PNP. Q2 possibly BCV47 NPN.
    upload_2018-7-24_12-2-45.png
    upload_2018-7-24_12-5-47.png

    I would appreciate any help on this.

    Thanks
     
  2. dendad

    Distinguished Member

    Feb 20, 2016
    2,640
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    [​IMG]
    What about this circuit configuration?
    Wit suitable components I expect it would work ok.
     
  3. seanstevens

    Thread Starter Well-Known Member

    Sep 22, 2009
    53
    2
    Thanks dendad, I had seen this circuit and yes I expect it to work but 4 resistors in this circuit as oppos to 1, not that I am counting ;-)

    I am more interested to find out how my circuit above works and why I cant even find any version of it on the net, where as the other 2 NPN transistors seem to be pretty common.
     
  4. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    The compound PNP-NPN transistor, sometimes called a Sziklai pair (mostly in audio circles, it seems), is quite common in power circuitry. One of its merits is high current gain without the two-Vbe drop of a simple Darlington emitter follower. In the configuration shown it behaves much like a single NPN transistor with high current gain. You can find lots of info on the web.

    None of your zener-based circuits will ever have better than mediocre performance. They are open-loop (no feedback) which means the variation in component characteristics, either initial or with change in input voltage, load, temperature or aging are not "nulled."

    When you simulate the circuits, look carefully at currents and voltages throughout the circuit. Looking only at the output voltage helps very little in understanding what is going on. Instead of using a static load and input voltage, vary them. A triangle wave is often a good thing to use - directly for input voltage and indirectly as a controlled resistance for the load.

    Dendad's circuit is much superior because there is some degree of feedback. The feedback gain isn't high, but it is sufficient to improve performance.
     
  5. Kjeldgaard

    Member

    Apr 7, 2016
    347
    126
    The circuit below looks right.
    But there are no decoupling capacitors on input and output?
     
  6. dl324

    AAC Fanatic!

    Mar 30, 2015
    8,258
    1,989
    I wouldn't consider any of the circuits you posted to be good voltage regulators. The BE junctions aren't in a feedback loop, so output voltage will vary with current; load regulation will be poor.
     
  7. crutschow

    Expert

    Mar 14, 2008
    21,996
    6,358
    Then I suspect a wiring error in your breadboard.
    Trace out all connections against the schematic.
    Particularly check the transistor pinout.
     
  8. crutschow

    Expert

    Mar 14, 2008
    21,996
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    Are you concerned about the output voltage regulation with change in load current?
     
  9. seanstevens

    Thread Starter Well-Known Member

    Sep 22, 2009
    53
    2
    Thank you for everyone's input.

    @ebp - Yes you said the merits and that is useful. However, I havent seen this configuration before and have just Googled Sziklai pair and now i can read all about it. If I dont get any joy out of this will consider switching to dendad' regulator. Thank you.

    @Kjeldgaard; there are a couple of decoupling caps i have just left them out sorry.

    @dl324; appreciate your concern re load variation but for what it is being used which is a fix load of about 60mA max i have no concerns.

    @crutschow; I have gone all over the little vero board that I built it on and have changed the 2 transistors and still no joy, then built another on a breadboard no luck with that either. Still investigating. No, load is about 60mA tops and fairly constant.
     
  10. seanstevens

    Thread Starter Well-Known Member

    Sep 22, 2009
    53
    2
    It seems something had gone wrong during the testing and both transistors had been damaged. I have now made 2 more and both are operating OK.

    Thank you for everyone's input and suggestions, as always on this forum, its been educational .
     
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