Connect with us

Transistor series voltage regulator

Discussion in 'Power Electronics' started by anditechnovire, Jun 24, 2020.

Scroll to continue with content
  1. anditechnovire

    anditechnovire

    5
    0
    Jun 11, 2020
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Rs passes a current which generates a voltage across the zener diode. This can be used as a voltage regulator on its own but if too much current is taken by the load, then no current will pass through the zener and the voltage will drop.
    Adding a transistor means that only base current loads the zener circuit. With a transistor gain of say 100 then a lot more current can be taken by Rl.
    The diagram shows a npn transistor so it is likely to be made of silicon. Such transistors need about 0.7V between emitter and base to turn them on so the output will be 0.7V less than the zener voltage. This is known as an emitter follower.The higher current passes through the transistor to the collector.
     
    Martaine2005 and anditechnovire like this.
  3. anditechnovire

    anditechnovire

    5
    0
    Jun 11, 2020
    Ok thanks, but how does the transistor actually regulate the voltage in this case. As in when the voltage increase or decrease?
     
  4. Bluejets

    Bluejets

    4,257
    907
    Oct 5, 2014
    Re-read duke37 first sentence in #2 above.
     
    anditechnovire likes this.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    10,544
    2,349
    Nov 17, 2011
    anditechnovire likes this.
  6. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Such a regulator will not increase or decrease the voltage. it is intended to maintain a constant voltage and will only change a little when the load is changed. To change the voltage, the transistor can be driven through a potentiometer or the zener can be changed for one with a different voltage. A better circuit if you wish to vary the voltage is to use a regulator chip. I use a LM317T.
     
    anditechnovire likes this.
  7. WHONOES

    WHONOES

    952
    248
    May 20, 2017
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    10,544
    2,349
    Nov 17, 2011
    Increase:no. Decrease: definitely yes.
    Of course yes. But not suitable for understanding transistor operation.

    Oh. btw: My post #10000 :cool:
     
    Martaine2005 likes this.
  9. anditechnovire

    anditechnovire

    5
    0
    Jun 11, 2020
    Ok, let me really make my point clear.
    In this kind of regulation, I normally read that when the output voltage decrease for any reason, then the BE voltage will be increased and causes the transistor to perform more. As a result, the output voltage will be maintained at a stable level.

    When the output voltage increases, then the BE voltage will be decreased and causes the transistor to perform less. As a result, the output voltage will be maintained at a stable level.

    This is really the area that is bothering me.
    How on Earth will the BE voltage decrease or increase to compensate with the output voltage?
     
  10. bertus

    bertus Moderator

    1,145
    433
    Nov 8, 2019
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    10,544
    2,349
    Nov 17, 2011
    This circuit is not really a regulator as there is no true feedback control loop.
    I repeat the image from the original post for discussion:
    [​IMG]
    The idea is that both Vz and Vbe are comparatively independent of load current and, to a lesser part, of input voltage:

    The differential (Vin-Vz) drives a current Is through Rs: Is = (Vin-Vz)/Rs. This current is split into one part Iz going through the zener diode and anther part Ib going into the base of Q1.
    The part Iz will create a stable zener voltage Vz.. A change in Iz will result in a small change in Vz:
    upload_2020-6-26_18-5-0.png
    (sourced from here with modifications).
    Therefore for practical purposes Vz = constant.

    Equally the base emitter voltage Vbe of an NPN transistor is, above a certain minimum current only weakly dependent from emitter current (emitter current being the sum of base current and collector current!).
    upload_2020-6-26_18-11-16.png
    (sourced from here with modifications).
    Therefore, again, for practical purposes Vbe = constant.

    In total, again for practical purposes, Vout = Vz + Vbe = constant.

    Note: practical purposes here means within and as long as the resulting tolerances of Vout are acceptable in the application.

    This is the most basic explanation I can think of for this kind of voltage "regulator". A more detailed consideration by a network analysis using the transistor and diode equations is left to you. You've been given a few links to study.
     
    anditechnovire and bertus like this.
  12. Ratch

    Ratch

    1,088
    331
    Mar 10, 2013
    What question are you asking? Are you asking how a BJT works? Or, are you asking how the submitted circuit works? A naked BJT is a voltage controlled current regulator. Additional components can be added to make a circuit to do other things. Please specify what your area of concern is.
    Ratch
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    I like to think of this as a common collector amplifier. The input signal in the base caused a slightly lower voltage to appear at the collector, but this output can supply much more current than is drawn from the input source.

    Your circuit is not conventionally drawn as a common collector amplifier, but all that is required is a modest rearrangement of the components with exactly the same connections.

    In this case the input is a constant (or almost constant) voltage, and the output voltage is the input voltage less the base-emitter voltage drop.

    As the load current varies, the b-e drop will change slightly, affecting the output voltage.

    As long as you are not concerned with these small changes, feedback is not required.
     
  14. WHONOES

    WHONOES

    952
    248
    May 20, 2017
    I like to think of it as an Emitter follower as that is exactly what it does. The Emitter will always follow what ever is on the base minus, of course, the base emitter voltage.
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Well, "emitter follower" describes the function, and "common collector" describes the topology of the same thing.

    When I was learning this stuff, I found it easier to picture it in my head as "common collector" and remember the characteristics. But I'll concede that in this case, describing it functionally may have been better.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-