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Voltage Regulator

Discussion in 'General Electronics Discussion' started by jethro1, Feb 4, 2013.

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  1. jethro1

    jethro1

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    Feb 4, 2013
    Hello to all.

    I am hoping that someone can offer some advice as to how a voltage regulator works. I understand the theory of it (controls flow of current to the field coil as needed to maintain output voltage from the stator) but not exactly what is going on inside, and when. I also understand transistors, diodes, capacitors and resistors.

    Is there someone here that is smart enough to break this down for me?


    [​IMG]
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    More current is allowed to flow into the field winding as the voltage drops.

    As the voltage rises, the field current is reduced.

    The zener diode is sensing the voltage and providing the signal used to lower the field current.
     
  3. jethro1

    jethro1

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    Feb 4, 2013
    Thanks for the reply. However, I am in search of more detailed information.

    Such as, what are the capacitors used for in this application, what are the resistors doing, what triggers the first transistor, which I think then triggers the other transistor, when does the zener diode come into play, etc.

    In other words, not what the device as an assembly does but what is going on inside on a step by step basis.

    Thanks,
     
  4. john monks

    john monks

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    Mar 9, 2012
    Let's call the PNP transistor that is connected to the zener diode Q1 and the other transistor Q2. Now the alternator output is connected to the voltage regulator and also the battery.
    When the output is putting out 12 volts Q1 is off because the voltage across the zener diode, maybe a 6 volt diode, does not have enough voltage across it to turn on because of the voltage divider set up by the two resistors connected to it. Say that voltage is 6 volts to ground so Q1 does not get any forward current on the base therefore you get no current on the collector. Q2 emitter is connected to the 12 volt line through a series diode connected to it.Thus the emitter voltage is about 11 volts and thus about 10 volts on the base turning on the transistor and thus energiesing the field winding inside the alternator. The field winding is connected between the collector of Q2 and ground. This causes the output of the alternator to start putting out more current raising the voltage on the alternator output.
    Now let's say the output voltage raises to 15 volts. The opposite situation occurs. The zener diode turns on because the voltage across it increases to more than 6 volts. Thus Q1 base turns on because the base voltage moves negative relative to the emitter voltage, turning on Q1.
    This causes the collector voltage in increase turning "off" Q2 because its base voltage goes more positive relative to its emitter.
    Thus the collector voltage goes to zero volts removing all voltage from the field winding and thus all the current. Then the alternator stops putting out current and the the voltage is reduced.

    The capacitors slow down the circuit to prevent oscillations by effectively reducing the transistors frequency response.

    The resistor from the base of Q1 to the emitter is to cause Q1 to shut off when the zener voltage across the zener diode is not reached.

    The resistor from Q2 base to ground is to turn on Q2 when no current is flowing through the collector of Q1

    The diode from Q2 collector is to prevent and inductive kick, that is high voltage coming back from the field winding and wiping out Q2.

    The diode from Q2 to the alternator output is to provide enough voltage between Q2 base and the alternator output to prevent residual voltage across Q1 from turning on Q2 when it should be off.

    If you want I will explain more thoroughly.
     
    Last edited: Feb 5, 2013
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    John, what do you think about the series R-C from the collector of Q2 to the base of Q1? This is actually positive feedback. I have an idea what it does, but I was hoping someone would know for sure.
     
  6. john monks

    john monks

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    Mar 9, 2012
    For the R-C I can only take an educated guess. It might be to prevent Q2 from staying too long in the active region thereby reducing lost energy through heat. The circuit will also help prevent the field winding voltage from swinging too wildly thus helping to prevent radio interference.
    So the voltage appears to be a switching regulator.
     
  7. duke37

    duke37

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    Jan 9, 2011
    I would guess that it is a switching regulator which switches rapidly to reduce the dissipation in the output transistor.
    The output will be 0V or 12V which will be too much for Q1 so the resistor is included to reduce the feedback drive.
    I imagine that the circuit will produce a pulse width modulated output running at quite a high frequency which will be smoothed by the inductance of the alternator field.
     
  8. jethro1

    jethro1

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    Feb 4, 2013
    Thanks much everyone. Your inputs are appreciated. Not being much of an electronics type I am trying to digest the information provided. I might be back with more questions but I certainly thank you for now.

    Although used in a "nominal" 12 volt system this particular regulator attempts to maintain an output voltage of about 14.6 volts. If this has a bearing on anything.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The voltage it attempts to maintain is dependant on the component values. Without them, a lot of discussion becomes speculation.

    The zener value, for example, will be critical in setting the regulation voltage.
     
  10. jethro1

    jethro1

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    Feb 4, 2013
    Steve, thanks again.

    I know that some voltage regulators are adjustable. I am assuming (yes, I know) that a variable resistor is used somewhere in the circuit to increase or decrease the turn on turn off of the transistors so as to change the field current which then affects the output of the alternator?

    Are the terms "turn on" and "turn off" the correct ones to use when describing a transistor allowing or not allowing current to flow from the emitter to the collector? I was thinking of the transistor as a switch in which case I might have used the terms "open" or "closed". I might need to adjust my terminology.
     
  11. john monks

    john monks

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    Mar 9, 2012
    The voltage can be adjusted by making one of the resistors connected to the right of the zener diode adjustable.
    The terms "turn on" and "turn off" are good terms for describing transistors that are either saturated or not conducting at all. A saturated transistor is in full conduction with very little voltage between the collector and the emitter, generally less than a volt.
    When a transistor is not conducting it means that no current is flowing to or from the collector.
    In this regulator circuit I am speculating that it is a switching regulator to the transistors are either full on or off most of the time. The current through the field winding would be smoothed out because of the inductance as duke37 says. The parallel diode would provide the current path when Q2 is switched off. This makes sense because you do not want to waste energy in a car.
    But on the other hand the transistors could be partially on or off with about 6 volts across them with a substantial amount of current going through them. This would be very wasteful but some voltage regulators are made just this way generally for receiver circuits where switching noise from a switching regulator would be a big nuisance.
     
  12. jethro1

    jethro1

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    Feb 4, 2013
    Hello John,

    Yes, this regulator is supposed to be a switching type where there is either current flow, or no current flow, the the alternator field. Thus, it is an electronic version of the earlier type that used a relay to open or close a relay contact a bajillion times a second to maintain proper field current flow so as to maintain a constant alternator output voltage.

    It almost seems like Q1 is being turned on in order to turn on Q2 in order to allow current to flow through the field coil.

    Is there any particular reason why just one transistor could not be made to work in this manner? For example, Qx turned off, no current flow. Qx turned on, current flow.

    After reading up on them a bit, at least I am learning quite a lont about transistors that I did not know before.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Q1 turns on to turn Q2 Off.

    A single transistor would settle into a linear mode where it would be dissipating (i.e. wasting) quite a bit of power.
     
  14. john monks

    john monks

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    Mar 9, 2012
    To expand on what Steve says in order to make an oscillator, thus a switching regulator, you need a circuit to create a 360 degree phase shift so as to create an instability.
    The short answer is that you could use a transistor and a relay but I assume two transistors is cheaper.
    As you look at the circuit you will see that a negative transition on the base of Q1 will cause a positive transition on the collector. This transition feeds the base of Q2 and so the collector of Q2 goes negative. And this transition is fed right back to the base of Q1 completing the 360 degree loop.
    All this is to prevent any transistor from being half-way "ON" and wasting energy.
     
    Last edited: Feb 8, 2013
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