What role R3 plays in this circuit?

Discussion in 'General Electronics Chat' started by samy555, Oct 10, 2015.

  1. samy555

    Thread Starter Active Member

    May 24, 2010
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    Hello

    I read in a book
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    [​IMG]

    My question is: what is the function of the R3? What if I took it away from the circuit?


    Thank you
     
  2. ErnieM

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    Apr 24, 2011
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    R3 (and R4) are doing about the same thing. With R4 especially, it is giving a path for C to B leakage currents to go besides going back into B and again to E. these leakage currents increase with temperature, so that resistor helps keep the device off when it should be off.

    R3 has an additional task of letting the Zener currents have a path around the transistor when the Zener is "off," remembering that off is only relative and currents do flow.

    If you leave them both out the circuit may work just fine. Or not. Or if you build 100 units only 80 seem to work. Or you find none work unless you buy your parts from certain manufacturers.

    Thus it is good practice to have a resistor there to catch small currents and insure the transistor stays off.
     
  3. samy555

    Thread Starter Active Member

    May 24, 2010
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    Thank you
    I did not understand much
    I wish more specific answer
     
  4. MikeML

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    Oct 2, 2009
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    It may be "improved", but that circuit is still a poor second cousin to a LM78xx or a LM317...

    Zeners do not regulate well unless the current through them is like 10mA or higher. If R3 were not there, the base current of Q2 (same as the Zener current) could be so low as to prevent the Zener from regulating well. Shunting the base-to emitter-junction of Q2 with R3 raises the current that would otherwise flow in the Zener.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    I don't see how you can get an answer much more specific than Ernie gave you. :confused:
     
  6. ErnieM

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    Well perhaps someone else could explain it another way our friend may comprehend it better.
     
  7. bertus

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    Apr 5, 2008
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  8. AnalogKid

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    Aug 1, 2013
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    R4 is critical to the operation of the circuit, not possibly optional as implied in post #2. Without R4, there will not be any output voltage. R4 and Q2 form a variable voltage divider. You can think of the node at Q1's base in two different ways. Both of them combined define the circuit. One way is that R4 supplies *all* of the base current for Q1, and Q2 shunts some of this current away to GND under the control of the output and the reference diode. Another way is that Q1 is an emitter follower circuit, and R4/Q2 form a variable voltage divider at its input, again under the control of the output. Either way, R4 ensures that Q1 can turn on and supply output current.

    R3 assures that Q2 turns *off*, a very different circuit function. The circuit will operate without it, but the transient response will be worse. Transient response is the ability of a circuit to keep up with changes going on around it. For example, if you suddenly increase the load on the power supply, called a step increase, the output voltage will decrease. This causes voltage at the top of the zener to decrease, which causes the voltage at the bottom of the zener, Q2 base, to decrease (you hope). This partially turns off Q2, increasing the base current available to Q1, turning it on more, making more current to the output, and increasing the output voltage to handle the increased load. But this works quickly only if Q2 turns partially off fast enough. Note that when the output voltage decreases under load and the voltage at the zener cathode decreases, the voltage at the zener anode does not necessarily decrease along with it. If there is nothing to maintain enough current through the zener for it to act as a zener - that is, maintain a constant voltage from cathode to anode - then the diode will just not conduct, and the voltage across it will change rather than remain constant. The zener diode junction capacitance works to help the circuit function without R3 installed, but the regulation is much better, especially during transients, if the zener has an approximately constant current through it t all times. R3 is the part that assures this. Also, the Q2 base-emitter junction capacitance stores charge and prevents Q2 from turning off rapidly, and a resistor to GND bleeds off this charge much more rapidly than letting it dissipate through internal leakage currents. R3 is the part that assures this.

    ak
     
  9. ErnieM

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    Apr 24, 2011
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    I agree with AnalogKids. Or reaction of my post concerning R4.
     
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