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| What is the purpose of this 3rd transistor ? |
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| Nusa:
--- Quote from: Beamin on April 28, 2019, 01:37:26 am --- --- Quote from: Nerull on April 28, 2019, 12:15:49 am ---The circuit in the OP uses an 8.2V zener, so would have an approximate output voltage of 7.6V. --- End quote --- But the input is 12v so how would it drop to 7.6 that would be a lot of heat given 9 LEDs for lighting. Does this circuit give a constant voltage drop or does it oscillate with a RC timing circuit formed on the emitter by C7501 and R7501and the two caps on the output/collector (C7507 C7508) smooth it out? Or is this putting the transistor in a semi on state (linear vs saturation?) --- End quote --- Six LED's, not nine. Those other 3 components are resistors. If you assume the circuit was designed to provide 20mA to each LED, what is the total current draw when the LED's are lit? At that current, how much power would be lost to heat to drop the voltage that much? Is that actually a large enough number to qualify as the same "a lot" you were thinking of before doing this exercise? |
| Nerull:
--- Quote from: Beamin on April 28, 2019, 01:37:26 am --- --- Quote from: Nerull on April 28, 2019, 12:15:49 am ---The circuit in the OP uses an 8.2V zener, so would have an approximate output voltage of 7.6V. --- End quote --- But the input is 12v so how would it drop to 7.6 that would be a lot of heat given 9 LEDs for lighting. Does this circuit give a constant voltage drop or does it oscillate with a RC timing circuit formed on the emitter by C7501 and R7501and the two caps on the output/collector (C7507 C7508) smooth it out? Or is this putting the transistor in a semi on state (linear vs saturation?) --- End quote --- No, it doesn't' oscillate. Its a trivially simple series regulator. You are vastly overthinking how it functions. |
| james_s:
Maybe it would help to draw it differently, first draw the zener and resistor between Vcc and Gnd, that will give you a reference voltage. Then draw an emitter follower vertically between Vcc and Gnd, then tie the output of the zener reference to the base of the transistor. |
| Beamin:
--- Quote from: james_s on April 28, 2019, 04:15:54 am ---Maybe it would help to draw it differently, first draw the zener and resistor between Vcc and Gnd, that will give you a reference voltage. Then draw an emitter follower vertically between Vcc and Gnd, then tie the output of the zener reference to the base of the transistor. --- End quote --- Is this the same principle? Is this whats inside an op amp to make it always try to equal out? |
| xavier60:
This page has good progressive explanations starting with a simple zenner shunt regulator and its limitations, http://www.ko4bb.com/e102/e102-4.php You need to also understand basic electronics. |
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