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Topic: Multivibrators (Sorry girls, nothing to get excited at here... :) ) (Read 1 time) previous topic - next topic



Dec 03, 2012, 11:55 am Last Edit: Dec 03, 2012, 12:05 pm by retrolefty Reason: 1
Don't tell me magic

I was told in the Air Force when we studied this circuit that it indeed was magic.  ;)

That is if your question really is: upon initial power up for this circuit which transistor will switch on first, Q1 or Q2? And will that always be the case upon each powering up event? Lets let the peanut gallery chew on that one for awhile.

Possible answers:

A:   Q1 always.
B:   Q2 always.
C:   It's always random if either Q1 or Q2 starts first.
D:   It can be either Q1 or Q2 but it will be the same transistor starting first for each specific circuit built.
E:   Some other magic will decide which transistor starts first.


A transistor doesn't have a "minus" side.  It has a base, collector, and emitter.  Read up on how a transistor works and you'll understand why the resistors are connected the way they are.  And that link that Riva gave will explain in plenty detail how the circuit actually works, how the capacitors work and how they affect the circuit.  Google itself might not answer your question directly, however it will give you plenty of references of where you can find the answer.

transistors? i know how they work...

re read my question...


The key to understanding a multi vibrator is to know that when one end of a capacitor changes the voltage it is at, the other end also changes by the same ammount.
So if you have 5V across a capacitor and you change the positave end to zero the the other end of the capacitor instantly goes to minus 5V.

This is the basis of a lot of circuits like negitave rail generators and voltage multipliers.


1. On applying voltage, at the start, do both caps start charging..

Both charge up, but at different rate. The one that gets charged up first flips the other transistor first.

2. Resistor 2 and 4 are both connected to the Minus side of the capacitors yet connected to a positive + rail... huh?   why?

Those capacitors see reverse polarities so the polarity marking is likely to indicate that they are larger electrolytics. They should be non-polarity capacitors.

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