# Astable Multivibrator not switching

1. ## Astable Multivibrator not switching

I am having problems with circuit, that I designed.
** R5=R6 Are calculated for hfe=306 measured. Designed for 20V -> 680 ohm -> U/R -> 20/680 = 0,0294A. Thats what transistor should turn on. With hfe 306 -> 0,0294/306 -> 0,000096 A. Thats minimal current for turn it ON. So to bypass turning second transistor On when capacitor rise 0,7V - resistor is calculated for 20V -> 20/0,000096 = 208kOhms. With resistors connected, there is no switching and i got nice smell from burning resistor R1 when rising voltage to 20V.
Problem is not resistors. I can change them later. Does abyone know why its not oscilating?
Thanks very much

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2. ## Re: Astable Multivibrator not switching

My two yen:
These classic circuits require an unbalance to start oscillating. In the good old days, transistor's hfe would vary significantly, and would take care of the unbalance automatically.
Nowadays the hfe spread may be tighter. I've seen some designs, which they provide slightly different base resistor values for that purpose.

Having said this............I've built MANY of these circuits over the years, and start oscillating very easily. I don't understand your R5, R6 calculations and ignore why you chose base resistors R5, R6 = 680 ohm. They are at least a couple of orders of magnitude too low for 20 volt operation.

3. ## Re: Astable Multivibrator not switching

Originally Posted by schmitt trigger

My two yen:
These classic circuits require an unbalance to start oscillating. In the good old days, transistor's hfe would vary significantly, and would take care of the unbalance automatically.
Nowadays the hfe spread may be tighter. I've seen some designs, which they provide slightly different base resistor values for that purpose.

Having said this............I've built MANY of these circuits over the years, and start oscillating very easily. I don't understand your R5, R6 calculations and ignore why you chose base resistors R5, R6 = 680 ohm. They are at least a couple of orders of magnitude too low for 20 volt operation.
I added r5/r6 becouse circuitry works before only when voltage was 0.5-1V. i though that when one capacitor rise to voltage of VBE than other transistor turn on. I got saw tooth wave not square wave.

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4. ## Re: Astable Multivibrator not switching

Hi,

You ask for help for a circuit that does not work. But you show a circuit without part values.
But even after you have been asked to give the part values ... you don't. Why?

What do you expect?

Klaus

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5. ## Re: Astable Multivibrator not switching

1) R5 and R6 should be replaced with wires.
2) R2 and R3 are calculated to cause the transistors to saturate, not to be a linear amplifier, then hFE is not used. Make R2 and R3 about 10 times the values of R1 and R4.
3) The capacitors cause the emitter-base voltage of the transistor to go to a fairly high negative voltage that is far beyond the maximum allowed voltage of only 6V so protection diodes are needed like this:

6. ## Re: Astable Multivibrator not switching

R1-R4 = 680 Ohms
R5-R6 = 220kOhm

7. ## Re: Astable Multivibrator not switching

Silly values.

R5 and R6 serve no purpose but to reduce the available current to the transistor bases. Remove them from in series with the bases and instead place them as bias resistors between bases and VCC.
My preference to prevent negative excursions on the transistor bases would be diodes from the bases to ground, cathode end to the bases.

Brian.

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8. ## Re: Astable Multivibrator not switching

R5 and R6 serve a purpose, at least limiting the negative base current if Vcc is larger than about 7V.

9. ## Re: Astable Multivibrator not switching

I posted my simulations on the other forum:

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