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Astable Multivibrator

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BrownOut

Banned
Never mind... I understand the new and improved version. I was getting cross-eyed from looking at the ckt for too long ;)
 
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sheldonstv

New Member
here is a page you need to have a look at if you arent familiar with this circuit....do not be scared.....
Astable Multivibrator
 

audioguru

Well-Known Member
Most Helpful Member
The Astable Multivibrator circuit at the Play Hookey site is correct.
Many sites have the polarity of the capacitors backwards and/or the supply voltage higher than 6V which causes the transistors to have avalanche breakdown.
 

BrownOut

Banned
To my knowledge, there is nothing inherent to the bi-multi that causes avalanche for supplies over 6v.
 

Nigel Goodwin

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To my knowledge, there is nothing inherent to the bi-multi that causes avalanche for supplies over 6v.
You need to seriously expand your knowledge then - it's an EXTREMELY common and well known effect - roughly 9V or above requires extra components to prevent it happening, a couple of protection diodes will do.
 

BrownOut

Banned
You need to seriously expand your knowledge then
That normally requres an explanation of the process, rather than just gratuitous ramark and restatement. As I've already written, I've never found much use for this circuit outside experimentaion and illustrative demonstrations. I never really gave it much thought.

But it's OK, I did the analysis, and I understand how reverse voltages build up. Still don't have much use for it though.
 
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audioguru

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To my knowledge, there is nothing inherent to the bi-multi that causes avalanche for supplies over 6v.
The Japanese guy also talks about the transistors breaking down if the power supply voltage for the Astable Multivibrator is more than 5V or 6V.
The max allowed reverse voltage for the emitter-base junction of a silicon transistor is 5V to 7V. The base of each transistor in an astable multivibrator is driven negative.

Diodes can be added in series with each emitter to stop avalanche breakdown.
 

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BrownOut

Banned
The Japanese guy also talks about the transistors breaking down if the power supply voltage for the Astable Multivibrator is more than 5V or 6V.
I don't watch Japanese TV. Thanks alot for the explanation. I haven't built or thought about Multi's in about 20 years! And most of my transistor work has been linear.

I guess I need to go back and start building some of these basic circuits again. Think I'll start with a JFET relaxation oscillator :)
 

audioguru

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I don't watch Japanese TV.
His Astable Multivibrator circuit and explanation was posted in this thread.

There is a 12VDC to 120VAC inverter project that uses an Astable Multivibrator. It doesn't work because its capacitors have backwards polarity and the transistors have avalanche breakdown which causes the capacitors to explode even if their polarity is corrected. The inverter schematic is here:
 

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