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Astable Multivibrator not switching

MacIntoshCZ

Member
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
 

Nigel Goodwin

Super Moderator
Most Helpful Member
If your supply is 20V, then it will instantly destroy the transistors as you have no Vbe protection on them. Replace the blown transitors, and drop the voltage to 5 or 6V, no more.

What are R5 and R6 for anyway?.

You're also rather vague about the component values?, if R1 is smoking it's obviously far too low a value.

What frequency are you trying to get out of it?.
 

crutschow

Well-Known Member
Most Helpful Member
Which resistor value is which is confusing.
Please post a schematic with all the resistor values next to the resistors.
 

MacIntoshCZ

Member
If your supply is 20V, then it will instantly destroy the transistors as you have no Vbe protection on them. Replace the blown transitors, and drop the voltage to 5 or 6V, no more.

What are R5 and R6 for anyway?.

You're also rather vague about the component values?, if R1 is smoking it's obviously far too low a value.

What frequency are you trying to get out of it?.
Trying to get 50khz 50% duty cycle. Transistors are not blown. Vbe was at 0,7V when I measured voltage. resistors are OK. Tested for seconds
R5-R6 are for bypassing 0,7V. When C1/C2 charging and there are no R5/R6 resistors after 0,7V it will turn second transistor On and discharge.
 
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ronsimpson

Well-Known Member
Most Helpful Member
I think the resistors values (ratio) is not good. You calculated for a transistor gain of 300. (post #1) I think you should change to gain of 100 and try again.
120322
Every time I made one of these R2,3 was a high value. and I never had R5,6.
120323
 
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ronsimpson

Well-Known Member
Most Helpful Member
Yes but that is to just turn on the transistors. You need to turn them on hard.
Post #6 last picture shows a ratio of 100. Many examples I found are 30 to 50:1.
 

MacIntoshCZ

Member
I think the resistors values (ratio) is not good. You calculated for a transistor gain of 300. (post #1) I think you should change to gain of 100 and try again.
View attachment 120322
Every time I made one of these R2,3 was a high value. and I never had R5,6.
View attachment 120323
When i created it for first time. I have n o r5/R6 . But it works only when vcc was 0,5-1V. After 1V it stop oscilating and one transistor was always on.
 

MacIntoshCZ

Member
Yes but that is to just turn on the transistors. You need to turn them on hard.
Post #6 last picture shows a ratio of 100. Many examples I found are 30 to 50:1.
Second capacitor is charging through base of other transistor. It should double current from begining
 

MacIntoshCZ

Member
+ ¨¨¨¨
View attachment 120324Here is an example of 15:1.
+ One transistor is turned ON always when R5/R6 is connected. If there is no R5/R6 it start to act similary to this when voltage is 0,5-1V. So a tried to calculate R5/R6 to turn second transistor on when capacitor charge to 20V not 0,7V.

Its from this article:
 
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ronsimpson

Well-Known Member
Most Helpful Member
Green trace is collector voltage 0.2V to 5V.
Blue trace is base voltage -4.2V to +0.67V. (ramp sawtooth on base is mostly negative)
Time is in mS/div.
120326
 

unclejed613

Well-Known Member
Most Helpful Member
** R5=R6 Are calculated for hfe=306 measured.
the transistors saturate in a multi, so the Hfe is actually going to be around 10. it's an oscillator, so all that's required for starting is a gain above 1. R5 and R6 are what is keeping your circuit, because they are limiting the base current to the point you can't drive the transistors into conduction. R1 and R4 should be appropriate for whatever VCC you are operating at, and whatever collector current is well within the transistor's limits. anywhere from 470 to 1.5k should be ok. R2 and R3 should be okay anywhere from about 10k to 47k to keep the base currrent limited. you would calculate the capacitor value from an RC time constant using R2. you're looking for 10 microseconds for each side, so the caps will be 1uF (assuming R2, R3 are 10k. i have built tons of these multis in the past, and one thing i've noticed is they like electrolytic caps better than solid types like ceramics. it may be when the circuit is loaded by an LED or something, it may need some reverse current through one of the electrolytics to get the multi started.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Trying to get 50khz 50% duty cycle. Transistors are not blown. Vbe was at 0,7V when I measured voltage. resistors are OK. Tested for seconds
R5-R6 are for bypassing 0,7V. When C1/C2 charging and there are no R5/R6 resistors after 0,7V it will turn second transistor On and discharge.
Well now you've mentioned your resistor values, that explains it - as I've already said, R5 and R6 shouldn't be there, but their presence (as well as stopping it working at all) has also prevented the transistors blowing.

As I've already said - DON'T use more than 6V to power it, or the transistors WILL blow when wired and working correctly.

As others have already said, your other resistor values are crazy as well - with no calculations at all, I'd try 1K in the collectors, 10K for the base resistors, 0.01uF and a 6V supply - and see what frequency you get. Tweak from there to get the frequency you want, once you've got it oscillating.

A multivibrator is a very simple and reliable circuit, but yours was all wrong, with spurious unwanted resistors, wrong values, and too high a supply voltage.
 

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