astable multivibrator

[muraliboopathi]

Hi... I m doing a project on astable multivibrator. I chose 100mf capacitors and four 10kohm resistors and used LED as load. When I switched the power supply both LED s were on... Theoritically only one LED has to be in ON condition but both the LED were ON. Could anyone explain wat is wrong in my project.

 

[iimagine]

Theoritically only one LED has to be in ON condition but both the LED were ON. Could anyone explain wat is wrong in my project.

Theoretically both transistors were assumed to have different hfe and VBE, resistance error ect... causing one to turn on faster than the other. In this case you are probably having everything perfectly matched! Try replace one bias resistor with a value different from the other bias resistor, or if you have a diode, connect it in series with one bias resistor, leave the other one alone.

 

[duke37]

Show us the circuit including the supply voltage and the active devices.

I presume you mean 100µF not 100mF, they would be a BIG capacitors.

 

[BobK]

More likely it is wired wrong. Also, you need a resistor in series with the LED load, you did not mention having one. Please post a schematic and pictures.

Bob

 

[muraliboopathi]

I use 100 micro farad capacitors...

 

[muraliboopathi]

I put a 10kohm resistor to base of the transistor and i use 12v dc supply... Tat 10kohm resistor drops around 11.2 volts and 0.8v is given to base... So i thought there s no need to put a resistor in series.

 

[iimagine]

No one said anything about adding another resistor in series

Assuming that you wired everything correctly, and that you are using the correct circuit commonly posted everywhere. Change ONE of the resistor at the base of ONE of the two transistors to any value other than 10k will do, something like 15k, 20k ect... if it doesnt work then check your wiring again, dont forget them LED resistors!

 

[BobK]

The resistors you are asked to add are between the LED and V+. These are needed to limit the current in the LEDs.

Bob

 

[duke37]

You refuse to show us the schematic with component details.

A 12V supply is too high on a simple cross coupled multivibrator, it generates a negative emitter/base voltage which should not be more than 5V.

iimagine
I do not think that the circuit could be so accurately balanced that it would not start.

 

[muraliboopathi]

Thanks to everyone. Its working now. I hv replaced one of the transistors's base resistor from 10kohm to 15kohm and its working. Bt I hv a doubt. I used two 10kohm resistors to limit the voltage that s given to base of the transistors. And I used another two 10kohm resistors for the capacitors to discharge through those resistors. Theoritically those two resistors and capacitors should act as time constant. And when both LED's were ON i should have changed those two time constant resistors to a another value. Bt as iimagine told i changed one of tha transistor's base resistors from 10kohm to 15kohm and its working now. I dnt knw why. What is anything to do with transistor's base resistor.

 

[muraliboopathi]

Hello bob sir.I have a doubt.i used to 10 kohm resistor to drop the +12v supply voltage to 0.68v and it is given to taransistor's collector. If I put one more resistor in series with LED, will not tat resistor drop any voltage?.i understand tat to limit the current which is going to LED I need to put a resistor. Bt my fear is tat wil tat resistor drop any voltage.

 

[(*steve*)]

muraliboopathi, I think we're still waiting for a circuit diagram. In addition to that, a picture showing how you've constructed it would be useful.

Without these, there's not much we can do to help.

 

[iimagine]

Because your transistors are well matched they both turned on at the same time. And since they are both on, neither will discharge each other capacitor, thats why the circuit was not oscillating
May I ask what kind of transistors are you using?

PS:
By changing one of the base resistor, you are applying a slight voltage changes to that base, this little voltage variation will cause one transistor to turn on faster/slower than the other, it has nothing to do with timing

 

[(*steve*)]

Because your transistors are well matched they both turned on at the same time. And since they are both on, neither will discharge each other capacitor, thats why the circuit was not oscillating
May I ask what kind of transistors are you using?

If this were true, simply warming one transistor between your fingers for a couple of seconds would likely allow it to start when power was reapplied.

Changing one transistor for another random sample should also do it.

I think that this explanation is unlikely.

 

[iimagine]

I think that this explanation is unlikely.

How do you explain that he got it to work by changing one of the resistor value to 15k as i suggested then?

 

[muraliboopathi]

I used bd139 transistor

 

[CDRIVE]

How do you explain that he got it to work by changing one of the resistor value to 15k as i suggested then?

That's a difficult question to answer. Since he won't post the schematic everything suggested and hypothesized is pure guesswork.

Chris