Need Solution for Astable multivibrator circuit for 24V

[satiz]

Hi there,

I have a 12V astable multivibrator circuit but, I need this to operate in 24V what are the changes I have to do?
Without changing the transistor. LED Quantity is not a matte for me I can add more.

Thanks in Advane
Regards
Satiz

 

[d123]

Hi,

You just need to do a little maths regarding the base resistors and collector resistors. This tutorial called Astable Multivibrator runs you through the circuit operation and calculations needed nicely, it's easy to understand. Rule-of-thumb would be to double the base and collector resistor values as 24V is twice 12V. ...And maybe the trimpot, I haven't seen that before in a multivibrator so I'm not sure what purpose it serves.

 

[d123]

Hi again satiz,

Doing rough calculations and excluding the trimpot from the circuit and the subsequent values, and assuming LEDs are around 1.3V to 1.5V...

For same as 12V circuit, where ~10mA LED current and t1 = t2 = 0.83 seconds:

R2 = R3 = 22k (if ic/ib = 10), C1 = C2 = 55uF (57uF is 0.865 seconds)
Or
R2 = R3 = 110k (if ic/ib = 50), C1 = C2 = 11uF
R1 = R4 = 2.2k

I guess trimpot 50k + 4.7k is for ic/ib = 55...

Hope that helps, if I'm right.

 

[KlausST]

Hi,

what about using a 12V zener in series with the power supply?
Or using a LM7812?

--> no need to change any value of the existing circuit.

Klaus

 

[crutschow]

I believe you need to add a diode in series with each transistor base to avoid excess reverse bias on their base-emitter junctions.