PWM input voltage compensation

[Boscoe]

I'm currently using the PWM circuit shown below. I will have a varying input from 24V to 0V and I need some positive feedback to compensate for the voltage dropping to 12V. How can i do this? Replace the pot with 2 transistors and bias them appropriately? If i use BJTs (I was thinking BC547 and equivalent) will I need the two diodes?

Thanks
Boscoe

[luka]

what drops to 12v? why don't you just make this circuit, and before pin8 and 4, make 12v regulator, so that you'll input 24v to it (and drive motor with such voltage too), while supply only 12v to 555 which drives fet

[Boscoe]

Sorry I should of been more specific. the design goal of this is to save energy so lnear regualtors are out of the question. The 555 is going the be driven by batteries but the motor is going to be driven by a bank of capacitors so compensation for their voltage degeneration is needed, I have designed another circuit to switch the capacitors over to battery power once they reach a critical voltage.

Thanks

[Aucosticraft]

Originally Posted by Boscoe

Sorry I should of been more specific. the design goal of this is to save energy so lnear regualtors are out of the question. The 555 is going the be driven by batteries but the motor is going to be driven by a bank of capacitors so compensation for their voltage degeneration is needed, I have designed another circuit to switch the capacitors over to battery power once they reach a critical voltage.

Thanks

I think your requirement can be easily achieved using sg3524. Check it datasheet. If you need further guidance Ping me back.

[johansen]

dropping the voltage delivered to the 555 to 12v with a linear regulator will reduce the joules dumped into the fet and save you some power because the energy needed to switch the fet rises with voltage squared.