555 circuit 24v down to 14v

[magudaman]

I’m not real experienced with building my own electronics but have used the 555 PWM for some bike lighting (11.8 down to 6v). what I want to do now is build a circuit to drop the voltage of this 24v Nimh pack down to around 12-14v so I can use it with my 12v electronics. I do not own an oscilloscope so can’t check how clean this power is going to be and I would like to run items that normally would run on a cigarette lighter. Does anyone if this would work at all, if I need to clean up the power to prevent damage to electronics, or is there a better way?

[jackinnj]

use the 555 to make a "buck regulator" (as you have done already) -- but to make it quiet drop the voltage to in the buck regulator to 16 V, then use a linear regulator like an LM317 or a 7812 to "post-regulate" to 12V -- this is an old technique but it works.

[magudaman]

Well I run into a sort of problem there...i was hoping to build the circuit to handle 10 amps. I wanted to use a Mosfet in the 555 timer to keep it high efficiency and to throw a regulator in the mix kind of take that efficiency away at those higher amperages. Sorry I didn’t mention the 10 amp thing that sort of changes everything. Any other ideas?

[sivan_and]

This circuit will be an ideal... got it from the web...input upto 40 Vo;lts o/p adjustble from 5 to 15 Volts @ 10Amp.
Goodluck
Sivanand

[consort_ee_um]

sivans circuit would work but if you want to stay with 555's, how about this:
http://www.edn.com/article/CA257052.html
You would need to beef up the Mosfet and the 1N5817 for 10A out

[sivan_and]

But the vcc limit of IC 555 makes this less versatile....

[sivan_and]

What about using Maxim MAX 724 ? two numbers in parallel with synchronised RC

[jackinnj]

Originally posted by magudaman
Well I run into a sort of problem there...i was hoping to build the circuit to handle 10 amps. I wanted to use a Mosfet in the 555 timer to keep it high efficiency and to throw a regulator in the mix kind of take that efficiency away at those higher amperages. Sorry I didn’t mention the 10 amp thing that sort of changes everything. Any other ideas?

Isn't the 555 a 16 V CC max device anyway? switching regulator chips are very inexpensive, and high input voltage applications are becoming ubiquitous.

A 555 is a pretty crude device to use to begin with for this application -- it will work, but not prettily -- you can't vary the duty cycle to "regulate" the output. the switching transients are nasty, high energy bursts of energy which will interfere with audio and radio circuitry. Thus you have to "snub" the diode in the circuit -- OnSemi and International Rectifier's websites have the maths you need to do this.

while it isn't the most elegant procedure, you can put a couple of switching chips in parallel which augments the current handling capacity.

Google "active snubber" there are several designs which pop up immediately.