I currently have a 24v circuit that pull only about 1/4 amp. Lets say I want to be able to run this circuit for 10 minutes off of a supercapacitor. I have not found any formulas that seem to link amp/hour with capacitance. Any suggested equations or places to start looking?

Thanks!

 

Try using the time constant C = R x C - I suspect your requirements are FAR above what a super capacitor can provide though.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

The equation for a constant current is
C= (I x t)/V
where I is the current, t is the time in seconds, and V is the amount of voltage drop you can tolerate before the circuit quits.

For example for 1/4 amp, 10 minutes, and a 4V drop (24V to 20V) the required capacitance is 37.5 Farads, a rather large capacitor.

 

And a high voltage one too, most supercaps are low voltage.

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com

 

United Chemi-con makes some very large capacitors. Some are used in cars. After a car crash you may not have 12 volts to make the electric windows work. They make capacitors that hold enough charge to run the window motors once.

Why not use a battery?

 

The charge cycle only has a matter of seconds..... I dont think a battery would last more than a few weeks after only getting charge in 10 second intervals.

 

Years ago I had some 5F low voltage capacitors. The charge and discharge curves are not like a capacitor but like a battery. I think they are actually batteries.

The UCC capacitors are real capacitors. I know. You need to look at the current rating on the caps. You may not get them charged in 10 seconds.

 

Might have a high ESR too. They're generally designed for clock backup.

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com

 

I've got over 1000 supercaps in stock at the moment

The temptation to hook them all up in parallel rather than selling them is immense .....

 

To recharge them in 10 seconds after supplying 1/4A for 10 minutes would require a 15A charge current.