Supercapacitor Sizing

[eld101]

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!

 

[Nigel Goodwin]

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

 

[crutschow]

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.

 

[blueroomelectronics]

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

 

[ronsimpson]

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?

 

[eld101]

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.

 

[ronsimpson]

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.

 

[blueroomelectronics]

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

 

[picbits]

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 .....

 

[crutschow]

You may not get them charged in 10 seconds.

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