Math help please...

[Ioannis]

I am designing a project based on nano-Watt PIC like the 12F683. It is going to be powered by a supercap of 1Farad and I want to calculate the time it will work on the cap. The data are:

Working voltage: 4.2 to 2.5 Volts
Capacitor: 1 Farad
Current: 208 micro amps

As this needs Integration to calculate the time the cap lasts, I am little confused. Can anyone help me with this?

I came up with the equation in the attached file. Is it correct?

Thanks,
Ioannis

[Acetronics2]

Hi, Ioannis

Q ( Coulombs ) = I (A ) * t ( s )= C ( F ) * U ( V )

...

( 4.2 - 2.5 ) * 1 = 208 E-6 x t

>> t = 8173 s ...

That's all !

Nice general equation, indeed ... may I laugh a little ???

If your current draw was constant ... but just have a look to the curves at the end of the datasheet ...

You'll have some extra time !!!

BUT you must take into account the Supercap self discharge current ....

A look at the datasheet ... shows .... 315 µA ( Horror !!! )

http://www1.produktinfo.conrad.com/datenblaetter/450000-474999/473120-da-01-en-Gold-Cap.pdf

So, You'll have some minus time ...


1.7 / ( 208 + 315 ) E-6 = 3250 Sec ...

Just hope an hour !!!

Alain

[Ioannis]

Hi Alain.

Thanks for the reply. I was not too rusty after all!

Be well my friend and thanks again.
Ioannis