Constant Current discharge

[andy257]

Hello All,

I am using a supercap as a backup battery to power a computer when the power fails. The circuit only has to run for about 10 seconds before the power fails all together.

Now i know the computer consumes about 1.5A when running and it can have a voltage range of 9V to 12V. However if i connect my computer to a fully charged supercap as the capacitor dicharges the voltage falls exponentially. Therefore i would also expect the current to follow the same curve.

I was considering using a constant current circuit to keep the current at 1.5A over the voltage range from 12V to 9V then disconnect the PC from the cap. I have no idea if this will work. The entire circuit will be ran off the capacitor voltage. Therefore the opamp in my constant current source will have its +ve rail tied to the capacitor output and will fall at the same rate. It wont be stable.

Does anyone have any circuit ideas for this and would it work?

Thanks

 

[RCinFLA]

Discharge of 1.5 amps for 10 seconds from 12v to 9v requires a 5 farad cap.

 

[andy257]

Yes i know it is. I have one.

My question was related to drawing a continuous 1.5A from the cap. If you treat the PC as 1.5A@12V = 8R. Exponential voltage through a constant resistance = exponential current. I do not know what my pc will do if the 1.5A drops with below this value.

 

[mneary]

Your PC isn't likely to be a resistive load. It probably has a switching power supply that needs to always deliver the same wattage to the load. If we say for example the load is 18W, the power supply would want 1.5A when the input is 12V but needs 2A to deliver the same power from 9V.

 

[andy257]

Your PC isn't likely to be a resistive load. It probably has a switching power supply that needs to always deliver the same wattage to the load. If we say for example the load is 18W, the power supply would want 1.5A when the input is 12V but needs 2A to deliver the same power from 9V.

Thanks for the reply. Can you see any problems with just running the pc straight from the battery without a constant current?

 

[mneary]

You need to keep the battery charged with a regulated voltage, not constant current.

As noted above, 5 farads will run your computer for about 10 seconds.