Capacitor overcharge protection

[AndyC1983]

Hi,

I work as a lab technician in a school and we are looking to buy a 5F capacitor. The problem is that its maximum voltage is only 5.5V and me / a kid / a teacher will inevitably connect it to a power supply above this. As they are quite expensive I would like to build a protection circuit around the capacitor to stop the capacitor charging when the voltage gets above, say 5V.

I have thought of using a parallel diode that has a threshold voltage around 5V somehow switching a transistor that is in series with the capacitor but am not really sure how to achieve this.

The added constraint is that it should be possible to measure the voltage across the capacitor as it charges from a d.c.supply and discharges through a (separately connected) resistor without the protection circuit affecting the charge / discharge curves.

The capacitor won't be used for any a.c. circuits.

Any help would be very much appreciated.

Thanks,

Andy.

 

[PTaylor]

There are several ways to limit the voltage. Easiest would be a zener diode. Probably 5.1V.
Also could use an LM7805 voltage regulator.

 

[(*steve*)]

The issue with a parallel zener is that if someone hits it with a power supply capable of supplying a lot of current, the zener will go short circuit.

That's not so bad, except that the capacitor will then discharge through the short circuit with a HUGE current causing it to explode.

After it has exploded, the zener is now open, the power supply charges the capacitor to well over 5.5V and the capacitor dies (not sure if it will be explosive, but there will be a lot of energy to dissipate)

Since it's going to be in the hands of those who may be keen to create their own light show, I'd go for something more robust.

My idea is to connect a mosfet in series with the capacitor. I would use a battery (say a 9V battery) via a (say) 100k resistor to keep the mosfet turned on. An open collector "voltage detector" across the capacitor could pull the gate to ground if the capacitor voltage exceeded 5V.

I would also place a fuse in series with the capacitor to help prevent people from discharging (or charging) it too rapidly.

There's still holes in this. For example, the mosfet will fail short circuit, which will leave you unprotected, and you need a way to reset the circuit if it gets over charged (perhaps you can push a button to connect a resistor across the capacitor.

The fuse should be connected directly to the capacitor, and a zener (sightly higher voltage than the voltage detector) placed after the fuse can act sacrificially to blow the fuse as a last resort.

I'd place the entire thing inside a clear case so the students can see the capacitor, but can't get at it.