Prevent over voltage on a Cap

Hi Guys

i am looking for a more sophistocated way of preventing someone supplying a capacitor outside its given voltage.

eg 25V capacitor

my method was to use a zener and a relay in series to prevent anyone supplying a voltage greater than 25V

using a zener say 10V and then using a relay where the contacts pull in around 15V. Once a voltage over 25V is applied the relay opens and no more voltage can be applied to the capacitor.

I was thinking this would be possible with an op-amp somehow sensing the voltage across the capacitor. any ideas?

Andy

andy257 said:

Hi Guys

i am looking for a more sophistocated way of preventing someone supplying a capacitor outside its given voltage.

eg 25V capacitor

my method was to use a zener and a relay in series to prevent anyone supplying a voltage greater than 25V

using a zener say 10V and then using a relay where the contacts pull in around 15V. Once a voltage over 25V is applied the relay opens and no more voltage can be applied to the capacitor.

I was thinking this would be possible with an op-amp somehow sensing the voltage across the capacitor. any ideas?

Andy

Click to expand...

Is the over voltage transient in nature or sustained? If it is to be sustained, consider just using a properly rated voltage for the capacitor.

If it is transient in nature, the zener diode alone may be sufficient - as it can be briefly over-loaded power wise without degradation. A relay takes time to engage (several milliseconds for mechanical) and may miss a transient phenomenon altogether.

Revealing how & why the capacitor could see over voltage could shed some light on your solution.

well basically i am using a large capacitor to supply me a high pulse current otherwise unobtainable from my power supply.

Charging the capacitor and then using the stored charge to generate the current i need through a resistor.

The capacitor i have is a 63V capacitor. If for some reason the operator connects the capacitor to a higher voltage greater than 63 volts the result could be dangerous.

therefore i want to provide a safety net to prevent anyone applying a voltage greater than 63V.

The capacitor is constantly connected to the supply but only triggered to uncharge when the operator requires.

andy257 said:

well basically i am using a large capacitor to supply me a high pulse current otherwise unobtainable from my power supply.

Charging the capacitor and then using the stored charge to generate the current i need through a resistor.

The capacitor i have is a 63V capacitor. If for some reason the operator connects the capacitor to a higher voltage greater than 63 volts the result could be dangerous.

therefore i want to provide a safety net to prevent anyone applying a voltage greater than 63V.

The capacitor is constantly connected to the supply but only triggered to uncharge when the operator requires.

Click to expand...

So the over-voltage condition is sustained.

If you do not know what the operator will connect to it (including reverse polarity by goofing up the leads!), then you ought to provide a more robust protection scheme. Fusing the input would be a good idea. Look up over voltage crowbar circuits. In those schemes, a zener diode conducts during an overvoltage condition and sets an SCR type device whereby high current is allowed to flow until the fuse opens. You can protect against reverse polarity with a diode bridge input scheme.

The relay idea may oscillate. When the over voltage condition opens the relay, the over voltage will be removed? Then the relay will close again and the over voltage condition re-appears and the relay opens.. this could continue in this oscillatory way until the user disconnects. It also will stress the relay.

Posting a schematic of your ideas could be helpful.