Capacitor used as switch, safety question

BoB_BoT

Hi all,
Came across a cctv system the other day that someone asked me to have a look at. The unit has no ON button other than the on off switch for the psu. But on the switch jumpers there was a capacitor. I can kinda understand in my head how this works to turn on the computer when the psu switch is flipped.

Has anyone seen this before and is it safe? Just had an image in my head of the capictor blowing due to power build up.

Would also like to know how it full works

Thanks,
-Rob

Khannie

I'd imagine that capacitor is to prevent sparking by providing a somewhat slowed power-on more than anything else. I am guessing though. My electronics basics are very rusty at this stage. You couldn't use a capacitor as a power switch safely though. It should just discharge (again if I remember my electronics correctly).

BoB_BoT

that's what I was thinking. That power would build up while the system is powered on.

If I knew how the actual switch worked for the mainboard it might a help. The circuit has to be made then broken for the mainboard to power on. I'd also imagine it would power down after 6/7 seconds of the circuit being closed. So it has me confused. It would work if the capacitor took a small charge, then discharged, making and breaking the circuit. But at the same time, it would be constantly doing this, would that in turn not send the computer into shutdown mode or sleep mode repeatedly?

Thinking about it, it must be safe, as it hasn't exploded in 3 years of use :P
Would still be interested to know the workings behind it.

Ghost Train

Like Khannie says its just to stop the sparking. You make a connection by joining and breaking two contacts, which causes a spark. This spark isn't good for the switch and eventually damages the contact points in the switch. With a capacitor across the contacts, instead of the energy generating a spark, the energy charges the capacitor.

Capacitor isn't like a light bulb and wont burn out (unless its put under a lot of stress, too much voltage or heat etc). Its just two metal surfaces with insulator in between. When a voltage is applied across the two contacts it charges up, when the voltage is removed it discharges, either by itself or through the circuit.

10-10-20

There is another use for a capacitor in a circuit such as this. That is a timed delay.
A capacitor will charge at a constant rate depending on the current flow into it, so you can use this delay to hold a pin (usually RST - reset) of a controller chip low (or high, as required) to allow the incoming power to settle before powering on the system.

I've used them for electronic detection systems where you want a very short pulse elongated.

Anyhow, as long as the capacitor is specified correctly, there is no chance of it exploding. You may check this by looking at the voltage rating on the side of the capacitor can. It will usually say 10v or 16v etc.

stylers

sounds like the capacitor was being used to power up the PC during power on. the capacitor would "look" like a momentary switch to the motherboard, when powered up. it probably is only seeing 5 volts, so safe enough i would think. On most motherboards nowadays, there is an option in the bios to set the power state after mains power returns from failure..

Zapho

Its grand, its hard enough to blow a capacitor unless your really ramp up the voltage and as Stylers says, PC power on buttons wouldn't supply more than 5V, I'd be suprised if it was over 3V. Anyway, if it does blow, it'll just pop, leaving a gap in the circuit and the PC just won't power on anymore. It won't destroy anything in the PC, so you've nothing to worry about! But at least you've recognised it, so if something does prevent the PC from powering up, you've got an idea what it is.

Capt'n Midnight

On the off chance, you sometimes have RFI suppression capaciators on the mains lines, this is what you are talkng about ... keep well away


on some circuits there is a capacitor/resistor on the CPU reset switch to stop it starting up before the power is stable or some *waves hands" thing like that

mathias

Anyway, if it does blow, it'll just pop, leaving a gap in the circuit and the PC just won't power on anymore. It won't destroy anything in the PC,

That very much depends on the type of capacitor , Electrolytics can spray corrosive/toxic substances on explosion which can cause havoc to the rest of the electronics.

Its also not very nice if any of it got on your skin or in your eyes , but you'd have to be around when it popped for that to happen.

Regarding the function of the capacitor above , its most likely for suppression , the switch may just be closing a control circuit that activates a relay as opposed to actually making the power circuit itself , in which case the inductive load needs the capacitor to stop sparks as mentioned above. It can also help prevent relay chatter.