safest way to charge a big capacitor

[bloody-orc]

Hi there!

Witch is the safest way to charge a capacitor using a lab PSU. What I intend to do, is some sparks for 8th graders. so charge a 10 000uF capacitor @ 12V and then discharge it rapidly using a metal plate 1m diameter sparks everywhere! Me likes, little children cry, me like even more!

 

[ericgibbs]

hi,

If your lab psu is rated at say 1Amp and its set for 12Vout,use a 12/1 =12hm: resistor, [I would use a 27R, 10Watt]

Saw TV news that Estonia has been a victim of a cyber attack, hope its all back to normal.

 

[HarveyH42]

Nice Halloween reference, 'witch'...

Anyway, 10k uf (best you could find?) should only take a second to charge. I'd be more concerned about safely discharging it. Shouldn't be much of a shock hazard, but the flaming-hot metal flying about might be a concern.

Would definately try it a few times be for the presentation. Would be kind of cool to include something that indicates the the charging of the capacitor, so you could show the spark at various levels (just an excuse to do it several times), add a little drama and anticipation.

And of course you can play 'Here, catch' with which ever child is most in need of some hands-on experience... (likely get in trouble for shocking a student, even if they deserved it).

 

[Leftyretro]

Hi there!

Witch is the safest way to charge a capacitor using a lab PSU. What I intend to do, is some sparks for 8th graders. so charge a 10 000uF capacitor @ 12V and then discharge it rapidly using a metal plate 1m diameter sparks everywhere! Me likes, little children cry, me like even more!

Charge the cap through a series connected resistor. The charge time will be the time constant of C x R to reach 62.5% of charge voltage. This will prevent large surge current from limiting or possibly damaging the power supply. After the cap has reached full PS voltage, disconnect cap from the PS.

It's not a very safe procedure to discharge a cap with a direct short as it is possible for a large cap to explode if internal pressure or heat exceeds the vent cap capacity.

Lefty

 

[bloody-orc]

So the correct way to connect the thing is:

PSU+ --- 10kResistor (as many watts as I can find) --- Cap+ -- Cap- ---PSU-

And across the capacitor I connect a volt-meter to show charge rate. right?



and ofcourse I will test it before class... I have to find the correct metal plate to test it with

 

[Nigel Goodwin]

So the correct way to connect the thing is:

10kResistor (as many watts as I can find)

Try applying ohms law, if you use a 10K resistor you will only need a tiny wattage - and it will take a LONG time to charge.

If you put a 12 ohm resistor directly across 12V it will pass 1A, and dissipate 12W (12V x 1A) - assuming you used such a resistor it would only dissipate the full 12W for a short time, and drop off as the capacitor charges.

 

[ericgibbs]

So the correct way to connect the thing is:

PSU+ --- 10kResistor (as many watts as I can find) --- Cap+ -- Cap- ---PSU-

And across the capacitor I connect a volt-meter to show charge rate. right?

Hi Rain,
I would still suggest using a 27R say 10W resistor.

If you think about a bog standard bench PSU, many use a 10,000uF at 16Vwkg [or higher] , how many PSU's would use a high value charge resistor???

Stay within say, the PSU's Imax/2 rating and that shouldn't cause the PSU any problems.

With a low value resistor you wouldnt need a meter to measure the Vcap.

As Lefty says, abusing the 10,000uF by applying a dead short is not a good idea.
If you must demonstrate the discharge, I would suggest that you do so inside a transparent safety box.

 

[bloody-orc]

OK thanks for all the answers and ideas. I'll start small and move higher with both capacitance and voltage.

Is it so, that if I take a capacitor with ratings of say 16V and 1000uF, then I can charge it with anything from 0 to 15V (1V left over, just to be on the safe side) and the capacitor will fill the same. It just charges itself up with less voltage in it. correct?

 

[ericgibbs]

OK thanks for all the answers and ideas. I'll start small and move higher with both capacitance and voltage.

Is it so, that if I take a capacitor with ratings of say 16V and 1000uF, then I can charge it with anything from 0 to 15V (1V left over, just to be on the safe side) and the capacitor will fill the same. It just charges itself up with less voltage in it. correct?

hi,
The 16V is maximum voltage rating for that capacitor, if its rated a 16Vdc working.

Dont exceed the rated working voltage of any capacitor, it may self destruct!

 

[Nigel Goodwin]

Dont exceed the rated working voltage of any capacitor, it may self destruct!

I don't think there's any 'may' about it!

Modern capacitors are VERY strictly rated, even a slight over voltage tends to make them die!.

 

[ericgibbs]

I don't think there's any 'may' about it!

Modern capacitors are VERY strictly rated, even a slight over voltage tends to make them die!.

hi,
I was mainly concerned about his young spectators, dont want to harm the kids.

I have accidentally overstressed caps, some go 'puff!' others 'bang' and spew out their contents, tants make 'crack and then a lovely glowing fireball that runs around the bench, just before it drops into your lap.

Take care Rain!

 

[HarveyH42]

We did extensive testing on the voltage ratings on electrolytic capacitors in high school electronics shop, every time the instructor left to get a fresh cup of coffee... Kind of like fire crackers, bigger is better . Definately not a good idea without some physical barrier (plexiglass?).

 

[Krumlink]

I just connect my caps up + to + and - to - and have bunches of fun melting copper tape and copper sided boards

I have the following setup:
total capacitance=90,000uf
Voltage In:30V
Joules of energy: 40.5 Joules!!!

They are in parallel, like for a coil gun