I bought some large capacitance, low voltage capacitors, 1300F and 5V. I also bought a AC/DC converter for charging them. I tested the converter and it seems to be giving about 7V. I have two questions concerning this:

1. I am just taking the two leads from the converter and connecting them to the voltmeter but while I was reading through VOL 1 here I think I remember something saying that you had to put a load or something between the terminals so that it could accurately read the voltage. Is this true or did I misremember something?

2. Say this converter is really giving 7V, if I just watched the capacitor charge and stopped it around 4.5V would there be any damage or any reason to not do this?

Thanks guys!

 

Why don't you just put a resistor between the converter and the capacitor? It will limit the output current of the converter to something it can handle when the output is shorted, ie. 0V on a big capacitor. It will also help you protect the capacitor. You will still have to watch the capacitor and stop the charging!

 

Why don't you just put a resistor between the converter and the capacitor? It will limit the output current of the converter to something it can handle when the output is shorted, ie. 0V on a big capacitor. It will also help you protect the capacitor. You will still have to watch the capacitor and stop the charging!

Not necessarily a good idea - a converter designed to charge a very large capacitor could output many amps, and most resistors would overheat, you'd probably need about a 10 ohm resistor, according to I^2*R that might dissipate 10W at 1A, not to mention higher currents, or short circuit current!

 

Because of what Tom said is the reason why I didn't try the resistor, I could have gone a bought one but I'd like to hear the answers to the first two questions also.

Actually I just checked the current and its about half what it says on the converter(and the voltage is about twice what it says, I feel like there's a connection here). The current is about 86mA which seems really low to me.

 

Because of what Tom said is the reason why I didn't try the resistor, I could have gone a bought one but I'd like to hear the answers to the first two questions also.

Actually I just checked the current and its about half what it says on the converter(and the voltage is about twice what it says, I feel like there's a connection here). The current is about 86mA which seems really low to me.

86mA will take a few weeks to charge a 1300F cap. Sounds like you have a faulty unit.

 

Not necessarily a good idea - a converter designed to charge a very large capacitor could output many amps, and most resistors would overheat, you'd probably need about a 10 ohm resistor, according to I^2*R that might dissipate 10W at 1A, not to mention higher currents, or short circuit current!

If the voltage of the supply is 7V, even into a short-circuit the maximum current into a 10 ohm resistance would be 700mA, with a dissipation of 4.9W; P=V^2/R. You could probably get away with a 6Watt wire-wound.

More of an issue would be the time constant; 10 ohms with 1300F gives RC = 13000 seconds, or 3.6 hours - it would take a bit longer to reach 5V. You would hardly want to be vigilant all that time, so I think some kind of automatic limiter is called for.

 

I found another converter which says .7A but I think it's a little lower than that. It seems to take about 2 hours to fully charge, its a little hard to tell because I've been testing what it can do everyone in a while and it looses some charge during that.

I have another question. When I have it charging and I check the voltage, it is about one volt higher than when I check the voltage of the capacitor not while charging. Which one should I use as my fully charged to 5Vs limit?

 

Don't charge it to more than 5v.

Be careful with it when it's even partially charged, as it's capable of putting out a large amount of current. If you accidentally short the terminals, it will do some damage, and you may get burned.

 

Don't charge it to more than 5v.

So when it's not charging should be the time I measure it?

 

No, measure it when it's charging.

 

Use a few diodes in series with power supply to ensure the voltage doesn't go over 5V. Otherwise:

 

@Sargent What's weird is that the difference between the charging and non-charging voltages varies sometimes. Like before I said there was about a one volt difference(IE charging was 5V and not charging was 4V) then I messed around with it and it lost about 2 volts. Then I started changing it again and the difference is now only about 1/4 of a volt.

@thatoneguy So I should just put a between the positive terminal of the capacitor and the current source and another between the negative terminal and the ground?

 

Use a few diodes in series with power supply to ensure the voltage doesn't go over 5V. Otherwise:

This will dissipate the same power as a resistor.