# cheap way to charge a cap bank

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#### Joeshmoe

Joined Jan 12, 2007
66
I need to charge cap banks ranging from 2400uf-32000uf i need to charge them quickly in seconds. i have access to a radioshack (but they dont seem to have transformers). I want to charge the caps to 400v if possible i would like to vary to voltage but i cant spend to much on this.

#### thingmaker3

Joined May 16, 2005
5,083
Make sure the caps are rated for not less than your working voltage, preferably rated for much more than your working voltage. If you plan to charge to 400Vdc, the caps should be rated for not less than 480Vdc.

Charge time for a capacitor is determined by resistance in series with the capacitor, including the capacitor's own "equivalent series resistance." Time for full charge, in seconds, is 5*C*R, where C = capacitance in Farads, and R = resistance in ohms. The series resistance you use should be no less than required to limit current to levels your charging source can handle. Example: if your power supply can safely source 3.5A, then your series resistor should be not less than 400/3.5 = 120 ohms (rounded up to commonly available value). Charge time for your 35000uF bank would then be 5*120*.035 = 21 seconds.

Try to find a surplus dealer in your area for components to build your charger supply. Also try online auctions.

By the way... what exactly do you plan for those 25.6 kilojoules of energy eek once you've so quickly stored them? Nothing mischievious, I trust?

#### Joeshmoe

Joined Jan 12, 2007
66
im working on a coilgun..... iv been using photoflash chargers to charge the caps but it gets annoying cuz they take forever to charge and i want to be able to charge quickly so testing will be quicker... what would be a simple way to make a power supply i would want more than 3.5A so the large bank can charge a little faster... thanx for the info

#### thingmaker3

Joined May 16, 2005
5,083

Let's start at the beginning. What size breakers do you have on your wall outlets, and will you be using 110/120Vac or 220/240vac? In other words, how many watts ya gotts?

#### Joeshmoe

Joined Jan 12, 2007
66
well thats the problem... first im a school and dont know the breaker size i do know that they warned us that the breakers are older and not to use microwaves or air conditioners it is 110/120V

#### kubeek

Joined Sep 20, 2005
5,791
May I ask how old are you? This voltage we are talking about is quite dangerous for a school student.

#### Joeshmoe

Joined Jan 12, 2007
66
dont worry iv been handling these for a while and know that voltage isnt the issue it is the current and seeing that i am using hundreds of amps i understand the dangers I am 19 and at a university

#### kubeek

Joined Sep 20, 2005
5,791
With that amount of energy it will probably be more a coil cannon than coil gun.
So first: can you list the capacitors you have - capacity,voltage rating, and any other parameters you know like ESR?
Then also it would be good to know what power supplies do you have? (maybe two PC supplies could do it)
Are you going to have it like a bench test device, or a portable thing?

For example you have the 32mf/400V cap, so charged to 350V it holds 1960J. If you want to charge it within 10 seconds, you need a 196W power supply (at least).
So creating a coilgun to be taken with you is quite a problem, because you need big batteries to have more than a few shots (a 12V battery pack with 2000mAh holds 24Wh, that´s 86,4kJ, which makes 44 shots, that is not that bad but in real it will be much worse because of inefficiency).

#### Joeshmoe

Joined Jan 12, 2007
66
i have 4 caps at 400v 3900uf 105c and 4 at 420v 8000uf +/-20% -40/+85c
and 1 450v at 2400 it says MDF but isnt that the same as uf

#### Joeshmoe

Joined Jan 12, 2007
66
for now i want a test bench device for testing so a speedy charge will make testing quicker ultimately i would like a portable device

#### kubeek

Joined Sep 20, 2005
5,791
You said you want it cheap, so do you have any cheap source of electronic parts, like some recycling place for computers etc.?
If so, what do they have?

#### Joeshmoe

Joined Jan 12, 2007
66
i dont know of a place like that around me... what kind of things should i look out for? can i mod a pc power supply to give out the right voltage

#### thingmaker3

Joined May 16, 2005
5,083
im a school and dont know the breaker size i do know that they warned us that the breakers are older and not to use microwaves or air conditioners it is 110/120V
Based on the prohibitions, I'll guess they're 10 Amp breakers. If we use a standard safety margin, we should refrain from drawing more than 8 Amps. Your charger will be able to o/p 4.5 Amps if you can find the perfect transformer. That will give you a charge time of around 16 seconds for the 32000 uF bank.

With that amount of energy it will probably be more a coil cannon than coil gun.
Most coilguns will convert less than 2% of the capacitor charge into projectile kinetic energy. I've never heard of one that can get above 11%. (Granted, the universe is chock full of things I've not heard of yet.) I don't expect Joe's project will be much more dangerous than a squirrel rifle.

#### Joeshmoe

Joined Jan 12, 2007
66
he was just kiddin around, so far from testing i havent seen huge differences between the 24000 uf bank and and smaller ones about half their size. now i havent done extensive testing because it just takes so long to charge. I also think that i may need a larger projectile. But my first step is to figure out this charging thing so testing will go a lot quicker.

#### Joeshmoe

Joined Jan 12, 2007
66
oh and 16 seconds would be great and i dont even know if im gonna be using the 32000uf bank because the caps in it are huge

#### kubeek

Joined Sep 20, 2005
5,791
Thingmaker: you missed that the cap is 3200uf, not 32000uf. Then the time would be 1.6 seconds?

#### thingmaker3

Joined May 16, 2005
5,083
I've certainly misplaced my share of decimal points in the past, but I have this one pinned firmly in place.

From the first post:
I need to charge cap banks ranging from 2400uf-32000uf.
From the ninth post:
and 4 at 420v 8000uf +/-20% -40/+85c
Smaller cap banks will, of course, charge more quickly. 5 times the TC for full charge, just like the basic lessons teach.

#### Joeshmoe

Joined Jan 12, 2007
66
kubeek im sorry but my last post had a typo one bank is indeed 32,000

#### Joeshmoe

Joined Jan 12, 2007
66
so how can i make power supply for my project

#### kubeek

Joined Sep 20, 2005
5,791
There are many ways to do the supply, but there are very few cheap.

You could mod some pc supplies, but this voltage could be a problem, you will have difficulties to fit that many turns to the original core.

You could also use a voltage multiplier straight from the wall outlet, but this is even more dangerous. You would have to have some insulated double-pole switch so you can switch the charger on without touching anything else, then switch it off and carefully disconnect the charged cap.
If you have 110V circuits, then simple doubler will give cca 310V, which should be more than enough for a start. Be aware that the doubler needs non-polarised caps, the best would be motor starter caps.

With two 10uF caps in the doubler the 2200uF cap is charged to maximal voltage in about 5 seconds.

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