Fast 450V 4700uF Capacitor charger help

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Old 09-21-14, 08:17 AM
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Question Fast 450V 4700uF Capacitor charger help

Hi,

I would need a portable system capable of charging a 450V 4700uF Capacitor to 99% in about 2-3 seconds (or less if possible). The system needs to be portable and contain its own power (a battery of somekind) and weigh less than 2kg. Is this possible?

I have done some googling but i am not very learned in electronics. To me it seems that with very low resistance the capacitor could charge arbitrarily fast? but then the current would probably melt something?

Thanks in advance, i will check this post as often as possible.
 
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Old 09-21-14, 09:39 AM
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arbitrarily fast? Typical batteries have enough internal resistance that they will not discharge fast enough to damage your capacitor.

Is this an expensive or unique capacitor? 4700uF seems like the kind of part you get at radio shack. If it is cheap enough to replace connect it to an alkaline battery AA or other battery (get a battery holder at radio shack while they are still in business) and then remove the battery and test the terminals with a volt meter, or if you have a small speaker or buzzer connect it to that and hear it discharge. There are voltmeters that test for capacitance so you can see if you damaged the capacitor.

HowStuffWorks "Farad"

You might also consider getting one of those learn electronics kits at Radio Shack.

What you are interested just requires basic arithmetic and simple test gear assuming the capacitor is easy to replace.
 
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Old 09-21-14, 09:59 AM
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Welcome to the forums.

Portable and fast don't go hand in hand here.
Depending on what voltage you want to charge that cap to determines what you need to charge it.
What voltage do you want to charge to ?
How often are you going to want to recharge ?
 
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Old 09-21-14, 12:04 PM
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Hi thanks for the replies,

@george: it is not unique nor expensive i got it from ebay. If i understood capacitors correctly how fast they will charge depends on the supply voltage so a AA battery would take forever to charge one like this to 450V? Im getting a multimeter from ebay next week so that should let me do the testing you suggested.

@pjmax:
Thank you, i need it to discharge as much energy as possible so i guess im looking to charge it to the maximum 450V? Its going to be an experiment so it will see some days of intense use every now and then (and long pauses in between).

Thank you
 
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Old 09-21-14, 12:07 PM
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I'll try to do the math but I'm pretty sure there isn't going to be a very handy "portable" solution.
 
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Old 09-21-14, 01:09 PM
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Do you think we would be less likely to help you if we knew what you were building? Why hobble our ability to help you by not fully explaining what you are trying to do? Heck, we might even be able to suggest a better way to do it.
 
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Old 09-21-14, 01:16 PM
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I agree that knowing exactly what the energy will be transferred to would allow us to possibly offer more.
An example is that if the energy from the capacitor is dissipated too quickly there would be a tendency for the foil wraps to blow holes in the insulating paper/composition separator.
 
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Old 09-21-14, 01:26 PM
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sorry i should have explained more:

The system is for a portable coilgun i intend to make. It will be a single stage one but i was thinking of making a clip to it so i would like it to be able to fire at least once every second or so. I was looking at this: Portable 1.25kJ Coilgun - Delta-V Engineering for ideas but it states a charge time of 30sec which is completely unacceptable for my purposes. although mine has about two fifths of the energy of that one, it would still have a charge time of at least 10 sec. That gun is pretty small however and i would accept a bigger model if thats what it takes. a normal Assault rifle weighs something like 3-4kg i guess so thats the target (upper limit). On the same webpage theres a 8 stage coilmachinegun that doesnt use a capacitor but it seems to be a lot harder to make so im going so start with a single stage model as i am a complete beginner thanks for the replies!

this first gun is an experiment and based on what i learn im hopefully going to be able to do something more advanced next time.
 
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Old 09-21-14, 03:09 PM
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I thought my potato gun was bada**.

You need to factor in the fact that if the capacitor is charged or cycled too fast it will overheat.
This project needs some serious electronics design.
 
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Old 09-21-14, 05:25 PM
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You do know that 450 volts can be lethal?

You would need a power supply to get 450 volts from a battery. The higher power of your supply will decrease the charge time but I would not think 4700 uf would take much current to charge quickly.
 
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Old 09-22-14, 05:08 AM
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Yeah im aware and will do my best not to zap myself. What do you mean by powesupply? A transformer?
Yeah its the badassery that got to me 👍
 
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Old 09-22-14, 07:13 AM
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The 450 volts is the peak voltage that may be put across the capacitor. You should allow a safety margin which would mean applying no more than 400 volts.

Also, what voltage do you want to draw from the capacitor after it is charged. You charge the capacitor to this voltage. At the other end of the scale, you do not get meaningful information (other than possibly verifying a defective capacitor) by connecting a 1-1/2 volt battery to the capacitor and testing the results when you want 200 to 400 volts to do things with. The total amount of energy held by the capacitor is greater when you charge it "to a higher voltage." Meanwhile, 4700 microfarad capacitors are used in a variety of applications with a variety of working voltages where it is not necessary for the capacitor to hold the maximum possible amount of energy for any length of time limited only by the maximum voltage rating.

It is okay to talk about a 99% charge at 1-1/2 volts which is not the same thing as a 99% charge at 400 volts.

Your charger has a maximum load in amperes (from its 200 to 400 volt or so output terminals), or batteries (sufficient number in series to yield 200 to 400 volts or so) have a maximum recommended load, and/or the capacitor has a maximum charging rate. Start with the smallest of these amperes values. When charging the capacitor you assume it's a dead short so you put a resistor in the line to limit the current from the charger to the value you chose to start with. (In any portion of a circuit, amperes equals voltage divided by resistance at all times.) Also take note of the number of watts as determined by the supply voltage times the chosen starting amperes. If this number of watts exceeds the watts rating of the resistor, choose a higher resistor value (you must recalculate using a yet smaller amperes value that equals voltage divided by resistance) until you get a watts (dissipated in the resistor) value not greater than the resistor watts rating so you do not overheat the resistor.

With the capacitor fully discharged, turning on the charger will start by putting the maximum voltage across the resistor, maximum watts dissipated in the resistor, and nearly zero volts across the capacitor. As the capacitor charges up the voltage across the capacitor will increase (the charge in the capacitor opposing the voltage applied by the charger) the voltage across the resistor will decrease, the watts dissipated in the resistor will decrease, and the current in the circuit will decrease.



The resistor discussed here is in the charging circuit only and is not part of the application (bazooka, flash gun, etc.).
 

Last edited by AllanJ; 09-22-14 at 08:25 AM.
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Old 09-22-14, 08:40 AM
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Thanks allan that really clarified things. So i will test different voltages and resistors hoping i find one that charges it fast enough. Which part of the circuit would get damaged if i didnt have a resistor before the capacitor? Thanks for the briefing
 
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Old 09-22-14, 11:14 AM
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Your power supply would have a transformer and circuit to produce a high voltage. It would be similar to the power supply in a photo flash unit. As a mater of fact that would work but would take much longer than you want to charge the capacitor. I think the flash units charge to 400 volts and use about 500 uf. cap.
 
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Old 09-22-14, 12:07 PM
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@ DMCman : ok sounds good i will check up how a photoflash charger works.

i did some research into what kind of resistors there are and found this WDBR5-47RKT - WELWYN - RESISTOR, POWER PLANAR, 47R, 5KW | Farnell UK
which states that it has a 5kW power rating but also that it has a 270W power rating with heatsink?? Is it 5kW or 270W ??
that one would be far sufficient if i would get the charger to charge at 400V.
 
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Old 09-22-14, 12:54 PM
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I would guess 270 watts continuous power but 5 Kw is the max peak power (very short time). I would get your power supply first. If it only puts out 10 watts you don't need that high power resistor.
 
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Old 09-22-14, 03:02 PM
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The time-of-charge, from 0 volts to maximum volts, is comprised of 5 "time-constants". if the charging time is 2 seconds, then the T-C value = 2/5 = .4

The value of the resistor in series with the capacitor is R = TC /Farads , or R = .4 / Farads

A micro-farad 1 / 1,000,000. 4700 micro-farads =4700 / 1,000,000 = 47 / 10000 = 47 x 1 x10-4 =.0047 Farads R = .4/.0047
 
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Old 09-22-14, 03:39 PM
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So it looks like you need 85 ohms and a 400 volt power supply with a peak current of 4.7 amps (when the cap is zero volts). That is a pretty big power supply.
 
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Old 09-23-14, 01:46 PM
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Hi again thanks for the replies

DC-DC HV Boost Converter
I thought maybe this would cut it if i get a 30V battery to power it? it says 90W charging power for the 24V and 36W for the 12V battery so a 30V should maybe get close to 150W ? which would ideally charge the 376J battery in precisely 2.5 sec or so? It looks pretty difficult to build but theres somewhat step by step ish instructions and i might find a friend who could help me build it.

any thoughts?
 
 

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