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Need advice on a coilgun charger

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lhl_henrylui

Mon Jun 28 2010, 07:02AM

Registered Member #1498 Joined: Thu May 22 2008, 07:08AM
Location: Hong Kong
Posts: 72

I've got six 2200uF 450V electrolytic capacitors and I intend to connect them in parallel to make a 13200uF 450V capacitor bank for my first coil gun.

I'm thinking what type of charger I should use. I've thought of a voltage doubler and a transformer.

If I use a voltage doubler, as the mains voltage here is 220V, the doubled voltage will be 440V and the peak voltage will be 440 x sqrt(2) = 622.25V but that is going to destroy my capacitors.

So, I've got two solutions and I want to know whether they are suitable and which one is the best:

1. Use a variac as the power supply of the voltage doubler and adjust the input voltage to 159 rmsV so the peak output voltage is 450V
2. Use the 220V mains as the power supply of the voltage doubler and add a circuit to monitor the voltage of the capacitor bank. The circuit automatically stops the capacitor bank from being charged when its voltage reaches 450V.

Also, I'm concerned about the vast amount of current flow when the capacitor bank is being charged, so I have to add a resistor and some problems arise here. For example, if I want to limit the charging current to 2A, the resistor will be 450V/2A=225ohms and the maximum power will be 900W!

That is impossible because the resistors with the highest power I can find here is 100W. If I choose 100W the maximum current will be reduced to 0.22A, which will take quite a long time to charge my capacitor.

Some say it is good to use light bulb as the resistor, but won't it blow out immediately? Can I connect a few light bulbs in series so that they won't blow out? And what should the rating of the light bulbs be?

GhostNull

Mon Jun 28 2010, 07:15AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

With the light bulbs, if you put them in before the doubler they should limit current to their rated power level. They will not blow out because they are being used under their nominal voltage. You would put them in series if you wanted to reduce the power level or run them at a higher voltage than what they are rated for.

klugesmith

Mon Jun 28 2010, 05:56PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

lhl_henrylui wrote ...
So, I've got two solutions and I want to know whether they are suitable and which one is the best:

1. Use a variac as the power supply of the voltage doubler and adjust the input voltage to 159 rmsV so the peak output voltage is 450V
2. Use the 220V mains as the power supply of the voltage doubler and add a circuit to monitor the voltage of the capacitor bank. The circuit automatically stops the capacitor bank from being charged
when its voltage reaches 450V.

For option 1, be sure to allow for mains voltage excursions 5% or 10% above nominal.
Option 2 is risky for a beginner.
How about option 3, if you are set on using mains: skip the doubler. Get some coilgun & switch experience with your bank charged to 311 volts DC.

As for current limiting, a nominal 220V lamp (on primary side) is ideal. Can't blow it up. And as capacitors charge, the lamp will dim and have many times less resistance, greatly reducing the charging "time constant".

[edit] also search the forum and the internet for advice on "forming" electrolytic capacitors.
They can be damaged by charging to full rated voltage immediately after months of storage without charge.
Another reason why reliably engineered designs generally de-rate components, as a matter of experience and judgement.

lhl_henrylui

Sun Jul 04 2010, 04:21PM

Registered Member #1498 Joined: Thu May 22 2008, 07:08AM
Location: Hong Kong
Posts: 72

Klugesmith wrote ...

[edit] also search the forum and the internet for advice on "forming" electrolytic capacitors.
They can be damaged by charging to full rated voltage immediately after months of storage without charge.
Another reason why reliably engineered designs generally de-rate components, as a matter of experience and judgement.

Could you please give me some more information about how the capacitors are damaged and how I can protect them? As we all know, the capacitors for coilguns are really expensive and we don't want to pay too much to replace the broken ones. So, reliability of the capacitors is critical.

Also, I've chosen the third option, i.e. Charging the capacitors directly by the mains. I want to get more experience from this before using a higher voltage. However, I don't know what should the rating of the rectifier be as I don't know the maximum current. For example, if I use a 100W light bulb the current will be (220^2)/100=484 Ohms. However, a light bulb's resistance reaches its rated value when it is heated up and this means that the light bulb's resistance is much lower when the bulb is just turned on and this gigantic current might damage my capacitors. Is my deduction correct? And how should I estimate the maximum current?

GhostNull

Mon Jul 05 2010, 01:03AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

If you rescearch how electrolytic capacitors work, you'll know that they use a layer of oxide on one of the plates as a dielectric. This oxide layer is created by an electro-chemical (i don't know if thats the right word) reaction with the electrolyte, plate and voltage. When sitting around doing nothing for a long time as NOS (new old stock) and surplus tend to do the oxide layer degrades meaning the capacitor is prone to failure. In order to if this the oxide layer must be reformed by reforming. Also because of the was electrolytic capacitors work they are polorised and will not work/fail if reverse polorised. This is why reverse polarity portection must be accounted for.

Yes your deduction is correct

To find the "cold resistance" simply get your DMM and measure the resistance of the bulb when it is cold. If the turn on current is greater than the current rating of the capacitor(s) than you might need to put a few in series.

Hope that helps
-Ken

TheMerovingian

Mon Jul 05 2010, 10:34AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I used, with good results, a ZVS flyback converter with low resistance secondary to charge the capacitors, switched down pulling down the fet gates using two 2n3904's. It was very compact and powerul, I was forced to ballast it because during charging it blowed up my 10A fuses (12V SLA), so i ballasted it (capacityvely on the secondary) for a 50W approximately.

DerAlbi

Mon Jul 05 2010, 09:08PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I am building a big charger right now. its controlled via 2 AVRs and the energy comes from 2paralleled 2sieries transformers from 450W-PC-Suplys.
Two transformers are supplied from one fullbridge at once and the secondary side is connected in series. With 2 fullbridges that give 450V each (350 needed) with a hell current

charging my 54mF in notime. Secondary side is limited to 10A and primary (15V LiPo) is limited to 70A. I have Added some securitysystems for programmable overrurrent/overvoltage/shortcircuit/general failure shutdown and discarging security (discarges caps automatically on shutdown or via ÂµC). all in all it has over ~15 opamp that measuring something.

Security is controlled completely analog and does not depend on the AVRs - at least if the AVRs are failing the security systems check this

This week i will solder the components. wish goo luck to me

The software will be hard too.
Size is 2x 16x10cm stacked on each other. If the design works as wanted, and someone is interested, i will share it.

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