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Ideas for a Charger for These Capacitors

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KLH

Tue May 26 2009, 12:59PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

Considering that you want to use the parts you have on hand, I would recommend using the flyback transformer in a standard flyback charger. There are already many projects and information on this site that can help you design the charger. Creating a charger using this topology would allow you to use a battery or a low voltage power transformer as a power source, along with the added benefits of portability.

If you still choose to use a mains charger with a diode tripler, then you should calculate the reactance of the capacitors at your particular mains frequency, so that currents would be limited to safe and predictable levels. A calculator can be found here:

On the previous remarks: Hobbyists in general seem to be relying on mains chargers for their capacitors. It is easy to see why; the mains charger, regardless of whether it uses diode multipliers or step-up transformers, is very cheap and reliable. However, I would still like to see a proliferation of powerful, efficient, and portable chargers. A portable charger would provide more interesting applications, with countless possibilities just waiting to be explored. (Unfortunately, I have been unable to do my part and contribute due to recent circumstances at school. Now that summer approaches, I should be able to pick up my work again... )

Wolfram

Tue May 26 2009, 04:36PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

A flyback is an exceptionally poor choice for charging low voltage capacitors. If you can get 10mA from a flyback for any length of time (I don't know if this is realistic), charging 7 12000ÂµF caps in parallel to 400 volts is going to take just under an hour theoretically, probably more in practice since the capacitors have a leakage current. If the capacitors are in series, then it is somewhat more feasible, but still not ideal, and electrolytics in series have their own problems.

Anders M.

KLH

Tue May 26 2009, 06:37PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

Anders M. wrote ...

A flyback is an exceptionally poor choice for charging low voltage capacitors. If you can get 10mA from a flyback for any length of time (I don't know if this is realistic), charging 7 12000ÂµF caps in parallel to 400 volts is going to take just under an hour theoretically, probably more in practice since the capacitors have a leakage current. If the capacitors are in series, then it is somewhat more feasible, but still not ideal, and electrolytics in series have their own problems.

This scenario only applies if you look at the power supply as a source of DC. A flyback charger (whether it uses a single inductor or a flyback transformer), in the sense that I was describing, does not output DC; instead, it repeatedly outputs pulses of a given amount of energy. With properly selected components, flyback chargers can be built with energy transfer rates from 10 J / sec all the way up to 2000J / sec. (2000 J / sec seems to be the upper limit for practical purposes using lead-acid batteries. Going higher would require better batteries and more advanced inductors which are not readily available.)

Z28Fistergod

Tue May 26 2009, 08:07PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Using the parts on hand would require him to use the TV flyback, which probably wouldn't last too long at 2000 watts. The way I see it the flyback would most likely appear as a constant current source at voltatges of 400V and below.

Using this formula C=Q/V >> C=AT/V >> T=CV/A

Where C is capacitance, V is voltage, A is the charging current, and T is time in seconds.

T for a charging current of 10mA is 56 minutes
T for a charging current of 100mA is 5.6 minutes
T for a charging current of 1A is 33.6 seconds
I think this is how Anders came up with his number.

So unless you can make a flyback supply with an output of 1A I don't think this would be practical. If you want a portable charger I would recommend a flyback charger with a transformer wound to your specs. But I don't think the railgun is going to be too portable with a capacitor bank that large anyway.

rp181

Tue May 26 2009, 08:37PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

I do not use current limiting resistors, the fuse does not trip past a 15 amp outlet. The diodes do not have to be large, as mine worked with 20A diodes. Right now, I am using ISOTOP 120A diodes, but only because It was the only thing I had at the time, and the screw terminals.
You could also use the flyback core, and wind a new secondary for a suitable voltage, using a normal driver.

Herr Zapp

Tue May 26 2009, 09:35PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Killah573 -

Nerdywarrier clearly stated that his flyback transformer was salvaged from a television, and asked if it was suitable for use as a simple cap charger for a 400V electrolytic capacitor.

The simple answer is "NO".

It's just not rational to suggest the use of a 20kV+, very low current flyback system to charge a low-voltage capacitor.

Certainly cap chargers capable of achieving 2kJ/sec rates exist, but they will NOT use a flyback transformer anything similar to one found in a TV or video monitor!

Regards,
Herr Zapp

Mathias

Tue May 26 2009, 10:24PM

Registered Member #1381 Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74

Don't want to sound too negative , but why does someone who ask a question like "how to charge the capacitors up" (with a high voltage , low current transformer that was around) would possible want to start a complex project like for instance: building a (working) high powered Rail gun?

Sure thing anybody could do it, if that person has acquired the knowledge needed for doing so. But for now this doesn't seem's to be that case.

Maybe start with something smaller and simpler.

Or good luck with the rail gun.

rp181

Wed May 27 2009, 12:05AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I agree, playing with that much energy is a bit much, especially if you don't know how to charge it. At least start with a coilgun, even if it is camera flash. A railgun will be difficult to get to work unless you have mechanical skills, or know people that do. A rail gun is more of a mechanical feat than electrical.

KLH

Wed May 27 2009, 06:50PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Unfortunately, it seems I failed to make something clear:

The TV flyback transformer cannot be used with its current windings to charge a low-voltage capacitor.

You are correct about the use of the transformer as-is. What I intended to state was that the core of the transformer could be removed and rewound with different windings and be used with a flyback circuit to transfer energy into the capacitor. If the core is big enough there would be a lot of room for experimentation, although parameters such as the effective Al value of the core should be found first.

Also, when I refer to "flyback circuits", I mean circuits that use the "flyback" properties of an inductor (the voltage "flys back" when the inductor discharges).

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