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Charging Cap bank with Flyback Transformer

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aarpcard

Thu Feb 03 2011, 01:43AM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44

I've been experimenting with using a rectified flyback transformer to charge my capacitor bank. I'm running into some problems.

The set up I'm using will output roughly 24000 volts at 6 milli amps. Here is the weird part. It will charge my individual capacitors (370 volts, 500mfd) in 2 seconds or less. However, my capacitor bank (which is just many of those capacitors in parallel) which is 370 volts, 5500mfd will take upwards of 10 minutes to fully charge.

My guess is that after a certain point, the current being stored in the capacitors becomes too much and starts leaching out backwards into the flyback. Do you think is the case? I mean it would be nearly 100 times faster to charge each capacitor individually lol.

Anyhow, I think perhaps I might need to put a High Voltage/Current diode in series with the capacitors to prevent this. Do you think that might fix it? Any response would be appreciated! Thanks in advance!

Inducktion

Thu Feb 03 2011, 03:19AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

That sound's like you're gonna make your caps go boom with that much voltage going in, but I'm probably wrong.

aarpcard

Thu Feb 03 2011, 03:27AM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44

I was worried about that at first, but after seeing other people do it without problem, I figured it'd be ok.

I just carefully watch until they reach their charged state and then cut the power. For a single cap it's a second or two. For the bank, it's several minutes. This is the problem I'm trying to fix.

The reason, there isn't a big risk of explosion is because while the voltage is insanely high, the amps are minuscule - only 6 milli amps. If it were 24000 volts and 1 amp, then that would be a completely different story.

So any ideas? Think a high current/high voltage diode might work?

Proud Mary

Thu Feb 03 2011, 11:00AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It is the electrolytic capacitors that are leaking. The more you connect in parallel, the greater the total leakage current will be.

In the very first instant you connect your supply to an uncharged capacitor, the supply 'sees' the capacitor as a dead short. As the supply cannot deliver an infinite current what happens? The voltage falls to nil in that instant, and then gradually starts climbing as less and less current is required as the capacitor charges. In a perfect capacitor, which does not leak, no current will be drawn when the capacitor is fully charged.

The only reason you have not destroyed your capacitors with your 24kV is because your power supply has such a high impedance that the output voltage falls to a very low level just through the effort of supplying your capacitors' leakage current.

Anyway, it's time for you to learn about the RC time constant which will set you up for a proper understanding of how capacitors charge up through resistors.

Sulaiman

Thu Feb 03 2011, 01:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Try dis-connecting the secondary and connect the capacitor bank via a diode (e.g. UF4006) to the primary .... much faster charging .... using the transformer just as an inductor.

GhostNull

Thu Feb 03 2011, 11:49PM

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

It's hard to understand what you are trying to do and what the problem is.

Are you trying to build something like this:

?

What is your current setup?
Do you have a schematic? (That might help in under standing the problem)
When you say "diode in series with the capacitors" do you mean from the output of the flyback to the capacitors?
How are you controlling charging?

aarpcard

Fri Feb 04 2011, 12:56AM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44

Thanks for the replies! As soon as I get home I'm going to try the suggestions. As for a schematic, this is basically what my circuit is:

Add a bridge rectifier to the secondary and that's my circuit for the most part. Really simple. And GhostNull, that's exactly what I mean when I'm referring to the diode. Think it'll work?

As for controlling charging, I haven't worked on a discrete system for that yet. That's my next step after I get this problem solved. Currently I'm using a volt meter to tell when the voltage gets to the right level and then I disconnect the capacitor bank from the flyback.

GhostNull

Fri Feb 04 2011, 06:26AM

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

Oh No! This won't work well! You need to do some research D:

Problems:
1) Your single transistor flyback driver (in the schematic) is very inefficient but foremost extremely unreliable. It is only really suited as an introductory circuit for HV beginners. You need a better way of driving the flyback transformer, e.g. a ZVS circuit.

2) 24000 * 0.006 = 144 watts. You can't hope to get 144 watts output from a single transistor driver. Again, you need a better driver.

3) A 24000v transformer to charge 370v capacitors. It will work, if you can stop control charging properly but it won't be very efficient. People using flybacks to charge their capacitors wind their own secondaries in order to get lower voltage, higher current and better efficiency. You also run a higher risk of accidentally charging the capacitors past their voltage rating and damaging them and if in a bank causing an explosion; If one capacitor fails, then all the other capacitors will discharge through it and boom!

I also would discourge manual charge control.

Answers:
For the current leaking back into the flyback transformer, that makes no sense. The amount of current stored in a capacitor has nothing to do with it's ability to leach back into the flyback transformer. I'm guessing you're thinking that the capacitor will be charged to a point which it will have enough power to push current back from the capacitor, which would be completely wrong. The movement of electricity is dictated by the electro motive force (voltage) not the current. Even then the rectifying diode would stop current from flowing back through the flyback secondary.

In regards to the diode to stop the leaching back, just as mentioned above, If the flyback transformer in rectified then there will be a diode there already.

It's good you've got some ideas and you want to try make a coilgun, but you need to do some more research!!

Hope this helps! Have fun and stay safe!
-Ken

aarpcard

Fri Feb 04 2011, 07:53PM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44

thanks for the tips. Looks like I'll be looking more into this. BTW, I already have a working coil gun - decently powerful too for what it is. I'm just looking into alternate charging solutions.

Rustycan

Mon Apr 04 2011, 04:37PM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

just found this on the web

if the AC output connected to a transformer then bridge rectifier, could it charge a cap bank?

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