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Charging 1.7kj Cap Bank from batteries

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FastMHz

Wed Mar 08 2006, 10:11PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I would like to find the easiest way to convert 12v DC into 700v-720v DC so that I can more easily launch my ringrockets from a field without an AC power source. It needs a bit of current, say 0.5a, so the bank charges in a few seconds rather than minutes. When I say "easiest" I mean with fewest components without having to etch a PCB.

Sulaiman

Wed Mar 08 2006, 11:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

There are many circuit possibilities,
the limitations will probably come from the battery.
e.g. 1700 Joules in 10 seconds = 170W at 100% efficiency
for a 12 V battery that's 14.17 Amps
probably nearer to 20 Amps considering losses.
That's a good NiCd pack or a large/heavy lead-acid battery.
(anyone have a good alternative for high-current batteries ? )

All things considered, it would probably be best to get a commercial 12V invertor to use the same design as indoors.(eBay?)
The weight will be less than the Battery.
An invertor is always 'nice to have'

EDIT
So compromise between
-Energy stored in Capacitor Bank
-Time to charge Capacitor Bank
-number of firings between Battery recharge

Your requirements are very similar to high power Xenon flash requirements, and almost every energy pulse discharge project on this forum!
-Cost/Weight of Battery

Bjørn

Wed Mar 08 2006, 11:45PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Modern lithium and NiMh based batteries can take those currents and more without any problems. Have a look at sites like http://www.ezonemag.com/ for high power battery packs.

A 230V inverter with a voltage doubler sounds like very good idea. Some of them are easily adjustable within a certain voltage range.

FastMHz

Wed Mar 08 2006, 11:51PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Thanks for the replies guys! I was beginning to think an inverter was probably easiest too, and they're cheap on ebay which is good. I'll be using a small 12v@5a GelCell to charge, it can handle high current draws for short periods.

The best batteries on the planet are Lithium Polymer....they're also most expensive...but you can see there capabilities in my RC project here. I won't be using batteries like this

Dr. Shark

Thu Mar 09 2006, 08:56AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

A few NiMh racing packs are probably the power source of choice, they take abuse well (LiPo does not), and they are cheap.
If you feel like experimenting you could try rigging up a simple blocking oscillator using the transformer from an old PC power supply in reverse and a single transistor (e.g. from the same PC PSU). the circuit is basically this

there was also a thread about that on the old board, but I can't find that now. Efficiency probably wont be that great, but hey, just chip in another battery pack and you probably still save weight compared to using a "proper" inverter

FastMHz

Thu Mar 09 2006, 06:37PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

That has got to be the cheapest, simplest inverter I've ever seen! I can certainly find uses for that - thanks for sharing it! I'm a little doubtful as to its usefulness for charging such a huge cap bank in about 20 seconds due to current draw though.

Dr. Shark

Fri Mar 10 2006, 09:00AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

If you use an ATX PSU transformer, which is good for at least 200W, you will probably get a bit more current than with the ferrite bead

This suggestion acutally came from Dan (EVR) or Mike (Mikes Electric Stuff), aparently he managed to get quite a bit of power through. You need a few extra components if you do this, like a Zener to protect the transistors base, some snubbing components etc.

mikeselectricstuff

Sun Mar 12 2006, 09:52PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

A while ago I was looking at simple, quick, crude and brutal ways to get a few hundred volts at high current (for an argon laser pointer project I never got round to finishing). I found that a simple blocking oscillator based on a PC power supply transformer worked pretty well. For 700 volts you might need to stick a doubler on the output.

rupidust

Sun Mar 12 2006, 11:17PM

Banned
Registered Member #110 Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85

FastMHz wrote ...

I would like to find the easiest way to convert 12v DC into 700v-720v DC

Is 12v absolutely necessary? To charge 1700J in 20 seconds requires an 85 watt charger. I got a 100 watt booster that uses 24v. Uses 24v so that the current consumption is lower. It just a booster converter, real small size and parts count.

FastMHz

Mon Mar 13 2006, 12:26AM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

12v isn't mandatory, but preferable since that's what vehicles use. I've already ordered a cheap 140w inverter off ebay and will use that. Thanks for the suggestions!

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