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Anyone know where i can find a transformer to convert 240v AC 50hz to roughly 400V AC, 10-50W

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jpoopdog

Thu Apr 18 2013, 12:47AM

Registered Member #6188 Joined: Wed Aug 15 2012, 08:34AM
Location:
Posts: 23

Hi

i came up with a stupid simple idea for a coilgun charger, as i browsed ebay. I saw that its possible to buy a 75W 50W continuous inverter for $6-10.
That led me to think, this can convert up to 240v AC, it wouldnt be that much harder to get 400v, it would be very efficient, and relatively small enough for portable use. and it could be done for very little. im currently saving up for a decent soldering station, so until then i cant build the circuit i have in mind using my coilcraft inductor, but i want to do experimentations in coil design and general capacitor bank stuff ,using something like this, an inverter and transformer, inverter being safer than running off mains.

Problem now is i cant find any transformers that can do this,
does anyone know where i can find some relatively cheap transformers that can convert 240v to anywhere between 370v - 480v, anywhere from 10w - 50w? and still be reasonably sized that its practical to use in a portable design?

thanks

klugesmith

Thu Apr 18 2013, 03:17AM

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

From 240 volt (RMS) sinusoidal source, you can charge a cap to about 340 volts with a full wave bridge,
and to about 680 volts with a simple voltage doubler.

I think the majority of 400 V rated capacitors in consumer products around you operate at around 340 V, with a reasonable and proper design margin.

Why do you want to use such high (and thus hazardous) voltages?
Electrolytic capacitor size, weight, and cost per joule are not very sensitive to the voltage rating.
A coilgun coil can be rewound with shorter and thicker wire, in the same volume,
to get the -same- magnetic performance and pulse timing with lower voltage and higher current.

Those inexpensive inverters put out a "modified sine wave",
whose RMS and peak voltages are supposed to roughly match those of mains at the same nominal voltage.
Peak voltage is what you care about.

An internet search for "modified sine wave" turns up many pictures whose peak voltage and duty cycle are drawn wrong. Get the cheap inverter and measure its actual peak voltage. Can do that by measuring the voltage on a small value capacitor charged with a single diode. Then you will already have half of the first voltage doubler configuration in link above. Try the same with a mains plug (and adult supervision).
If the cheap inverter is like mine, you can tweak the output voltage by changing a resistor inside -- maybe just by changing a trimpot adjustment.

jpoopdog

Thu Apr 18 2013, 06:12AM

Registered Member #6188 Joined: Wed Aug 15 2012, 08:34AM
Location:
Posts: 23

interesting.

I knew that 240v in theory would still charge a capacitor that high, but i thought there would be problems/struggle with doing that using full wave DC. Also i was under the asumption that voltage doublers couldnt handle that much current.

The capacitors i have in mind are rated 330v, so this is good, though i have others pulled from ATX supplies or TV's i forget which one, which are rated 400v.

Ill buy the thing now and experiment with it, ill see if i can increase voltage by doing something like changing the frequency perhaps. Thanks

Steve Conner

Thu Apr 18 2013, 09:33AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Transformers like this are commonly used in industrial control panels.

jpoopdog

Thu Apr 18 2013, 10:36PM

Registered Member #6188 Joined: Wed Aug 15 2012, 08:34AM
Location:
Posts: 23

ok, so are there any downsides to using a voltage doubler? to get the voltage up? as in will it at all restrict current flow in any way?

Steve Conner

Fri Apr 19 2013, 09:57AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think with a cheap inverter, you will want to restrict the current flow somewhat. If anything a voltage doubler wouldn't restrict it enough.

jpoopdog

Fri Apr 19 2013, 11:13PM

Registered Member #6188 Joined: Wed Aug 15 2012, 08:34AM
Location:
Posts: 23

so voltage doubler is that efficient huh, wow.
why should i be limiting the current though?
Since ill likely be using NIcads, or at best a small SLA, either one cant produce the 75W max, i doubt they could even do 50W.

Ill be using a voltage comparator setup to switch the thing off once the target voltage is reached, plus ill have a cheapo volt meter on there also in order to calibrate the thing, ontop of seeing the voltage.

So explain why i should restrict the current?

PSCG

Sat Apr 20 2013, 08:04AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136

Can we adapt this circuit as a "variable" current limiter?

1366444928 3792 FT1630 Spark Eliminator Circuit

The idea is that it takes some time to charge the capacitor C1 (depending upon C1 and R1, R2)
and reach the threshold where T1 will start conducting.

DC - AC converters are designed for a maximum load and with protection against short circuiting and overload. If a discharged capacitor is connected to the output, it will show up as a short circuit , possibly triggering the protection. So, every time you try to charge the main capacitor, it will "lock up" and require to cycle it off and on.

Fraggle

Sat Apr 20 2013, 03:50PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Got a nice core if you want to wind...

jpoopdog

Sun Apr 21 2013, 12:19PM

Registered Member #6188 Joined: Wed Aug 15 2012, 08:34AM
Location:
Posts: 23

no thanks.

Hey ive been wondering. since a discharged capacitor will act as a short, what if i use the 12v input to charge the cap to 12v before the inverter takes over? would that have any effect at all?

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