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Some questions about transformers.

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LAN_403

Yanom

Fri Jul 13 2012, 05:10PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

So I think I know how transformers work, but I had a few questions -

How much power does a transformer consume if the primary side is hooked up to an A/C power source, but the secondary is left unconnected?

I was looking for a power transformer that I could plug into the wall to boost the power from 120VAC to 240VAC. Is it acceptable to plug the primary side straight into the wall, with no resistors?

Is it possible to make your own transformers or do you have to buy manufactured ones?

radiotech

Fri Jul 13 2012, 05:47PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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The transformer consumes power depending on how large it is. You can measure
the wattage, or you can ask the manufacturer.

Costs can be substantial if you just energize it and forget it. In addition,
a large transformer will have a poor power factor and may cost you in penalties
on the power bill.

Transformers are best connected by the utility company on their side of the
meter.

1 % of maximum output, loss, unloaded, might be typical for a 10 kW unit.

Yanom

Fri Jul 13 2012, 05:57PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

radiotech wrote ...

The transformer consumes power depending on how large it is. You can measure
the wattage, or you can ask the manufacturer.

1 % of maximum output, loss, unloaded, might be typical for a 10 kW unit.

so I assume that's because there's current flowing through the primary side, drawing amperage from the wall?

...

what if I built the world's crappiest transformer, having only one turn of wire on each side. Then I plugged one side into the wall. Due to the low resistance of such a short wire, would that not draw a ton of amperage from the wall?

Inducktion

Fri Jul 13 2012, 06:27PM

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

It would indeed. The reason transformers do not blow up when you plug them into the wall is due to inductance + impedence. When you have so much inductance it prevents too much current from flowing, and thus prevents the transformer from ...exploding.

You need a TON of turns of wire on an iron core transformer to prevent it from getting hot, or self destructing..

Yanom

Fri Jul 13 2012, 06:58PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

Inducktion wrote ...

You need a TON of turns of wire on an iron core transformer to prevent it from getting hot, or self destructing..

So in that case, I probably shouldn't try to build my own transformer?
Does anyone know were I can get one that'l boost the 120VAC wall socket up to 240v? I built a Cockroft-Walton Generator, but it's not supplying the current I need (for the exact same reason a transformer needs lots of wire - I had to put a resistor on it so that it won't draw too much current and destroy itself)

Harry

Fri Jul 13 2012, 07:12PM

Registered Member #4081 Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139

A US to UK travel transformer might work but would be pretty expensive. You could build your own SMPS, which would be very complicated but a great learning platform. Or a boost converter. There are a lot of options, what kind of power output were you hoping for?

Mattski

Fri Jul 13 2012, 07:35PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

A simple voltage doubler such as one of these:

will not draw substantial current once its capacitors are charged, though it depends on what load is connected to it. If you short it then it will draw as much current as it can.

Those are much easier and cheaper than making your own mains transformer.

Yanom

Fri Jul 13 2012, 11:08PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

Harry wrote ...

what kind of power output were you hoping for?

enough to charge up a 2500uF 250V cap bank in a reasonable time.

wrote ...
A simple voltage doubler such as one of these: will not draw substantial current once its capacitors are charged, though it depends on what load is connected to it.

I've built a simple two-stage Cockroft-Walton Generator, which is a lot like that thing.

wrote ...
If you short it then it will draw as much current as it can.

AAaaaaand here's the problem. My C-W Generator has diodes rated for 1 AMP, so I had to put a small resistor in series with the mains input to prevent the current from going over 1 amp. However, when I connected my cap bank, that resistor blew out, because it was only rated for 1 watt and it apparently experienced a lot more than that. Apparently charging up a capacitor draws a lot of current - but how much exactly? What's the formula for that?
Anyway, here's the circuit I built:
11jol1t

dude_500

Fri Jul 13 2012, 11:50PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

You should be fine to drop the resistor, a 10uF capacitor connected directly to mains would only pull about 750mA peak current, so the multiplier can never draw more than that. 1A diodes are fine for this.

This is draw from the wall, if you were to short your output after the multiplier has already been charged you could have a current surge through diodes as the multiplier discharges, but with only 10uF the diodes may survive. Regardless, you shouldn't plan on shorting it out. This does mean that you have to connect the capacitor to charge before plugging it in, but that would be an obvious safety requirement anyways.

Yanom

Sat Jul 14 2012, 01:09AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

wrote ...

a 10uF capacitor connected directly to mains would only pull about 750mA peak current

what is the formula that determines this?

wrote ...
This does mean that you have to connect the capacitor to charge before plugging it in.

I'm confused as to what you mean by this. Firstly, you mean the big 2520uF cap bank that I want to charge up, right? And secondly, you mean hook up the leads from the powersupply to the capacitor before flipping the switch? So it doesn't arc?

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