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Hi,

I want to plug this transformer I have into the wall to be able to use its output to power several of my other projects. I need a way to limit the current from the mains power supply into the transformer, it has to be limited to about 1/2 an amp. I really don't want to lose any voltage though, and when I use a resistor to limit the current it sucks down all the voltage and overheats. I really need some help with this.

Thanks

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## Discussions

You don't need to limit the input current. The transformer will only draw as much current as needed by the load to which it is attached.

If you really want to build a current-limiting circuit, you should do so on the output side (so you're dealing with low voltage components). 0.5A mains will be either 60A (North America) or 120A (Europe etc.). Are you really powering a project which needs that much current?

Well, when I didn't limit the input current, the transformer caught on fire. Also, this is a step up transformer (1200V) so i should be getting .05 amps out the other side. My projects are high voltage. I really need this to work. I was using this transformer to operate a small spark gap by charging a capacitor to a high voltage and letting it arc across the gap once it was built up high enough.

If it helps I was using a 25 Ohm, 20 Watt resistor to limit current on the other side.

Any advice on what went wrong with this would be appreciated.

Oh, sorry. The picture in my head was trying to do current-limiting on the input side of a wall-wart. In your case, the mains voltage is the lower-voltage side, so your idea is correct.

And I do see your problems (there are two). First, resistors do not limit current. They will draw infinite current if possible, heat up and fry. Second, you're charging a cap to drive a spark gap. Neither of those are intrinsically fixed-current, so they too will draw as much current as they can (overloading your transformer).

Look up "current-limiting circuit" in Google and you'll find a lot of useful information. Since you're running off mains input, you will need to spec the circuit using appropriate high power elements, but other than that it's a well-understood problem.

I run a circuit simulator to test my ideas. This is the circuit I came up with to solve this problem. It looks easier to build then a current limiting one (not talking about fuses, that would be very easy) Take a look and tell me what you think. My program says its ok, but I want second opinion. According to the simulation it should draw no more than .44 amps from the main at any one point. The only real problem is that the 250 ohm resistor connected to the main has to dissipate 49.5 watts of heat.

Use a 50 watt incandescent bulb as your 250 ohm resistor. It has the added advantange of telling you when you have power connected.

Also add a fuse on the primary side regardless of which current limiting circuit you use, to protect the transformer and save burning down your house.