I am looking at building my own power supply, at higher voltages then I ever really dealt with. Mainly for testing DC motors etc.

I am looking to get a 1KW transformer, at 120V to drop it down to 60V. I only plan on using a 45-50V limit on the maximum voltage however so I can smooth out the AC a little better (ie a capacitor at the 60V to ease the Full Bridge Rectifier spikes and then drop down to 45V to eliminate most of the spiking).

But getting a transformer rated for 120V to 60V is proving difficult (looks like a 200$ purchase). As such, I just wanted to confirm that using a 240V to 120V transformer (can be had for 50$) I can effectively do the same thing, as the voltage is a function of the number of coils on the primary and the number on the secondary, therefor if the ratios are the same it should work out?

Dimitri

I can't think of any reason why it wouldn't work. But you have to keep in mind that you'll need a transformer rated for twice the power, so 2kW instead of 1kW.

For 60V rectified DC you want a 42V secondary not 60V.

Lee.

A 240 VAC to 120 VAC step-down transformer will work fine.


That would be true for a light load where the filter capacitor(s) could charge to the peak (42 * 1.414). If he wants to load the power supply to close to the 1KW rating of the transformer, he will need a higher input voltage to the bridge rectum fryer to compensate for capacitor discharge as the AC cycle crosses through zero.

BTW, even though 60 volts is within the definition of "low voltage" (the threshold in the USA is 100 volts peak or 70.7 VAC RMS) a 60 volt supply driven by a 1 KW transformer could still give you a pretty good wallop.


You mean twice the current rating, not KW. He wants 1KW, so if the transformer is designed to step 240 down to 120 and he's driving the primary with 120, his secondary current doubles at full load (1000/60 ~ 16.6 A).

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

Same thing, isn't it ? If it can handle 16.6A on the secondary, it should be rated 120*16.6 = 2kW.

It's also true for a heavy load. For a particular ripple voltage at a particular load you only need to specify the resevoir capacitors correctly. You don't need to raise the secondary voltage as more current will be drawn during the forward conduction period.

Lee.

Interesting, thanks for the information guys, I have been reading more into it and the design of variable power supplies. There is more to it then it seemed at these higher power ratings where componates for them are more difficult to find, none the less, I will come up with something, looking at the reference designs for switch mode power supplies.

Also realized that I should recitify, to a go from AC to DC then at a higher frequency Pulse Modulate it through another transformer/recitifyer system to control the voltage. As this seems to be a popular method for variable DC power output. As the higher frequency you go through the secondary transformer, the less of a power rating it needs. Making the componate footprint smaller.

Dimitri

What you are describing is a switch-mode power supply.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!