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# High current power supply

## High current power supply

(OP)
This may sound a bit bizarre.
I have in mind of building a powerful power supply in terms of current.
I rewired the secondary of microwave oven transformer. I used 1.5 mm^2 cable.
At 240v on the input I got about 36v at the output. Being a 700w transformer I am assuming it will give up to 19amps of current on the output side. I will limit the current by a 15amp fuse. In turn I intend to rectify the voltage and with 36v AC this would give me about 48v DC.
The question is if would like to be able to vary the output voltage what is the best way to do it?
I found some AC voltage controllers which do this by PWM. If I stick this at the input of the transformer will the whole thing work? The voltage input for the PWM module is 110 - 240.

Now for the bizarre bit: I intend to use this to blow/bust some electronic circuits and components. I could apply 240V directly but it just blows. The fun is to see it gradually blow.

### RE: High current power supply

(OP)
I was told that ac voltage is RMS and the peak would be RMS voltage x sqr of 2 which is about 1.4.
I already did a trial and 24v ac gives me about 32v dc using a bridge rectifier.

### RE: High current power supply

(36 x 1.41)-1.4 = 49V or there-abouts.

To vary it plug it into a Variac and "dial-a-voltage".

Keith Cress
kcress - http://www.flaminsystems.com

### RE: High current power supply

While you are getting the voltage ratio, I am not sure that the winding is able to handle the current.
A Variac (variable transformer) on the input would be easiest.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: High current power supply

(OP)
Thanks for the feedback. A variac is certainly a good way to control input voltage.
If I use a PWM voltage regulator will it work also?

### RE: High current power supply

PWM? Not as well because transformers like that don't like the pulsating waveform PWM brings. On the finer points a Variac running this transformer is NOT a voltage regulator it is an open loop "voltage setter". If the load current can vary the the voltage will vary too.
Do need a regulator? What kind you use varies greatly based on a dozen or more criteria which we can't help you with the scant info you've provided.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: High current power supply

Hmmm...if I had to guess, it sounds like you're trying to make a 48V battery charger?

You've taken the (high voltage) secondary out of a microwave oven transformer, and wound your own secondary on its core with 1.5 mm2 magnet wire? That's about 15 gauge wire- and it will get VERY hot if it is used to pull 19 A continuously, even if the core and primary are good for that kind of power.

Remember that when you full wave rectify an AC waveform, you get pulsed DC output. You either need a big filter capacitor on the output, or your load needs to behave like one (which a battery more or less does- depending on its type of course). While this is normal for a flooded lead acid battery (none of the chargers I've seen for these have any output filtering caps), some types of batteries don't like being used as if they were capacitors...And if you have big output filtering caps to handle that 19A worth of current, you will still have to manage the problem of charging them in the first place. They will act as a dead short the moment you turn on the power supply for the 1st time, drawing an enormous instantaneous current. You'll likely need a means to limit that current, other than just relying on the transformer to limit that current to a reasonable level.

As to varying the input voltage- a variac (variable autotransformer) is the simplest means, but as noted it is open rather than closed loop- the voltage you get will depend on the current you draw.

Feeding a PWM circuit into an inductor like a transformer primary is possible, but not unless the PWM circuit is actually designed to manage that inductance. If not, the PWM device is likely to generate very high voltage spikes and a dead unit fairly quickly. Most PWM devices that are cheap and cheerful are really only useful for resistive loads. And no, you absolutely cannot just put a triac-type lighting dimmer on the primary...

### RE: High current power supply

If you are testing to destruction without a lot of instrumentation voltage regulation may not be that important.

Use a simple Variac and a bridge rectifier. Connect a voltmeter and start to run the voltage up. Note the voltage indicated when the magic smoke show starts. Generally when an electronic device fails due to over voltage, it is the peak voltage that starts the breakdown.
Multiply your indicated voltage by 1.414 to get the peak voltage.
A current limiting resister and a fuse may be a good idea.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: High current power supply

Bear in mind that a lot of things don't gradually "blow." We used to try and blow traces on ICs and it turned out that you needed both voltage and SHARP transitions to get an actual "blow." Often, if the transition wasn't sharp enough, the trace would handle the same current that it couldn't handle with a sharp transition.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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