Help me build a power supply.

[atvdave]

I need some help. I’m trying to build a small, compact, and lightweight adjustable power supply for a DIY project. I work with electric switchgear / breakers. We have large, heavy AC/DC adjustable power supply’s that we use to test the breakers, however I’m attempting to build a small more portable version of the power supply for on-sight jobs to make it easier on myself and others.

First off I know I can just buy an auto transformer (a veriac, a $100 or so) and just build a simple AC to DC converter, however, I’ve used a veriac for many years and have never found one small, or light enough to carry around with me in my tool bag.

What I need to do…. I need some variable AC from around 90vac to 120vac. My DC needs to be from around 48VDC to around 140VDC.
I made a schematic of a simple AC through circuit, and an AC to DC rectifier, but would like to know how to make it adjustable with low cost components, light weight, and be able to fit into a small package, (meaning no auto transformer).

Every time I do a search on the net, it always points me to the auto transformers, and maybe that’s the only way, that’s why I’m asking here.

 

[1911ly]

What kind of currant levels are you working with?

 

[atvdave]

less than 20A

 

[1911ly]

PM me your # and I can go over a few things with you.

 

[remauto1187]

right before the AC + DC outputs just stick a potentiometer in series in each output. The pots will have to be rated for the amount power you have at the ouput (in watts).

 

[Cameramonkey]

You forgot the flux capacitor. (I got nuttin)

 

[Mr Evilwrench]

I have variacs down to 5A. They're a bit smaller than my fist, and not very heavy. That's no problem. I've built a variable AC supply around one of those and an isolation transformer, for ramping up old vacuum tube equipment. If you don't need regulated DC output, the full wave bridge will be fine, but if you want it smooth you might want a bit more capacitor on it. You shouldn't even need a switch, just wire up two sets of banana plugs or whatever and have AC and DC simultaneously.

 

[Scout]

You are going to need a step up transformer before the rectifier in order to get 140 VDC.

 

[Mr Evilwrench]

The variacs usually have some stepup available, interestingly about 140V.

 

[atvdave]

No I don't have to have regulated DC but I like the boost it gives, I also need to get up to around 140VDC... I tested the DC part of the circuit today at work with a 110uf 350vdc cap on the out put. With out the cap I got 117VDC out, with the Cap I got 178VDC out.

For years I've just been using a suicide cord to tune/set the neutral on DC motors (AC curve method). If you look at my print you will see that the AC side of it is nothing more than a pass through, I'd just like to make it variable. Making the DC variable is easy, but making the AC end a safe variable is a bit more complicated.

I've talked to 1911ly and he had some good idea's. I think the 5A variac will be a bit small. The charging motors on some of the larger breakers draw some high amps. 20A may be a bit of over kill, but I want it to last.

 

[Mr Evilwrench]

Oh, you can get bigger ones, 5A is just the kind I've used. The size and weight should be proportional to the current rating. The reason you're seeing a difference in the voltage with and without the capacitor is the ripple. If you look at it on an oscilloscope, without the capacitor it will go all the way to zero between peaks, so the average voltage is far less. The capacitor will hold the voltage up between peaks, but it depends on the load. If you load it heavily, it won't hold it up like it will with no load. The larger the capacitor, the better it will hold it up. The voltage rating out of the wall is what's called RMS (root mean squared) which is a mathemagical way to equate it to DC. The peak is actually about 170V each way, but due to the sinusoidal wave, the power is equivalent to a 120VDC source. Sounds like for your application regulation would be way overkill. I mean, I could make you one that will hold to .001% voltage at as many amps as you want, but the applications for something like that are pretty specialized, and it would cost you.

 

[1911ly]

I have some 10-11 amp commercial variacs. I can hook you up Dave.

 

[Lectric102002]

Dave - Is this for secondary injection testing ? Just curious.

 

[atvdave]

Dave - Is this for secondary injection testing ? Just curious.

No... we use a High Current test set for that as the currents we use get up to around 30KVA +.

This is for testing the charging ckt, closing ckt, trip ckt, and the anti-pump ckt.

And yes, I'm not a expert but I do know all about the regulated vs. unregulated part of it on the DC end. I just need help on the AC end, and would like to stay away from a transformer.

I also have a few variacs laying around the shop that I could use but thy are just too large and heavy for my liking. I'm wanting to make this thing as small & light as possible.

 

[CountryBoy19]

Is there any reason you can't use a light-dimmer switch for you AC variable output? I have to say I'm not positive how they work, but I know that for hot-wire foam cutters etc people make them variable by tagging a dimmer switch with a AC > DC transformer in the 12 VDC range. Using the dimmer varies the input to the transformer, which varies the output, voila!!!

 

[Mr Evilwrench]

A light dimmer is a triac circuit that cuts off the AC at a point in the sine wave proportional to the "dim". It's a really ugly wave, and the only reason it works without destroying the lights is the inductance of the load. What we really need to know here is the exact specifications of the power supply that's needed. It won't be possible to make something truly useful unless we know that. Voltage range required, and current required. From the sound of it, though, a variac is just the ticket, but that is in fact a transformer. Don't fear it! Be Bold!

 

[JettaKnight]

Is there any reason you can't use a light-dimmer switch for you AC variable output? I have to say I'm not positive how they work, but I know that for hot-wire foam cutters etc people make them variable by tagging a dimmer switch with a AC > DC transformer in the 12 VDC range. Using the dimmer varies the input to the transformer, which varies the output, voila!!!

Yes. The load the OP has is capacitive. Those dimmers are designed for resistive and inductive loads. Big difference.

As to the voltage, the reason you measure two different voltages dependent on if the cap is in, is because the meter looks at the RMS (or average) voltage of this rectified sine wave, whereas the capacitor is going to bring the DC voltage up near the peak voltage.

 

[Mr Evilwrench]

Am I being too autistic, or was that sort of what I said?

 

[remauto1187]

Am I being too autistic, or was that sort of what I said?

Autistic? No. Territorial...yes. Afraid he is going to pee farther than you?

 

[Mr Evilwrench]

Oh, I can pee pretty far; all the way to the electric fence if I have to.