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Want to build psu 0-30vdc 30a 125v source

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Dardan

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I'm new here, and not real skilled in electronics. So please bear with me.

I have been hunting a schematic for the above psu for over a year and thus far have found only commercial units. The cost of commercial is prohibitive at this time. I think I have about all the components on hand, but not the savy to calculate and lay out the circuitry from scratch.

I realize this size psu would require a lot of transformer, or multiple trans in parallel? Viable to have variable constant voltage and variable constant current control?

In all likelyhood, I could buy better than I can build but I wouldn't learn anything from that.

If I can acomplish this it would come in handy both on my electronics bench and my garage too.

If anyone can guide me to a schematic, help me figure one out, or show me why I should give away my soldering iron and go back to the wrenches, it would be greatly appreciated.
 
Howdy,

The 0-30V is easy, as well as the variable current, but do you really need this at 30A?? That's a huge bench supply for horsing around with. Most lab type bench supplies can source 2A or 5A, but 30A requires some mondo transformer.

Why the high current requirement?
 
A power supply of that size is a serious project. If you make it as a linear supply, with current limiting, the heat sink will have to be rated at over 1kw.

You would probably need to use two transformers to get the current rating that you need. 1kW transformers are available, but seem to be 40V or more.

What is is powering? With that amount of power, regulating the voltage is difficult and if that can be avoided it will save a lot of effort.
 
This is really DIY

Pick off 120vac upstream of the breakers in your panel, then going into a half-wave fan-cooled heatsinked diode gives you ~50vdc.
Since we want upstream resistance you can parallel many small diodes using skinny wires to do the current sharing.
Fuse this with a 30A fuse or breaker with an interrupting current rating equal to that of the other breakers in your panel, or 240,000 amps.

You need a (50-30)v/30A = 2/3Ω, 900w load resistor. An oil-cooled coiled coat hanger wire, or a thinner iron wire, should do it; iron has 6x the resistance of copper. As long as you have fluid cooling, 7' of #30 or 70' of #20 copper should be usable for the resistor. HVAC places throw out Nichrome resistors like these, used in elec. furnaces. Bulky, but they are free and they work.

The problem is the 30vdc goes to much higher values when no load current is drawn. You'll need some kind of shunt regulator, maybe coupled with relays that raise the 2/3Ω to higher values in a step-wise fashion. The shunt element only needs to handle the input current minus the load current.

Or get transformers from discarded auto battery chargers, several of them.
I got a lot of free stuff by advertising "XXX you consider not worth fixing" with XXX being lawnmowers, VCRs, edgers, test equipment, motorcycles, etc.
For $120 you can cobble together smaller transformers from Hosfelt. Depending on the voltage/current needed at the moment, these would be switched in and out to reduce heat sink size.

This is the unregulated part of the supply.
 
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I'm new here, and not real skilled in electronics. So please bear with me.

I have been hunting a schematic for the above psu for over a year and thus far have found only commercial units. The cost of commercial is prohibitive at this time. I think I have about all the components on hand, but not the savy to calculate and lay out the circuitry from scratch.

I realize this size psu would require a lot of transformer, or multiple trans in parallel? Viable to have variable constant voltage and variable constant current control?

In all likelyhood, I could buy better than I can build but I wouldn't learn anything from that.

If I can acomplish this it would come in handy both on my electronics bench and my garage too.

If anyone can guide me to a schematic, help me figure one out, or show me why I should give away my soldering iron and go back to the wrenches, it would be greatly appreciated.

If you could explain your use for such specifications we might be able to steer you to other ways to accomplish what you need to accomplish. Is this for battery charging?

As stated already the demanding specs you have given will take a real hit on efficiency (lots of generated heat) unless you went with some kind of tracking pre-regulator. If you just need lots of adjustable 0-30vdc power, then running a large 24VAC transformer with a variac, then driving a bridge rectifier and larger filter caps is probably the best approach.

Lefty
 
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Want to build 0-30v 30a 125v source

My intended usage varies from minor low power electronics projects, to charging a capacitor bank to fire a cap discharge spot welder, to elecrtolisis derusting parts in the garage, to battery substitute, to trying a little electroplating, to what ever.

I have on hand several power supplys that are rated @ 26v @ 6a to canablize (37v & 7.5v sec on transformers) I wondered if I could parallel 4 or 5 of these transformers after the bridge rectifiers.

I was fortunatly able to access a lot of telephone battery charger parts and heatsinks at a scrap yard a few years ago. So high power components are not a shortage if they are tame enough to use. Plus a bunch of lower power psu parts.( ITT/ North Power systems used to scrap there.)

I was able to build a bridge rectifier for my 225a buz box welder that is heavy enough to not not need a fan. 4, 250a @300v diodes. Love that salvage.

Maby a small unit for the electronics bench and something heavier for other use would be more practical. At least I can find lots of schematics for lower amp supplys.

I tinker with a little bit of everything and thought this might make a handy and useful project.

40+ years ago I was into electronics till an unpleasant experience with a tv picture tube cathode lead shorted my enthusiasum. It's been about long enough to almost forget that experience and revive my curiosity again. A big Jacob's Ladder sounds like fun. I almost know enough to be dangerous.
 
I have on hand several power supplys that are rated @ 26v @ 6a to canablize (37v & 7.5v sec on transformers) I wondered if I could parallel 4 or 5 of these transformers after the bridge rectifiers.
Yes, figure out the internal impedances first. You're halfway there, cost-wise.
And if you make the whole thing modular you'll have great versatility, mixing and matching regulators with xformers with power needs.

The best shock I ever got barefoot on a concrete floor was when I tried to push an auto radio vibrator into the 4 prong tube socket of an ancient radio I rescued from the scrap yard. It didn't quite fit so I was pushing and rocking the thing back and forth.
The guy standing next to me didn't feel a thing!
 
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I thought that I could use a LM317 regulator controled with a pot for my votage control and trigger large power transistors or scrs to handle the current. Concept seems simple ( to me) but I don't have a clue when it comes to calculating the values or selecting the right components.

I'll try to find a data sheet for my transformers and go from there.

A vibrator into a 4 pin socket eh, that would be en-lightning.
 
I thought that I could use a LM317 regulator controled with a pot for my votage control and trigger large power transistors or scrs to handle the current.

Fsmyth's link above shows precisely this, albeit with an LM723 instead.
 
At this sort of size I would seriously be considering a switched mode power supply, even if this meant using a 24V transformer with a buck on the secondary rather than an online unit.
 
Want to build 0-30v 30a 125v source

Appearantly, a couple of my posts are lost in the nether-world, I'm not real skilled with computers either. I'll try again.

I've done a passel of reading and I just don't comprehend how to check the impedance of my psu transformers. The 5 units that I have to canibalize/modify are marked "use forced air cooling above 250 watts" and ID'd as 28v @ 10a output, 5.3a @ 120v input. Fan cooling is not a problem, if needed.

It seems (to me) that it would be realitivly simple to modify the LM723 regulator circuit part of the pcb, or patch in a supplimental board to be controled by an LM317 regulator, for varriable voltage control.

Now the real question: Is it feasable to regulate say, 4 of these psu from one LM317 control board, feed the outputs together to attain my 30a output (with a little reserve), and manualy, progessively, switch on (in paralell) the individual psus as my amp demand required me to? Or would the outputs buck each other and raise havoc? I can forego constant output amp control.
 
I've done a passel of reading and I just don't comprehend how to check the impedance of my psu transformers.
28v @ 10a output

If they are identical you don't need the impedance.

But, if the 28v reads 30v open circuited, then, roughly, (30-28)v/10A = 2/10 Ω. This is the Thevenin equivalent impedance, and the 30v is the Thevenin equivalent voltage.

Now you know the short circuit current: 30v/0.2 = 150A.

But something is wrong. 636w in and 280w out? They can't be that inefficient.
 
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It seems (to me) that it would be realitivly simple to modify the LM723 regulator circuit part of the pcb, or patch in a supplimental board to be controled by an LM317 regulator, for varriable voltage control.

It seems you may be fixated on using the LM317? Why not use the LM723 as intended? It too is an adjustable regulator, and probably more suited to such large, linear type supplies than the LM317...
 
Want to build 0-30v 30a 125v source

I'm more familiar with the LM317, it has higher heat & amp ratings, and most of all, I did not realize the LM723 had that range of variable voltage capability. Untill I was prodded to look it up. DUH

The circuit board has a 312M trim pot to tweak the voltage +/- about 3v. Seemingly, I should be able to sub that with a pot that will allow me to adj the 2-30v range I'm after, and maybe a couple of resistors on the sensing feeds. I'll have to pull a pc board and schetch out the schematic.
On the power in-effiecency, I'l have to do a load/draw check to see the reality.

Yes, these 'take-out' psus are the same model and appear identical.

There is an electronics dismantle/salvage place about 10 miles from me that has loads of this kind of stuff and their prices are reasonable.

Thanks folks, I'm still learning and havent seen smoke yet.
 
...the LM317 has higher heat & amp ratings

Right, but that doesn't really matter here as you need to drive a bunch of pass transistors anyway, akin to the schematic that Fsmyth showed you. The LM317 was designed to do an amp and a half without additional components (although one can add pass transistors to it, certainly). Conversely, the LM723 was designed mostly for use with pass transistors.

The circuit board has a 312M trim pot to tweak the voltage +/- about 3v. Seemingly, I should be able to sub that with a pot that will allow me to adj the 2-30v range I'm after,

Right. Grep the web for LM723 circuits, you'll find many that show how to do the full voltage range.
 
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