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New guy to the forums needing some help making a 0-18VDC regulated variable power sup

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Mike86H

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I am looking at making a 0-18VDC regulated variable power supply with a current of around 1.5-2A. I have been scouring the internet for the last 2 days and am having quite a bit of trouble finding something that I think would be suitable that's diagrammed in a way that I can readily understand and comprehend. (I'm not real good with knowing how to replace components or add potentiometers/rheostats and the like.)

I've built electrical circuits plenty of times before, but back then it was mainly guitar related components (amps/effects pedals), and its even been a while since I worked on those. But I can generally read schematics and I know how to solder and all that, so I dont feel too over my head.

What I am trying to put together is a power supply unit for running tattoo machines. I need it to operate at between 0/3-18V. Im thinking that regulated power would be better than unregulated, although I've seen some unregulated power supplies used for my intended purpose. And considering that this would be used for extended periods of times I would prefer that it have a fan wired in somewhere as well.

Im not sure if this is the right forum to post this question, and if not I apologize in advance, but Im hoping that someone can help me out with this.
 
There are a lot of regulators around that a pretty simple and inexpensive but don't go all the way down to zero volts - only to about 1.25. I suspect this might work. Do you?
 
Yeah, 1.25V would work. I was thinking about using the LM317 regulator which I think has the 1.25 minimum rating. One concern I do have though is the current rating. I know that it is recommended to have close to 2A on a tattoo power supply, but I think the LM317 is rated at 1.5A. I think that would work but is the amperage current listed on a product the max amperage under max voltage? If that makes any sense?
 
Would the 350 be a direct replacement to the 317? That would work alot better than the 317. As for the chassis, I was thinking of making my own out of sheet metal, similar to a guitar amp chassis, and then mounting the components, insulating using rubber washers or something.
 
OK.

Yes just like the 317 - works the same way just higher power.
I'm thinking something like this to get from line voltage down 24 volts then just use the 350 for the adjustment. Let me check around for a heat sink so you can get an idea of how big it will be. There are better ways to do this but they are also more complex, so lets go a little further. Maybe someone has a ready made one they have seen. Most I have seen are pretty expensive.

**broken link removed**
 
So with something like that, would I plug it into whatever contained the circuit for voltage regulation?

Sorry to seem like a total newbie here, I just figured that I would have to buy and build from the transformer through the rectifier and filter caps.
 
Mike86H said:
So with something like that, would I plug it into whatever contained the circuit for voltage regulation?

Sorry to seem like a total newbie here, I just figured that I would have to buy and build from the transformer through the rectifier and filter caps.
Click to expand...

Yes, you could just cut the plug off and that would go to the regulator. Switch mode power supplies are so cheap now that you can't build an old fashion one as cheap. :D
Click to expand...
 
So it would look like this:
 

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So the pot or rheostat would go on the output side of that correct? I wouldn't have to replace a resistor with the pot? Or is the annotation at the bottom referring to replacing a resistor with a pot?
 
If you buy the power supply there here is a pretty good deal on a fan.

**broken link removed**
 
Ok, so I found a schematic for basically the exact power supply that I want to build. Its an old 1970's design, but in its original form, its a tank. An unregulated, some would say dirty, unbreakable, tank. I've priced components this morning from Mouser, which as luck would have it is right down the road from my house, and its only going to be a few dollars more for me to build the power circuit as oppose to buying one pre-made, especially once you figure in shipping, then the price is almost even across the board. So heres the schematic:

**broken link removed**

The biggest problem I can see with this design is that it uses no voltage regulator and would probably have a pretty choppy sine wave on an oscilloscope. I dont have a scope to test it on, but I want as close to flat on the waves as I can get. Also, the above schematic has "U1" as the only means of varying voltage, it just seems a bit crude. Obviously its functional, but still crude.

What I wondering is, if I basically cut off the circuit between the filtering cap and the rheostat, like this:
**broken link removed**
could I then just go in and run the VR circuit that RonV posted?

Or, since the original schematic also has the part of the circuit to power the LED lamp, could I just cut out "U1", like so:
**broken link removed**
and finish out the circuit, just removing "U1" that way I can still run the lamp from there? Would "C2" still need to be left in, or would running "C1" and "C2" in parallel at that point be a bad idea?

Sorry for sounding like an absolute beginner here, Im sure I should know all of this stuff by now, but looking at schematics for too long and trying to comprehend them fully sometimes gives me a headache. And as my dad always said, two heads are better than one. Also, please forgive the crap-tastic MS Paint skills. :D
 
U1 is a 1 AMP 12 V 3-terminal voltage regulator and usually will require a heatsink, heatsink grease and a mounting kit (The kit consists of an insulating collar, bevel washer, screw, nut, and eclectically insulating thermally conductive pad).

The raw DC has to be at least 3 V higher than the regulated voltage, same as for an LM317T.
 
But wouldnt that regulator limit the power to only 12V since its non variable?

That schematic was found on a video on YouTube where the guy talks about rebuilding the old power supply using more modern parts, but when he actually shows the internals, I dont see any circuitry for a voltage regulator. I asked him and he said that the schematic was just a generic power supply schematic that mimicked the original design. Although now I dont see it. Maybe in the power in and rectifying portions.

So lets say that I am using RonV's VR circuit, I would just cut off the schematic I posted after the filter caps and then add in his circuit?
 
One problem no one has mentioned is the power dissipation in the regulator. An LM350 may be able to supply 2 amps under some conditions but not as you're thinking about it.

Say you supply the regulator with 20 volts which will give you about 18 volts maximum. If you adjust the voltage down to 2 volts and you're drawing 2 amps from the circuit, the power dissipated by the regulator would be

(Voltage drop across regulator) x (current) = 16 x 2 = 32 watts. Ever burned your hand on a 60 watt bulb? This is a lot of heat to get rid of!

The switching supply somebody posted here is your best bet. Find a wall wart at the thrift store rated for around the maximum voltage you need and more than 2 amps – any larger value is fine. Use this to supply the module and you'll be set.

For some background information, check out this article: A Look At Switching Power Supplies

To step back even further and fill in some big holes, you might look for the basic ARRL general electronics book.
 
You had mentioned that the power supply was non-regulated. I'm telling you it is at 12V.

The regulator has to dissipate (Vin-Vout)*Iout Watts of heat since it's a linear regulator.

So lets just say (22-2)*2 or 40 Watts as an example just picking numbers. That's a lot of power. The point is, the thing is going to get HOT!

So, wide-range becomes a problem.

I have a LM317 (1.5 A), see **broken link removed** based regulator (commercial) that varies the transformer secondary at the same time. Mine is on 24/7 to power a transistor radio.

You could still add another LM317 to get any voltage you want. Just set the output of the fixed supply to at least 3V higher.

So there is a move to "switching regulators" which have efficiency numbers from nearly 80 to 98% or much less heat. They come in buck (lowe voltage), boost(Higher voltage) and buck-boost.

The gizmo that I pointed out was a switching regulator.

The LM350 is the 3A big brother. In both cases it takes two resistors to set the output voltage, one can be variable.
 
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