View Full Version : Need Help! Power Supply

03-24-2011, 12:35 AM
I'm building a polyester ribbon cutter to help my wife out.
It needs to make 7 12" nichrome cuts simultaneously across the ribbons.

I've wired up an old PC power supply and it is supplying about 10 amps at 12V. But it is not hot enough, though it almost is.

I have 2 rails (+ and -) running 48" and the nichrome wires are stretched between them at 6" intervals.

Any suggestions on how to improve my power supply to heat my wires to red?

I'm open to buying a solution (< $200) and/or making one myself.


03-24-2011, 05:26 AM
if you're using a PC supply
+5, -5, +12, -12 are all available
so you could get 17 volts by using +12 and -5
or 24 volts by connecting between +12 and -12
just remember to connect the grounds together!


03-24-2011, 11:00 PM
Thanks for the reply mmcp42.

My PSU does not have a -5v (white wire) available so that is out.

And every time I combine a negative voltage wire (-12v blue) with a positive voltage wire (+12v yellow) the PSU shuts down like with a short (and I do have all of the grounds connected together.). Using any positive voltage wire with the ground across my nichrome wires works fine.

I'm thinking that my older PSU just can't do it. For $100 I can get a newer 950W that will give me 4 12v 20A rails. But that won't necessarily be adjustable power, which gives me a little concern.

Would a picture help here?

Any suggestions?

03-25-2011, 04:39 AM
here's what I had in mind
not connect +12 and -12, but connect the heater wire between thm:

03-25-2011, 09:33 AM
If the output is maintaining 12v but is not providing the current, (not hot enough) then you need to increase the voltage of the supply, I would recommend a testing with a power supply made from a variac to find out just what voltage you need.
Careful with testing this way as a variac is not isolated from the supply.
Also if it is purely a resistive load, you do not neccessarilly need DC, AC from a transformer secondary will work.

03-28-2011, 08:23 AM
I ran across this at Frys.
FRYS.com*|*PHILMORE 48-1205 (http://www.frys.com/product/4638758)
I don't think that I could build it for much less.

yes that is exactly what I did. I got 24 volts until I connected the wire and then the PSU would shut down.

03-28-2011, 10:13 AM
That is the Variac I mentioned, but take care as it is not an isolated supply, if the socket you plug it into is not wired correctly it is possible for one side of the output to be 120v with reference to ground instead of 0v.
This can be checked easily with a meter when you plug it in.

03-28-2011, 11:00 AM
A variac supplying a small arc welder would work.
The welder will give you the safe isolation.
Make sure the welder is a crumby transformer type. Not a fancy switchmode one.
PC power supplies are not designed for this sort of operation.
Mixing 5v and 12v in series will lead to problems too.

03-28-2011, 07:10 PM
I picked one up at a local shop, they even matched the Frys price.

Hooked it up, verified that the voltage did actually vary (so my house outlet must be wired correctly).

I then hooked it across my seven 12" NiChrome wires and they lit up like a Christmas tree. Exactly what I was hoping for. Best $70 I've spent in a while.

Any suggestions on what to do with the green wire? My aparatus is primarily wood, plexiglass and aluminum, so not much metal to attach it to.

The total voltage, to get all seven wires orange is about 20V, much less than the 130V that it can do.

Thanks again,




03-28-2011, 07:16 PM
Hooked it up, verified that the voltage did actually vary (so my house outlet must be wired correctly).
Any suggestions on what to do with the green wire? My aparatus is primarily wood, plexiglass and aluminum, so not much metal to attach it to.


The voltage will vary regardless, the thing to confirm is if neither of the 20v connections reads 120v to ground, (green wire).
If this checks OK and your set up is predominantly wood and plexiglass, you could attach the ground wire to the aluminum frame as a precaution.

03-28-2011, 07:25 PM
If you don't use an isolating transformer, just make sure you TOUCH nothing metal, and keep your fingers well away from the GREEN wire if you must touch something.:nono:
Turn it on after it is set up and TOUCH NOTHING.

If you hook the green wire to the frame youy will get sparks and problems where you don't want them!!!
Some personal safety advice from an electrician with a multimeter is important, if we are to see more posts from you!!

03-28-2011, 07:39 PM
Might be a good idea to check the current draw, as your variac is supplying
that current from18-20% of its windings which might produce some local
overheating of the variac winding. It would have to be done hot as nichrome
resistance is temp-variable.

Back to comp PS, the -12v rail is always only a few amps at most, nothing like
what the +12v will supply, hence bridging a load across them that draws more
than ~40-50W will trip the power supply, as you found out.

03-28-2011, 07:45 PM
As I mentioned it is easy to confirm, the voltage from one of the 20v conductors should be close to 0vAC between it and the ground conductor, the other, 20vac, if this so then it should be safe to operate.
This link shows an auto transformer which a variac is, and if you notice, the conductor at the bottom should be the neutral conductor as it is common with the secondary (20v in your case) common.
This is the one that should be at the same potential as ground.
General Types of Transformers (http://www.thomasnet.com/articles/electrical-power-generation/transformer-types)
If your common reads 120v to ground then this would present a safety problem.

03-29-2011, 12:00 AM
Such great advice, thank you everyone! My wife also thanks you as she will be the operator. (Nothing worse than dealing with a wife that just got a shock.) :argue:

Just a few more questions...

To test the current draw, I have a 10A multimeter. Power everything on with it inline between the transformer and the unit and read the current?

I did notice a slight smell coming off the transformer, but only after I left everything on for 10 minutes. I just thought that it might be breaking in (burning off the factory oil, for example). But at 30 seconds of use at a time it seems fine.

Now that I've posted my design, is there a better way of accomplishing the same thing? I'm using two 4' aluminum 1/2" angle with the seven wires stretched over them and tensioned with a spring. The transformer connects its wires to each "bus" and the "buses" carry the voltage to each of the seven wires. A NiChrome ladder if you will.

The 8" aluminum channels in the middle are there to hold the ribbon in place while the hot wires come in 1/8" behind them and do the cutting.

I'm always open to suggestions and look forward to improving as I go. (I'm already going to insulate the top of all of the metal nuts and bolts that are in contact with the wires.)

Thank you so much,

03-29-2011, 10:27 AM
You can measure the current in the 120v lead or one of the 20v leads, the Variac you linked to was 500va which means if you were say drawing 10amps on the 20v side, this would be within specs at 200va.

03-29-2011, 08:31 PM
Minor quibble: variac 500VA rating is at 130Vac, ie a 4-4.5 amp draw
from the full autoformer. That suggests the wire winding is so rated,
so one would not want to greatly exceed this current draw at any
voltage as the limiting factor is the wire gauge. I suspect an 8-10 amp
draw would be 'ok' but not indefinitely as there will be local heating.
That is why I suggested a current draw check as the nichrome is all
in parallel. If the wife is going to use this a lot it might be left on for
unknown periods, hence the concern. Occasional 5-10 minute sessions
likely no problem. Timer bell?` As to current check: meter in series,
variac at zero and sloowly turn dial upto 20VAC, keeping a sharp eye
on the meter. There will be an inrush as nichrome resistance goes up
with temperature.

03-29-2011, 10:54 PM
I will check the amp draw tomorrow.

I did check all of the outlets and they appear fine. I used both my multimeter and one of those little yellow thingers. That is a relief.

And yes I will add a momentary switch next to the handle, so that it is only on while she is closing it to do the cutting. (less than 10-20 seconds)

I so appreciate the help and advice.

BTW, I'm also working on a ribbon roller (think coil winder) which is more CNC related with several axis and stepper motors. (I put it aside temporarily to work on this.)

Thanks again,

04-04-2011, 10:02 AM
I still wonder why anyone bothers using things like a variac or other complex circuitry to control the current in NiCr (resistance) wire.

All you need is to add resistance (more NiCr wire) in series with the cutting wire to give you the current flow and temperature you want.

Take the wire out of an old toaster, popcorn popper, heater, or other electric heat appliance and use it - you can tap any length you want of it to get the desired result. I've been doing it that way AND SUCESSFULLY for many years.

I had someone build me some kind of SCR(?) circuit box to use and it was always a bit of a PITA.

Look up Ohm's law. :rolleyes:

04-04-2011, 12:23 PM
To me that is a rather crude and backward step, you are introducing non-useful heat.
A Variac is hardly 'complex circuitry'?

04-04-2011, 10:51 PM
I don't think "crude" is the word and so what if it is?

It's reliable.

It's cheap.

It works.

Just seems like some people here aren't aware of Ohm's law. Also, I shake my head at those here that insist on using stainless (non-resistance) wire that requires prodigious current and control.

But if people want to make something complicated out of something that isn't, then hey, go right ahead; it's a free country. Myself, I prefer something easy.

And a little extra heat ain't a big problem in my book. Never has been; never will be.

I'll be using such a setup this week. Works like a charm.

04-20-2011, 08:12 PM
First of all, I don't like seeing a variac in anything that isn't completely shielded and grounded, or enclosed in non-conductor at all times. If some little kid turns that knob when nobody is looking those rails could be running 120V next time you power it up. Somebody touches a rail then and they might as well stick their fingers in an outlet. In fact, the assumption should always be that those rails have potential 120 (or 130) on them with a variac.

Second, your requirement is for about 20 V. You don't need continuously adjustable voltage up to 130. You don't need a transformer that puts out more than 24 V, which is a very common voltage, and considerably safer for this kind of device. The only thing you needed to find out is how many amps you needed and then to buy a 24V transformer to suit.

So why can't you measure the amperage? Put an ammeter in series (NOT parallel) with one of your 7 heating elements. That should be well under 10 amps, so you won't blow your meter's internal fuse. Then multiply that ammeter reading by 7 to get the total amperage needed.

Then buy a 24V (or 23.6V) transformer rated about 25% or more above your total amperage needs. You can feed it with a dimmer or router speed control on the 120V side for some adjustment range. Put a fuse in line with both the input and the output. Then sell the Variac on eBay.

04-21-2011, 01:26 AM
Thanks for reading all of the posts. I agree with your concerns, and I have two little kids, not just one. :)

The variac provides not only variable power at the peak, but it is also used to "stretch" the wires pre and post cut. They are not happy about instant power. So the wife will turn it from 0 to 25, slowly, to warm up the wires, make the cut, and then slowly turn it from 25 to 0, and turn it off.

If there is something that I can do to make the existing solution more safe, please let me know. It is fused and human controls are in place to prevent the scenario that you describe.

At this point the unloading and the loading or the polyester ribbon takes 10 minutes, the cutting only takes less than a minute.

She is also aware that there is voltage present and doesn't touch anything metal while cutting. It is all about the procedures used, precautions included.

The solution is working great and she is very happy. I will time the before and after to calculate the man hours saved. But right now she is cutting nearly 500 3 foot ribbons a day into 6" sections. (that is 3500 cuts)

I will be making a simliar one that will make 4" sections (9 wires instead of 7).


04-21-2011, 02:58 AM
Since you are using a Variac I assume you are going direct to the heater wires. If you can determine the current load that you require you can install
a thermistor in series with one leg and this will allow you to just turn the
unit on and off and not worry about power surges. The thermistor is a high
resistance when cold and lets the power come on gradually. Once it is hot the
resistance drops and the wires will be at normal voltage. This will solve any
problems of turning the Variac up and down all day. Search for such a component would be suppliers such as allied radio, William B Allen in New Orleans, or Newark Electronics. You can find them on Google. Important is
the current load so you dont overload the thermistor. These components have been around for a long time and were used in early tv's to limimt the
current when first turned on. If you need a drawing I will make one for you.

04-21-2011, 11:32 AM
Sorry, but this isn't just for you but for others who may read this thread.

A variac without an isolation transformer is the wrong thing to use for this application, period.

20 or so volts which you've specified can be obtained much cheaper and more easily and much more safely with an ordinary step down power transformer that isolates the 20V side from the line side. These are available surplus at half to a third the cost of what you've paid, depending on the amperage needed.

But also, as pointed out by others, your lack of determination of the amperage draw in the first place may mean you are over spec on current with this Variac, which is another safety problem. Why haven't you measured the amperage draw of one element. You apparently have a 10 amp meter, which should be enough. Unless your running 70 amps through that Variac!

Anybody who touches the element supply voltage side of this cutting rig is connected to line -- hopefully to neutral, and not to hot. Lets hope nobody in the next twenty years plugs it into a mis-wired outlet.

The fact that you need to ramp up power to start could have been handled by a properly rated dimmer or router speed controller on the primary side of a normal power transformer.

Unlike a Variac, the supply side current of a step down transformer is a fraction of the secondary amperage, approximately proportional to the step down ratio. A Variac puts the same amperage through the high side as the output. In fact they're connected together. People keep trying to tell you this.

The fact that you didn't test amp draw with your earlier attempt with the first power transformer you tried (and burned out) should have been a warning not to just smoke test unknown circuit requirements, but actually plan the design of what is in reality a basically simple low voltage hot wire cutter power supply. This stuff is basic.

Measure your current at the very least if you are going to hang on to this design, and make sure the Variac is rated for it. Let us know what that is, and I'll find you a power transformer rated for the current online, then make your decision about whether to continue this way -- fair enough?

04-21-2011, 11:49 AM
As the previous post and as I suggested back in post #5 to just use the Variac to determine the voltage you need, you could put a 120/120 isolation transformer between the mains and the Variac.
But the preferred method would be a low voltage transformer, if you buy a toroidal type that is very close, then these are very easy to tailor to the exact voltage by adding or subtracting a couple of turns, usually ~2 turns/volt.
There is a guy on ebay that makes them, Company AnTek his ebay name is Jonango.

04-21-2011, 12:13 PM
As the previous post and as I suggested back in post #5 to just use the Variac to determine the voltage you need, you could put a 120/120 isolation transformer between the mains and the Variac.
But the preferred method would be a low voltage transformer, if you buy a toroidal type that is very close, then these are very easy to tailor to the exact voltage by adding or subtracting a couple of turns, usually ~2 turns/volt.
There is a guy on ebay that makes them, Company AnTek his ebay name is Jonango.

An isolation transformer of sufficient amperage is probably going to cost more than a proper power transformer in the first place. He wants/needs a variable supply so adjusting turns of a toroid is unnecessary. Just get a standard 24 V power transformer and put a Harbor Freight router speed controller on the input. They are enclosed, fused, have an on/off switch and a 3 prong plug and cord already wired in. What could be simpler and less expensive?

04-22-2011, 09:57 PM
Okay, if you want to hang onto that Variac, here's a suggestion:

1.) measure the amperage draw of your device.
2.) if under 10 amps, go here:


3.) Hook the primary (input) of that transformer on the output of your Variac. Hook the output of the new transformer to the cutter.

This will do three things:

a.) reduce the load on your Variac to 2 amps or less
b.) isolate your cutter from line
c.) limit the maximum voltage to the cutter to about 30 volts

You will find that now you're running near the upper end of the variac adjustment instead of the lower end, but that's fine, you should still have enough range -- with the specs you indicated earlier of about 20 volts working.

Cheap insurance at the price.

05-01-2011, 02:03 AM
vtcnc, I didn't take anything personal, you are absolutly right.

I tried measuring the amps, the MM was inline with the white wire attached to the unit (between the variac and the invention), but it didn't appear to register one way or the other, though the nichrome wires were heating up. (Not sure what the problem was.)

She normally runs at about 36 volts, when I pushed it to 48 volts (the wires got really hot) and the 4 amp fuse on the variac blew, which makes me think that the current limit is about at 4 amps. So a transformer of 30-40 volts and 4-6 amps should work just fine. Please recommend something.

I'll pick one of these up as well: Router Speed Control (http://www.harborfreight.com/router-speed-control-43060.html)

Thanks for the advice. Hopefully someone someday reads all of this and saves some bucks and frustration.


05-01-2011, 05:54 PM
vtcnc, So a transformer of 30-40 volts and 4-6 amps should work just fine. Please recommend something.

Need Help! Power Supply - CNCzone.com-The Largest Machinist Community on the net! (http://www.cnczone.com/forums/showthread.php?t=124822&goto=newpos)

Paul that transformer in the the ebay URL of that last post of mine is a 10 amp transformer, and it has an on-off switch and cord receptacle built into the frame. Only 20 bucks and the guy ships out fast -- I nabbed one myself this week and it came in just a few days. It checked out at 28 volts open circuit. (If you need more volts you could rectify it with a bridge rectifier and capacitor and it would probably put out about 35 volts DC. But I bet it's enough for your purposes as-is at about 24-28 V AC.)

BTW, I'm rectifying mine with another 10 bucks worth of ebay parts because my CNC foamcutter requires DC, not AC for heat control -- I don't need the extra volts, but the CNC board will cut the 35 V down to whatever is needed.

I can help if you need to figure out a rectifier circuit.

If I were you I'd try it straight AC first.

ps. I'm merely suggesting this stuff based on my own experiences and skill level -- you are on your own in terms of the safety and design of anything you build yourself. You could always hire this stuff out if you feel the need for professional wiring and design. This is a DIY user forum, so these are partial generalized suggestions only for working circuits and methods. Additional considerations like, fusing, shielding, wiring, insulating, warning labels, testing and workmanship are a builder's responsibility. I'm sure you know that but important for me to make clear in something like this.

05-01-2011, 06:18 PM
Another possibility if your cutter runs too cool with the stock transformer is to slightly increase the diameter of the cutting wire. This will cause it to draw more amps and heat up more, at a lower voltage. Don't overdo this or you will go beyond the 10 amp capacity of your transformer. Just slightly thicker wire makes a big difference in current drawn. Don't do anything extreme like doubling the thickness -- that would increase the amperage by 4 times. If you went up 40% in thickness it would double the amperage. So I'd say try to find wire about 10% thicker, and that ought to tune it for this transformer.

Of course I mean use a slightly thicker version of the same type of wire you are using now -- don't change from nichrome to stainless steel, or vice versa, because that would alter things drastically. I'm not sure what kind and size of wire you are using now.