[PaulEsch]

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.

Thanks,
Paul

[mmcp42]

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!

HTH

[PaulEsch]

Thanks for the reply mmcp42.

My PSU does not have a -5v (white wire) available so that is out.
http://www.wikihow.com/Convert-a-Com...b-Power-Supply

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?
Thanks,
Paul

[mmcp42]

here's what I had in mind
not connect +12 and -12, but connect the heater wire between thm:

[Al_The_Man]

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.
Al.

[PaulEsch]

I ran across this at Frys.
FRYS.com | PHILMORE 48-1205
I don't think that I could build it for much less.

mmcp42,
yes that is exactly what I did. I got 24 volts until I connected the wire and then the PSU would shut down.

[Al_The_Man]

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.
Al.

[neilw20]

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.

[PaulEsch]

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,
Paul

[Al_The_Man]

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.

[/IMG]

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.
Al.

[neilw20]

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!!

[sch]

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.

[Al_The_Man]

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
If your common reads 120v to ground then this would present a safety problem.
Al.

[PaulEsch]

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.

:idea:
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.
:idea:

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,
Paul

[Al_The_Man]

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.
Al.

[sch]

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.

[PaulEsch]

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,
Paul

[43220]

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.

[Al_The_Man]

To me that is a rather crude and backward step, you are introducing non-useful heat.
A Variac is hardly 'complex circuitry'?
Al.

[43220]

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.