Power Supply

[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.
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.