I'd like to power a short piece of hot wire as a formed shaped cutter for foam. I"ve worked out that I'll need a power supply that puts out about 0.5 volts at 6.5 amps, so, say 8 amps full range.

I can build a simple transformer-bridge rectifier-filter capacitor PSU, except for the fact that transformers are easy to find in the 6, 12, 24 volt range, but I don't know where I'd find something that put out fractional volts. Cost is a factor for me also. It would be great to get something surplus, or out of used equipment etc.

I had thought about even using the 3.3 V line out of a computer supply, but it is still too high in voltage -- I'd have to use very thin wire, and it wouldn't hold shape as a formed cutter. Also thought about nicads, but 1.25V is still high, and I'd have a short cutting time and have to recharge, etc.

I guess I could make an air resistor out of the same wire to reduce 3.3 down. But I was hoping someone here might come up with something more elegant, cheaper, or simpler.

Thanks in advance for any help/suggestions...!

EDIT:

I guess I'd need about .5 ohms in a resistor that could dissipate 30 watts to bring a 3.3V ATX power supply line down to .5 V for this wire cutter. Would paralleling two 20 watt 1 ohm resistors work to do that?

 

Sounds like a job for a pwm. The thermal time constant of the cutting wire is likely a few hundred ms, so pulsing the wire with 3.3V for a low duty cycle at a rate of ~100Hz would keep the average power where you need it. Even an 60Hz SCR phase control would likely do just fine.

 

Your concept of voltage and current needs modifying.
What you are saying is that your load is a heating wire of about 0.1 ohms.
What you need is a current supply that puts out, let's say, 5A.
Forget the voltage. That will take care of itself and self adjust to 0.5V.
So what you need is a 5A, 2.5W constant current supply.
You can up the ante to 10A, 10W if so desired.

You would be better off to select heater wire that will have more resistance
and hence give more heat for the same current.

 

Thanks MikeML -- Is there a schematic somewhere I could follow to make that?

Thanks Mr. Chips my wire is actually acting as a .067 ohm resistor. I don't believe I need a constant current supply. for this simple case.

Would my later edit adding a .5 ohm resistor in a constant voltage 3.3 V supply work?

Further question: When changing cutters -- if I were to change cutters---- to a longer or shorter length of the same wire, a cutter with more resistance would require higher voltage to drive an adequate amount of power through the wire, yes?

In my idea of how hot wires work: it is watts per inch that is the constant for the same cutting heat, not volts or amps alone. These have a relationship but can't be fixed. The current capacity of the circuit is a limiting value not a determining one, and the voltage is varied to provide the proper heat required.

 

Glad you know more than me.

 

Didn't mean to upset you.

 

A constant current supply will prevent your heating wire from changing temperature in destructive ways!

Hope this doesn't frighten you.

 

Thank you Kermit2 I've done a lot of wire cutting and haven't had any frightening experiences with the wire. In fact I've built a CNC foam wing cutter, which has worked very well for a couple of years. It has a variable voltage supply to adjust temperature of the wire which is 42 inches long and .011 stainless steel.

Can you explain what you mean by changing temperature in destructive ways? Do you mean overheating when it leaves the cut?

 

Literally thousands of R/C modelers build, use, and have used hot wire foam cutters for decades. Yet I know of no one using a fixed current supply for these. I'm not saying it hasn't been done if someone happens to have a current regulated bench supply, just that the great majority don't, and it's not necessary.

Look folks, in commercial foam cutting houses they commonly use a Variac with isolation to heat a long wire for slicing off big panels of styrofoam. As far as I know, A Variac isn't a constant current supply. You wouldn't want to hook DVM set to measure current across the terminals of that cutter unless you like blowing meter fuses.

Safety of such a cutter? Yes there are all kinds of safety concerns like limiting voltage with stops on the Variac, using fuses to limit current into the supply, properly sizing the wire and setting the voltage for that wire with calibrations, and isolating the wire from line with an isolation transformer somewhere in the circuit, etc. But......

My question wasn't "how do I make a safe power supply" I was asking for a simple circuit to achieve the parameters listed in the title and first post. Could we limit the discussion to the very specific electrical problem at hand?

No one has even answered my power resistor question.

 

-- Is there a schematic somewhere I could follow to make that?...

Pulse-width_modulation
http://en.wikipedia.org/wiki/Pulse-width_modulation
Phase Control
http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

 

-- Is there a schematic somewhere I could follow to make that?...

Pulse-width_modulation
http://en.wikipedia.org/wiki/Pulse-width_modulation
Phase Control
http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

 

Thank you MikeML. Getting closer.

re Wikipedia. PWM is used in R/C for servo control, and I do understand the concepts.

re. All about circuits. The phase control reference is a great text for explaining what goes on in a variety of circuits. And includes generalized schematics.

But I can't start specifying parts and values in a similar circuit that will give the effective power of a specified DC wattage through a specified DC resistance . I don't have the knowledge for the specifics and how to arrive at them. Since this is a more complicated circuit than a simple series resistor on a regulated supply, I guess you would probably make it variable to make it easier without any added complexity. But I still don't know the specifics to get in the ballpark of say 0 to 1 volt with an 8 amp capacity. Well the equivalent of those DC specs assuming a chopped supply.

 

What about a circuit with an adjustable voltage regulator IC?

 

Nobody has mentioned it yet, so I will: how about one transformer with its output driving a second transformer? Say you had 2 transformers rated to produce 12V off a 120V input. Your first stage would give you 12V, then the second would drop it down to 1.2V. Choose the voltages correctly, and you'd be all done, but make sure the second unit can handle the current you want!

That first transformer could be a variable autotransformer type (like a VariacTM). Then you'd be able to adjust the power according to the length and size of your cutting wire etc, and the second transformer would keep it safe by isolating the cutting wire from line voltage.

 

What about a circuit with an adjustable voltage regulator IC?

The problem will be power dissipation in the regulator. If Steve starts with 3.3V and wants to deliver ~0.5V to his load, the power dissipation in the regulator will be (3.3-0.5)*8 = 22.4 Watts. This could work, but would take a large heatsink.
Besides, most IC regulators will not adjust below 1.2V unless you have a dual polarity power supply.

Using a modern FET in a PWM circuit, the power dissipation in the switch could be < 1W

 

Ok, use a variac and a step down transformer.

Or a welder's power supply.

or this:
http://www.nichromewire.biz/power_supply_design.html

 

Your best idea is a constant current source using an op amp and mosfet follower, like this
and your hot wire is the resistor R2.


http://www.google.co.uk/imgres?q=co...90&start=35&ndsp=20&ved=1t:429,r:4,s:35,i:200

 

If you have any toroid transformer around, you can easily obtain low voltages by a few turns of thick wire.

 

Yet I know of no one using a fixed current supply for these.

They all are, in reality. Believe it or not, even a light bulb is a constant current device. It's internal resistance self-adjusts in response to heat, in a way that more-or-less miraculously ensures that the current, heat and light production are all nearly constant. Even a voltage drop at the bulb does not reduce brightness as much as you would otherwise expect, because the filament cools and draws more current to compensate.

My point is you absolutely need constant current to define the heating level. You're correct that, like using a light bulb, you may not need much fancy to accomplish that. As recommended above, if you supply a given current, voltage will take care of itself.

A power resistor is a wasteful way to accomplish limited current, but it will work. You'll need one rated for at least double the power dissipation that you want it to accomplish, and with heat sinking. It'll get hot and you'll want to be sure that it can't get near fingers, power cords, flammable stuff, etc.

 

I think it was back at post #3 I told op he needs a constant current supply.
But op thinks he knows more than we do.