Need advice on power supply circuit for foam cutter

I havent looked at the linke, so cant comment there.

All thats needed is a bit of nichrome heater element wire and a supply to run it, any low voltage supply that gets it hot but not burning. Thats it. (I wouldnt choose ss myself.)

Were you wanting to add some kind of accurate wire temp control?

NT

I havent looked at the linke, so cant comment there.

All thats needed is a bit of nichrome heater element wire and a supply to run it, any low voltage supply that gets it hot but not burning. Thats it. (I wouldnt choose ss myself.)

If yourea hobbeast youve probably got assorted wall warts and other supplies around.

Were you wanting to add some kind of accurate wire temp control?

NT

Brian, to avoid building anything, have you considered a variac and a step-down transformer. If you want to save money, you could probably pick these up for a few dollars at a hamfest flea market.

Stainless wire is good.

12 inch of 20thou is about 1.1ohms at room temperature and 1.21 ohms at 120degC. To reach 100degC needs about 2 amps, 200degC needs about 3.5amps. Hence power requirement for say 120degC cutting, would be in the region of (say) 2.5amps at 3V. (ie 8 Watts). The indicated circuits are over-the-top for this job. I would suggest just feeding the mains to a normal lamp dimmer, which then feeds your transformer primary. The transformer secondary being directly connected to the heating wire cutter. regards john

Just wondering if an adjustable current regulator would provide a more constant wire temperature? The wife bought a little cheapie hanheld cutter that has just enough wire for a D cell battery to heat it up, but the temperature drops quite a bit when when it engages the foam and of course as the battery runs down. She got tired of changing the battey so I hooked it up to the 5V output of an old PC power supply and a fixed resistor. At least she doesn't have to change the battery, but the temperature still varies all over the place depending on foam thickness and how fast she goes through it. Mike

No, constant voltage is better. When the temperature drops, so does the resistance, which causes the current to increase (when voltage is held constant), which is what you want to combat the temperature dropo. A constant current control would not benefit from this small feedback.

-Robert Scott Ypsilanti, Michigan

The ideal would be constant adjustable temp. ie the supply monitors wire R and adjusts supply to keep it steady, and the dial sets temp. Then you can set the temp perfectly for all plastics and workpieces.

NT

NT,

Would any of the listed circuits come close to the "ideal" setup you mentioned? If not how would that be accomplished? I know that I couldn't design such a circuit.

Thanks for the replies. Brian

As I said in the first post I have a 120VAC -> 12V 8amp transformer. Some of the replies mentioned using a variac. Could I use this transformer with the variac? Would it be a shock hazard?

I would like to use cutting bows that are from 8" to 24" long. (Maybe longer ones in the future). I just want a flexible system that would work well, be safe and not too expensive. In other words I don't want to assemble a circuit and not have it work later on. Or buy a variac use it for awhile and then later need to build a circuit. As I said, this really isn't my expertise.

Brian

Just my 2 cents worth.

Assuming that constant voltage is the way to go then the 2nd circuit utilizing the LM350K is your best choice of the three. The author states the current regulation is provided by P3. The truth is that there is no current regulation. The LM350k in this circuit is configured as a voltage regulator and P3 simply provides for adjusting the output voltage. 10k is a bit high for P3, 5K would be better. The data sheet for the LM350 says that 240ohms is ideal for R2. Although it really doesn't make that much difference, if you have to buy it, buy a 240ohm. He also describes R3, and I may be losing it, but I can find no R3 in the circuit and I don't see where one is needed. He is correct that the LM350K will get very very hot if the resistance of your heating element wire is low. Use the lowest voltage transformer you can and still get enough voltage to get the wire hot enough. If the wire resistance is less than about 5ohms then a 12.6V 5A transformer would be fine for 3A maximum current while generating much less heat.

The circuit at the first link could be much more efficient, but has problems. The biggest problem is that there is no feedback that is necessary to regulate the output voltage. The same is true of the 3rd circuit

Mike

I'd forget all that, just plug the wire into a transformer so it works. The other stuff is just making life a lot more difficult.

NT

Only if you leave the terminals exposed. :-)

If you have the resources to get a variac, or have one on hand, I'd say try this first - I did a little plastic-cutting experiment where I took a few inches of #36 NiCr wire, and clipped it to a 7VDC, 1A wall wart, and it cut styrofoam like hot butter:

(the gouge that looks like a raggedy hole that's been cut in half is a raggedy hole that's been cut in half - a remnant of an earlier experiment with styrofoam. :-) Hey! Hot copper pipe! - Oops, off on a tangent. Never mind. :-)

Of course, the wire flexed a little, and steel would probably need more current than nichrome to get the same temp, but that's pretty easy to test, given the variac, of course. :-)

Use a bunch of fuses, like 1A from the mains to the variac, 1A from the variac to the 12V transformer, and 10A on the transformer secondary. Seriously, it's to protect the transformer in case there's a short-circuit type of mishap with the cutting wire. :-)

Have Fun! Rich

Your transformer is about a 10:1 step-down. Among other things this is also the ratio of secondary to primary current. Someone mentioned 3 Amps through the wire as a good number. That makes for 0.3A in the primary or

0.3*90=27W (allowing for wire voltage drop). So place a standard bulb socket in series with transformer primary, a connect the hot wire directly across the secondary. Then use either a 15, 20, or 25Watt bulb in the socket for best effect. View in a fixed-width font such as Courier.

. . /--\\ . | | . |bulb| . | | . \\--/ 120:12 . __ ---- . --| |----------| |-------- . --|__|--. ---- )|| ------o . | 15,20,25 )||( / . | )||( \\ ss . | L M H )||( / wire . | )|| ------o . | watts )|| . '--------------------- . .