OT-Hot wire foam cutter/controlling output of power supply question

[challenger]

I have cobbled together a hot wire foam cutter. I use it to cut rigid insulating panels into "follower boards" which I use in some of my beehives during winter.
The cutter is made with 18" of 26 ga. Nichrome wire salvaged from a toaster or something-I forget exactly what appliance I got it from. I made a "bow" by heating and bending some 3/4" PVC pipe and I isolated the ends where the wire ends are and where the supply voltage goes to. The power supply I'm using is a regulated PS the is 24VDC/5A output. The problem I have is regulating the output so it doesn't make the wire too hot. Up until now I've been using a large 25 ohm rheostat which I paid a fair sum for a few years ago. This isn't the best way to control this circuit because I only need about 3 ohm added in series to the circuit to get the resistance proper for the right sire temperature. This uses a very small part of the rheostat which results in the rheostat getting pretty hot. Today I spent time adding another large 5 ohm resistor in parallel with the rheostat. This allowed me to put the wiper of the rheostat about 1/2 way which seems like something I can deal with. I found that I can control the wire temperature much better. My concern is is there is still a lot of heat coming from the rheostat and the resistor (an automotive ignition part).
Should this heat concern me? If not then I'll just make a metal box to put the resistors and PS into so there is no chance that parts will contact each other or contact me. I originally had the rheostat mounted to the bow and this got too hot for me to be happy with.
Thanks for any advice. I've looked far and wide for a better way to control the DC output but for a regulated DC output it seems like the adding of resistance is the only reliable way to do this. I don't want to buy another power supply so I need something that works with this one.

 

[jlevie]

If stuck with this power supply, which sounds to be rated for a bit more voltage than you need, you are stuck with using a resistor to limit current through the wire, A resistor, in this case, is simply a heating element and if what you are using is getting too hot you need a resistor and pot rated for higher wattage. You can calculate the wattage needed by knowing the resistance you are adding and the system voltage.

What you really need is a variable power supply, preferably an adjustable constant current type.

 

[Gordon Heaton]

Wire-wound ceramic resistors are commonly run at high temperatures. Way to hot to touch. Large wattage rheostats too. If the rheostat and resistor are rated at the power level (or better yet 150%) you are using, the heat is not a problem for the components.

 

[BobRenz]

Track down an e-bay Variac. It's a variable voltage transformer that will let you adjust the output voltage up to about 140 VAC. Common sizes are rated at 5 amp and at 10 amp.

 

[jkopel]

You don't need DC!
+1 on the variac, it is a really easy way to to set up for hot foam cutting.

 

[paul39]

You might also use a household light dimmer which are usually good for 600 watts. Keeping the auto resistor in series would cut down the wattage that the dimmer is controlling.

As stated above, resistance heaters don't care if they are getting AC or DC.

Paul

 

[doug8cat]

SIMPLE light dimmer a/c cord, I went with a little bling (hate the word) adding a pilot light and a\c volt meter. works like a charm. Cost ~5.00 and 45 minutes raiding the junk box, also works great for regulating the soldering iron and slip a piece of Kevlar over the nichrome and you have plexi bender.

 

[RayG]

I used to manufacture foam cutter power supplies (another lifetime), ideally what you need an adjustable current source, but a variac or light dimmer should work just fine as well.

The bow needs to have enough tension on the wire so that it doesn't droop and then have the center lag behind the bow, which leaves a curved cut surface instead of flat surface.

If you wanted to experiment a bit further, there are adjustable current sources on the market now-a-days for controlling LED lighting.

Ray

 

[Chip Chester]

A fan on the resistors will help dissipate the heat... or fan the flames.

Chip

 

[barky]

50 foot spool of wire from your stock shelf with a iron slug in it......

 

[DDoug]

I have built (2) hot wire cutters for PolyStyrene foam insulation board.
(1) a large 6' long bow, with the aforementioned tension spring to keep
the wire taught as the heat comes on.
(2) a bench model "jig saw" looking affair.

Both use the same power supply, a machine tool control xformer, 500 watt,
set to give about 24 vac out, with a standard household lamp dimmer on the input.

It needs to be better, and I thought I would find it here, in the O.P.'s request.

You adjust it, the wire is hot, but when the foam hit's it, it cools it down.

I have heard, what is needed is a "constant voltage supply" much like a MIG welder.
I would think a simple voltage regulator added to feed back the wire voltage (downstream of
the xformer) back to the lamp dimmer control
would do what is needed, but it is beyond my education level.

 

[TFPace]

Superior Electric Variac Powerstat Type 116 7 5 Amps 1 KVA Up to 140V Out | eBay

There are many more to pick from too. Lots of older Superior brand plus the Chinese knock offs too.

 

[kb1cjp]

What you want is a constant current source like tig or stick welding. The problem with constant voltage is as the wire heats up its resestance drops which if you leave the voltage constant means more power ie more heat until the wire fails if eh voltage is set too high. By u putting a series resistor inline you are limiting the current. I made something like this quick and dirty for a project a while ago. The bow was made from a piece of lexan plastic about 4"x36" scrap piece. The wire was a galvanized steel wire from the hardware store. I drilled a hole in each end of the lexan and pulled the wire tight bowing the plastic. Then I just used my test bench power supply and clip leads. I put the clip leads a couple inches in from the ends of the hot wire so the end wouldn't get hot and melt the lexan. I limited the current to an amp set the voltage up to max 60vdc and watched temperature. I then increased the current unit I was happy how it was cutting. I believe I ended up with 4-5 amps of DC and the voltgage fluctuated between 8-11 volts depending on demand. Just like stick welding, as you change your arc length the voltage fluctuates but the current remains constant.

Never thought about using AC, I'm sure the suggestions of the other posters would work. Although variacs just adjust voltage so I'm sure you could make it work, not sure it is ideal. I think PWM like a dimmer would uses would be better.

Brian

 

[steve-l]

Look on eBay for a 24V 10 amp PWM motor control typically used for motor speed control. Then use your 24V PS you already own. Cheap, small and light weight. Temperature will be controlled via a small pot on the control where you can adjust the temp as needed in use without causing any excessive heat by current limiting resisters. It's all about average power through the wire by changing the on/off time at the control. It is important to size the control for the maximum current expected, not the average. To do this, measure the cold resistance of the nickel chrome wire with an ohm meter. Maximum current is equal to the PS Voltage (24V) divided by the measured resistance. You should give yourself a 20% margin for reliability reasons. So if the wire is 3 ohms cold, max current would be 8 amps, hence my 10 amp control guess. As the wire heats, the resistance will increase slightly reducing current. Good luck.

 

[challenger]

Look on eBay for a 24V 10 amp PWM motor control typically used for motor speed control. Then use your 24V PS you already own. Cheap, small and light weight. Temperature will be controlled via a small pot on the control where you can adjust the temp as needed in use without causing any excessive heat by current limiting resisters. It's all about average power through the wire by changing the on/off time at the control. It is important to size the control for the maximum current expected, not the average. To do this, measure the cold resistance of the nickel chrome wire with an ohm meter. Maximum current is equal to the PS Voltage (24V) divided by the measured resistance. You should give yourself a 20% margin for reliability reasons. So if the wire is 3 ohms cold, max current would be 8 amps, hence my 10 amp control guess. As the wire heats, the resistance will increase slightly reducing current. Good luck.

This would be for an AC power source similar to a router speed controller yes???

 

[9100]

Connecting straight to a Variac will have the same problem, it will be right at the bottom of the range, just like your resistor. You do not want a constant current supply. Just the opposite, you want a constant voltage so that as the wire heats, its resistance will increase and it will draw less current, making a stable situation. I would use a Variac with a low voltage transformer, which would eliminate the shock hazard of working straight off a Variac.

Before you do that, look inside the power supply you have. Sometimes there is a voltage control inside the box. Also, it would be a lot cheaper to look for some higher resistance wire. How long a piece do you need. I have various types in stock and if I have a suitable one, I would send you a couple of feet gratis.

Bill

 

[challenger]

I used to manufacture foam cutter power supplies (another lifetime), ideally what you need an adjustable current source, but a variac or light dimmer should work just fine as well.

The bow needs to have enough tension on the wire so that it doesn't droop and then have the center lag behind the bow, which leaves a curved cut surface instead of flat surface.

If you wanted to experiment a bit further, there are adjustable current sources on the market now-a-days for controlling LED lighting.

Ray

I read about using an LED dimmer. These limit current by switching the power on & off I believe. Is this correct. I'll look for one with 24vdc rating.
Thanks

 

[challenger]

Connecting straight to a Variac will have the same problem, it will be right at the bottom of the range, just like your resistor. You do not want a constant current supply. Just the opposite, you want a constant voltage so that as the wire heats, its resistance will increase and it will draw less current, making a stable situation. I would use a Variac with a low voltage transformer, which would eliminate the shock hazard of working straight off a Variac.

Before you do that, look inside the power supply you have. Sometimes there is a voltage control inside the box. Also, it would be a lot cheaper to look for some higher resistance wire. How long a piece do you need. I have various types in stock and if I have a suitable one, I would send you a couple of feet gratis.

Bill

So the "danger" of using a varied is valid as I stated in an earlier post I take it?
Using a second, low voltage, transformer provides isolation from the line voltage. From my research it still seems to me that the ac dimmer is a poor application and isn't the power supply and control of choice for those that do a lot of cutting. I am not able to refute or confirm this-its just my take on all I've read.
I am not doing a lot of cutting but I am trying to use the components I already have so putting my existing components into a box, perhaps with a fan, will save me $ and the parts can get hot without any danged of anyone getting burned. That's the theory I am going with as of now anyway.
Thanks

 

[deltap]

I use a solid state welder to heat a 5' foam cutting wire. I believe it is an IGBT type welder, Miller 150 amp. I turn up the current control until the wire turns dull red and then back off a bit to about 10 amps. I would not purchase a welder just to cut foam, but it works great.

 

[RayG]

You do not want a constant current supply. Just the opposite, you want a constant voltage so that as the wire heats, its resistance will increase and it will draw less current, making a stable situation. I would use a Variac with a low voltage transformer, which would eliminate the shock hazard of working straight off a Variac.

Bill

I agree, Variac's should always be used with isolating transformers.

Either will work, constant voltage or constant current, constant current is better... the advantage of using a current source is that it's independant of bow size ( wire length ), the resistance doesn't change much at foam cutting temperatures, for nichrome wire it's about +3% at 300C. Also you can monitor the voltage at constant current to calculate (or control) the temperature if you want.

In the case we are discussing, it's 18" of 26 gage nichrome, which works out to about 4 ohms, and for 26 gage wire you want around 2 amps to get 300C, so that's 8 volts. and I^2R = 2*2*4=16 watts. ( at 300C the resistance will be 4.12 Ohms )

If you have a transformer with a 12V 4A secondary, and use a variac on the primary side that would work nicely.

The other idea of using the existing 24V 5A DC supply to drive a cheap ebay PWM motor controller, or dimmable 24V LED driver would work out just as well.

This is the cheapest I could find easily.

DC 12V 24V 3 2 A Motor Speed Control PWM Controller RC | eBay


Ray