Hi All,
Can anyone please recommend to me what I can use to power a hot wire for foam cutting, other than buying the feather lite or whatever the one advertised in MA is called and is quite expensive? Any help is greatly appreciated.

                                  T.I.A.,
                                        James

I built one with a transformer and a rheostat.

Hey Crist, Yeah, I always Google everything first, I didn't find diddley crap.  James

Will a model train transformer work?  James

OK, so I have a 24v, 55a dc power supply, what do I need to go ask for/ buy to control the power to the cutting wire/ wire temp. to get the proper cut? I'm sorry, even though I was a copier tech. for 5 years, many of these terms are Greek to me. Thanks for all the help so far, anyway what do I need to go buy?   James

I use Nichome wire. I think I got it from Sig.

I went out and looked and mine's made from parts from Radio Shack. Transformer is conversion from 120V 60 to 12 volt and a potentiometer.

Pretty easy, actually.

Go to the Barton site controlline.org.uk.  Search foam.  Scroll down to General - foam cutting.  Scroll down to post by Sleepy Gomez.  This is close to the Holy Grail for the non professional. 

Probably the oddest thing I ever heard of was the use of an R/C transmitter and a fairly large ESC hooked up to a foam bow. They used the throttle lever to control the power to the bow.
I think their power source was a car battery.

A Variac makes a good power supply.  Just make sure that it is wired properly.  Use a voltmeter and make sure that the two lines in the outlet have one at ground potential(reads near zero with respect to a good ground) and doesn't change with the dial)  the other power wire should go to ground when the dial is at zero and track up with the dial.  It is quite possible to wire up a Variac with 110v. on one wire and a variable voltage on the other.  Not safe at all, and don't trust the manufacturer, since in many applications it doesn't matter.

A standard light dimmer also works OK.  Wire a 200 watt lightbulb to it and use the other end of the bulb to the wire, and the ground wire on the dimmer to the other end of the wire.  Again, use a voltmeter and make sure that 110 v.does not appear on either of the connections to the wire.  The bulb makes a nice space heater in the winter, and acts as a fuse if the wire accidentally gets shorted out. 

For either one, make sure the wall outlet is protected by a GFI(ground fault interrupter) socket.

For cutting wire .012-.018 flying line, either solid or stranded works fine.  Nichrome stretches a lot more.  I bought a couple of miles of .010 Inconel quite a few years ago.  It outlasts anything and doesn't stretch much, only about 1/4in.on a 36 in. wire.  The downside is it requires more voltage, about 40v +- for a 36 in. wire.  24 v works OK if you don't rush the cutting speed.  I use a fixed, 24 v transformer for blocks and trimming.  If anyone wants to try some Inconel, send me a self-addressed, stamped envelope and I'll return 10 ft. or so.

Phil C

A Variac makes a good power supply.  Just make sure that it is wired properly.  Use a voltmeter and make sure that the two lines in the outlet have one at ground potential(reads near zero with respect to a good ground) and doesn't change with the dial)  the other power wire should go to ground when the dial is at zero and track up with the dial.  It is quite possible to wire up a Variac with 110v. on one wire and a variable voltage on the other.  Not safe at all, and don't trust the manufacturer, since in many applications it doesn't matter.

A standard light dimmer also works OK.  Wire a 200 watt lightbulb to it and use the other end of the bulb to the wire, and the ground wire on the dimmer to the other end of the wire.  Again, use a voltmeter and make sure that 110 v.does not appear on either of the connections to the wire.  The bulb makes a nice space heater in the winter, and acts as a fuse if the wire accidentally gets shorted out. 

For either one, make sure the wall outlet is protected by a GFI(ground fault interrupter) socket.

Phil's obviously still alive, so his recommended hardware isn't always immediately deadly.

But speaking as both an electronics engineer as the son of a former Fire Marshal, and as the son and brother of a pair of firefighters who have had to scrape dead or seriously injured people off of the remains of their Really Clever home-brew projects, you can spend just a little bit more money and be a lot safer.

Most Variacs are what are called "autotransformers".  They have one end (nominally the neutral end, not ground) connected in common to both the input side and the output side.  The other side is nominally connected to hot.

For the last 50 years or so, North American house wiring uses a three-wire system, with hot (nominally 110-125V), neutral (nominally ground), and safety ground (really ground, unless there's a wiring fault).  The neutral and ground wires are supposed to be connected together at the pole, and the hot wire is connected to hot on the pole transformer (there's a whole song and dance about how to get 240 for appliances -- look it up if you're that curious).  Sockets are polarized (that's the whole irritating "you can't plug a new radio into an old extension cord" problem).

The key word here is nominal.  Neutral wires are supposed to be close to ground, but either because of wiring faults in the house, or because a given socket is mis-wired, what is supposed to be neutral can have a few volts on it, or it can have 120V.  Worse, if the guy that wired your house was sloppy, some plugs will be backwards of others.  So your "Phil Approved" Variac or dimmer could be just fine in one socket, and give you a serious shock in another.

The physical arrangement that's much more likely to avoid electrical shock is to use some sort of isolation between you and the socket.  You can get isolation transformers to put on the 120V side of things, or you can run your Variac into a 25V transformer from RadioShack (25V, 2A, $14.49).  Using the step-down transformer means that you'll be using the upper range of the Variac, which will be lighter duty for that device, and (if you wire the low voltage side correctly) the 120V side can have all sorts of faults and you will never feel a thing.

I'm not so sure about the light dimmer -- they're not supposed to be happy working into inductive loads (like a transformer), but on the other hand I've heard reports from people that they work OK.  So a light dimmer into a step-down transformer may work, and it will be a lot safer than a light dimmer alone.  And dimmers are a lot cheaper than Variacs so it may be a worthwhile thing to just try one out.

Jeeze, I think I'll just forget about building planes.

Never!  It's a web forum -- dissension is to be expected.  Besides, as I said, Phil's been doing it his way for years so I can't speak out too loudly against it.

you can do this for about $25 bucks, or less if your scrounge. 

Variacs work well.  But as someone who works around power all Day, I would not use a variac alone. For my mind designing out the shock risk is worth an extra $10.

A good source is out of an old school battery charger, or a 120- 24 volt control transformer from a furnace guy.

http://www.rcuniverse.com/forum/m_8307147/anchors_8315575/mpage_1/key_/anchor/tm.htm#8315575

Harbor freight males a nice router speed control for about $20  feed that into an isolation transformer and your are all set. 

This isn't hard and there are many ways to skin this cat so don't give up.

Will a model train transformer work?  James

I used an American Flyer train transformer for a many years, but later decided it was foolhardy to risk an irreplaceable item like this when there is another way.  It worked OK up to about 20" cores.

Later I went to a Variac I bought off Ebay.  Much better!


More power!    Tim Allen

Or you could bid on mine http://www.ebay.com/itm/300903867098?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1586.l2649

Hey William,
No, I have the Power Lab 8,  does it have a foam cutting setting too? HOPEFULLY.

Thanks William, well can you recommend something for me to use? James  hey remember
I have your 12/ 24 VDC  55A power supply, I just need a way to regulate it to cut foam,any
ideas?

David, How do you control the temp of the wire?

I gotcha Tim

I think that it's important to have a low impedance power supply feeding the wire.  If you have a resistor in series with the wire (a potentiometer or a light bulb, for example), the voltage across the cutting wire will decrease as the wire cools and increase as the wire heats up.  That makes it hard to control the wire temperature, which you do with cutting speed.   Two ways I've used to get a low impedance supply are a light dimmer driving a hefty transformer and a garage-sale Variac driving a hefty transformer.  Old audio amplifiers are good sources of hefty transformers.  Mind you, I haven't made any quantitative measurements (I probably did, but I don't remember what they said nor where I put them), so this is kinda conjecture, but I learned how to cut foam well after several years of trying. 

The idea of using RC equipment is really cool.






Through-hole man trapped in a surface mount world

Dont see how the lightbulb can w

Quote from: Allan Perret on May 10, 2013, 08:44:00 AM

Dont see how the lightbulb can work as a fuse.  The bulb can take full voltage by itself and not blow.  A fault in any other part of the circuit will not cause the bulb to pass anymore amps than what it normally operates at, because of its internal resistance.

ork as a fuse.  The bulb can take full voltage by itself and not blow.  A fault in any other part of the circuit will not cause the bulb to pass anymore amps than what it normally operates at, because of its internal resistance.

Dont see how the lightbulb can work as a fuse.  The bulb can take full voltage by itself and not blow.  A fault in any other part of the circuit will not cause the bulb to pass anymore amps than what it normally operates at, because of its internal resistance.

.     

Well, all I know is I hooked the gizmo up, with 110 on one side of the wire.  An accidental short blew out the bulb.  Then I checked the voltages and fixed 'em to avoid further surprises.