6V Flashlight Battery/Hotwire cutter

[emoone]

Would a 6V flashlight battery power a 15" hot wire foam cutter?

Eddie

[jlk]

I don't think so. Watts is what you need and that is volts times amps. A flashlight battery probably can't put out the amps. A 12 volt car battery will give more voltage and more amps.

Personally you can build a 24 volt transformer with a reostat for about $20 using radio shack stuff. I have heard of guys using 6-8 cell nicad car packs and a speed contoller to regulate the power. The nicads put out more than the flashlight battery.

Jordan

[Duane]

I use my full size car battery charger, set at 2 amps, and it works ok.

[Mr.RC-CAM]

You will quickly go broke if you use standard flashlight batteries. Although "D" size Alkaline might work, they will last only for a few short minutes.

Save you money and protect the landfills -- build an AC transformer supply.

[Spam]

There are several options for a hot-wire cutting power supply.

The safest is a high-amperage 12-24V transformer (rated for at least 5-10A). And a cheapo Light Dimmer...

THe best is (IMHO) A Variac hooked up to said transformer. Variacs usually cost $45- $65, and can be found at many electronics suppliers, and industrial liquidators (Earl's, Gateway Electronics, etc.).

The Variac is a variable AC transformer that takes 110V input, and outputs 0->110VAC. WIth the step-down transformer, you effectively expand the control range (and make the voltage safer for us humans).

I've used the latter setup for quite some time, with up to a 36" bow without any problems.

If you can't find a Variac, Industrial Liquidators sells a 5A model for $55. Here's one from them on e-bay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1749438024

Best of Luck!

Spam

[emoone]

The only reason I asked was because I have a little 4" cutter that uses 2 "D" cell alkalines I bought at a hobby store. I think its designed to cut scenery foam for model train setups. I've used it to cut ribs for foam planes and to hack up the fuse of those $4 wally world gliders.

I was just wondering if a $2 6V battery might power a 15"-20" bow if 2 "D" cells could power a 4" bow.

I may look at car battery chargers the next time I'm at an auto supply store. Its probably worth spending the extra money.

Eddie

[drcrash]

It seems to me that a car battery charger may be the wrong kind of transformer for this application, if it's a smart trickle charger.

IIRC, battery chargers generally vary the voltage a bit to keep the current relatively constant. If the resistance goes down, and the current goes up, they turn the voltage down to maintain a relatively constant 2 amps (or 6, or whatever).

That's bad for a wire cutter, because when you cut stuff it cools the wire and that lowers the resistance of the wire. The more/faster you cut, the more it cools the wire, making it harder to keep cutting.

At that point, you'd like at least constant voltage, so that the wire will not cool any more than necessary. Ideally you'd like the voltage to go up when the resistance goes down, to maintain constant heat; a battery charger does the opposite.

A dumb transformer on a dimmer (or a variac) sounds like a better choice. Even better would be something that raised the voltage a bit when the resistance goes down.

One thing some people do is to put a light bulb in series with the hot wire. (Or maybe in series with the primary side of the transformer.)

The lightbulb acts as a big resistor and a current limiter; the total resistance never drops below the resistance of the light bulb, so if you can safely power the light bulb with your transformer, you can vary the length of hot wire without worrying about frying anything.

I got to thinking about that recently when I found out that the resistance of a light bulb varies tremendously as it heats up. (Like the hot wire in a cutter, only much more so.) When the light is off, its resistance is low, and when you turn it on, it goes way up very quickly.

It seems to me that this has the same problem as the car charger. When you cut, and cool the hot wire and lower its resistance, the current through the light bulb will go up. This will heat it up and increase its resistance, so it will "resist" the increase in current and your hot wire won't heat back up as fast as it would with a fixed resistor. (But I'm not sure I've got that figured out right.)

It would be nice to find some cheap way of getting the opposite effect, but I haven't thought of one yet.

Paul

[drcrash]

The safest is a high-amperage 12-24V transformer (rated for at least 5-10A). And a cheapo Light Dimmer...

Hmmm... the ampiest plain 12 or 24 volt transformers from RadioShack or seem to be the 12V 3A one. The upside is that it's only $10.

I'm sitting here with one of those and a dimmer, and realizing it's just not enough amps for any reasonable gauge of wire.

(I'd need a very fine wire to get enough resistance to use a high enough voltage to get the right watts at 3 amps. My understanding is you need 2-3 watts per inch of wire to cut foam.)

I'm thinking that it should work to use two 3-amp transformers in parallel as one 6-amp transformer. I could just run over to Radio Shack and buy another $10 transformer.

Anybody see a problem with that?

Paul

(BTW, I found a two-pack of 600-watt dimmers on sale for $5 at Home Depot a few days ago, in the lighting department. Much cheaper than the ones in the electrical department.)

[drcrash]

Ok, I made a bow and wired it up. The 2 Amp 24 V transformer on a dimmer seems to work fine with a .008 guitar string about 35 inches long. (I could have gone a bit over 3 feet with the guitar string, but my bow isn't quite that big.)

Another option (which came up in another thread) is to use a router speed control (12.49 on sale right now at Harbor Freight) and a wall-wart style plug-in transformer. Just plug the transformer into the router speed control's output socket, and you're ready to go, with no wiring work to speak of, except connnecting the bow. I'd have gone that way if I'd known it'd work, because it'd be handy to have the speed control for other things, like maybe my router.

[drcrash]

There are several options for a hot-wire cutting power supply.

The safest is a high-amperage 12-24V transformer (rated for at least 5-10A). And a cheapo Light Dimmer...


There seem to be two schools of thought about this. A lot of people say that you should have 4-6 amps, and dissipate about 2 or 3 watts per inch of wire. For that you probably want something like 21 or 22 gauge nichrome, or somewhat smaller stainless steel wire.

Then there are people who use thinner wire, like 26 gauge nichrome or a .008" stainless steel guitar string, and seem to get by fine on two amps or less, and a watt per inch or less.

I don't know how to resolve these competing views, but right now I'm going the cheap 2-amp route.

Other cheap power supply options I've come across:

Hefty used computer power supplies, which supply 5 and 12 volts, depending on which connections you use. You can't put them on a dimmer, because they're regulated. You just pick between 5 and 12 volts, for the approximate gauge and length of string, and fine-tune by alligator-clipping your connection to your string at different points to adjust the effective length of the string to match the voltage and give the right amps/watts.

(I'm thinking now that this probably works fine with a car charger, too, as long as you don't hit the point where the charger's adaptive regulation matters.)

A transformer from a junked microwave oven, used backwards as a step-down transformer (down to a few volts) rather than as a step-up transformer (to 2500 volts or so). I happen to have one, so I'll probably try putting it on my spare dimmer to get a low-volts/high-amps power supply for short thick bent wires.

[rs.led]

Well i anticipate is like this right, Personally you can body a 24 volt agent with a reostat for about $20 application radio berth stuff. I accept heard of guys application 6-8 corpuscle nicad car packs and a acceleration contoller to adapt the power. The nicads put out added than the flashlight battery.