DIY Hotwire Tech

[Derek_S]

So, picking up where we kinda left off...

What is wrong with an AC hotwire as opposed to DC....?

Lets say I make my howire powered through a light dimmer swithch. I use a 100W light bulb or so in series as a load (Watts might be adjusted depending on what I need). The hotwire is in series with the lamp. Will this work? I know it doesnt sound like the safest option, but otherwise, will it work?

[aeryck]

Subscribing to this, as I'm sure some great info will come. I also have another question on DIY hotwires - Does anyone know a good way to straighten out nichrome salvaged from an old hair dryer? =] Or will I never get the curl completely out for a good clean cut?

[Derek_S]

I dunno, but nichrome is pretty cheap on ebay, so it might not be worth your time to straighten it out.

[IBCrazy]

AC VS DC:

No difference, really. So long as the RMS (root mean squared) voltage is the same as the DC voltage you would have used, that's fine. so 12VAC is the same as 12VDC since the bow is purely resistve (at least at 60Hz).

Bow wire:

Nichrome is not needed. Any metal will do. For a 12V source (or car battery charger), the following wire types work well:

27-32" bow - .025 - .028" Stainless steel fishing leader or .025-.030 mig welding wire
38-44" bow - .035 - .050" Stainless steel fishing leader or .030-.035" mig welding wire
48-56" bow - .065-.075 stainless steel fishing leader or .045 - .050 mig welding wire

Anything more than 56" long will have consdierable sag in the wire. You will need to keep it tight and go slowly with minimal heat. Use a router control or simply the car charger's selector switch to control heat. You will also need a strong spring. It can be part of the heating circuit as well but since the resistance is so low it should not heat up much. Keep it tight as possible without stretching the wire.

-Alex

[aeryck]

Do you need an additional load on the circuit if not using nichrome? (Isnt the stainless or mig wire basically just a dead short?)

[Derek_S]

I have never used a load with mig wire (.030) but I run it off my 12v 10A charger. It still has enough resistance to get hot, it is just less than nichrome. I.e. I can heat up about 30" of mig 030 with the same power as 12" of nichrome, with probably comparable heat.

The only reason mention a load with using out of the socket AC is to help get the watts dissipated down to a more manageable level so the dimmer will have better resolution.

[Dionysos]

Hi,

I go with AC, nothing wrong with it. It hums a little.
... but 12V is barely sufficient. I have 30V AC, with 17Amps so that's a bit less than 600 Watt. Google/wiki: Rene Wire. good stuff. dont kink it, but other than that you can tension that thing very very much, it won't break.
In the other thread, there is this guy cutting a flying wing with one rib only. Seems nice but it has limitations. Ironically especially when you cut a flying wing. The cut is nice, sure, but with that method you cannot build-in any twist. This is of utmost importance when you want a performing flying wing. So, yep, it works with "normal" planes, but that's about it. I use 2 jigs and instead moving the bow I move the piece of foam. very nice.

As for the circuit: Get a transformer, 30-35V 10-15amps (not higher as at some point you still wanna live and see your baby fly instead of dying from an electrical shock) and a simple phase-angle dimmer will do.

Cheers,
Dio

[IBCrazy]

Do you need an additional load on the circuit if not using nichrome? (Isnt the stainless or mig wire basically just a dead short?)

Not really. Conductors have a certain conductive capability that vaires with temperature. As the heat increases, so does the resistance. Nichrome wire changes resistance very little which is why it is used so often. Steel and stainless steel have a fairly low conductivity when compared to copper or aluminum. With copper, the wire would be a dead short. With .030 steel, it's about 0.8 Ohms for a 32" run. So for a 12V battery charger it puts out about 14-15 Amps at full capactity which translates to about 180Watts. This will rip through foam very fast and thus I don't recommend that much heat. If you turn the charger to the 6V setting, it dissipates 40-50 watts, which cuts foam rather well.

What you are looking for is power dissipation. As the wire length goes up, so must the thickness or the resistance will increase to the point that there is not enough wattage to melt the foam.

Remember P = V^2/R

Since V is more or less constant, you must adjust R for proper heat. Shoot for 1.25 Watts per inch of bow for EPS, and 1.5 watts per inch for EPP.

-Alex

[Dionysos]

Oh, and DON'T BULLSHIT AROUND. Let the socket be a socked but don't stick your wire in there.

Otherwise: may you roast in heaven... Dude, not wise, not sensible!!!

Cheers

[IBCrazy]

^ LOL! Voltage is 120VAC here in the states, not 220V like it is in many countries. Touching it hurts, but rarely is it fatal. I've been zapped countless numbers of times during my carpentry days... which would probably explain a few things... In fact, in the NEC from the 1940's they told you to use your hands and sense a tingle. If no tingle, use your tongue! Can you imagine that? That was the National Electric Code!

However, straight 120V would be extremely dangerous like you said, but mainly from a shorting perspective. However touching it certainly wouldn't feel very good.

-Alex