NiChrome questions

We do? Maybe this is why my woodturning seems so difficult...

PK

PS: I am not sure about those RC guys either...are they NiChrome experts for some reason?

REpository.

Let's assume you need 3 feet for the sake of the math below

When resistance is fixed per foot, and the manufacturer doesn't know how many feet you have, they can't use watts. They could use watts per foot, but that's essentially amps. Constant amps gives a predictable watts/foot regardless of the length of the wire. For the same amps (and thus temperature), and twice longer wire uses twice the watts (and needs twice the voltage).

3 amps for a 3 ohm (3 foot) wire is 9 volts (E=IR). You choose the voltage in order to create the appropriate current.

If you put a 3 ohm wire on a 120VAC outlet, you'll get 40 amps (4800 watts) for a few seconds. If you try this, use a movie camera so we can all share the excitement ;-)

You want a variable voltage power supply rated at, say, twice the amperage you'll actually need, and high enough volts to generate the amperage your project will call for. You adjust the voltage to result in the current needed. Make sure the supply has an ammeter on it, or use your own.

The average light dimmer is a pulse width modulator suitable only for

120VAC circuits.

If you want 3 amps across 3 ohms, with a 120VAC circuit, 3 amps at

120VAC is 360 watts. You'd need a couple lightbulbs in parallel to get that, or a floodlight. I suppose you could build a box with, say, 3-6 100W bulbs each with their own light switch, to provide variable current. Each bulb adds ~ 0.8 amps.

RC aircraft wings are often cut with hot wires through foam.

"DJ Delorie" wrote: (clip) REpository. (clip) ^^^^^^^^^^^^ SUPpository is funnier.

:)

No plans to try this any time soon.

Yep. Priced the Variacs. :( I tried using the dual variable power supply (3A/37V) that I built 20 years ago. I smoked the PS before the wire got hot. Fortunately I only fried the output caps and not the transformer. (I love the smell of burning Tantalum in the morning.)

Thanks. This may be the easiest solution.

I thought so, but each group has it's own culture. I really have to bite my tongue when posting in the Cat group. Especially after coming from rec.woodworking.

Watts ? No. The required wattage for 3Amps (400F) depends on the length of the wire. Fir example 1 foot (1 ohm) would need 3 V to drive 3A through the wire - Watts = I^2 * R or Watts = E * I Wattage needed would 9 Watts at

3V to give you 400F for 1 Foot.

For longer pieces of wire work the math: Given: Current (I) 3A Length of wire: Whatever you need Resistance: 1 ohm/foot

Voltage (or Electromotive force E) R = 1 (ohm /foot) * Length needed (in feet) E = I * R

Power = E * I

If you're using 2' of wire with 1 ohm/foot then you're getting you've got 12A with a 24V transformer, and drawing 288 watts, and the 500w lamp dimmer should be ok. You'd need to bring the voltage down to 6V for 3A with a power of 9W.

These currents are on the secondary side of the transformer - the primary (120V side) the currents would be approximately 1/6 as much.

I've seen the

It's also the most dangerous, because you have exposed metal with

120VAC on it. If ANYTHING happens, you get electrocuted. DO NOT USE WITHOUT A GROUND FAULT INTERRUPTER. And rubber gloves. And make out your will first. And have someone nearby ready to pull the plug. I'm not kidding.

Better yet, just don't do it.

Ahhh...I am just old enough that I only know of tissue covered balsa wood planes; didn't know ppl were fabricating their own foam ones, but of course it makes perfect sense.

Also old enough to have used djgcc a fair bit; many many thanks for rescuing me from the perils of the crap commercial compliers of the day (not that things are so much better today).

PK

It depends on the plane. WHen I was doing it, foam cores and carbon fiber spars were used for gliders and covered spar-n-rib was used for powered and/or high performance planes.

Well, djgpp is much better today :-)

A nichrome suppository is funny??

High Pucker Factor!

rec.crafts.woodturning is a knowledge base for NiChrome??

I must have NiCHrome filtered out.. never seen it mentioned here..

Is that at room temperature or at 400 degrees? Nichrome, as all metals, change resistance as temperature rises. Therefore, you'll have higher currents (Amps) at lower temps, so your supply needs to have adequate reserve to heat the wire. As it reaches the higher temps, the current will self limit according to voltage available. ================================

================================ The normal Ohms law rules won't work here since the resistance is variable as well as the Voltage. (Ohm's law is valid, just not as you'd normally think of it in linear terms). ================================

=================================== If you could find an old filament transformer like used in old tube type radios, etc., it would probably get you into the desired range, but you'd need to find out more about the cold to hot characteristics of the Nichrome.

25°C.

The adjustment factor is only 1.037 for the temp I want.

Probably not as frequently as the RC group. I believe you guys use it to make "hot pens" to sign your work.

Thanks Mike, I get the V=I*R. Although it's been a good number of years (decades) since my basic electronics course. It just surprises me that the amperage specified per foot, would not be for a given voltage. For example: I tried to test my wire using a variable power supply (3A/35V) a built a number of years ago. But the wire never did start to glow before the PS smoked itself. Everything past the regulators got fried. The regulators have thermal protection and there is a fuse on the mains side, so I guess the parts (diodes/caps) were underrated. It should not be a big deal to fix these components. But I'm surprised I did not get some colour out of the wire at 3A/35V.

Thanks.

why the heck do you want to use a nicrome wire for this purpose? How sharp a bend do you need? is a 1/2 inch radius OK - if yes, use coper pipe with a halogen bulb inside. Or take apart a toaster or something and use that heating element. If you use either a lamp or a 115V element, you can put a lamp dimmer in series and control the power with the dimmer. measure temp with a thermometer and adjust as requred.