"Mike" wrote in message
nk.net...
Bill Stock wrote:
This is somewhat OT, but you appear to be the biggest suppository of
knowledge when it comes to NiChrome.

I want to build a Hot Wire to bend some Acrylic. The wire in question is
#22
and approximately 1 ohm per foot.

Most of the temperature tables I've seen for NiChrome wire show the
relationship between Amps and Temperature. But shouldn't this really be
Watts? For example the tables show that I need about 3 Amps for the
400°F
that I require. The tables I've seen don't actually show temps that low.
But
assuming the tables are referring to 120V, that would mean 360 Watts or
15
Amps at 24 Volts? This can't be right?

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.

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.

For longer pieces of wire work the math:
Given: Current (I) 3A
Length of wi 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





The second problem is the power supply. Assuming I'm confused about the
Watts and 3 amps at 24 Volts will suffice. The average light dimmer will
only handle about 500 Watts, but if I short out (2 Ohms) the transformer
secondary I will fry the dimmer and/or transformer (DAMHIKT).

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
light bulb in series trick, but even a 200 Watt bulb on the Transformer
secondary would not give me enough Amps to reach my temperature.
Although I
suppose a 200 Watt bulb on the Transformer primary would give me 8 Amps
at
24V?

There's obviously something I don't understand here. I would appreciate
it
if someone could lead me out of the darkness.