I'm trying to assemble a Power supply for a Nichrome wire heater

Thread Starter

birckcmi

Joined Jan 1, 2018
98
Wayneh, I agree with your concern. I have no intention of rewinding anything. I'm hoping to be able to put 2 or 3 new or used off-the-shelf assemblies into a series that will heat the nichrome wire safely and reliably. Regarding your comments: Thanks for providing that analysis of the problem. I haven't, in fact, been taking resistance into consideration, and I need to know more about the interaction of voltage and amperage before I can go any further with this. I need to go over your formulas and di324's to nail down what I need and what I can afford. This has been a big help.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
How do you know that the temperature of the wire will be 400°F?

What is the length of the wire?
What is the wire gauge?
What is the ambient temperature?
How is the wire mounted?
How is heat removed from the wire? Conduction, convection, radiation?
 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
MrChips,You are asking the right questions. That estimate of the temperature is from a reliable source, but I'm not sure I understand how it was arrived at. All the other details are up to me, but I need to know more about powering the wire before I decide them.
 

MrChips

Joined Oct 2, 2009
21,666
There are many instructions available on the web on building you own plastic strip bender. Here is one as an example:


 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
MrChips, yes, I've watched most of them. That one, I noticed, has the wire, and thus the bare power source, suspended less than 1/4" away from an aluminum channel and a steel hinge. I think it was Instructables that included one video description of a DIY plastic bender made with 2 coiled heater wires from a toaster oven suspended over a wide sheet of polished brass, included to act as a "heat shield" for the plywood beneath it(!). The video on the left end of the row describes a much classier construction, but it too winds up with the wire 1/4" from hinges and aluminum channel. Am I missing something here? Seems to me putting bare power wires, steel hinges, and aluminum (or brass) in such close proximity is a recipe for human toast.
I'm hoping to improve on all three of them.
 

MrChips

Joined Oct 2, 2009
21,666
Yes. Electrical safety, as well as fire hazard prevention) is paramount.
Watch this one. (I do not recommend how he connected power and ground to the extension cord.)

 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
Thanks, MrChips. I hadn't seen that one, but I have a plastic bending board on that pattern. It works, but I'm hoping to make some improvements with the new one-like a bender that heats up within 2-3 minutes, is power-adjustable, and can be put away, out of sight, more easily. BTW, it appears that it would be worth my while to take a look at the TAP store, maybe for a heat controller. Thanks.
 

MrChips

Joined Oct 2, 2009
21,666
There is no simple way to control the temperature without actually measuring the temperature and using a power controller.

I think the simplest solution is to use a light dimmer switch.
Startup at full power. After a few minutes, turn the power down. Use a hand held IR digital thermometer to measure temperature.

Do a test bend with scrap piece of plastic.

1578426916432.png
 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
MrChips, thanks. It's looking cheaper and simpler. It appears that the job could be done with a 24-volt 30- Amp DC power supply followed by a 30- Amp DC stage light dimmer? Then calibrate it myself, by temperature? So it won't require a 150-volt variac?
 

MrChips

Joined Oct 2, 2009
21,666
I like simple solutions.
I don't know about 30A DC dimmers.
I can buy a 120VAC 600W dimmer at any hardware store for under $10. I would stick with a 120VAC solution. Nothing else to buy besides the heater wire and some connecting cables.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
MrChips, Hi. I did some digging online and was able to find some of the info required for designing this circuit. To answer your first series of questions: Length is 2 ft; Gauge of wire is 20 AWG; Ambient is 63˚F; So far, the wire is mounted between two screw-eyes on a plywood base; Heat is removed by radiation. According to most sources I could find, a foot of 22ga. Nichrome wire possesses 1 ohm of resistivity, and 4.5amps would get the two-foot wire up to 800˚F. Another source claims that it would take 3.2 amps. I think the two-foot length would act as a 2-ohm resistor. Finally I ran into a Nichrome wire Calculator at EasyCalculation.com that gave me the following stats: Nichrome C wire 22AWG, 61cm long, to achieve 400˚F, would have a total resistance of 2.114 ohms, require 17.8 watts, 2.9 Amps, and 6.1 volts. By the same calculations, 61 cm of 20 AWG, to get to 800˚F, would generate 1.3 ohms, require 52.36 watts, 6.3 amps, and 8.3 volts. The website cites examples such as toasters and hairdryers, which would suggest that it is based on AC. Does that sound right to you? How do I drop an AC source voltage of 120V down to 8.3? And by the way, most of the stuff I read pointed out that the resistance in the wire doesn't vary much as the heat goes up, nor does the increasing length make much of a difference. So how does changing the voltage, e.g., with a variac or a dimmer or an AC regulator, change the amperage? The charts all indicate what amperage is required to get to any particular temperature; they don't say anything about voltage. Meanwhile, variacs, etc., are all calibrated in volts. I have yet to find an amp regulator. What is the connection?
Sorry to drop all this on you. I hope some good will come of it.
 

MrChips

Joined Oct 2, 2009
21,666
Volts and amps are not independent. They are interrelated by Ohm's Law.
Use any one of the three equations to calculate the unknown third parameter when the other two are known.

I = V / R
V = I x R
R = V / I

Similarly, power is determined by knowing any two of the three parameters.

P = I x V
P = I x I x R
P = V x V / R

To arrive at a desired temperature you need power, assuming that all other things in the environment are constant.

A 1200W space heater would need 10A @ 120VAC. In other words, the heater wire would be R = V / I = 120V / 10A = 12Ω.

If your nichrome wire is 10Ω,
then a 24V supply would deliver 24V / 10Ω = 2.4 A.
The power would be 24V x 2.4A = 57.6W

If the nichrome wire is 2Ω,
then a 10V supply would deliver 10V / 2Ω = 5A
The power would be 10V x 5A = 50W

If 50W will work for your application then aim for 100W.
A 15V supply would deliver 15V / 2Ω = 7.5A
Power = 15V x 7.5A = 112.5W

So you need a 15V / 7A power supply.

There are a number of options:
1) Use a variable DC power supply, approx. 15VDC 10A
2) Use a variac. Make sure you start from 0% and don't exceed the max wattage of the wire.
3) Use a light dimmer. Make sure you start from 0% and don't exceed the max wattage of the wire.
4) Use a 15VAC / 7A step-down transformer and a dimmer switch between 120VAC mains and the transformer.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
Thank you. That spells it out. Two last questions: Am I correct in thinking that for a two-foot-long wire, the resistance in ohms is simply two times the resistivity, for three feet it's 3X the resistivity, etc.? Second, I haven't seen too many downstream devices that include conventional transformers [for isolation]. Would a step-down or step-up transformer serve as an isolation transformer? Anyway, thank you for going over this and spelling out the options.
 

MrChips

Joined Oct 2, 2009
21,666
Be careful of how you use the term "resistivity" since it has an exact definition.

Reference:
https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

Resistance is determined by the following equation:

1578522308328.png

where,
R = resistance in ohms
ρ = resistivity
l = length in metre
A = area in metre squared

Hence the units of resistivity would be ohms x metre squared / metre

1578522653104.png

If we know that the nichrome wire is size 20 AWG, then we can look up the resistance per unit length.

Reference:
https://temcoindustrial.com/product-guides/wire-cable-and-accessories/resistance-and-non-resistance-wire/nichrome-wire

The resistance of 20AWG Nichrome 60 is 0.66Ω per foot.
Hence multiply 0.66Ω times the number of feet to get the resistance:
1 ft is 0.66Ω
2 ft is 1.32Ω
3 ft is 1.98Ω

A transformer is isolated if there is no connection between the primary winding and secondary winding.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
MrChips, Thanks. I was using the term wrong. I have the Temco chart and I will use that. Your calculation clears up my question about wire length-v-resistance.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
98
MrChips, One of the points you made in the discussion was "Be careful not to exceed the maximum wattage of the wire". Sounds like good advice, especially considering the capability of a variac. I couldn't find a table of wattage for wire gauges online, so worked with the stats I could assemble. Maximum working temperature for 2 feet of 20 AWG wire is 2000˚F, which would take 17.5A. That, in turn, would amount to 404 Watts as a working wattage for 20AWG wire. Does that sound right? Another stat I was able to find is that, according to the Makrolon guide to working with polycarbonate, working temperature for bending is 350-365˚F. That works out to 3.8 Amps, and less than 2 Watts. I think any of your suggestions for circuits could deliver that.
 
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Thread Starter

birckcmi

Joined Jan 1, 2018
98
shortbus, Quite true. And I have a brake for that. But I wind up working with 1/4" and even thicker Lexan. I guess I'll find out how thick I can go.
 

shortbus

Joined Sep 30, 2009
7,849
shortbus, Quite true. And I have a brake for that. But I wind up working with 1/4" and even thicker Lexan. I guess I'll find out how thick I can go.
I've done 1/4". You just need to pull the hold down fingers/top clamp back farther than the actual thickness of the material. But then the bend gets a bigger radius when bending.
 
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