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I am working on making a vacuum forming machine, and I need to power a significant length of nichrome wire off of a 120v AC wall socket. I have very little experience in electronics, but I have talked to a few people who are experienced in electronics, and they have suggested using mosfets or PWM for powering long nichrome wire coils. I have done some calculations on what I need to power the wire, and it has very specific voltage and amperage readings (304V at 2.4A to reach 1000deg F). Are those hard numbers? I have talked to my instructor, and they have said that for that length, I should literally just plug a wall outlet into a dimmer switch, and route the dimmer directly to the coiled nichrome. From there I could control the temperature with relative ease. I feel like that is a really bad idea, and that there should be some extra safeguards. I have done a ton of research online but either I am too inexperienced in electronics, or there isn't that much documentation on this specific problem.

The wire is 28g Nichrome 80 4.09ohms resistance/ft

https://www.amazon.com/gp/product/B07CHTT73J/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

I realize it would be easiest to just buy a space heater from amazon, but it seems like building out a circuit and figuring out all the stuff I need is a valuable experience.

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  • \$\begingroup\$ You should show your calculations. Also, do you really want to control temperature or power? By understanding voltage and current you can control power, but if you want to control temperature then you need to talk about the thermal environment, mass to be heated, etc. \$\endgroup\$ – Elliot Alderson Sep 19 '18 at 19:41
  • \$\begingroup\$ @ElliotAlderson in vacuum forming the mass of plastic is usually negligible. Even heat distribution is more important. Temperature control is quite problematic, since it requires some kind of remote sensor. Hobbyists usually go with trial and error approach. I wonder if infrared thermometer will work or it will pick up mostly reflected radiation from the heater \$\endgroup\$ – Maple Sep 19 '18 at 20:38
  • \$\begingroup\$ @ElliotAlderson Controlling temperature open loop? That doesn't sound good. Maybe a closed loop system with a thermometer would be exactly what OP wants. \$\endgroup\$ – Harry Svensson Sep 19 '18 at 23:32
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    \$\begingroup\$ @HarrySvensson Agreed. I wanted the OP to think hard about this question. \$\endgroup\$ – Elliot Alderson Sep 20 '18 at 0:13
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but I have talked to a few people who are experienced in electronics, and they have suggested using mosfets or PWM for powering long nichrome wire coils.

That is what I would use to design a circuit to control a nichrome coil too.

But as your instructor already mentioned, you don't need to design your own circuit. Read on...

(304V at 2.4A to reach 1000deg F) Are those hard numbers?

Well they are if you want it to be 1000deg F. Wall power is 120V (in your country) so when you plug this coil in it'll be more like 120V at 0.9A (maybe more than 0.9, but less than 2.4) and in my estimation based on your original numbers, about 250-400 deg F.

A shorter length of wire will actually create more heat, due to the lower resistance. Find a length that will create the right amount of heat with 120V. Or, if that's too short (and doesn't reach far enough), you could find the length that creates half as much heat, but then use two of them in parallel.

I have talked to my instructor, and they have said that for that length, I should literally just plug a wall outlet into a dimmer switch, and route the dimmer directly to the coiled nichrome. From there I could control the temperature with relative ease.

Yup. That's what a dimmer does really.

I feel like that is a really bad idea, and that there should be some extra safeguards.

I can see three main dangers here:

  • Don't put flammable items nearby because they could catch fire (duh)
  • Don't touch any live parts, including the nichrome wire while it's plugged in because you'll get shocked
  • Make sure the wire isn't touching anything conductive, including itself - that would be a so-called short circuit which would increase the current.

I think the first two are easily solved with some common sense - make sure all the live wiring is in a place where you won't accidentally touch it, and where there's nothing flammable.

To mitigate the third possibility, if you don't trust your apparatus, you can add a fuse in series with the nichrome wire. I'm not really qualified to suggest which type of fuse.

You could also double-check the resistance with a multimeter if you want (while not plugged in) to make sure you haven't shorted it out. The resistance should be more-or-less the same as what you calculated.

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  • \$\begingroup\$ How does 1000deg F translate into 120V 0.9A of necessary power? \$\endgroup\$ – Wesley Lee Sep 19 '18 at 22:59
  • \$\begingroup\$ @WesleyLee It doesn't, but 0.9A is what you get when you plug a 133 ohm resistance into 120V. \$\endgroup\$ – immibis Sep 19 '18 at 23:07
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    \$\begingroup\$ But you said "those are hard numbers if you want 1000F". Ah, wait a minute...did you read hard as difficult? I read hard as fixed or finalized. \$\endgroup\$ – Elliot Alderson Sep 20 '18 at 0:15
  • \$\begingroup\$ @ElliotAlderson They are the fixed numbers you need to get 1000 degF with this wire. But you're not going to get these hard numbers that you need for 1000 degF, so you'd better redesign it so you can get 1000 degF with the numbers you do have. \$\endgroup\$ – immibis Sep 20 '18 at 0:52
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    \$\begingroup\$ You should make sure that the dimmer is rated for a high enough current, and a GFCI would not be a bad idea. \$\endgroup\$ – Someone Somewhere Sep 20 '18 at 10:21
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MOSFETS are too complicated (relatively) for a simple task like this, all you need for a heating element is an SCR control, hence the suggestion of a "dimmer" because most dimmers are just a simple triac device. You could also use a simple cheap SSR (Solid State Relay) that does Time Proportional Control, that way you could automate this if necessary, i.e. use a PID loop controller to maintain the temperature directly.

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I feel like that is a really bad idea, and that there should be some extra safeguards.

You are absolutely right. The problem with any coil directly connected to mains is that accidentally touching any part of it can be dangerous. And, unlike space heaters, the coils are usually exposed over large surface area.

So, apart from obvious mechanical solutions, like grounded metal enclosure with protective screen or grille, I'd like to suggest transformer isolation as additional safety measure. There are two possible configurations.

First, you can put 1:1 transformer before the dimmer. Pros: any off-the-shelf dimmer can be used; Touching the wire in one place is relatively safe. Cons: It is still dangerous to touch the wire in two places far apart.

Second, you can put transformer after the dimmer. In this case you'd need a dimmer capable of driving inductive loads. Look for off-the-shelf motor or fan controllers, or design your own, a lot of related questions on this site.

Now, in this configuration you can use transformer with different ratio and reduce output voltage to much safer 24-40V. Recalculate nichrome length for new voltage. If it is too short to cover required heater size use multiple wires in parallel. Make sure your transformer secondary can support the current and the dimmer can provide the power. Pros: much safer in use; some off-the-shelf components can be used. Cons: somewhat more complicated.

Finally, if your choice of nichrome wire is not finalized, I'd recommend you to take a look at ceramic heaters designed for vacuum forming. They could be more expensive, but they are much safer than open nichrome coil. A simple dimmer will work great with them, so the total cost could be actually lower.

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