I'm slowly finishing a nichrome wire plastic bender. One source of information, the Jacobs website, pointed out the need for grounding as a safety measure. Fine. I want to ground the circuit, but the diagram supplied on the Jacobs site shows the ground wire starting at the middle of the nichrome wire itself, smack between the positive and negative poles. My proposed solution is to wrap or clamp the ground wire, sheathed in high-temperature insulation, at mid-nichrome, with some built-in slack, and run it back to the ground prong on the three-prong plug. I've also designed in a fuse and fuse holder for the power wiring. Can anyone in the forum tell me if that is likely-or unlikely-to serve the purpose of grounding the circuit? Am I missing something?

 

You don't need to ground it if you're using isolated low voltage AC or DC.

 

the ground prong on the three-prong plug

I hope that's not a mains plug. Powering your wire directly from the mains would be highly dangerous, with or without grounding.

Am I missing something?

You could be missing your life if you don't use a mains-isolated supply.

 

I failed to include some information. No, I would not be powering the wire straight from the mains. I'm working up two power sources, one using a "router speed control" 120VAC, 15amps, with a 3-prong plug, the other a 24VDC power supply also maxing out at 15amps, with its own grounded wall plug. I included a non-schematic drawing of the circuit. So is there any way other than connecting the ground at mid-nichrome wire to ground the circuit?

 

Seems to me the first way, using the router speed control, is dangerous. There is no isolation from the AC line voltage. In that circuit, ground should be connected to the neutral end of the nichrome wire....and your life insurance paid up.

Using the DC power brick should at least be non-lethal.

 

a "router speed control" 120VAC, 15amps

Agree with Jon. Even if you grounded one part of the wire there would be a hazardous voltage elsewhere on the wire.
The 24V supply, providing it is mains-isolated, would be safer.

 

If your speed controller is a simple phase angle controller the load is not isolated from the incomming mains supply. Grounding any point on the resistance wire is the same as connecting the mains neutral to that point. You will still have the mains voltage between the ends of the wire but it will be switched off for part of each cycle. looking at the wire tables for nichrome wire to get 8 amps with 120 volts will require about 7.5 meters of wire.
(A bit less for 22 gauge , a bit more for 20 gauge.
Using the 24 volt DC power supply I assume it will be an isolated output so there is no need to ground it.

Les.

 

Thanks for the information. I previously built a DC plastic bender that worked, but had wiring problems, so I'm trying to do a better job this time. Sounds like I should stick to using DC, in spite of people, even on this forum, telling me to use AC. I presume that using the AC-DC power supply (or brick?) will isolate the circuit enough? As well as relieving me of the need for a ground? Then I'll stick with DC. Just for my information, though, what would be required to isolate the AC circuit? Just a step-down transformer? Thank all of you for the critiques. I'm working alone, so I take everything you-all have offered seriously.

 

If your speed controller is a simple phase angle controller the load is not isolated from the incomming mains supply. Grounding any point on the resistance wire is the same as connecting the mains neutral to that point. You will still have the mains voltage between the ends of the wire but it will be switched off for part of each cycle. looking at the wire tables for nichrome wire to get 8 amps with 120 volts will require about 7.5 meters of wire.
(A bit less for 22 gauge , a bit more for 20 gauge.
Using the 24 volt DC power supply I assume it will be an isolated output so there is no need to ground it.

Les.

Dear Les, I decided on what I would need of wire and current by using the Jacobs Online Nichrome wire temperature calculator, which told me that, to come up with 800˚F along a 12-inch wire would require 20 AWG wire @ 6.3 amps. Does that not make sense to you?

 

I just checked the resistance per metre at room temperature. I don't know how to calculate the rate of heat energy loss aginst wire temperature so I can;t say if 6.3 amps through 20 AWG will raise it's temperature to 800 F.
It does not matter if you use AC or DC. You just require it to be isolated from the mains and be a safe voltage. (Say less than 50 volts.) It is probably cheaper to use AC as you can just use phase angle control on the primary or secondary to adjust the power. If you choode to do it on the primary you can just use a light dimer.

Les.

 

I don't think a light dimmer supplying a transformer is a happy arrangement.

 

Dear Les and Jon, OK. I will stick to DC for the current plastic bender design, but I will put together a schematic of a possible AC version, with the safety measures, including an isolation transformer, everyone has brought up here built into it.

 

Dear Les and Jon, OK. I will stick to DC for the current plastic bender design, but I will put together a schematic of a possible AC version, with the safety measures, including an isolation transformer, everyone has brought up here built into it.

Not just isolation, but step-down as well. Getting the voltage on the wire below the generally accepted 50V is half the safety measures. In that configuration it is even marginally safer than DC since if something goes wrong with the DC version, and a high voltage DC appears on the wire, grabbing it may mean not being able to let it go while AC doesn’t lock up the muscles like that.

 

Thank you for the comment. Yes, I was thinking in terms of a step-down transformer. The answers I've got in this forum have iimpressed on me the advantages of lowering the working voltage in general.

 

.... the advantages of lowering the working voltage in general.

Staying alive is a biggie for most people. You should stay below 50v. With an isolating transformer it's simpler to run AC - that is what I have done in the past.

 

The plastic bender itself is complete, leaving me with the power supply to figure out. I know I can make the circuit work by dropping from 120AC to 24VDC and using a PWM speed control or a dimmer switch. So if I put 24 volts into a (120VAC) dimmer switch that is rated at a paltry 600 Watts max at 120AC, can I safely depend on reciprocity to deliver 7-10 amps at the nichrome wire? I run into a similar problem trying to figure out isolation transformers. Most of them are step-down transformers with multiple secondary outputs, but none of those (including 24VAC) are rated at over 2 amps. Can someone clear this up for me?

 

10A X 24V = 240W,
what is the Power ( VA ) rating of the Transformer ?
Using a Triac on the Primary of a Transformer will cause additional HEAT to be generated,
so the Transformer should be comfortably over-rated for the expected Load.
.
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10A X 24V = 240W,
what is the Power ( VA ) rating of the Transformer ?
Using a Triac on the Primary of a Transformer will cause additional HEAT to be generated,
so the Transformer should be comfortably over-rated for the expected Load.
.Thanks for the speedy reply. The Primary inputs are 120, 208 and 240; the secondary is rated at 24V, and 40VA.
So the power is rated at 40VA ?
.
.

 

which told me that, to come up with 800˚F along a 12-inch wire would require 20 AWG wire @ 6.3 amps.

That's what I read but a 12 inch 20 AWG nichrome wire has a resistance of only 0.63 ohms. If that is true then the voltage required to force 6.3 amps through the wire is 6.3 amps X .63 ohms = 3.97 volts or 4 volts consuming 25.2 watts

 

I don't know what the VA-rating of your Transformer is,
but it should be roughly twice what You need if it will be run for more than ~5-minutes or so.

You have the choice of input Voltages,
If You are operating on a nominal 120-Volts,
then you can use the higher-Voltage connections to
reduce the Output-Voltage, and therefore the Current.
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.
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