variac/hot wire question

[will_newton]

Hi!

I was wondering about using a VARIAC to power a hot-wire cutter. I've added a rectifier, so I have DC output. But I read somewhere that I should use an isolation transformer to prevent electrocution (such as a ground loop).

Any opinions or sources for a suitable isolation transformer. I have since read about using two transformers piggy backed. 120 vac to 12 vdc to 12 vdc to 120 vac.

Help save the life of a fellow r'cer!

[bluesky123]

I'm using a simpliest wire cutting setup for many years: a light dimmer connected in series with an old clothing iron (retired by my wife after an iron upgrade) with the foam cutting wire inserted in the split of one of the power wires. In other words, powe comes from one pin of the AC outlet through the dimmer to the iron, then back from the iron to the cutting wire and back to the second pin of the AC outlet. The iron draws some curreny (5A for a 600W iron), which can be regulated by the dimmer. The maximum voltage between any two exposed pieces of wire is only as large as needed to heat the cutting wire (you'll always have this voltage applied to the wire, no matter what circuit does that).
Simple and reliable setup!

:)
Boris

[Dan Baldwin]

It's common to use Variacs to power hot wires. AC works fine for hot wires, so a rectifier is not necessary unless you need the DC for something else. Electrocution is a real concern with Variacs since they are an autoformer. In other words, they are not isolated from the 120 VAC power. As long as the variac is plugged in such that the lead that is common to both input and output is at ground potential, there should be little danger of electrocution. You can check that by turning the variac to 0 volts, plug it in and turn it on, then check the voltage between the output leads and an earth ground. There should be no voltage. If the variac is plugged in backwards, there will be 120 VAC, which would cause a hazard. If you are using a fairly short wire, the voltage necessary to heat the wire will be low; typically 25 volts or less.


Dan

[will_newton]

thanks guys,

I'll give it a test. I use the DC current for titanium anodizing, which uses higher voltages. I didn't want a shock doing that either!

Will

[john whitehead]

:eek: thanks guys,

I'll give it a test. I use the DC current for titanium anodizing, which uses higher voltages. I didn't want a shock doing that either!

Will Please...don't do it..use a variac that is. Getting zapped even with your LOW VOLTAGE??? USA mains is not a nice thought. You will find a standard car battery charger with a large electrolytic capacitor hung across the +&- terminals will propably give you enough JOLTS!! :eek: Or why not buy a couple of cheap 12 volt 2 amp transformers and wire the secondary's in series. Light dimmers and electric irons are a sure stairway to heaven! When it comes to AC mains anywhere on the planet....for your lifes sake use a low voltage isolation transformer. The alternative..increase your life insurance!!! I have a variac on the shed floor...it stays there just to keep the door open on the odd occassion when it's not raining here. My foam cutter is simply two 6-0-6 tapped transfomers wired is series. So up to 24 volts in 6 JOLT steps is much kinder to the old ticker and FULLY ISOLATED.
All the best John. West Midlands UK (220/240 JOLTS)

[will_newton]

hmmm... some conflicting info here.

any more opinions? :D

[slipstick]

If you want to be really safe a variac driving another transformer is the right answer. No need for rectifiers, AC is fine, the heatling element doesn't care. It can be either an isolation transformer (115 to 115) or a stepdown, say 115V to 40V. You can then control the final output with the variac.

I don't know suppliers in your part of the world but a quick search on EBay(US) for "isolation transformer" finds quite a few.

Steve

[Dan Baldwin]

I work with all of the EPS foam molders in Southern California, and many of the EPS fabricators in the area, and they all use variacs directly driving their small hot wire setups.

When I had to make a small hot wire machine for a customer (down cutter), I used a variac driving a 24 volt transformer to make absolutely certain it would not be a shock hazard.

For my own hot wire harp, I use a 24 volt transformer which is too much voltage for my wire, then I have a simple chopper to reduce the duty cycle and therefore the power going to the wire.

An automotive battery charger would probably be fine, and you really don't need a capacitor across the output. It wouldn't drive a very long wire, and the voltage would be fixed, but you can change the length of the wire to change the heat of the wire. Make sure the wire is too long, then attach an alligator clip to the wire wherever it needs to be to get the correct temperature.

Dan

[Dan Baldwin]

I just tried hooking my automotive battery charger up to my harp. I was able to cut foam with a 24 inch long hot wire, although it wasn't very hot. I use #26 steel wire so I don't run it very hot anyway.

Dan

[will_newton]

dan- what's a chopper? and do you mean stepping down to 24v AC or DC?

Thanks guys,
Will

[Chippie]

Ukktor,
A chopper is a means of reducing the power in a circit by altering the on/of time...similar in operation to a speed controller...Which leads on to...

Why not use a speed controller for a brushed motor to control your wire?

If you build a small servo tester circuit, use that to drive the esc and power it from say 3 gel batteries..? Safe as house in terms of electic shock :)

[Dan Baldwin]

Chippie is correct. A chopper is just a switch, a FET in this case, that turns on and off quickly. Mine operates at 60 hz, and I control the amount of time it's turned on from 0% to 100% to control the heat of the wire. I am rectifying and filtering the 24 volts, but that is only because of the simple circuit I'm using. The voltage going to the wire is pulses of 24 vdc.

The hot wire doesn't care if it's AC and DC. Do you already have a variac?

Dan

[Andrew0820]

I'm doing essentially the same thing Dan is -- my power source is a converted PC power supply and I put together a simple PWM circuit (chopper, if you will). Since I fly mostly 1/2A, the cores are seldom over 18" although I've cut a few at 24". I use .018 304 stainless wire. Like Dan, I can run a 0% - 100% duty cycle, but most cuts are around 45 to 55% at about 110 Hz. Input is pure DC.

[will_newton]

dan-yes I have one. The AC output wire from the variac runs to a box with a rectifier, which then goes to a voltmeter and output leads.

I use this to anodize titanium. I use a digital volt meter so I can control the titanium's color accurately. The leads run into a bath of TSP (electrolyte), one connected to a large piece of Ti and the other connected to the piece to be anodized. Apply the desired voltage for the color. An interference film forms on the Ti in the color you want.

The original post I saw online was something to the effect of mr. whiteheads above. "YOUR VARIAC CAN KILL YOU!!!"

Even though I have never had a problem, I thought someone here could shed some light, before i did.

Will

[rick.benjamin]

9 milliamperes passing through a person's core can throw a heart into ventricular fibrillation, which is bad. Also can stop the heart (heart surgeons do this).

Electricians and other smart folks respect and admire the wonder of electricity, and seek to keep those nasty jolts in the wire. Keep one hand in your pocket, remove all jewelry, avoid metal glasses frames, insulate, insulate, insulate.

Regards
Rick

[rick.benjamin]

I built a cutter power supply from information located at:
http://www.utahflyers.org/

Also used my brushed ESC, receiver, xmtr to power a cutter.
Used a car NiCad pack Worked good.

[sfsjkid]

...The original post I saw online was something to the effect of mr. whiteheads above. "YOUR VARIAC CAN KILL YOU!!!"


He is right, under the right conditions it can. I'd vote for the isolation transformer or the PC power supply. At work I've accidentally touched a live 600V40A circuit and a 20KV10mA circuit, the isolation the one handed rule saved me and only felt uncomfortable tingles.

[BlazerB52]

Just found this thread. Have a question about my setup I am using.

Took a router speed control and plugged in a car battery charger to power homemade table top cutter. Set at 6v dc and 2 amps. Control heat with the router speed control. Cuts thick foam nice. But I was wondering if I am going to be okay with this. Don't want to burn something up or get shocked. Knowledge of electrical limited to enough to get myself into trouble. Will add pic of table and setup later.

[Dan Baldwin]

As long as I 1) made sure the variac was polarized correctly when it is connected to the wall, 2) was careful to never turn the voltage over about 24 volts, and 3) always treated it like it might be live, I personally wouldn't hesitate to drive a hot wire directly from a variac. If you're still concerned about the shock hazard, I suggest driving a 24 volt, perhaps 5 amp transformer from the variac. Then all you have to do is connect the AC output of the 24 volt transformer directly to the hot wire, and you use the variac to control the heat of the wire.

Dan

[ZIPPER]

I have used a veriac for years and (touch wood) have had no problems "YET".
I put a foot switch on to mine so I can have both hands free, and if I get a boot form the wire, then my foot should come off the switch as I am flying through the air.


Glenn

[xtal]

why not just use a light dimmer and 24 volt transformer ???????????
Its looks like that would work ..... you have control and isolation ........

...oh yea I would not use a floor switch ...because sometimes you don't move
,,,just giggle rapidly.....
I would consider using a knee switch though [ if standing]

Lets face it doing stupid things and inattention is potentionally harmful .....

[AndyOne]

why not just use a light dimmer and 24 volt transformer ???????????
Its looks like that would work ..... you have control and isolation ........
....Lets face it doing stupid things and inattention is potentionally harmful .....

Xtal,

I looked into ways of making a power supply like this about 30 years ago and discovered that a dimmer switch doesn't like working into an inductive circuit such as a transformer.

The best way is to use a variac from domestic AC supply is with an transformer that isolates and steps down. Just isolating won't keep you safe if you turn up the volts up too much which is far too easy to do.

I agree on the safety issue but if you can absolutely guarantee that you will never ever make a mistake with it "go ahead and make my day" use a variac in the raw!

"A man who never made a mistake, never made anything"

Andy.

[ZAGNUT]

Xtal,

I looked into ways of making a power supply like this about 30 years ago and discovered that a dimmer switch doesn't like working into an inductive circuit such as a transformer.



plenty of modern dimmers out there designed to work transformers for low voltage lighting.

i use a big 22V 11A toroid with a dimmer on the 220 side and get great results. a pair of easier to find 12 volt toroids would do the job just fine.



dave

[alexcmag]

My cutter is a 110V -> 24V 5A transformer.

Dimmer is 110V on input.

The output of transformer is wired to a 1 meter 25AWG NiCr wire.

It works fine with most foam.

When choosing equipment to make cutter I read somewhere that you need 1Watt per inch of NiCr wire.

[ImaBiggles]

Hi!

I was wondering about using a VARIAC to power a hot-wire cutter. I've added a rectifier, so I have DC output. But I read somewhere that I should use an isolation transformer to prevent electrocution (such as a ground loop).

Any opinions or sources for a suitable isolation transformer. I have since read about using two transformers piggy backed. 120 vac to 12 vdc to 12 vdc to 120 vac.

Help save the life of a fellow r'cer!

I use a variac hooked to a car charger - works well and gives ya the isolation you need as you are now just messing with low voltage DC 0-30amps.

[will_newton]

wow, great info guys. thanks so much!

As long as I'm asking, what about a source/parts list/plan for making the bow?

Thanks,
Will