Foam Cutter Help Needed

[ybanrab]

Hi All,
I built a foam cutter this weekend but I'm having problems powering it propperly. I'm using a light Dimmer and a Transformer circuit that I've seen others mention. Because the cutter is a tabletop profile cutter, the hotwire is only about 4 inches long.
Under no load, the circuit outputs 5 volts at the lowest setting, rising to 11.5 at full.
When the hotwire is in the circuit, there's only about 0.5 volts there. The Dimmer buzzes quite loudly when the hotwire is in place and the wite does not get moticeably warmer.
I tried doubling the length of the wire and at full power, the wire will JUST cut a small chunk of foam.
I'm using Nichrome wire, and I'm fairly sure I've used it successfully before.
When I directly attach a 12v source to the cutting bow, it gets hot enough to glow red, and the (perhaps dodgy) ammeter on the 12v source reads about 2 amps.
The transformer is marked 240v in and 11.5v out and is pretty chunky, about 3 inches long, wide and high.

Does anyone have any idea about what I'm doing wrong, or any advice?

I've attached a circuit diagram, and a diagram of what I'm making.

[R. C. Day]

Ybanrab,
I also use a dimmer switch to vary the A/C delivery to my power supply. I am using a 10A battery charger. I have built four cutting bows, the shortest at 12 inches and the longest at 48 inches.

I have not had any problem adjusting mine to the wire length, however I am using .020 safety wire (from my full scale aviation days).

You said that the dimmer is talking to you, is it rated for 220VAC? I am not versed on the power systems out side the US. I think that your dimmer is not sharing the power with your transformer.

I had a similar problem tring to use an electric train transformer, the transformer would not share.

Good Luck on your quest, I will check back tomorrow.

R. C.

[small.planes]

It sounds like something is going into current limit. :(
a 4" length of nicrome has very little resistance, but the bow, which is presumably a couple of feet long has enough for the psu to drive into. Try getting a big wattage resistor and putting it in series with the short length of nichrome. I assume your PSU is not just a transformer, but also must have some other stuff in it?


Dave

[ybanrab]

Thanks both of you for replying.

R.C. I'll see if I can get a battery charger and see if that works. My dad still works on full scale aviation, I'll ask him if he can get hold of some safety wire. I'm guessing it comes in sifferent sizes?

Dave, I'll try adding a Resistor in series like you suggested. The PSU is just an old transformer, nothing else. The dimmer is rated for low voltage lighting systems, which apparently means it's able to drive transformers.

I'll report back in a day or two and let you know how I get on.


Barny

[Ollie]

The short length of nichrome wire has such a low resistance that it results in too much current at the high end of the voltage range. I suspect that the high current has overloaded something and may have destroyed a component inside the dimmer. All your components should be protected by fuses at or under the components current rating.

The dimmer circuit chops the sinusoidal wave form of the mains current, changing it to a variable shaped sawtooth wave form. At a range of settings of the dimmer, the wave form will have the same peak voltage but the current will be switched off for up to half the wave form, reducing the heating effect up to 50%. Unless your volt meter is root mean square (RMS) responding, it will give misleading readings when used on a sawtooth wave form. Most voltmeters today using transistor or integrated circuit amplifiers are peak responding and RMS calibrated on the assumption of a sinusoidal wave form. Such a voltmeter can't give useful readings unless it is recalibrated for the specific sawtooth waveform being measured. An RMS responding voltmeter like an old fashoned iron vane style voltmeter will give useful readings for a variable sawtooth type wave form.

For a resistive load like the wire, the RMS voltage across the wire times the RMS current through the wire is equal to the watts of heating in the wire.

Stainless steel cutting wire will last longer because it is stronger and less brittle than nichrome wire. Because SS wire is stronger, you can use a smaller diameter wire with more resistance. The smaller diameter will also result in a smaller kerf.

[vintage1]

I am surprised no one has spotted the obvious - you should not use a dimmer in the primary of a transformer. It will work in a way that I am struggling to predict, but almost certainly not the way you want it to .

First of all, check that the wire gets hot without the dimmer. That will prove the transformer.

THEN build your own dimmer with extremely high current (30A) Triacs, and stick it on the OTHER side of the transformer.

If this is all too much, throw the transformer away, and BUY from an electrical wholesalers, a 'dimmable electronic transformer' for low voltage spotlights. That WILL work with a standard dimmer, and will drive up to maybe 20A reliably, if you get a 250W 'transformer'. Note these are not transformers, they are switched mode power supplies, but what the heck.

[mr_wood]

easiest solution would be to make your own potentiometer - if you take a long piece of wood (1m) with a nail either end and connect the nails with a piece of nichrome wire. Then connect the +ve output from the transformer to one of the nails.

Take the cutter and connect the -ve to the transformer. Now the cleaver bit .... if you connect the +ve from the cutter using an aligator clip to the nichrome wire between the nails you can vary the current by moving the clip. Nice :)

Should work and is very simple - don't worry about a dimmer. I use this method and it works fine - except I use steel wire not nichrome.

Cheers
Tim

[ybanrab]

Tim,
Thanks for the suggestion, I'll give that a go too tomorrow. I think that should work fine. I'll post back on what I find out.

Barny

[steve lewin]

Originally posted by vintage1
I am surprised no one has spotted the obvious - you should not use a dimmer in the primary of a transformer. It will work in a way that I am struggling to predict, but almost certainly not the way you want it to .

Even though standard lamp dimmers are not designed to work with inductive loads (and a transformer primary is surely nothing else) the several I've tried have actually all worked without any fuss at all. Obviously the waveforms are far from sinusoidal as Ollie says and with all those voltage spikes flying about I never use them up to their full rating but in the real world they give less problems than a purely theoretical view would suggest.

Steve

[ybanrab]

My dad warned me about the inductive load problem and when I looked in the local Wickes, there were 2 dimmers, one for normal lights (£5) and one rated for driving Low voltage Lighting, which I think means that it will be capable of driving inductive loads.

I'll have a try of all the methods mentioned here and see what I can get to work.

Again, thanks to all of you for taking the time to help.

Barny

[ybanrab]

Well, I got a little time to play with the cutter this weekend. I took the dimmer out of the circuit, and used a chunky wire-wound 20ohm resistor and got the wire just about hot enough to cut foam. I've ordered a few more low value resistors, so I can play with the temperature a bit more precisely. I'll also have a go at putting the dimmer back in, with the resistors and see if that works.
Thanks everyone for your help, I'll post an update when I know more.

Barny

[steve lewin]

One of the simpler ways to drive a foam cutting wire is from a 12V battery or NiCd/NiMH battery via an ESC. That's what I do most of the time. It saves all that messing round with mains electricity.

The ESC can be driven from a simple servo tester and you have excellent control of the wire temp. And many of us already have everything needed lying round. I use an old 20A ESC which is *HUGE* by today's standards but who cares when it's never going to leave the ground ;).

Steve

[ybanrab]

Good point Steve,
I'm sure I've got a spare 20A ESC kicking around somewhere, and I've got a 12v power supply available as well, I'll give that a go too. I could use a PC PSU as the power supply if I want to make it mains powered (more portable).

Thanks

Barny