"Hot-knife"/Thermal cutter power supply design

Question

I'm undertaking a DIY rope cutter project to avoid paying for the ~$150 store bought ones.

Many resources online told me to use a light dimmer + transformer configuration and I had planned to go a step further and build the dimmer too, using the circuit from this site: (replacing the light with a transformer)

http://www.electronicecircuits.com/electronic-circuits/filament-light-dimmer-circuit

However, upon reading a few posts here it has been suggested that dimmer circuits don't mix well with inductive loads such as transformers so I'm looking for alternatives.

I played around with a current division circuit using parallel resistors and a variable resistor however the high power would fry the resistors by a long shot.

Unfortunately this is where my education draws a blank and I open it up to any suggestions from those more educated than myself.

On a final note, please don't suggest using a bench power supply as this is not financially viable at this point in time, also needs to run off 240VAC (Australia.)

======================================================== EDIT

Blade arrived in the post today, it is advertised as a 60W but I would like to test it.

I also bit the bullet and grabbed a benchtop power supply, 0-30V 0-20A.

I thought I could find the current-voltage combination by connecting the blade across the supplies terminals however when I do so the voltage refuses to climb above 0.6V (pushing current up to 20A)

Using P=VI this means I can't get the power above 12W, much lower than the ~60W needed to heat the blade.

I'm inexperienced with benchtop supplies, is this normal? Are there any workarounds?

Answer 1

Most hot-knife tools use a fairly beefy metal cutting tip. This is often a strip of wide nichrome ribbon - wide for mechanical strength and nichrome to make it easier to heat. You can use materials other than nichrome but these materials usually require significantly more current.

Because the cutting tip / cutting head is both small and beefy, its resistance is quite low. You therefore need a fairly-low voltage but lots of current.

To build your won power supply / controller, you need to start at the cutting tip. Determine what size of cutting tip you need. Then either figure out or measure how much current it's going to take to get it hot enough to cut your material. When you have an estimate of both the resistance of the cutting tip and the current required, you can choose a suitable operating voltage.

Most hot-knife cutting tools that I've seen and worked with use anywhere from 5 to 25 Amps at voltages ranging from about 1 to 3 Volts.

Note that I am talking about a hot-knife cutter here. There is a similar class of tools that use a long wire instead of a short ribbon for cutting. These are used for cutting foam board and sheets. Because the cutting element isn't small and beefy, these usually require less current but significantly higher voltage. But the principles are the same.

When you have determined what voltage and current you need, either purchase or build the power transformer that you need. There is a plethora of DIY articles that will show you how to re-purpose the power transformer from an old microwave oven for this and similar uses.

Finally, you need some form of current control to set the temperature of the cutting tip. Because the total power involved is relatively small (500 Watts or less), a triac-based dimmer works well. Again, you can either build or purchase your own control. However, I find that the speed controls used for ceiling fans work well in this application. They are usually quite inexpensive and because they are designed for inductive loads, have the appropriate snubber circuit that allows them to work with your transformer.

The dimmer controls that I've purchased from eBay have worked extremely well and they cost me significantly-less than what I could build them for. In fact, the price for the completely assembled and working units that I purchased recently was less than what I could have purchased the individual parts for.

Answer 2

You do not need to mess with your power outlets to build a thermal knife/cutter. Thermal knives operate by using the Joule effect to provide enough energy to the material to cut in order to heat it up past its fusion point until it melts (or up to its flash point until it burns).

You need Joule dissipation power: $$P_J=RI^2$$

A high resistance wire will give you more heating power, but will require a higher voltage to provide the same amount of current. Looking at the above formula, if you have reached the maximum voltage of your supply, stop there, as current is more important than resistance.

Resistance of a wire : $$R=\rho*L/A$$ Where rho is the resistivity of the conducting material, L the length, and A the cross section area. L can be increased by winding the wire around the material, and A decreased using higher AWG gauge wire. Don't forget to remove the insulation of the wire, as it represents a thermal resistance that will decrease the power making its way out of the wire.

Your first requirement is the required amount of energy to cut the rope. This energy is $$E_{melt}=m\Delta T C+mH_{fusion}$$ Where the first part is the amount of energy to heat the material up to its fusion/flash point (m is the mass of that material, $\Delta T$ is the difference in temperature, C the specific thermal capacity), and the second part the amount of energy to complete the fusion/combustion process ($H_{fusion} is the specific heat/enthalpy of fusion$). Note that if the thermal resistance of the material to cut is not negligible (as we can expect from a rope), this formula will give you an upper bound as the heating power will not diffuse throughout the material, and heat a specific spot instead. This assumes no losses to the environment though, I advise proper thermal insulation around the cutter, and to use thermal paste between the cutter and the material.

Fire up Excel, and figure out what you need. I think a 2A DC wall wart could do the trick for you with suitable wire. If you use a transformer for higher voltages and/or currents, do not forget that Joule heating is in function of RMS current.

Answer 3

There are half dozen soldering guns for sale on Amazon (and elsewhere no doubt) that range in price from US$14 to US$84 that would only require you to possibly modify the supplied tip or purchasing a replacement tip for a professional rope cutter. These cheap soldering guns are usually not suited for heavy or continuous use but should work very well in intermittent service.

http://www.amazon.com/Pit-Bull-CHIG001302UL-Soldering-5-Piece/dp/B000KPW84G/ref=sr_1_12?s=hi&ie=UTF8&qid=1436130366&sr=1-12&keywords=gun

The rope cutters start at US$34 and the replacement tips are under US$10 You will save a lot of money and expense to just purchase consumer parts rather than making your own unless you want something more robust or enjoy the learning experience.

Remember to be careful of mains shocks if you do not have transformer isolation in your design.

http://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Dtools&field-keywords=rope+cutter&rh=n%3A228013%2Ck%3Arope+cutter

Answer 4

Here's what I've made and been using for years.

Surplus microwave transformer from older type m/w oven (newer ones dispense with the heavy transformer).

Remove high voltage secondary.

Replace with - in my case, 4 turns of 'electric stove wire' cannibalized from a discarded unit - capable of high current capacity, and heat resistant coating.

For blade, I've used anything from SS ribbon cut from old 3.5¨ diskette part (gets very soft when red hot) to a short, 8 cm piece of worn out hacksaw blade (very stiff, low R compared to SS). This last works well on our 120 V at 40 A, though not red hot. haven't tried it on our 240 V, but should be fine as well.

I never bothered with 'temp' controls.

I used overwhelmingly for cutting/sealing ends of my many sailing/rockclimbing synthetic lines/ropes.

It's not rocket science.