Electronic – Design of a switchable dual voltage power supply

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.

Is this any good?

from a:-

It's circuit diagrams for the Korad (US) /Tenma (UK) series of bench PSUs. Thousands of people have them. They switch the windings as you change output voltage, which you can actually hear quite easily. With two SPDT relays, it might be able to switch in four tap settings. They also automatically switch off the output as you change level. This might be the key to your question.