I have a basic CNC machine for cutting EPS foam which uses about 9.5in/23.5cm long straight 28 gauge Nichrome wire as the cutting element. The nichrome wire is powered independently with a 220V-36V 5A rated step down transformer with a dimmer in series on the mains side. The CNC have no way of knowing the temperature of the wire.
Using this calculator, I found that I needed 5.2V at 1.6A (8.4W) to heat the wire at 600F/316C which is the recommended cutting temperature. But from my experience I found that supplying 2.5V - 3V AC (measuring across the wire with a cheap multimeter) is about right for cutting the foams. I am unable to measure the current or temperature while heating as I do not have the tools for those. Below are a few statistics of voltage and resistivity of the wire:
ACVoltageAcrossWire WireResistance Temperature 0V 2 Ohm Cold 2.5V-3V 30-90 Ohm Good 4V 200-400 Ohm Glowing
The problem with this setup is that - in my place, mains power is fluctuating very much causing the wire to unevenly heat up during cutting and is almost impossible to control with the dimmer as the machine is moving at 700 - 1000 mm/s. During a single cut, sometimes the wire would heat up too high that it eats up too much of the foam; and then the wire would be too cool that it would start dragging the foam causing lots of wastage.
I tried using a 12V 2A rated SMPS with an XL4015 buck converter (voltage and current adjustable) to power the wire. When I tested with a piece of 24 gauge coiled nichrome wire (about 10 Ohm), it worked fine. So, I tried it on the machine’s wire but couldn’t get it to work. The buck converter was very hot and is not working anymore. The machine supplier also told me that they prefer the transformer over SMPS as they had more problems with the later.
So, I was thinking, if I somehow get the output from the transformer rectified and regulate the voltage, it would solve my problem. But as I have very little knowledge in electronics, I don’t really have an idea how to go about it. I googled around for the designs or ideas, but couldn’t find anything that could fit my specific needs (apologize if I had missed any). I have a some linear regulators like 7805, 7809 & 7812, few LM317, TIP122, BT131-600 and other discrete components in my salvaged inventory. I would be very thankful if anyone could suggest a cheap & easy circuit for my problem.