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A project log for Hot wire foam cutter

A power supply, bow and table for cutting foam with a hot wire

QuinnQuinn 02/22/2021 at 22:520 Comments

Following the DIY route, I started digging through stock for what I needed.  I already had the variac transformer and meters.

Variac

I'm going to use this 1.75A variac, wired for 0 - 130V output.

Transformer

I'm going to use the second ups transformer, with 17.5VAC center tapped output.

Meters

I mentioned the meters before.  I'm going to use this module I pulled out of some old electronics.  I need to use an external 0.01ohm shunt for the current, and a resistive divider for the voltage, but otherwise super easy to use.

Input breaker

Because I want to make this robust in case of accidental abuse such as short circuit, I need some protection.  The variac is probably the first thing to go in a short circuit situation as it's limited to 1.75A.  The transformer has a current limit probably about 23A, but with a measured ratio about 7.5:1, the variac can be exceeded.  1.75A input means a max output about 13A.  I did find a 1.5A AC breaker which will work well in this application.  This limits output to 11.25A, but gives a bit of margin.  It's important to note when using breakers that they have a time delay aspect to them, which varies by how much the current is exceeded.  In this case as the device being protected is a transformer, it matches well, because it's failure also has that ratio because it's failure mode is generally overheating causing windings to melt.

Output breaker

I didn't have a 12A low voltage DC breaker, but did have a 10A 28VDC breaker that I'll use.  Gives further margin, which is plenty for this use case.

I considered fuses for both of these, but having the breakers decided they are more convenient.  I could have omitted the output breaker as the input should always trip first, but decided to include as I had it.

Switches

I needed 3 switches for this project.

The first is simply an input power switch. SPST 1.75A is sufficient.

The second controls the output, and selects between turning the output on, and enabling the remote output switch.  Functionally this is just in parallel to the remote switch, and only controls the current to the relay coil, so a SPST, low current switch is sufficient

The last is a remote switch, which can be located on the bow/table to make it easier to use.  I needed a momentary SPST pushbutton for this, and it is just wired in parallel with the main output switch.

Continuing in the retro industrial robust design, I pulled out a pair of large toggles.  For the remote, I found a round one, so wires can be soldered to it, and heat shrink applied around the button body and wire make an easy handheld button.

Knob

The variac takes a bit more force to turn than a normal pot, so needed at least a moderate sized knob.  I pulled out three options fitting the more retro look.  I'll decide which to use when I get to the end.

Output connector

I pulled out a ganged pair of binding posts/banana jacks for the output.  I expect I'll mostly use the banana jacks, but the convenience of the binding posts in case I need a custom setup is nice.

Remote connector

For robustness, and because I like the connectors, I went with a pair of banana sockets.  I have plenty of dual banana plugs, so will just mount one on the end of the switch wire.

Power input

Lamps

For a bit more safety to indicate it is on, I wanted a couple indicators.  To keep in the styling, and because I didn't really care about the power use, I pulled out a matching pair of red and green incandescent bulb indicators.  

The green will be for the input switch on(though this is also indicated by the meter being in), which will simply be wired to the 12V supply output.

The red will be for output on, and simply wired across the relay coil.  I'll add a 33ohm in series to knock the voltage down a little to extend the lifespan.  They are plenty bright regardless.

Control supply

The meters require 5vdc, max 140mA.  I wanted to use a 12v relay, and the indicator bulbs were all 12V, so I dug out a small switching supply with 5/12 volt outputs.  This probably came out of an external CD ROM drive or hard drive and will cover all the needs.  I don't know the current on it, but this should be well within it's rating.

Relay

I'm going with a 12VDC coil relay as I have a number of higher current switching ones

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