AC voltage control for hot knife

[Joe Schwindt]

I am trying to build a hot knife for cutting foam in a unique shape. what is the best way to regulate the voltage and current to the inconel bars i am using? do i need to have load resistors and extra circuitry? any help i can get would be great.

 

[Russlk]

Take a look at the Weller soldering gun, it is just a transformer with short circuit secondary.

 

[Phasor]

{deleted: brain fart}

 

[Oznog]

I am trying to build a hot knife for cutting foam in a unique shape. what is the best way to regulate the voltage and current to the inconel bars i am using? do i need to have load resistors and extra circuitry? any help i can get would be great.

You could use an SCR but only if the desired average voltage is most of 120v, trying to drive say 6v @ several amps with just SCR phase control may not work too well.

Load resistors can generate a lot of power if you drop so much voltage across them. Actually, hot wire works best with a regulated voltage source, their resistance goes up as they heat up, so this lowers the current and thus it more or less self-regulates its temp.

You do not mention your desired voltage or current. We cannot give you much help without these details.

There probably won't be a way around this other than a transformer of some type. The cheapest thing may be a surplus power supply with close to the desired voltage and current.

 

[stevez]

Joe - sounds like you need to know where to begin in terms of voltage, current, etc. Ultimately you need to find out how much of a load the knife presents and then you need to establish how much power to apply in order to run at the temperatures desired. That might take some experimentation unless - you can copy another design or the manufacturer can provide some guidance.

I have some nichrome wire that I was thinking of using for a similar purpose. I'd start by taking a very long section of it and applying some power from a variable supply or variac (a variable transformer). I'd establish the current/voltages it took to get the wire to glow a dull red (assumption is that dull red is more than hot enough). I might have to reduce the length or add length. I'd take the information and adjust for length then work from there.

If you know the approximate resistance of the bar - and can predict the resistance at some temperature it will give you a start. What will be missing knowing how much power you need to get to temperature.

Example: if the bar has a resistance of 1 ohm and you apply 12 vdc a current of 12 amps can be expected, unless the resistance of the bar changes as it warms up. If the current were 12 amps the power applied would be 144 watts. What we can't tell yet is whether or not this is too much or too little power - unless you have some sense of that.

Until you've established the voltage/current it's tough to proceed further. There are a variety of ways to control the power - electronic such as SCRs, Triacs, etc. A variac is a manual means of adjusting AC voltage.

 

[Someone Electro]

the stupid simple soluton wod be puting an 100W varable resistor.

I once neded to cut styrofome and came up whith an iron wire that is coneted directly to a 15V trasfomer (can provide 13A for 5 mins whithout geting serusly hot.thats almost 200W)

I coneted an thin wire and it got yelow hot! (made some light too but it melted)