Hot Wire Foam Cutter project, by Tony van Roon (VA3AVR)
  LM350K pin-out
Parts List:
  BR1 = Bridge Rectifier, 80V - 10A      C1 = 1000uF/63V, electrolytic
  IC1 = LM350K, adjustable regulator     C2 = 0.1uF/100V, ceramic
Meter = 5A scale                         C3 = 1uF, 63V
   T1 = Transformer, 18-22V, 5A        Plug = 3-wire power cord
   R1 = 1K8, 1 Watt, 5%                  S1 = On-Off switch
   R2 = 220 ohm, 1/2 Watt, 5%            D1 = 1N4001
   R3 = 0.1-0.33 ohm, 20W, WW, 5%      Led1 = Led, Red, 5mm
   P1 = 10K, potentiometer             Fuse = 110V, 3A, slow-blow              

Misc: Fuse holder, wire, solder, case, knob for P1, Red & Black Banana Jacks
Hot Wire Foam Cutter project, by Tony van Roon (VA3AVR) Description:
Transformer T1 rectifies the 18 to 22 Volts AC via the bridge rectifier (BR1) to about 26 or 32 Volts DC. T1 is a regular type without "CT" or center-tap. If your transformer is above 22VAC, you need to modify the circuit to protect the LM350K from over-voltage (33V max). Check the datasheet for more info.

Capacitor C1 filters this DC voltage smooth of most ripple together with C2 for high frequencies. The value of C1 was lowered to 1000uF from the previous version, since a foam cutter basically doesn't care what the ripple is. Even a 470uF type can be used.

The LM350K (IC1) is a TO-3 case type and needs to be cooled, or it will get extremely hot. Using a large coolrib with a fan, the LM350K can easily handle 3 amps and up to 35Vdc. If mounted on a large coolrib + a cpu fan, it can provide up to 4.5A until the internal overprotection of the LM350 shuts it down. Use silicon compound for a good connection to the coolrib.
The LM338K can provide up to 5A continuous current. Which may be overkill in your application.

Led1 and R1 is your "on-off" power indicator and can be omitted if you don't want to use it.
The general purpose diode D1 takes care of possible 'feedback' spikes to the regulator. I indicated a 1N4001 type but you can pretty much use any type in the 1N400x series.

Current regulation is provided via the 10K potentiometer P1. R3 can be in the range of 0.1 ohm to 0.33 ohm. The prototype used a 0.1 model but anything in that range should work fine. C3 provides additional filtering.

The analog 5A meter can be expensive and can be omitted if you wish. Just 'eye-ball' the current by observing the quality of a 'sample' foam-cut, or use a multimeter in the 10A setting in series with the '+' output wire.

This unit will work as shown with NiChrome wire at around 3 amps or less. Any other type of wire may require modifying the circuit for more current. Hence use the LM338K and a 8Amp transformer. Whatever you use, the wire needs to be at least 11 ohms with a 22-24VAC transformer.

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Copyright © 2005 Tony van Roon