Foam Cutting Power Supply

My good friend Don has put in a lot of effort and completed a number of circuit diagrams plus a text overview on how to build a variable controlled power supply that utilises 555 timer circuitry.  I have made the linked pictures quite detailed so that they can be easily read. So if you wish to download the larger linked files you will have to be patient, as they can be as large as 400kb in size. Well that's enough from me, and it's onto Don...

Donald Bladier: To me he's been an Electronics Legend

CIRCUIT DETAILS FOR A HOT WIRE FOAM CUTTER

By Don Bladier

This circuit uses the principle of applying a pulse width modulated 24 volt supply to the cutting wire.

If you were to apply zero volts to the wire it wouldn’t heat up at all.  Similarly if you were to apply full 24 volts to the wire then the wire would go up in smoke in a hurry. However, if you were to apply the 24 volts to the wire in short bursts and you could control the period of these bursts then you could control the heat of the wire very effectively. This is what this controller does.

At the lowest setting the 24 volts is applied for only 5% of the time, and at the highest setting the 24 volts is applied for 95% of the time. The time referred to is the time between pulses and in this circuit it is about 1000 cycles per second. Now, if you have a look at the circuit diagram, the circuitry to the left of the opto-coupler (4n35) is the timer section.

Click on to view. Note: large file to download.

This is a fixed frequency, variable pulse width generator. The pulse duration is controlled by the 2 5.1k resistors and the 100k potentiometer connected to pin 6 of the timer I.C.  The 5.1 k resistor fixes the end points of the range of the 100k pot.  In this case the range is from 5% to 95%. The circuitry to the right of the opto-coupler is the high current switching circuitry. This circuit has been used in an industrial electronics system for many years and is very reliable. I have added a second 2n3055 output transistor to share the output load and to save the one transistor from overheating. Probably not needed but the extra cost is only about $1.00 so why not.

The 2n3055 collectors (case) feed the wire cutter via a point 1 ohm resistor. If you measure the voltage drop across this resistor then you will get an indication of the heat setting. That is, if you had a long cutting wire and you advanced the control pot till the wire cut the foam at a nice speed, take note of the meter reading. Now if you use a shorter length of wire, and set the control for the same meter reading, then the cutting temperature should be the same as before.  A long wire equals high resistance, and needs say a 50% pulse width to get to the required temperature. In this case the meter may read say 0.25 volts. Alternatively, a short wire equals low resistance, and you may only need 10% pulse width to get the same temperature. The meter will again read 0.25 volts. 

The 12-volt regulator lm340t 12, drops and regulates the voltage to the timing circuit. The circuit for the power supply is a standard full wave bridge rectifier circuit with a couple of filter capacitors to smooth out the 24 volts. I used a toroidal transformer because I had it. You could use any other type but it should be about 100va rating. The one I used had a dual 120v primary winding and a dual 18 volt secondary winding at 2.78 amps each winding. This was nice, and I put the primaries in series, (we have a 240 volt AC supply in Australia) for 120 volt supplies put the primaries in parallel.  I parallelled the secondaries to give me 18 volts at 5.56amps don’t forget to fit a fuse to the primary circuit.  For 240v AC mains supply, use a 1 amp fuse. For a 110/120 volt AC mains supply, use a fuse rated at 2 amp.  

Click on to view. Note: large file to download.

Now to the construction
I would build it into a metal box. The 12 volt regulator and the two 2n3055 transistors should be mounted on the rear of the box make sure that you use insulating washer kits to mount these 3 bits because they shouldn’t make contact with the metal of the box. They are mounted to the box so that the box can act as a heat sink.

Note!  If you are not familiar with your country’s wiring regulations then you should get a qualified person to at least  make up the power supply section for you

The control circuit is made on a piece of perforated strip board. I have numbered the tracks 0 to 40 along the length and lettered them A to T. Across the width there are some crosses marked on the drawing, this is where you should cut the tracks. Use a 5/32 drill and twist it in your fingers. Check with a magnifying glass for whiskers of copper that may short tracks after you have cut them. 

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The cuts are at locations D7, D13, D20, K6, I8, J9, L9, I13, J14, K13, L14, I18, J18, K18, L18, H21 I22, J22, E25, H28, J28, and L28. Be aware that the drawing is of the component side of the board. The tracks run lengthways on the board. Solid lines running vertical on the drawing are jumper wires and these should be fitted to the board before any components are put on.  The 555 timer and the opto-coupler are mounted into I.C. sockets. No need to insulate the jumper wires bare tinned copper wire is O.K. Fit insulated wires to the connection points for the control pot, the 24 volt + and 0 volt  power  supply,  the output transistor base connections, and the voltage regulator.  Note that these wires can be small hook up wires as they do not carry much current.

In heavier gauge wire connect:

A. The 24 volt + from the power supply to the output terminal (red).  

B.  The 0 volt from the power supply to the emitters of the 2n3055  transistors. 

C.  The wire from the output terminal (black) to the .05 ohm resistor.  (the .05 ohm resistor is mounted on the bottom of the box.) 

D. The other end of the .05 ohm resistor to the collectors of the 2n3055 transistors. The collector is the case of the transistor,  (don’t forget that the transistor cases must be insulated from the metal of the cabinet.) 

These 4 wires need to be heavier gauge than hook up wire as they are carrying the full current to the cutting wire and we don’t need any voltage drop in these wires. When finished, mount the stripboard on insulated stand off pillars. Connect the leads to the control pot, the voltage regulator, the 2n3055 bases, and the 24 volt power supply + and 0 volts. The meter just connects across the .05 ohm resistor.  This meter can be a 1 ma 1000 ohm per volt unit or something similar. To calibrate the meter, connect a 5 or 10 amp meter in series with the cutter wire and select the resistor value to give a full scale deflection of the 1 ma meter when the amp meter is showing 5 amps. This resistor value should be about 220 ohms for a 1000 ohm per volt meter movement. 

Click on to view. Note: large file to download.

If you have access to an oscilloscope then looking at pin 3 of the 555 timer should show the switching from 0 volts to 12 volts with a pulse width of from 5% to 95%. If you are not getting this then do not proceed until you do.  By the way, the control pot should be set up so that anticlockwise is minimum pulse and clockwise is maximum pulse. If it is reversed then it is only a matter of reversing the two outer leads to the pot. If all is well, connect up the wire cutter and look at the waveform at the black output terminal.  This should be switching from 0 volts to +24 volts. 

Using the unit 

With the control pot in the anticlockwise position, connect your cutter wire to the red and black terminals. Advance the control pot clockwise until the wire will cut the foam at the speed you require.  Take note of the meter reading. Now if you change to a smaller cutter frame, and you set the meter to read the same value, the cutter should be at the same temperature as before. This is providing that you are using the same type and gauge of wire in both cutters.

PARTS LIST  POWER SUPPLY

Transformer, Farnell part no. 149 992
Bridge rectifier,  Farnell  740 238

Capacitors 2200 uf 50 volt, Farnell  698 751

PARTS LIST  CONTROL CIRCUIT

STRIPBOARD FARNELL 451 058

4N35   “   326 057 555

TIMER  “   409 364

BC327    434 589

BC337    434 772

2N3055     361 963

LM340T12    413 150

100 K LINEAR POT   350 102

.05 OHM RESISTOR,  R.S.COMPONENTS  PART NO 160 641

 RESISTORS REQUIRED, ¼ WATT
3K3  3
5K1  2
4K7  1
15K  1
6K8  1
1K5  1
½ WATT, 100 OHM 2
2 WATT 560 OHM  1 

CAPACITORS REQUIRED
ELECTROLYTIC 47 UF  25 VOLT  1
POLYESTER  150 NF  50 VOLT 1
10 NF  50 VOLT  1

OTHER THINGS
2N3055 MOUNTING KIT (INSULATED)  2
LM340T 12 “  “   “   1
I.C.SOCKET 8 PIN     1
I.C.SOCKET 6 PIN     1
RED OUTPUT TERMINAL    1
BLACK OUTPUT TERMINAL   1
KNOB  WITH POINTER  TO SUIT POT  1
INSULATED  STAND OFF PILLARS AND SCREWS 4
FUSE HOLDER, PANEL MOUNTED    1
1 MA, 1000 OHM PER VOLT METER MOVEMENT 1 

Note: Farnell and RS Components are electronics suppliers in Australia and other parts of the world. The electronic codes/descriptions used however should be common world wide.

If you are interested in detailed views of Don's finished unit, click on the below picture to take you to a selection of shots: 

Click on to take you to a selection of shots...

Thanks Don for your contribution! 

If anyone has any queries about the above, I can forward them onto Don verbally as he does not have Internet access. Note that this can take some extra time. 

This page was last updated on Tuesday June 26, 2001.