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WITH INBUILT HEAT CONTROLLER If you’ve ever tried to cut polystyrene (especially!) and polyurethane materials using a saw, razor blade or knife, you’ll know that the results are invariably less than satisfactory. If you are after a clean, precise cut, a hot-wire cutter is the answer. The hot wire actually melts the material and results in a very neat, very fine cut, without the thousands of bits of foam flakes you normally get. by John Clarke F or modelling, hobby and furniture upholstery work, a hot wire cutter is a must-have. No more material deformation, no more jagged edges and crooked cuts, no more beads of polystyrene broken off and flying about – and the cut is so much more accurate into the bargain. But wait, there’s more: this SILICON CHIP Hot Wire Cutter includes a controller to allow the wire temperature to be adjusted to produce a clean cut regardless of the thickness or even the type of material being cut. It suits a variety of low-meltingpoint “thermoplastics” but with polystyrene it really comes into its own. There are two common forms of polystyrene – the beaded type, popular as packaging material and as the “beans” inside beanbags. When all those beads of polystyrene are extruded into a block, we get the type of “foam” we’re so familiar with. Extruded polystyrene has an enormous variety of uses. It’s widely found in consumer goods packaging, it’s used in modelling, it forms the basis for 26  Silicon Chip surfboards and other floating aids and is used as an insulator – sometimes on its own but more often “sandwiched” between two tougher materials, as on its own it’s quite brittle. Believe it or not, the letters in the photo above were actually cut (using our new Hot Wire Cutter, of course!) from offcuts of 50mm-thick Polystyrene foam used as part of the cladding on the home of one of our staff members. Polyurethane, at least in the form we are talking about, is often called “foam rubber”, though of course there is no rubber in it. Its most common usage is for padding in furniture and even car seats. It’s also shaped into many products such as bedding underlays. In its “crumbled” (or crumbed) form it too is used extensively as a packaging material. Both types of plastic have a relatively low melting point of around 170 - 240°C and both are delightfully easy to cut with a hot-wire cutter. Other types of plastic that could be cut with a hot wire cutter include PET (eg, soft drink bottles), ABS (eg, “plastic” cases and parts) and clear or coloured Acrylic or Perspex. We’ll have more to say about cutting these different plastics later. Our hot wire cutter design Hot wire cutters are relatively simple and comprise a frame that supports a length of heated resistance wire which is kept taut by some form of spring. The wire needs to be taut so that the cut is straight and the wire does not bend while cutting the material. Tensioning is also required to maintain wire rigidity, as the wire expands when heated. A power source is required to provide the energy to heat up the wire. This can be sourced from a battery, or via a low voltage supply derived from the mains. Previous tests show that you need up to 100W per meter for cutting polystyrene and polyurethane. (You might remember an earlier hot wire cutter, published in the April 2000 issue. This one is more elegant and has its own variable power supply siliconchip.com.au Do we really need to tell you not to touch the hot wire when the cutter is in operation? The wire is HOT. You will get burnt! so it is much more versatile when it comes to material to be cut.) Ideally, a means to adjust the power applied to the wire is necessary so that the wire temperature is correct. If too high, it can cause melting or burning of the material and ultimately the melting (and eventual snapping) of the cutting wire. If too low, the material will not melt and therefore not cut and the cutting wire will be strained. The power is adjusted to give the best cut for the type of material without too much curling at the cutting edge. The heat setting also sets the rate at which the material can be fed through the cutter. Again, if it is too low, the material needs to be pushed harder to cut and this too may cause the cutting wire to break. Refinements to this cutter include a plinth and adjustable edge guide so that sliding along this straight edge can cut the material straight. Some cutters include automatic feed so that the cut is consistent along the length. When feeding by hand, any hesitation in feeding the material will cause siliconchip.com.au excess melting. Feed the material too fast and the wire will tend to bow. The bowing is caused by the wire’s inability to melt the material at the rate that the material is fed and hence cutting is slowed or halted. The solution is to feed the material more slowly or to increase the power fed to the wire. It’s wise to practise on pieces of scrap material before trusting your skill on real work! Our cutter is a hand-fed unit suitable for hobbyists making models and general plastic cutting. The actual size of the cutter depends on the size of material that needs to be cut. For upholstery work, a cutter that has more than 450mm wire length may be required and with a similar throat size, so it has the ability to cut wide work. Modelling work may only require a short length of wire at say 150mm long. Hot Wire Cutter Controller OK, now all that is out of the way, let’s see how to make a practical Hot Wire Cutter Controller. We’ll look at the actual cutter shortly. Ours is housed in a small box containing the circuitry mounted on a single PC board. The only controls are a power switch and “temperature” knob. These are located on the top of the box. A DC socket is for the power in while power out is via leads that pass through a cable gland. These leads connect to the hot wire. The temperature knob doesn’t actually control the wire temperature as such, rather it works by controlling the rate at which power to the resistive wire is switched on and off which in turn controls the average power applied. This average power sets a constant temperature in the wire. At full setting for the hot wire controller (fully clockwise), power is delivered continuously to the hot wire, providing the maximum power. As the control is wound anticlockwise, the percentage of time that the power is delivered to the hot wire is reduced. At the mid point adjustment setting, for example, the controller applies power to the wire for half the time and so power is 50%. December 2010  27 7–17V DC INPUT* + – A F1 6A D1 D1 1N4004 K R1* 100 100nF A * SEE TEXT FOR POWERING FROM 5–7V OR 17–24V (R1 = 330  0.5W FOR 17–24V INPUT) 2.2k K A TO CUTTING WIRE A D2,D3: 1N4148 LED2 K  K ZD1, ZD2 POWER S1 A 100nF 2.2k K A LED1 A  ZD1 12V 1W 100 F 16V 10nF K 7 6 8 K C B 4 E 3 IC1 7555 2 1 Q1 BC337 K A 5 D2 10nF D3 E B 2010 HOT WIRE CUTTER CONTROLLER K A Q3 IRF540 ZD2 16V 1W A IRF540 BC327, BC337 LEDS VR1 10k LIN S K Q2 BC327 POWER LEVEL SC  G C A K D 10 B E G C D D S Fig. 1: the hot wire cutter controller sets the wire temperature by varying the on/off power ratio, switched by Q3. The controller can adjust the power from essentially fully off through to fully on allowing a full range of heat adjustment for the hot wire. The circuit A CMOS version of a 555 timer (IC1) and a power Mosfet (Q3) plus a few extra components are used for power switching. IC1 is arranged as an oscillator with the 10nF capacitor at pins 2 and 6 charged and discharged via the pin 3 output through diodes D2 and D3 and VR1. With the 10nF capacitor discharged, pin 3 will be high at close to the supply voltage and the capacitor charges via diode D2 and the section of VR1 between the cathode (K) of D2 and the wiper of VR1. When the voltage reaches 2/3rds the supply voltage this is detected by the threshold input at pin 6. The pin 3 output then goes low at close to 0V. Now the 10nF capacitor discharges via diode D3 and the section of VR1 between the anode of D3 and the wiper of VR1. The capacitor continues to discharge until its voltage reaches 1/3rd the supply. This voltage is detected by the trigger input at pin 2. The pin 3 output then goes high and the charging of the capacitor restarts. If potentiometer VR1 is set to mid28  Silicon Chip way, there is a similar resistance between the wiper and the cathode of D2 and the wiper and the anode of D3. The capacitor charges and discharges over a similar time and so pin 3 is high for about the same time it is low providing a 50% duty cycle. When VR1 is set so the wiper is fully toward the cathode of D3, the 10nF capacitor charges very quickly, directly via D2 and so the pin 3 output is only high for a brief period. The period during which the pin 3 output is low is much longer due to discharge via the full VR1 resistance. In a similar way when the wiper of VR1 is set fully toward the anode of D3, pin 3 is low for a short period as it discharges the capacitor directly via D3. Charging is via D2 and the full VR1 resistance. Frequency of operation remains the same regardless of the position for VR1 since the frequency is the inverse of the total period for when pin 3 is both low and high. The total resistance of VR1 and the 10nF capacitor sets this period, which is about 69s (0.693 x 10nF x 10kΩ) and frequency is the inverse of this, about 14kHz. The output (pin 3) drives buffer transistors Q1 and Q2. When pin 3 is high, Q1 is switched on to drive the gate of Mosfet Q3 via the 10# resistor. When pin 3 goes low, Q2 switches on to discharge the gate of Q3 via the 10Ω resistor. The 16V zener diode ZD2 prevents the gate going beyond the safe operating voltage for the Mosfet device. Mosfet Q3 drives the resistance wire between the plus supply and the drain. Indicator LED2 lights when Q3 is on and its brightness is depends on the duty cycle of the switching. Full brightness is when the Mosfet is continuously switched on. The power indicator LED1 lights to show when power to the circuit is connected via switch S1. Diode D1 provides reverse polarity protection while the R1 resistor limits current to the oscillator circuit, regulated to 12V by zener diode, ZD1. This conducts when the input supply is above 12.6V. The zener is required to prevent IC1 being powered by more than its absolute maximum voltage of 15V for the LMC555CN. The circuit as shown is designed for a supply between 7V and 17V but it can be used with lower voltages down to 5V and up to 24V with some minor changes. We do not recommend controlling over 5A. Other voltage operation If you plan to operate the controller with a supply that is between 17V and 24V, then R1 should be changed from 100Ω to 330Ω 1/2W to reduce the siliconchip.com.au NOWEVEN S! H TURE T I W FEA E R O M SCREENSCOPE – THE GENUINE, STAND-ALONE, REAL-TIME OSCILLOSCOPE Version 2 now available with a function generator, FFT and X-Y mode vector drawing! (do not confuse with inferior USB “scopes” which can’t do what the Screenscope can!). Se Screene the review Scope i SILICONn Jan 2010 CHIP! ONLY $539 (inc GST) Here’s what you get: A genuine digital scope that is ready in seconds! 50MHz 240MSPS real-time sampling 3 channels - 2x 8-bit and 1x 1-bit input FFT in dBVrms, dBm (50, 75, 100, 300 600 Ohm termination) with selectable window +, --, x and -- math functions and memories Auto and manual measurements using markers USB host - save waveforms as .txt or .csv Save screen shots as .bmp Easy fast uploads of new firmware revisions Perfect with widescreen monitors (but fine with just about any old computer monitor!) Very easy operation - just single mouse clicks for controls .. and you can easily move waveforms and objects directly And so much more (see our website for full specs) And now with new upgrades: with a money-back guarantee! ScreenScope is now even better, with extra features, extra performance. Just look at the upgrades (below left) and you will agree. Screenscope is the new type of scope you are going to love to take anywhere and use anywhere. All you need is a mouse and virtually any computer monitor. You don’t need a PC and it’s fun to use! : Optional function generator turns Screenscope into a complete electronics lab! Signal paths from input right up to wave drawing entirely in hardware for greater speeds*. FFT calculation now performed in hardware greatly improves FFT trace rates* siliconchip.com.au New XY mode vector drawing and dot joining performed in hardware keeps up with traces* *If you already own a Screenscope, existing models can be upgraded with new firmware to take advantage of these new levels of performance! CALL NOW: (03) 9714 8597 www.screenscopetraces.com December 2010  29 S1 (REAR) VR1 (REAR) D1 10nF 4004 10111181 100nF 21+ ZD1 12V LED1 100nF 10nF IRF540 16V 10 Q3 LINK1 4148 4148 2.2k D4 D3 A – ZD2 TO HOT WIRE D N G TU O 100 F DC INPUT SOCKET 2.2k R1* IC1 7555 + A LED2 Q2 F1 (6A) Q1 RETTU C power dissipation in the 12V zener diode. No other changes are necessary. Normally we wouldn’t recommend operating with voltages lower than 7V but there might be situations where this is necessary. To do so, changes are necessary so that the gate drive to Mosfet Q3 is sufficient for the device to switch on fully. To allow this Q3 is changed to a logic level type, the IRL540N Logic level Mosfet. (The IRL540N is available from Futurlec, www.futurlec.com). Also, replace D1 with a wire link and change Zener diodes ZD1 and ZD2 to 9V 1W types. Note that reverse polarity protection without Diode D1 relies on ZD1 conducting with reverse supply. R1 remains at 100Ω as shown and current is limited to 64mA or less through the 100Ω resistor. This resistor should be 1/2W rated. ERI W T O H Fig.2: same-size component layout, with a matching photo below. plastic case is actually upside-down – ie, the base of the case becomes the front panel and the lid is on the bottom. This means that the switch S1 and the potentiometer VR1 are mounted through the base of the case. The PC board mounts with the components facing the base of the case. The board is shaped so the corner pillars are cleared and so the PC board sits on the internal side supports in the box. When the lid is in position, the PC board is held tightly in place. Begin construction by making sure the board fits into the case and then checking the PC board for breaks in tracks or shorts between tracks and pads. Repair if necessary. Check the sizes of the holes are correct for each component to fit in position. The screw terminal holes are 1.25mm in diameter compared to the 0.9mm holes for the ICs, resistors and diodes. Larger holes again are used for the fuse clips. Assembly can begin by inserting the resistors and wire link. When inserting the resistors, use the resistor colour code table and/or a digital multimeter to confirm each resistor value. The diodes can now be installed - these are all polarised, so must be mounted with the orientation as shown. Note that there are three different diode packages: take care! Mosfet transistor Q3 mounts horizontally on its heatsink and both the transistor and heatsink are held in place with a 6mm M3 screw and nut. Bend the leads at right angles to suit the holes in the PC board and secure it to the heatsink and board with the screw and nut before soldering the leads in place. PC stakes can be installed for the three terminals used for wiring to VR1 and for the power switch S1 and the DC socket and hot wire connections. Construction The Hot Wire Cutter Controller is constructed on a PC board coded 18112101, measuring 63.5 x 85mm. The PC board is mounted so that the 30  Silicon Chip Here’s how it all looks just before the pot and DC socket are screwed into position and the board is pushed back into the case, ready for mounting. siliconchip.com.au placed at diagonal corners. The other two lid screw positions are used to secure the upsidedown case (with lid) to the baseplate of the Hot Wire Cutter using M3 x 30mm screws inserted from the underside of the baseplate. The lid can be used as a template for the hole positions for drilling into the baseplate. Note that when using M3 screws, the corner pillars of the box need to be tapped for an M3 thread. This can be done (preferably) using an M3 tap, or if you don’t have one, using an M3 screw that has a filed notch along one side of the thread to provide a thread cutting edge. The remaining two corner pillars can be left untapped so the supplied securing screws can be used. The completed PC board “folds” down into the bottom of the case so that the case lid becomes the new base. Only two screws hold the lid on; the other two holes are used to secure the Controller to the Hot Wire Cutter baseboard. IC1 can be mounted on a DIP-8 socket or directly onto the PC board. Make sure the socket and IC are installed with the correct orientation. Orientation is with the notch positioned as shown. Transistors Q1, the BC337 and Q2, the BC327, can now be soldered in place. If a clear or translucent box is used, the LEDs are mounted inside the box with their tops about 20mm above the PC board surface. If a non-see-through box is used, the LEDs must be mounted high enough – the top of the LED about 25mm above the PC board – for them to peek through the base of the box (which becomes the front panel). Take care with the LED orientation. The anode has the longer lead. Capacitors can be mounted next, again ensuring the electrolytic types are oriented correctly. Fuse clips for the fuse F1 can be installed noting that each clip has an end stop to prevent the fuse sliding out. These end stops are oriented to be at the outside of the fuse. Usually it is easier to clip the fuse in the fuse clips first and then place the clips into the PC board. That way they will be oriented correctly. Finishing off The front panel label can be used as a guide to the hole positions for the switch and the potentiometer. The DC socket is located on the side of the case roughly above where IC1 is positioned. Note that the DC socket could be a 2-pin DIN socket instead to suit the 4A current when Cuprothal is used as the resistance wire. Additionally, the plug connector for the supply would need to be changed to a DIN right angle plug. At the outlet end of the box is placed the cable gland for the hot wire cutter connections. When soldering the wires from the switch and potentiometer to the PC board, use heatshrink tubing over all connections except the switch terminals. Wires connecting to the switch terminals need to be soldered to the side of each terminal with the lead exiting from the terminal side. This is because the switch sits almost on top of the PC board, when assembled in the box. We secured the lid onto the case with only two screws siliconchip.com.au Parts list – Hot Wire Cutter Controller 1 UB5 box 83 x 54 x 31mm, translucent blue or clear (or black/grey – see text) 1 front panel label 78 x 50mm 1 PC board coded 18112101, measuring 63.5 x 85mm 1 2.5mm DC bulkhead socket (or 1 2-pin DIN plug and 2-pin DIN socket –recommended for 4A use) 1 SPST mini rocker switch (S1) 1 knob to suit VR1 1 mini TO-220 heatsink 19 x 19 x 9.5mm 2 M205 PC board fuse clips 1 6A M205 fuse 1 cable gland for 3-6.5mm cable 1 10mm M3 screw & nut (for Q3 and the heatsink) 9 PC stakes 1 100mm length of light gauge red hookup wire 1 50mm length of light gauge green hookup wire 1 50mm length of light gauge white hookup wire 1 100mm length of 24 x 0.2mm figure-8 wire Semiconductors 1 ICM7555IPA or LMC555CN CMOS timer (IC1) 1 IRF540 100V 32A N-channel Mosfet (Q3) 1 BC337 NPN transistor (Q1) 1 BC327 PNP transistor (Q2) 1 12V 1W zener diode 1N4742 (ZD1) 1 16V 1W zener diode 1N4745 (ZD2) 1 1N4004 1A diode (D1) 2 1N4148 switching diodes (D2, D3) 2 3mm LEDs (LED1 – red, LED2 – green) Capacitors 1 100F 16V PC electrolytic 2 100nF MKT polyester 2 10nF MKT polyester (code 104, 100n or 0.1) (code 103, 10n or 0.01) Resistors (0.25W 1%) 2 2.2kΩ (4-band code red red red brown) 1 100Ω (4-band code brown black brown brown) 1 10Ω (4-band code brown black black brown) 1 10kΩ 16mm potentiometer (VR1) December 2010  31 Building the Hot Wire Cutter Perhaps the best description of our Hot Wire Cutter is of a miniature gallows, albeit without the hangman’s noose. As they say, a picture (and a diagram!) are worth a thousand words, so we’ll save a few thousand by referring to the picture and diagram of our prototype cutter. They are pretty-much self explanatory. For this particular size cutter a 9V 3A plugpack is suitable. You may care to change the dimensions if required, bearing in mind the comments about wire length and power requirements. Ours uses a 240mm length of 0.315mm Nichrome 80 wire. With this length the cutter can cut up to about a 230mm height of material; much thicker than you would normally expect to cut. Additionally it can cut material in up to 240mm wide sections. The cutter is made from dressed radiata pine. A flat 19mmthick baseplate, 500 x 240mm, supports two uprights (280 x 19 x 12mm) that in turn support a 370 x 19 x 12mm lever arm. This arm is pivoted at the top of the upright, while an extension spring provides the tension for the wire at the opposite end of the arm. The baseplate sits on four mounting feet to allow room for the wiring and for the connectors to the lower hot wire attachment. The lever arm pivots on a 6mm x 50mm bolt passing through the uprights, with 6mm washers between the lever arm and the uprights. The nut is not tightened up fully, so Parts list – Hot Wire Cutter with a 240mm wire length 1 1m length of 19 x 12mm DAR (dressed all round) pine 1 500mm length of 30 x 12mm DAR pine 1 240 x 19 x 500mm pine or MDF board 1 extension spring, 9.525mm diameter x 95.26mm length x 1.041mm (eg, Century Spring Corporation C-215, available from Bunnings Hardware) 4 screw-on equipment mounting feet 30mm diameter (eg Jaycar HP-0830) 4 wood screws to suit equipment feet mounting 1 brass plated screw eye 3mm gauge 30mm long (13mm OD eyelet) 6 brass plated screw eyes 1.6mm gauge 15mm length (6.5mm OD eyelet) 2 50mm M6 galvanised screws, with nuts 6 M6 washers 2 30mm M3 screws to secure the controller box to baseplate 6 crimp eyelets with 5.3mm ID hole and 6.6mm cable entry 2 8G x 12mm round head screws for timber 2 6G x 25mm countersunk head screws for timber 1 1m length of 24 x 0.2mm Fig-8 wire Resistance wire – see controller text 110mm Pivot 19 mm Lever Arm 370 x 19 x 12mm Crimp eyelet connector 135mm 20mm n sio ten Ex Upright 6.5mm OD screw eye 13mm OD screw eye rin sp g 19mm ~230mm Hot Wire mm 12 Uprights or ts pp Su mm 36 100mm C L ts or Eyelet crimp connector m Su m pp 30 19mm To Hot Wire controller 100mm Baseplate 500 x 240 x 19mm Mounting Feet 32  Silicon Chip 6.5mm OD Screw eye soldered to crimp section Second eyelet section 6mm Uprights 280 x 19 x 12mm (2 off) Supports 30 x 12mm (3 off) 500mm siliconchip.com.au Here’s some close-up detail of sections of our Hot Wire Cutter, including an enlargement of the way the wire itself is terminated. It’s a similar arrangement at the bottom end. At right is the end-on view of the spring assembly and pivot. While below right is the back of the baseboard, showing the Controller connecting wire and the four non-slip mounting feet. that the arm has free movement (we didn’t use one but a locknut might be in order here). One end of the tension spring attaches to the end of the lever arm with a screw hook while the opposite end connects between the two uprights via another 6mm x 50mm bolt. Connectors Connectors for the hot wire itself are made using crimp eyelets and a 1.6mm gauge screw eye with a 6.5mm OD. The crimp section of the crimp eyelet has a small hole drilled through it and the screw eye is inserted into this hole and is soldered in place. To add strength to the assembly, the eyelet section of a second crimp eyelet is removed from its crimp section and soldered on top of the main crimp eyelet. The resulting connector is secured to the underside of the horizontal beam using an 8g x 12mm screw. Another crimp eyelet is also secured with this screw and is for connection to the wire that leads to the Hot Wire Controller. The wire is supported using four small screw eyes spaced along the lever arm and down the upright, as shown in the diagram. For the lower hot wire connection, the construction is the same only that the assembly mounts beneath the baseplate that protrudes into a 10mm hole. The hot wire wraps around the screw eyelet a couple of times and then around itself a few times to attach the wire at each terminal. The power wire leading to the hot wire controller passes under the baseplate and then through a hole to access the controller. When positioning the hot wire terminal at the top horizontal beam, it should be such that the wire sits vertical when siliconchip.com.au connected to the lower baseplate terminal. Tension on the wire needs to be about 700g. This could be measured but we found the easiest way was with the “twang” test – when properly tensioned, plucking the cold wire should result in a note somewhere around middle “C” – about 260Hz (if you don’t have a piano or keyboard, Wikipedia has a note you can play [http://en.wikipedia.org/ wiki/C_(musical_note)]. Spring tension sets the wire tension and can be set by the positioning of the lower M6 bolt. Spring tension will be greater than 700g. This is because the pivot point (or fulcrum) is not centred on the beam. For our design the distance between the fulcrum and the hot wire is almost 250mm and the horizontal distance between the fulcrum and the spring attachment on the beam is 105mm. As a consequence the spring is tensioned by about 700g x 250/105mm. This amounts to about 1.66kg. Using the dimensions shown in the diagram, with a 230mm length of cutting wire (ie, fitted length) the specified 95.25mmlong spring is stretched to approximately 150mm. December 2010  33 Hot Wire Cutter: Resistance Wire and Power Requirements T he type of wire and the wire length used in a Hot Wire Cutter determines the power requirements for the supply that drives it. For 100W per meter, a 500mm length of wire requires up to 50W of power while a 150mm wire length only requires 15W of power. How this translates into voltage and current is dependent on the actual wire used for the wire cutter. We know that the power is voltage multiplied by the current but the value of current is dependent upon the wire resistance. Several types of resistance wire could be used but the two types of wire we recommend are Cuprothal 49 and Nichrome 80. Both are about 0.315mm in diameter, which provides a fine cutting edge for accurate cuts. Cuprothal 49 has a melting point of 1280°C and maximum continuous operation at 600°C. It is an alloy that comprises 44% Nickel with 55-56% Copper. Other metals in the alloy include about 1% Magnesium and 0.5% iron. Cuprothal 49 is corrosion resistant and is used for precision resistors due to its very low change in resistance with temperature. The ‘49’ designation refers to the resistance of 0.49Ωmm2/m value Nichrome 80 has a melting point of 1400°C and maximum continuous operating temperature at 1200°C. Nichrome 80 is an alloy of 80% nickel and 20% chromium. It is also resistant to corrosion and is generally used for heating elements such as toasters and hairdryers. The ‘80’ value refers to the proportion of Nickel in the alloy. Melting points for Cuprothal 49 and Nichrome 80 are well above the melting points for Polystyrene and Polyurethane. More information on these alloys can be found at www.kanthal. com/products/materials-in-wire-and-strip-form/wire/resistanceheating-wire-and-resistance-wire/ Note that the Nichrome 80 manufactured by this company is called Nikrothal 80. Resistance wire sources Dick Smith Electronics (www.dse.com.au) sell both Cuprothal and Nichrome wire. They are 28B&S/AWG (about 0.32mm in diameter) and are 4m in length. The catalog number is W3200 for the Cuprothal and W3205 for the Nichrome wire. Wire resistance for the W3200 is 6.08Ω/m and for the W3205, 13.4Ω/m. Jaycar Electronics (www.jaycar.com.au) sell the Nichrome wire with catalog number WW-4040. It is 28B&S at 0.315mm in diameter and 4m long with a resistance of 13.77Ω/m. Jaycar do not stock Cuprothal wire. Why the Dick Smith Electronics Nichrome wire has a slightly lower resistance per meter compared to the Jaycar Nichrome wire is possibly due to a slightly larger wire thickness or slightly different alloy composition. The different resistance values do not affect the current and voltage requirements to drive the wire to any noticeable degree. For our calculations we used 6.08Ω/m for the Cuprothal wire and 11.4Ω/m for the Nichrome wire. For Cuprothal we calculate the required current and voltage noting that the power requirement is 100W/m and that power is the voltage squared divided by the resistance. The required voltage is therefore the square root of the power multiplied by the resistance. A similar formula for power is the current squared multiplied by the resistance. In this case the current is the square root of the power divided by the resistance. These calculate to a current requirement of about 4.05A and 24.6V for a 1m length of wire. For shorter lengths of wire, the current 34  Silicon Chip requirement remains at 4.05A while the voltage is reduced proportionately. For example, a 500mm length of wire requires 12.3V at 4.05A. For the DSE Nichrome wire at 13.4Ω/m calculations set the current at 2.73A and 36.6V/m. For the Jaycar Nichrome wire at 13.77Ω/m this equates to a current of 2.69A and 37.1V/m. For different wire use these formulas to find the required voltage and current for a 1m length of the wire.  I= power requirement per metre  the wire resistance in Ω/m) V= (power requirement per metre x wire length in     metres2 x the wire resistance in Ω/m) Note that the power requirement per metre is 100W. Also note again that the current (I) does not change with length because the resistance changes at the same rate as the power requirement. So for example a 500mm wire length requires half the power compared to 1m and so is 50W. The resistance is also halved compared to the 1m length. Using different wire We do not recommend using other wire for the wire cutter. Cuprothal and Nichrome wire are resistant to corrosion – this is something to take into account because when the wire is heated, corrosion is accelerated. Corrosion in this application is the formation of oxides of the wire alloy by reaction with oxygen in the air. Having said that, some readers may wish to use resistance wire that they may on hand or pearhaps is possibly easier to obtain. For example, one possible alternative is stainless steel wire such as that used in boating and fishing equipment. A typical stainless steel wire has a resistance of 0.9Ωmm2/m, although this is dependent upon the grade. A 0.315mm diameter length of the wire has an area of 0.0779mm2 and so 1m of wire will have a resistance of 11.54Ω. This resistance is calculated by dividing the wire area into the Ωmm2/m value. Current requirements for this wire would be 2.9A at 34V. Using a shorter length of this wire will set the required voltage to a lower value. A thicker gauge wire will increase the current requirement but lower the voltage requirement. It would be wise to measure the wire resistance to ensure it is suitable for a hot wire cutter application before purchasing. Other wire may not have a suitable resistance. When the wire resistance is too high the voltage needs to be excessively high. Alternatively, when the wire resistance is too low, the current will be excessively high. For example, steel piano wire typically has a resistance of 0.118Ωmm2/m so 0.315mm wire will have a resistance of 1.51Ω/m. The wire would require just over 8A for a 1m length at a voltage of just over 12V. This is a high current and is not suited for our Hot Wire Cutter Controller. Additionally, the steel wire is liable to corrode at the elevated temperatures of a wire cutter. A similar result is for a steel guitar string. We measured a light gauge E4 steel string for an acoustic guitar at 1.5Ω for a 660mm length. This is 2.27Ω/m. Its diameter was around 0.3mm. Table 1 shows a list of standard switchmode power supplies suitable for driving the shown Cuprothal and Nichrome wire lengths. These power supplies are either in plugpack form or as in-line power units. Alternative supplies include bench power supplies of a suitable current and voltage rating and batteries. For example, a 12V lead acid battery could be used as a 12V siliconchip.com.au supply for the 487mm and 328mm wire lengths shown in the table. The wire length does not need to be as precise as shown. A 519mm wire length as expressed in the table could be plus or minus 5% or about 25mm longer or shorter without changing the cutting effectiveness of the wire cutter. Wire length Current <at> Standard Wire full supply switchmode type voltage power supply rating Wire size: 28B&S (or AWG) or 0.315mm in diameter 973mm* 811mm* 656mm* 770mm* 519mm* 487mm 410mm 365mm 328mm 304mm 246mm 205mm 164mm* 203mm* 137mm* 4.05A<at>24V 4.05A<at>20V 2.73A<at>24V 4.05A<at>19V 2.73A<at>19V 4.05A<at>12V 2.73A<at>15V 4.05A<at>9V 2.73A<at>12V 4.05A<at>7.5V 2.73A<at>9V 2.73A<at>7.5V 2.73A<at>6V 4.05A<at>5V 2.73A<at>5V 24V 5A 20V 5A 24V 3A 19V 5A 19V 3.2A 12V 5A 15V 3A 9V 5A 12V 3A 7.5V 5A 9V 3A 7.5V 3A 6V 3A 5V 5A 5V 3A Cuprothal Cuprothal Nichrome Cuprothal Nichrome Cuprothal Nichrome Cuprothal Nichrome Cuprothal Nichrome Nichrome Nichrome Cuprothal Nichrome *See note in text concerning use of the Hot Wire Cutter    Controller below 7V and above 17V. Table 1: standard switchmode supplies suitable for driving the indicated wire lengths and type for 100W/m. This power rating is suited for cutting Polystyrene and Polyurethane. A 24V lead acid battery can be used for the 973mm and 656mm lengths. Similarly a 12V lead acid battery can be used with the 487mm and 328mm lengths. Below is a list of the switchmode supplies listed in Table 1 from Altronics (www.altronics.com.au) and Jaycar (www.jaycar.com.au). 24V 24V 20V 19V 19V 18V 12V 12V 12V 12V 9V 9V 7.5V 6V 5V 5V 5V 5A 4.2A 5A 5A 3.2A 5A 5.4A 5A 5A 3A 3A 3A 3A 3A 3A 3A 3A siliconchip.com.au Altronics Altronics Altronics Altronics Jaycar Altronics Altronics Jaycar Jaycar Altronics Altronics Jaycar Altronics Altronics Jaycar Altronics Altronics M 8973 M 8996 M 8996 M 8996 MP-3246 M 8996 M 8939 GH-1379 MP-3242 M 8987A* M 8987A* MP-3496 M 8987A* M 8987A* MP-3480 M 8987A* M 8909A *Multivoltage/ current outputs Cutting other plastic types While the 100W/m power into the wire is suitable for Polystyrene and Polyurethane, the cut tends to be slow with other plastics such as PET, ABS and Acrylic (or Perspex). For these, power requirement could be set higher for a faster cut. With power set at 180W/m, this has the wire glowing red hot. We recommend using Nichrome 80 wire due to its high continuous operating temperature. We do not recommend using Cuprothal at 180W/m. At the 180W/m power setting, you can cut a PET bottle in half and cut long plastic IC carriers into separate sections suited for packaging individual ICs. When cutting ABS, Acrylic or Perspex, the edges will generally be a little rough and if clean edges are needed may require finishing with abrasive paper or a file. Cutting rate is about 1mm per second at full power. We also tested the wire cutter for cutting Nylon, such as used for PC board standoffs and for screws. This proved unsuccessful since the cut resealed itself as the wire passed through the material. Wire Length Current <at> Standard Wire Type full supply switchmode voltage power supply rating Wire size: 28B&S (or AWG) or 0.315mm in diameter 489mm* 3.67A<at>24V 24V 5A Nichrome 408mm* 3.67A<at>20V 20V 5A Nichrome 387mm* 3.67A<at>19V 19V 5A Nichrome 244mm 3.67A<at>12V 12V 5A Nichrome 183mm 3.67A<at>9V 9V 5A Nichrome 152mm 3.67A<at>7.5V 7.5V 5A Nichrome 102mm* 3.67A<at>5V 5V 5A Nichrome *See note in text concerning use of the Hot Wire Cutter    Controller below 7V and above 17V. Table 2: suitable switchmode supplies to drive the hot wire at 180W/m for a given length. A 24V and 12V lead acid battery could be used for the 489mm and 244mm wire lengths respectively. Other power supplies? As we mentioned earlier, a 12V (or perhaps two 12V) lead-acid batteries could be used for the power supply in many instances. But if you have an old computer power supply, it might be possible to press that into service. Almost invariably, they have two individual outputs, 5V and 12V, (definitely not linkable for 17V!) and are usually rated at a minimum of 150W (~12A); some are much higher. Of course, the bulk of a computer supply is a consideration. An alternative, much smaller, supply you might like to consider is one intended for a computer external hard disk drive or indeed a laptop. Generally these are rated at between 12V and 19V or so with currents from 2-5A and due to the huge numbers made, are often very low in cost. Just beware, however, that some are not all that marvellous when it comes to quality control (or maybe even quality!): not long ago we purchased a couple of 12V external HDD supplies via the internet and one of them, in the words of that old Hillaire Belloc poem, “exploded with a loud report” the moment it was plugged into the mains. (OK, so together they only cost us $7.50 including postage from China . . . what did we expect?) SC December 2010  35