Radio Transmitter

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Friday, June 5, 2009

Foam Cutting Power Supply

After seeing other modelers building their model wings from plastic foam, I decided that I wanted to do the same. Building your wings from foam covered with 1/16 in. balsa can produce a strong and light wing that could be difficult to duplicate with the standard balsa rib construction, especially if the wing had a duel tapered, symmetrical airfoil. The standard way to cut foam is with the Hot Wire technique, using steel or nichrome wire through which an electrical current flows to heat the wire.

However, the methods that many use to get the wire hot leaves something to be desired. The most common method I saw used was to connect a 12volt battery charger to 4 or 5 feet of nichrome wire which was tied to some kind of a bow. Using the variable charging rate, you could control (to a limited degree) the temperature of the wire and thus the speed of the cut. But if you cannot accurately control the heat, you'll get many poor cuts. Some have connected a series of light bulbs in line with the wall service of 115 volts AC.

It works, but WOW, is it ever dangerous! Terrible shock hazard! I've even seen some connect the nichrome wire across a 12 volt car battery, also very dangerous. Over the years there have been several schematics listed in the model magazine for building a hot wire foam cutter power supply. All of them worked, I'm sure. Some were very simple, but left little heat control, and others were complex and expensive. Heat control is the secret for making good foam cuts. Also a good transformer is important for removing the electrical shock hazard that threatens the modeler in his shop. A good current limiting feature also makes the device safe from high current burns, which some auto mechanics have suffered when working with large 12 volt batteries.

The following circuit is a simplification of several older designs. This design uses readily available parts, is easy to build, has total temperature control for both a long bow (48") and a short bow (24"), and has served me well for the last 15 years. Many of the planes that I fly are my own design and I build most of them with foam wings, foam turtle decks, foam stabs, etc, usually with dual tapered, symmetrical designs. The short bow is valuable for sculpting foam pieces into various shapes, as it can be held in one hand and the foam sample in the other.

The first step in building one of these foam cutters is to take the Bill of Materials to your local Radio Shack and search for the parts. I picked this source because of shopping convenience and the total cost is a little above $30. Also get a small copper clad circuit board (CB), about 3 by 4 inches or larger in size. If you chose not to make the circuit board, you can solder the parts together using electrical stand-offs. The first order of business is to mount the switches, the potentiometer, the Red and Black electrical posts (#274-662), and the red indicator light on the front panel of the component box according to the picture and illustration. Next mount the transformer (#273-1512), fuse holder (#270-364), and electrical cord (#278-1255) in the box as shown in photo. Put some rubber feet on the bottom of the box (also from Radio Shack) so that it won't scratch your wife's end table when you take it to show her what a great craftsman you are.

The circuit is a simple AC Triac voltage control circuit similar to the ones used to control house lamps. The transformer provides the electrical isolation the makes this item safe to operate. The voltage at the bow will tingle a little, but will not harm the operator. The OFF/ON switch is a simple s.p.s.t. switch (#275-651). The "Long/Short" Bow selector switch is the same part number. Across the primary side of the transformer is mounted an indicator "ON" lamp (#272-712) which will light up when the unit is turned on. The temperature control is through the 5k ohm potentiometer R2 (#271-1714). The Triac gate current is controlled by R3, a 470 ohm, 1 watt resistor. This resistor is not part of radio Shack's inventory, therefore it may be required to solder two 1k ohm, 10 watt resistors in parallel. The capacitor, C1, is a 0.22microF disk (#272-1070). The 5 ohm, 20 watt resistor R1 is made of two 10 ohm, 10 watt resistors in parallel. They are large ceramic resistors mounted side by side. These resistors drop the voltage when the short 24" bow is being used. These resistors will get hot, don't touch!

Enclosed in this article is a actual size drawing of the circuit board (CB, 2.5" x 4"). Cut out this drawing and use it as a template, and paste it on the side opposite of the copper on the CB (circuit board) with some rubber cement. Next use a center punch to mark the center of each hole. Then drill the holes with the CB held tightly to some wood backing, making the four corner holes a 1/8" in dia and all the rest about 1/16" in dia. These smaller holes will be where you solder the components and wires. The larger holes are for the mounting bolts to hold the CB to the case. Cut the CB to the exact size as shown on the template (2.5" x 4"). Then, print out the copper side drawing and paste it on the copper side of the board. Use a sharp X-acto knife to remove thin strips of copper as shown. This will isolate the copper soldering pads from one another. Remove the paper. Insert the components in the CB on the side opposite the copper. Where the component leads stick out on the copper side, solder the component leads to the board being careful not to allow solder to bridge the cut lines in the copper. Cut off any excessive lead after soldering it. Bolt the Heat Sink on to the Triac with the fins pointing out. The Triac should be mounted in a vertical position, perpendicular to the CB. Next mount the CB in the box with 6-32 x 1" bolts and stand-offs. Finish soldering the connecting wires to the board before tightening the bolts. Drill 3 or 4 vent holes (1/4" dia) in the top of the box in the area above the Triac heat sink.

Plug the nichrome wire bow leads into the dual plug speaker connectors. It is best to trim the wire insulation on the wires back about 1/2 in. then tin the wire ends. After the wires are plugged in, turn the unit on and with the temperature control at half point and the heat switch set up to long. The wire should get hot to the touch almost immediately. If it doesn't, then examine the construction on the circuit board and wiring, and fix any errors found. After the unit is finished, bolt the top on and your and you're done.

When you use the foam cutter, be sure that the Bow switch is in the correct position. The switch must be in the "Short" position (down) if the 24" bow is used. Otherwise you may blow the fuse. Leave the unit in the "Long" bow position at all times unless you are using the short bow and you should have no problems. Before you turn the Foam Cutter on, turn the temperature (TEMP) control fully counter-clockwise, to minimum temperature. Turn the unit on with a bow plugged in and increase the temperature by turning the TEMP knob clockwise. The temperature of the wire increases almost immediately. With a piece of foam, test for the foam cutting temperature. Reduce the TEMP control until the cut is smooth with little foam evaporation around the wire. Remember, the smoothest cuts are made slowly. Spend some time practicing until your cuts are smooth. You will never go back to balsa ribs!

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