Electronics Lab – Styrofoam Saw

Recently, various model-making magazines have covered the topic of “thermal saws.” Some of the proposed circuits for controlling the cutting wire’s heat were quite complex. However, this report aims to show that there is an even simpler and more affordable way to achieve the same results without sacrificing the convenience of stepless heat regulation.

In an issue of the magazine “Modell” published by Villinger-Verlag, a suggestion was made—for the sake of simplicity—to control the cutting coil using a standard dimmer. Although it is briefly noted that the dimmer should be suitable for inductive loads, as I have observed in various discussions, people don’t give this much thought. I’ve even been told: “Then I’ll just connect the cutting wire directly to the dimmer.”

Why does the dimmer need to be suitable for inductive loads? A transformer is an inductor, not a “light bulb.” When switching inductive loads, switching spikes occur that can reach voltages many times higher than the original voltage; these then contaminate the power grid as interference signals. After all, the dimmer doesn’t just turn the transformer on once, as we do with a switch, but a hundred times per second. These so-called interference spikes also throw the dimmer out of sync, and then people wonder why fuses are blowing one after another or why the circuit is “acting up.”

As an electronics technician, I place great importance on personal safety. For this reason, I strongly advise against controlling the heating wire directly with the dimmer. As a reminder, I’d like to point out that voltages above 50 V are life-threatening.

So what can you do to get a good, affordable device? Department stores offer car battery chargers for very little money. This way, you can get an affordable device that has been tested and approved by the SEV for personal safety. What happens after the transformer is no longer of concern to the relevant authorities. So now you have the transformer, the housing, an ammeter to monitor the cutting wire’s temperature, a rectifier, and a circuit breaker. All you need now is a suitable circuit that operates on a voltage of approximately 18 V _DC.

P = 5 kOhm potentiometer, R1 = 100 Ohm ¼ W resistor, R2 = 22 Ohm ¼ W resistor, C = 0.1 uF 40 V plastic film capacitor, T = Q 4004 triac from ECC, 4 A/240 V, model with built-in trigger diode in a Thermotap housing.

Charger for conversion: Must be suitable for a 12-V car battery. Charging current = 4 A, e.g., Miocar charger for CHF 38.00 at Migros.

The circuit shown in the diagram works flawlessly up to a transformer voltage of 12 V. It is connected between the secondary output and the rectifier. The cost of the components is low, and the few components fit easily on a scrap piece of a Veroboard perforated board. The triac is screwed directly—and in an insulated manner—onto the metal housing of the former charger, or better yet, onto a separate heat sink. The advantages of the modified charger are obvious. More important than the low cost—including that of the charger—are operational reliability and personal safety. The secondary control not only ensures adequate protection but also prevents the

mains supply is not contaminated with inductive voltage spikes. With this control method, the circuit is purely resistive, meaning that switching spikes do not occur in the first place.

If the resulting hum is bothersome, you can connect a 2200 uF/40 V capacitor across the output terminals. Anyone who is reluctant to do the work involved in the conversion can purchase a ready-made unit from me for Fr. 85.

My address is:

Klaus-Dieter Preiss
Baumgarten 6, 8630 Tann-Rüti

Now all that’s left for me to do is to wish everyone success with their builds and a great flying season.

K.-D. Preiss, Best regards, Hinwil