Simple motor driver for tuning a Magnetic loop antenna.
pa0nhc 20180813

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This manually operated little box drives a small 12V / 0.5Adc motor and loop tuning capacitor.
The tune motor must be able to run forwards and backwards by changing the polarity of its supply voltages
If end stops will be used, they should change the motor polarity and thereby its rotation direction.

            Earlier version.

Downloads :




New simplified schematic 20180812.

This simple circuit is linear, and does not generate any radio noises. S1 sets the tuning motor speed to low or high. At high speed, the motor gets the full supply voltage. With my DC tune motor, reduction ratio and tuning capacitor, a band change from 80m to 40m takes only 15 to 20 seconds. For low tuning speed, 10W wire wound series resistor R1 lowers motor voltage and current. Its resistance value could be adapted for your particular situation. 

A cheap and small 12V 10VA transformer, low drop bridge rectifier Br, and buffer capacitor C5, generate 17Vdc (no load)  to 12Vdc (full load). Anti rattling capacitors C1 to C4 prevent radio interference, and Fr1, Fr2, C6 and C7 block possible RF energy on the power line and motor line. The three mains cord wires are wound three times through a big hi-u ferrite ring core Fr1. 

The mains ground wire is connected to the circuits mass. It leaks possible static voltages on the motor lines to ground. 

As the transformer warms up a little, even when the unit is in stand by, i suggest drilling a few 8mm ventilation holes into the right hand side and the front side of the  cabinet. Check that no 230Vac contacts could be toughed.

The motor only runs as long as switch S2 is pushed. This switch returns to "open" position when not pushed.

S3a/b changes the tuning direction, by inter changing the polarity of the motor lines. Wile one motor line is connected to mass, the other is connected to the positive drive voltage. 

        MOTOR LINES : 
Depending on the rotation direction of the motor, each motor line is : or grounded, or contains a positive drive voltage.
Both motor lines between tuner and antenna MUST therefore be floating, in other words : be fully insulated from mass etc.

With ON-ON switch S4, an external relay can be activated via output bus pin4, for instance for band switching.

On output bus pin2 is always +12V present for a 5W anti-condensation heather in the antenna tune box. 

The anti-condensation heather in the antenna tune box should be powered 24/7 . 

Even while 230Vac mains on-off switch S5 is present, the unit should always be connected to 240V~, and be switched ON.