Simple motor driver for tuning a Magnetic loop antenna.
pa0nhc 20180813 / 20181210

(Copyright) The use, copy and modification of all info on this site is only permitted for non-commercial purposes and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer /publisher.

(Text not completed).

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.

When a vacuum capacitor and end stops will be used, end switches should change the motor polarity
and the rotation direction of the motor.

            Earlier prototype.

Downloads :




New simplified schematic 20181213-34.

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, 14400 : 1 reduction ratio and 360 degr. tuning capacitor, a band change from 80m to 40m takes only abt. half a minute or less. For low tuning speed, wire wound series resistor R1 lowers motor voltage and current. Its resistance value must be adapted for your particular situation. To keep temperature low, R1 consists of two resistors in series. Speeed switch S1 is of the ON / NONE / ON type.

A cheap and small 12V 10VA transformer Tr, a low drop bridge rectifier Br, and a buffer capacitor C1, generate 17Vdc (no load)  to 12Vdc (full load). Anti rattling capacitors C2 to C5 prevent radio interference, and C6 and C7 short ciruit possible RF energy on the power line and motor line. 

Still it is wise, to wind the mains core, and the motor cable, several times through 29mm Fair-Rite MIX #31 ferrite cores. 

The transformer secondary is fused. This is essential, as a short circuit in the external motor cable can lead to overload of the transformer. 

The mains ground wire, and the motor cable -12V wire, are inter connected via the PCB mass planes. Possible static voltages on the motor lines now leak to the mains safety 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 into the front side, of the cabinet. Check that no 230Vac contacts could be toughed.

        MOTOR LINES : 
The motor only runs as long as ON / NONE / [ON] toggle switch S3 ("UP") or S4 ("DOWN") is pushed. These switches automatically return back  to the grounded  position, Then both motor lines are shorted together, and the motor stops immediately, without over shooting its present position.

S3 and S4 change the motor rotation 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. 

Depending on the rotation direction of the motor, each motor line is grounded, or contains a positive drive voltage.
Both motor lines between tuner and antenna MUST therefore be floating, in other words : be insulated from mass etc.
External 10k leakage resistors to antenna ground are permitted on all lines.

With ON / NONE / ON switch S2, an external relay can be activated via output bus pin4, for instance for antenna band switching.

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

S5 switches the mains power. But the unit should always be left switched ON, for powering the band switch relay, and the 5W anti-condensation heather in the antenna tuning box.

In case of a nearby thunderstorm, pull the motor plug and the mains plug.