Tune motor driver for "Mag" loop antennas.
pa0nhc 20150216 20180321

(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.

This simple design is a further developed variant of some other elsewhere published.
This motor driver can power a little 12 V / 0,5 Adc motor, for turning a 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 direction by changing the supply voltage polarity.

The system generates a square wave signal with constant amplitude, but is switch able between two different duty cycles. The use of a square wave in stead of pure DC has the advantage, that the motor torque is already high at low motor RPM. For optimizing the the control for the used motor (self inductance !), the oscillator frequency can be changed by changing the value of C3.

The motor runs as long S3 or S4 is pushed. ON-NONE-(ON) switches S3 or S4 will return into their default position when not pushed. When both switches are In their default position, the motor line is short circuited to mass. This will act like a brake, and the motor stops immediately, without overshooting the reached tuning position. LED 2 (red)  or LED3 (green) indicate a running motor and the tuning direction (up/down).

With ON-NONE-ON switch S1, motor speed is set to low or high, by changing the duty-cycle of the square wave output.

With ON-NONE-ON switch S2, an external  relay can be activated, for instance for band switching. If not needed, then omit this switch and its drill hole.

Although the unit could always be left switched-on, mains switch SW5 enables complete noise free reception by disconnecting power, as the circuit could theoretically generate some digital noise, even when the motor does not run.

T1 can be any PNP TO220 or SOT93 power/switching transistor. Extra cooling is not needed, as the transistor always is driven in saturation. Observe is pinning sequence, some transistors (like BD438) have to be soldered upside-down. D7 and D8 damp occurring voltage overshoot due to the motor inductance, and prevent damage if static voltages on the motor line should occur.

When the motor runs and draws current, the voltage drop over wire wound resistor R9 is used for "running motor" indication by LED 2 or LED3. If the motor circuit is disconnected, the LEDs will not light up. Also, R9 forms a low pass filter together with C6, to suppress radio equipment interference.

My advise is to insert an extra noise filter between the PCB output M1/M2, and the 5-pole DIN motor chassis bus, consisting of 5 to 10 turns in a Ui=850 ringcore, and 0.15uF foil capacitor over the bus contacts.

This factory made double sided PCB is available.
It must be connected to a mains transformer and an output filter

After soldering the connecting wires, the PCB can be screwed inside the lid of a small Hammond type 1554G ABS box, using the four switch ring nuts for fastening.

Connections Tr1 and Tr2 must be connected with a 12Vac / 10VA mains transformer, secondary fused with 0.8A slow blow.
Connections Rel1 and Rel2 can be connected with a (40-80m) band switch relay at the antenna.
Connections M1 and M2 should be connected to an extra low pass filter between PCB and chassis output bus. For instance 5-10 turns over an Ui=850 ring core, and a parallel capacitor of 0.15 uF, connected over the chassis bus output connection.

When the motor direction is changed, motor lines M1 and M2 contain or positive voltage, or is grounded. 
Both motor lines and motor connections MUST therefore be fully insulated from mass etc.

GROUNDING : A 3-wire power cable is advised. Connect the mass of the PCB to their safety ground, as static charges on the antenna system are possible and could cause flash-over from the tuner unit.

Schematic Partslist Drilling template