Taycol Model Marine Electric Motors
A long time ago I thought that there should be simple way to detect the polarity change from an ESC and switch a relay to provide reverse without the voltage drop associated with the diode technique.
My experiments were frustrating, however, until I found some really useful people on the ALLABOUTCIRCUITS forum, who were able to run circuit simulators, and provide an ideal set of component values. I can now present a simple circuit which works for all my Taycols under a wide variety of conditions, and looks like being the ideal way to enable reversing with a modern ESC.
Circuit description.
This is a small circuit of around 10 components. I built most of my test examples on a bit of Veroboard 2.5in x 1in. It needs a power input and 4 input/outputs for the switching. It will run from 3v-35v, and seems very resilient to any spikes or interference coming down the line from the motor.
The circuit takes the PWM outputs from the ESC, smooths them to DC with a resistor/capacitor pair, uses an OpAmp LM358 to compare them and drives a Mosfet IRF540 to switch a PCB-mounted relay. The relay is voltage-critical, of course, but I specify a 5-6v coil and add a resistor in series to drop the feed voltage to the correct level.
Block diagram is as follows:
The only complexity is in the relay connections. For Taycols with a reverse coil, the unit is placed in series with the motor and simply switches one line to the reverse coil, using a changeover single pole SPDT relay. For motors without a reversing coil, we use a crossover DPDT relay and place the unit across the armature connectors to switch polarity there.
So there are actually two circuits - with slight differences around the relay connections, as below:
Component list (all resistors 1/8 watt)
R1/2 - 33k
R3 - 680k
R4 - 10 ohms
C1/2 - 0.1Mfd
OpAmp -LM358P
Mosfet - IRF540N
PCB Connectors - 0.1 pitch (4)
JST power connector
Relay - SONGLE SRE-05DC-SC-2L or SRA-05DC-CL depending on circuit
The original Taycols would either have been reversed using a crossover switch if in a static installation, or by a crossover relay if in a radio-controlled installation. Relays were ubiquitous in early R/C circuitry - small currents had to drive large physical movement - but they are no longer a common thing to attach to the output of a modern radio.
As has been noted before, modern motor control is via speed controllers, and they assume a permanent magnet motor which requires reverse polarity to go backwards. To emulate an original control system, we must convert this into a crossover switch.
Reversing Solutions
Relay Reversing
The Changeover connections for a Taycol with reversing coils.The common line goes straight to the motor with a single connection to the unit for comparison, while the other line goes into the unit to be switched, and comes out as two line for the forward and reverse contacts.
For non-reversing coil Taycols, the motor is powered directly from the ESC, and the unit is connected across the coil and brush contacts - removing the links. The electronics remain the same, but the relay connections are different.
And here are the circuit diagrams. Click on them to get a .pdf. and go forward to the next page to see some hints on construction.
Optional - 36 ohm resistor to drop 12v to 5v - other feed voltages and relay coils may need different values. Or you could buy a relay with a coil voltage matching the one you wish to use...
