Speed 400 Speed Control

Specifications and User's Guide

This speed controller was designed for two reasons. First, I am a technical person (AKA geek), and didn't want to purchase all of the electronics. Second, I was looking for something to do with the Microchip, Inc. 8-pin PIC processor. It really was not designed to save money, although if several people go together, it can be built more cheaply than any commercially available controller. The controller should run any speed 400 motor using 6, 7, or 8 cells, and provides all of the common features. In addition, it has a non-linear control response that spreads out the high speed range.

-- Specifications --

Speed control: High rate controller with a 3 kilohertz PWM rate. The PWM duty cycle has a resolution of 2.1%, and is non-linear with the servo pulse width as described above. The on resistance at full throttle is 7 milliohms typical, and 10 milliohms max. The controller is designed for 10A continuous, with a momentary peak rating of over 100A. The design is quite conservative; if the controller gets cooling air, the continuous rating can be higher with no problems.
Prop brake: Shorts out the motor at the minimum throttle position and when the low battery voltage detector shuts off the motor. This greatly reduces windmilling and its associated drag, although the prop does continue to turn slowly with a direct drive setup.
Battery Eliminator Circuit (BEC): Provides 5V power for the receiver and servos, in addition to powering the speed control electronics. It can provide about 300 milliamperes continuously and over 500 milliamperes peak; this should be sufficient for a receiver and 2 or 3 micro servos. An option is also shown for a 1A peak rated BEC. This has been tested with a Hitec 555 receiver, and works well. However, if one analyzes the data sheet for the part used here, it is possible that there will be problems with a receiver which uses exceptionally low impedance filter capacitors. A correspondent says that most commercial units use the 1A part; however, I cannot guarantee it will work in all cases.
Low voltage detector: As designed, the BEC cuts off the motor and turns on the brake when the battery voltage goes below 6 volts. The pilot can re-start the motor by returning the throttle to minimum and advancing it again to get another short burst of power; however, the circuit will turn off the motor again when the battery drops below 6 volts.
Arming: If the speed control is powered up with the transmitter above the lowest setting, the speed controller is disabled until the throttle is returned to its down position. Note: like all safety features, one should not rely on it; always turn on the transmitter first and set the throttle to the down position before applying power to the speed control.
Valid Signal Detection: When the speed control is powered up, it requires 5 pulses representing the minimum throttle position before enabling the controller. In addition, if about 300 milliseconds go by without a valid servo pulse, the controller resets to its initial condition with the motor off. Note that, in a noisy environment, this has not proved very reliable.
Throttle Position Indicator: The speed controller has a red/green LED (D1 on the drawings). This LED shows red to indicate that the throttle is in the full OFF position, the brake is applied, and the controller is armed. The LED shows green to indicate that the throttle is in the full ON position. This is useful for setting the throttle trim and rate for different transmitters to place the range where you want it. I have been using the trim to center the range, and leaving the rate at its maximum; this results in about 3 clicks on each end of the range where the LED is lit. Note: this LED is just fine in the garage, but is useless in sunlight. You can drop R8 to as low as 180 ohms to try to make it brighter; I would prefer to keep the power drain low after the motor cuts off.
Optional Power Switch: By wiring the two pads shown as the connections for W1 through a low-power switch, the entire airplane can be turned on and off. The switch controls power only to the radio, servos, and speed controller electronics. However, the speed controller is designed to hold the motor OFF when it is not powered. This design is taken from RC car practice, where it is common. This does not provide the level of protection that an arming switch does; however, I think it is adequate for speed 400 use without an arming switch.

Note that there is no reverse polarity or short circuit protection; either of these features would either increase the size of the controller or reduce its performance. Reverse polarity is unlikely to damage the radio and servos; the regulator used in the BEC is designed for automotive applications, and blocks reverse voltages up to 30 volts. However, both of the power FETs (Q3 and Q4 in the schematic) will act like high-power forward biased diodes. The brake FET(Q3) will probably fail first; its diode is only rated at 16A continuous. If you get lucky, the bonding wire in the brake FET will blow, protecting everything else. Just don't do that.

-- Usage --

The Obvious: Connect the lead labeled +BAT to the positive lead of the battery, and connect the lead labeled -BAT to the negative lead of the battery. You will probably want to use a connector here. Connect the lead labeled +MOT to the positive terminal on the motor, and connect the lead labeled -MOT to the negative terminal on the motor. Put on the noise suppression capacitors described below first; it's easier that way. I don't think you need a connector for the motor.
Mounting Ears: The artwork provides for two 3/8" mounting ears. No one has found them useful; by the time the motor and battery are connected, the wire supports the controller just fine. The artwork provides marks to assist in cutting them off; I suggest you do so.
Receiver Overload: In some installations, bringing the transmitter near the airplane causes the motor to run and/or the servos to jump around. These are two separate problems. The motor running is caused by RF getting into the servo pulse wire and confusing the microprocessor. The fix involves soldering a surface mount capacitor between the servo pulse input and the ground pin on the processor. Because this involves handling a very small part and is not necessary for satisfactory operation, I have made it optional. The servos jumping around seems to have nothing directly to do with the speed controller. Instead, it seems to be due to the long wires between the receiver and the speed controller, followed by longer than necessary wires to the battery. If the wires are as short as is practical, the problem goes away, at least on the installations I have tried.
Motor Noise Suppression: I use three 0.1 microfarad ceramic capacitors tied to the motor terminals and case. One capacitor goes between the two motor terminals, the second goes between the + terminal and the case, and the third goes between the - terminal and the case. For the RS-380 motors I have been using, there is no problem soldering to the case; I just use a 25-watt iron with a well-tinned 1/8" chisel tip. The capacitors specified for C1 and C3 are quite suitable, but any ceramic capacitor should do. I don't actually know if three capacitors are needed; I copied this from the recommendations of RC car speed controller manufacturers.
Servo Connector Wiring: The pads used to connect the servo plug wires do not match any radio that I know of, so don't connect the wires in order. Find out which wires are the + and - power wires. Connect the + wire to the + connection on the servo pads. Connect the - wire to the - connection on the servo pads. Connect the wire carrying the servo signal pulse to the P connection on the servo pads. Double check everything; a mistake here can damage the receiver and/or servos.

May be copied as desired and modified as needed so long as this copyright notice is preserved.

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