RC Air Switch
Download Files
General
The RC Air Switch is a low frequency soft start motor control, with an adjustable battery voltage cut out. It will run from 6V to ~ 30 Vdc (not tested) with a current throughput of ~60Amps continuous.
How does it work?
Refer to the circuit diagram. D1, R5, C2 & R4 for the signal input circuit as explained in Simple RC Signal Input Fig 1., and with IC1:A and trim pot P1 make up the rest of Fig 4. L1 is used as a cheap voltage reference and does not glow. R3 provides power for L1 and P1. IC1 is a quad comparator that can handle voltages up to 36V and has an open collector output i.e. output can only be tied to ground. R2 provides voltage to the gates of the mosfets M1 to M5 to turn them on, and R1 is to make sure they are discharged when the circuit is turned off. This completes the soft start switch.
IC2, C3 and C4 provide a 5V regulated supply to the receiver.
The rest of the circuit is the low voltage cut out. L2, L3 and R7 provide a voltage reference of ~ 3V. R6, P2 and R8 give an adjustable range to compare the battery voltage with the 3V reference. Initially, C1 is discharged. When S1 is turned on, C1 holds Inputs 9 and 11 low on the LM339, while 8 and 10 rise quickly to 3V. Both outputs 13 and 14 are pulled to ground, 14 keeps C1 discharged and 13 discharges C2, which prevents the input signal from reaching input 5, and therefore the mosfets M1 to M5 are turned off and cannot be operated. When S2 is pressed momentarily, C1 is forced above the 3V reference and outputs 13 and 14 go open circuit. With Output 13 now open, the signal from the radio can charge C2. As soon as C2 gets above the voltage of Input 4 the mosfets turn on. You now have normal operation.
If the battery voltage gets low enough, inputs 9 and 11 will get lower than the 3V reference, and the device will revert to the locked down state. During throttle up the motor currents are quite high, and therefore the battery voltage dips substantially. C1 provides a filter that keeps the inputs 9 and 11 higher for short periods.
Notes
-
Each mosfet can handle around 10 to 12A of current. More can be added or some removed depending on the current draw. The actual brand of mosfet is not critical, however Phillips PHP60N06LT mosfets have a gate limiting zener built in preventing gate over voltage.
-
A Schottky Diode was not included in the original design, and is highly recommended. About a 4 amp one across the motor is fine.
-
S1 and S2 can be combined so that only 2 power wires need to be soldered on to the board (see picture). The S1 to S2 connection can be made externally.
-
The battery positive wire is soldered direct to the motor positive.
-
The battery positive, M- and B- wires need to be large enough to carry the current. Deans Wet Noodle (®) recommended.
Assembly
The components are well marked on the artwork, however you will have to look closely for the + sign on the electrolytics as they are over components next to them. There is a single link on the component side required. It is the line across IC1.
Adjustment and Usage
-
Connect to motor and battery.
-
Turn on power switch S1. Nothing should happen, other than servos connected to the radio receiver should now operate.
-
Move throttle to lowest point, keep well clear of the propeller and press S2 momentarily. Motor should be at rest. If motor bursts into power. move throttle and test if the throttle moves in the correct direction. If not, reverse throttle channel in transmitter.
-
Move throttle over full range to determine if throttle operates properly. Adjust P1 till throttle motor operates over full range.
-
If the motor only operated whilst S2 was pressed, adjust P2 to one end or the other, till motor remains on while adjusting P1 (step 4)
-
Connect a battery of the trip out voltage, or run one down on the circuit to the voltage required. Adjust P2 so that a trip occurs.
-
Connect a new battery and go flying, repeating steps 1 to 3 for normal operation.
Public Domain and disclaimer
This device, and any devices detailed in the "Free Stuff" pages is for public use and can be copied and distributed freely. The user accepts any responsibility for using these circuits. Don't contact me if you have a problem, you're on your own.
All other pages, circuits, photographs and documents on this website are the property of myself and are subject to international copyright laws.
Ian W. Armstrong