A Quick-and-Dirty DC Motor Controller

October 31, 2016

Years ago my pal Steve Titus wanted to experiment with some drone rotor designs. He needed a simple motor controller that could operate from 8V to 12V and deliver amperes of current to a DC motor used in radio-controlled aircraft ( figure 1 ).

I whipped out a design based on the classic TL494 PWM (pulse width modulation) chip ( figure 2 ). This Texas Instruments chip is second-sourced by ON Semi, as well as others. Distributors carry the chip in small quantities with pricing from 24 cents in 1000s to 60 cents each (2016). I learned to love this chip when I consulted to Teledyne. I designed power supplies for radar jammers on F-16 fighter jets. The TL494 came in a ceramic DIP package so you could use it on mil-spec projects or for ultra-reliable industrial applications. These days I would design in an SOIC for the small size and vibration resistance. The part even comes in TSSOP for micro-miniaturization.

To get the large amperage Steve required, I had the chip drive a large TO-3 metal-can PNP Darlington transistor that can handle 100V and 16A. Since the inductance of the motor and the wires connected to it could generate switching spikes even higher than 100V, its necessary to put a robust clamp diode across the motor ( figure 3 ).

Figure 4; The circuit applies pulses to the motor (red). This is based on the control voltage reaching a set threshold (yellow). The current into the motor gets up to 4 Amperes, and then falls through the off-time (green) . Small oscillations in current are due to the inductance of the motor and wiring interacting with the capacitance of the snubbing diode.

You effect motor speed control by chopping the DC input voltage into variable-width pulses. I picked C3 and R5 to give an operating frequency of 8.7kHz. If the IC doesn't chop at all, it applies full battery voltage to the motor. If it applies brief narrow pulses to the motor, the average voltage is low, and the motor runs slowly ( figure 4 ). With pulses that are vanishingly narrow there is no voltage applied to the motor, and it stops rotating. With no load on the motor, the voltage waveform can look squirrely as the back-EMF (electromotive force) of the motor creates voltage as a generator.

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While it might appear brutal to apply these pulses to the motor, remember that motors run on magnetism and magnetism is made with current, not voltage. So while the controller apples voltage pulses, the motor inductance turns this into current ramps that are proportional to the voltage.

For mechanical design I bought some heat sinks at a salvage yard that were already drilled to mount a TO-3 transistor. I used mica insulators and thermal grease when I mounted them.

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