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DC Motor Controllers: 12 Volt, 24 Volt, Variable Speed, Hi / Low Speed

Chris Greacen

Homebrew

©1995 Chris Greacen

Small motors which are on much of the day can use a butt-load of power. For example, a number of companies (Backwoods Solar, AEE, Electron Connection, etc) sell 12 and 16 inch diameter ventilation fans with DC motors. At 12 Volts the bigger fan draws 1 Amp and pushes 500 cfm. If it needs to be on 24 hours a day, that's 24 Amp-hours — the day's output of a couple 50 Watt solar panels. At 24 volts, the same fan pushes more air, but draws 2.75 Amps. That adds up to 66 Amp-hours a day — the daily output of about six 50 Watt panels. Clearly, if you need a fan that's on for much of the day, you'll want to be careful to get one that uses no more power than necessary. Surplus catalogs like C&H Sales have a plethora of DC motors, so there's good chance you'll find what you need.

The easiest solution is to get the motor that's exactly the right size, and avoid motor controllers altogether. If you want a motor with variable speed, or if you're set on a certain motor, but want to throttle back its speed and power consumption, you need a motor controller. You can buy these — AEE sells 12 and 24 Volt controllers for

20K<

$45. A client blew up one of the commercial 24 Volt controllers. He hired me to build him a replacement. The circuit I built has been in 24 hour-a-day service for 6 months, powering a 3 Amp fan. With power handling parts adequately heatsunk, the circuit will handle 10 Amps.

You can build this circuit, or its variations, for around $15 in parts in an evening or two at the workbench. Here's the design.

Basic Theme: 12 Volt Variable Speed Controller

The brains of the circuit is the NE555 timer chip, wired as a variable duty cycle (5% to 95%) astable multivibrator. What does that mean? It means it puts out a signal which goes from high to low (12 Volts to 0 Volts), hundreds of times a second. The chopped signal turns on and off a power field effect transistor (FET), which in turn switches the motor on and off, hundreds of times a second. "Duty cycle" is the percent of time the output is "high" versus the percent of time it is "low". You can vary the duty cycle of the 555 chip, and thus the speed of the motor, by turning the potentiometer attached to pin 7. (See figure 1)

Here's how the 555 part works: current flows through the upper 1.6K resistor, through the upper half of the potentiometer and wiper arm and right hand side diode, charging up the 0.1 ^F capacitor. Pin 6 senses when this capacitor is charged to 2/3 Vcc.At this point the output (pin 3) switches to LOW, and pin 7 becomes conductive to ground. Now the capacitor discharges through the left diode, lower 1.6K resistor, and lower half of the potentiometer, to ground via pin 7. When pin 2 senses that the capacitor is discharged to 1/3 Vcc, pin 7 stops conducting to ground, pin 3 (output) goes HIGH, and the cycle starts again. Turning the knob of the potentiometer makes the capacitor charge faster and discharge slower, or vice versa. This changes the proportion of "on" time of the output.

The output (pin 3) can continuously source or sink 150 mA of current, and instantaneously source and sink

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