PWM Motor/Light Controller
Pulse Width Modulator for 12 and 24 Volt applications
INTRODUCTION
A pulse width modulator (PWM) is a device that may be used as an efficient light dimmer or DC motor speed controller. The circuit described here is a general purpose device that can control DC devices which draw up to a few amps of current.
The circuit may be used in 12 Volt and 24 Volt systems with a few minor changes. This device has been used to control the brightness of an automotive tail lamp and as a motor speed control for small DC fans of the type used in computer power supplies. A PWM circuit works by making a square wave with a variable on-to-off ratio, the average on time may be varied from 0 to 100 percent. In this manner, a variable amount of power is transferred to the load. The main advantage of a PWM circuit over a resistive power controller is the efficiency, at a 50% level, the PWM will use about 50% of full power, almost all of which is transferred to the load, a resistive controller at 50% load power would consume about 71% of full power, 50% of the power goes to the load and the other 21% is wasted heating the dropping resistor. Load efficiency is almost always a critical factor in alternative energy systems. An additional advantage of pulse width modulation is that the pulses are at the full supply voltage and will produce more torque in a motor by being able to overcome the internal motor resistances more easily. Finally, in a PWM circuit, common small potentiometers may be used to control a wide variety of loads whereas large and expensive high power variable resistors are needed for resistive controllers. The main Disadvantages of PWM circuits are the added complexity and the possibility of generating radio frequency interference (RFI). RFI may be minimized by locating the controller near the load, using short leads, and in some cases, using additional filtering on the power supply leads. This circuit has some RFI bypassing and produced minimal interference with an AM radio that was located under a foot away. If additional filtering is needed, a car radio line choke may be placed in series with the DC power input, be sure not to exceed the current rating of the choke.
Parts
U1:LM324N quad op-amp
U2:78L12 12 volt regulator
Q1:IRF521 N channel MosFet
D1:1N4004 silicon diode
LED1 Red LED
C1: 0.01uF ceramic disc capacitor, 25V
C2-C5:0.1uF ceramic disk capacitor, 50V
R1-R4:100K 1/4W resistor
R5:47K 1/4W resistor
R6-R7:3.9K 1/4W resistor
R8:2.7K 1/4W resistor
VR1:10K linear potentiometer
F1:3 Amp, 28V DC fast blow fuse
S1:toggle switch, 5 Amps
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