PWM Dimmer/Motor Speed Controller


This is yet another
project born of necessity. It’s a simple circuit, but does exactly what
it’s designed to do – dim LED lights or control the speed of 12V DC motors. The circuit uses PWM to regulate the effective or average current through the LED
array, 12V incandescent lamp (such as a car headlight bulb) or DC
motor. The only difference between the two modes of operation is the
addition of a power diode for motor speed control, although a small
diode should be used for dimmers too, in case long leads are used which
will create an inductive back EMF when the MOSFET switches off.

Photo of Completed PWM Dimmer/Speed Control

Photo of Completed PWM Dimmer/Speed Control

The photo shows what a completed board looks like. Dimensions are 53
× 37mm, so it’s possible to install it into quite small spaces. The
parts used are readily available, and many subsitiutions are available
for both the MOSFET and power diode (the
latter is only needed for motor speed control). The opamps should not be
substituted, because the ones used were chosen for low power and their
ability to swing the output to the negative supply rail.

Note that if used as a motor speed controller, there is no feedback,
so motor speed will change with load. For many applications where DC
motors are used, constant speed regardless of load is not needed or
desirable, but it is up to you to decide if this will suit your needs.

Description

Note that if used as a motor speed controller, there is no feedback,
so motor speed will change with load. For many applications where DC
motors are used, constant speed regardless of load is not needed or
desirable, but it is up to you to decide if this will suit your needs.

PWM Waveform Generation

PWM Waveform Generation

Figure 2 shows how the PWM principle
works. The red trace is the triangle wave reference voltage, and the
green trace is the voltage from the pot. When the input voltage is
greater than the reference voltage, the MOSFET
turns on, and current flows in the load. Because the frequency is
relatively high (about 600Hz), we don’t see any flicker from the LEDs, but the tone is audible from a motor that’s PWM controlled. The PWM
signal is shown in blue. The average current through the load is
determined by the ratio of on-time to off-time, and when both are equal,
the average current is exactly half of that which would be drawn with
DC.

Dimmer/Speed Controller Schematic

Dimmer/Speed Controller Schematic

The circuit is shown in Figure e. U1 is the oscillator, and
generates a triangular waveform. R4 and R5 simply set a half voltage
reference, so the opamps can function around a 6V centre voltage. U2A is
an amplifier, and its output is a 10V peak to peak triangle wave that
is used by the comparator based on U2B. This circuit compares the
voltage from the pot with the triangle wave. If the input voltage is at
zero, the comparator’s output remains low, and the MOSFET is off. This is the zero setting.

In reality, the reference triangle waveform is from a minimum of
about 1.5V to a maximum of 9.5V, so there is a small section at each end
of the pot’s rotation where nothing happens. This is normal and
practical, since we want a well defined off and maximum setting. Because
of this range, for lighting applications, an industry standard 0-10V DC
control signal can be used to set the light level. C-BUS (as well as many other home automation systems) can provide 0-10V modules that can control the dimmer.

While a 1N4004 diode is shown for D2, this is only suitable if the
unit is used as a dimmer. For motor speed control, a high-current fast
recovery diode is needed, such as a HFA15TB60PBF ultra-fast HEXFRED
diode. There are many possibilities for the diode, so you can use
whatever is readily available that has suitable ratings. The diode
should be rated for at least half the full load current of the motor,
and the HFA15TB60PBF suggested is good for 15A continuous, so is fine
with motors drawing up to 30A.

Construction

While it’s certainly possible to build the dimmer on veroboard or
similar, it’s rather fiddly to make and mistakes are easily made. Also,
be aware that because of the current the circuit can handle, you will
need to use thick wires to reinforce some of the thin tracks. This is
even necessary for the PCB version. Naturally, I recommend the PCB, and this is available from ESP. The board is small – 53 × 37mm, and it carries everything, including the screw terminals. The PCB is double-sided with plated-through holes, and has solder masks on both sides.

The MOSFET will need a heatsink unless you are using the dimmer for light loads only. It is necessary to insulate the MOSFET from the heatsink in most cases, since the case of the transistor is the drain (PWM
output). For use at high current and possible high temperatures, the
heatsink may need to be larger than expected. Although the MOSFET should normally only dissipate about 2W or so at 10A, it will dissipate a lot more if it’s allowed to get hot. Switching MOSFETs will cheerfully go into thermal runaway and self destruct if they have inadequate heatsinking. You may also use an IGBT
(insulated gate bipolar transistor) – most should have the same
pinouts, and they do not suffer from the same thermal runaway problem as
MOSFETs.

As noted above, there are many different MOSFETs (or IGBTs) and fast diodes that are usable. The IRF540 MOSFET
is a good choice, and being rated 27A it has a generous safety margin.
There are many others that are equally suitable – in fact any switching MOSFET rated at 10A or more, and with a maximum voltage of more than 20V is quite ok.

Testing

Connect to a suitable 12V power supply. When powering up for the
first time, use a 100 ohm “safety” resisor in series with the positive
supply to limit the current if you have made a mistake in the wiring.
The total current drain is about 2.5mA with the pot fully off, rising to
12.5mA when fully on. Most of this current is in the LED, which is also fed from the PWM supply so you can see that everything is working without having to connect a load.

Make sure that the pot is fully anti-clockwise (minimum), and apply
power. You should measure no more than 0.25V across the safety resistor,
rising to 1.25V with the pot at maximum. If satisfactory, remove the
safety resistor and install a load. High intensity LED
strip lights can draw up to ~1.5A each, and this dimmer should be able
to drive up to 10 of them, depending on the capabilities of the power
supply and the size of the heatsink for the MOSFET.

source: http://sound.westhost.com/project126.htm

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