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Pulse Width Modulated (PWM) Controller for 12 Volt Motors pdf
#1

Pulse Width Modulated (PWM) Controller for 12 Volt Motors


.pdf   Pulse Width Modulated.pdf (Size: 236.92 KB / Downloads: 118)

INTRODUCTION

This electronic controller is designed to allow a user to vary the speed and power output
of a typical 12 volt motor such as a fuel pump, water injection pump or cooling fan. It
could also be used as a secondary injector controller. Other uses, robots and small
electric scooters and carts. Anywhere a 12 volt DC motor needs to vary speed or power.
This controller circuit allows setting a “Low speed” when full power is not needed and a
“Hi speed” for use when full power is needed. An additional feature included is a
“Progressive” feature that smoothly ramps speed up from Low speed to Hi speed based
on an input signal of 0-5 volts. This circuit will be offered both in kit form and fully
finished.
The inspiration was a request to control the speed of a large positive displacement fuel
pump. The pump was sized to allow full power of a boosted engine in excess of 600 Hp.
At idle or highway cruise, this same engine needs far less fuel yet the pump still normally
supplies the same amount of fuel. As a result the fuel gets recycled back to the fuel tank,
unnecessarily heating the fuel. This PWM controller circuit is intended to run the pump
at a low speed setting during low power and allow full pump speed when needed at high
engine power levels.

Motor Speed Control (Power Control)

Typically when most of us think about controlling the speed of a DC motor we think of
varying the voltage to the motor. This is normally done with a variable resistor and
provides a limited useful range of operation. The operational range is limited for most
applications primarily because torque drops off faster than the voltage drops. Most DC
motors cannot effectively operate with a very low voltage. This method also causes
overheating of the coils and eventual failure of the motor if operated too slowly.
Of course, DC motors have had speed controllers based on varying voltage for years, but
the range of low speed operation had to stay above the failure zone described above.
Additionally, the controlling resistors are large and dissipate a large percentage of energy
in the form of heat.
With the advent of solid state electronics in the 1950’s and 1960’s and this technology
becoming very affordable in the 1970’s & 80’s the use of pulse width modulation (PWM)
became much more practical. The basic concept is to keep the voltage at the full value
(in this case 12 volts) and simply vary the amount of time the voltage is applied to the
motor windings. Most PWM circuits use large transistors to simply allow power On &
Off, like a very fast switch. This sends a steady frequency of pulses into the motor
windings. When full power is needed one pulse ends just as the next pulse begins, 100%
modulation. At lower power settings the pulses are of shorter duration. When the pulse
is On as long as it is Off, the motor is operating at 50% modulation.

PWM Controller Features

This controller offers a basic “Hi Speed” and “Low Speed” setting and has the option to
use a “Progressive” increase between Low and Hi speed.
Low Speed is set with a trim pot inside the controller box. Normally when installing the
controller, this speed will be set depending on the minimum speed/load needed for the
motor. Normally the controller keeps the motor at this Lo Speed except when
Progressive is used and when Hi Speed is commanded (see below). Low Speed can vary
anywhere from 0% PWM to 100%.
Progressive control is commanded by a 0-5 volt input signal. This starts to increase
PWM% from the low speed setting as the 0-5 volt signal climbs. This signal can be
generated from a throttle position sensor, a Mass Air Flow sensor, a Manifold Absolute
Pressure sensor or any other way the user wants to create a 0-5 volt signal. This function
could be set to increase fuel pump power as turbo boost starts to climb (MAP sensor).
Or, if controlling a water injection pump, Low Speed could be set at zero PWM% and as
the TPS signal climbs it could increase PWM%, effectively increasing water flow to the
engine as engine load increases. This controller could even be used as a secondary
injector driver (several injectors could be driven in a batch mode, hi impedance only),
with Progressive control (0-100%) you could control their output for fuel or water with
the 0-5 volt signal. Progressive control adds enormous flexibility to the use of this
controller.

Circuit Specifications

This circuit is intended for use on a typical 12 volt automotive electrical system. Most of
these systems actually run at 13-14 volts. Some race cars use an extra cell in their battery
to achieve a higher voltage. If this will be in excess of 16 volts, we would need to use a
different diode in the power supply portion of the circuit (contact the MYO-P for this
change).
The prototype circuit is intended to run loads that draw up to 20 amps continuously.
Although the main mosfet transistor used is rated at 50 amps, under load, at working
temperature, don’t expect more tha n 20 amps. The wires leading to the mosfet are rated
to 26 amps so there is some margin but please respect the design. The “production”
circuit will have provision for at least a second main transistor and possibly a third
transistor. In this case the load could be 20 amps per transistor or a total of 60 amps.
The PWM controller should be mounted fairly close to the motor to be controlled.
Temperature can be an issue. The die cast aluminum housing that is suggested for use IS
the heat sink for the large mosfet transistor and the voltage regulator. The transistor will
operate up to 150 C internally but most other components on the circuit board are only
good to 70 C (180 F). For use in a very warm environment, specific selection of
components could get the heat tolerance up to about 100 C. Contact the designer if this is
needed. So a cooler location is better, in the fender well rather than on the firewall above
the motor of a car.

Fuse, Reverse Voltage and Voltage Spike Protection

There is protection for reverse polarity and keeping any voltage spikes from leaving the
circuit. The PCB has a 3 amp replaceable fuse mounted on it inside the PWM controller
case. If you test the reverse polarity feature you will most likely blow the fuse. That is
the real purpose of the fuse. Remember, this control circuit uses much less than one amp
for operation. Any voltage spike generated within the control circuit is clipped at 16
volts per normal automotive practice. Any spike trying to enter the circuit from outside
will also be clipped. This 16 volt spike protection is why use with a 14-16 volt battery
will require a different diode (D2).

Actual Circuit Schematic and Function

For the remainder of the discussion the actual circuit schematic will be built, component
by component. The component numbers will be the actual numbers used in the design,
so they won’t necessarily be in order of introduction. The function of various
components will be discussed with regard to this PWM Controller circuit. Much more
detail of “How and Why” for a specific component can be found in data sheets from the
manufacturer. Generic component names are used in this discussion, che cking the
component list for this circuit posted elsewhere on this web site will allow one to retrieve
the actual data sheet from the manufacturer.
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#2
Pulse width modulation (PWM) uses digital signals to control power applications, as well as being fairly easy to convert back to analog with minimal hardware.

Analog systems, such as linear power supplies, tend to generate a lot of heat because they are basically variable resistors that carry a lot of current. Digital systems generally do not generate as much heat. Almost all the heat generated by a switching device is during the transition (which is done quickly), while the device is neither on nor off, but intermediate. This is because the power follows the following formula:

P = E I, or Watts = Voltage X Current

If the voltage or current is near zero, then the power will be close to zero. PWM makes the most of this fact.

PWM can have many of the features of an analog control system, in which the digital signal can be freewheel. PWM does not have to capture data, although there are exceptions to this with the higher end controllers.

One of the parameters of any square wave is the duty cycle. Most square waves are 50%, this is the norm when discussing them, but they do not have to be symmetrical. The on time can be completely varied between the signal being off to be fully on, 0% to 100%, and all ranges between.

Examples of a 10%, 50% and 90% duty cycle are shown below. While the frequency is the same for each one, this is not a requirement.

[Image: 43003.png]
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Marked Categories : pulse modulating 12 volt motor control, pulsed with modulated injection driver, 12 volt pulse width injector control, pulse width modulator for 12 volt automotive application, pwm controler 12 volt fuel pump pulser, motor controller for two different size 12 volt motors,

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