EEPC: Basic Design

Electronic Engine Performance Controller

Basic Design

This page is devoted to a simple electronic circuit that is used as the center of a series of Electronic Engine Performance Controller (EEPC) applications. It consists of a basic LM339 quad comparator with some support circuitry to make it stable. The comparator compares a DC input signal to a preset threshold and produces an output dependent on how you configure it. This configuration will be discussed in detail below. Be sure to read the entire contents of these pages.

Description

The idea behind these circuits is to take a DC input voltage from a vehicle sensor (MAP, TPS, O2, etc) and make decisions based on their value. The reference voltage for each comparator has a range of 0.0V to 5.1V, which covers the output range of the before mentioned sensors. A higher reference voltage can be used if a wider range is required, but realize that a high reference voltage decreases the accuracy to which the reference can be adjusted. The comparators output is either 0V or 8V, which is used to control a driver transistor. The comparator can be configured two ways: as a positive response or a negative response circuit.

As a positive response circuit, the output of the circuit is on, when the input voltage is higher than the reference voltage. Here is the schematic that shows one of the four comparators inside an LM339 configured as a positive response comparator:

(click here for a larger image)

As a negative response circuit, the output is on when the input voltage drops below the reference voltage. Here is the schematic:

(click here for a larger image)

As you can see, the circuitry requires two power supplies: an 8V supply to power the comparator and a 5V supply for the reference. The seperate power supply is important to make the reference very stable to prevent oscillation. This is done with a simple 5.1V zener diode and resistor power supply, which can be used for all the references in the circuit. The schematic below shows an example all four comparators in the LM339 being used. Two (1 and 4) are setup as positive response circuits, while the other two (2 and 3) are negative response. It is layed out to show the physical pin locations on the chip. It also shows the 5V power supply for the reference (D1 and R5), but the 8V power supply and the driver transistors are not shown. Also missing is the input and output protection resistors and diodes, but these are included in the application schematics and are also shown and explained in a later section.

(click here for a larger image)

Theory Of Operation

The way the comparators work has already been discussed, but here I will explain why the various compnents are there:

R1 - R4: These are the reference voltage adjustments.
R5, D1: These provide the 5.1V reference voltage. A 270 ohm R5 provides about 11mA of current, which is more than enough for 4 comparator references. If your circuit has many more comparators or uses different pots (R1-R4), you may need to drop the value of R5 to increase the supply current, but each comparator and reference (if 47k pots are used) only requires about 0.4mA of current.
R7 - R10: These are in place to limit the input current to the comparator so that some feedback can be provided for hysteresis
R11 - R14: These are the feedback resistors that provide hysteresis to each comparator to prevent oscillation when the input voltage is very close to the reference voltage. With R7-R10 set to 10k and R11-R14 set to 1M, the total hysteresis is about 81.2mV (0.0812V), which is small enough to still provide very good response. If your input signal is noisey and the comparator oscillates, you can decrease value of the feedback resistor to increase the hysteresis voltage.
C1: This is a filter capacitor that is used to keep the LM339 chip from oscillating due to noise on the power supply. Each LM339 chip has to have its own filter capacitor and it should be put very close to the Vcc pin (pin 3).

Input/Output Protection

The protection circuitry for the comparators is an important part of it. In addition to the very noisy power in an automobile, many of the sensor signals that will go into this circuit are also subject to noise and voltage spikes. As such, some input protection is required. The circuit below is added to the signal input of the comparator circuits above. The series resistor limits the input current to the circuit, while the two diodes prevent the input signal from going above 8V or below 0V.

The outputs also need to be protected from a phenomena called "lash-back", when they are used to drive solenoids, relays, or motors. When the current to any of these device's is shut off, they return a voltage back to the circuit that is the opposite polarity and high voltage. This occurs because of the collapse of the electromagnetic field inside. A fairly strong diode is connected between the output and the 12V voltage input to suppress the spike. Note that this is NOT connected to the 8V supply voltage, but rather to the raw 12V from the vehicle's power.

The 8V Power Supply

You have two choices with regards to the 8V power supply. The best choice is to use a 7808 fixed-voltage regulator. These are very stable, easy-to-use, and fairly cheap regulators that can handle a lot of current and even have built-in overload protection. Only two capacitors are needed in addition to the regulator, so the circuit is very simple:

The second, and less desirable choice is another zener diode-type regulator. Although they are very stable and work well for low-current applications, they quickly become very inefficient at higher currents. The required circuit would use an 8.2V zener diode and would require 38 ohms of resistance to provide the necessary current (100mA or greater spikes are likely) at 12V. Since the vehicle's supply voltage can reach close to 15V at times, 1.2W of power can be dissipated through the resistor! Dissipating that much heat will eventually cause the resistor and/or zener diode to fail, causing either a voltage failure or a loss of regulation, respectively. In other words, don't use this method. :)

Parts

You will need to decide what kind of project cases, wire, and connectors to use, depending on which setup you choose. I have designed these circuits so that the parts can be purchased at your local Radio Shack, but I recommend purchasing them through another dealer, such as Mouser or Digikey for better prices.

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This page is maintained by Russell W. Knize and was last updated 04/20/99. Comments? Questions? Email minimopar@myrealbox.com.