This page provides basic information about voltage comparator
integrated circuits and is to act as reference material for other circuits. The
circuits shown are based on the LM339 Quad Voltage Comparator chip or the LM393
Dual Voltage Comparator chip. These devices are functionally identical. The
LM311 Voltage Comparator can be used for these applications as well and also has
a number of unique features.
Internal Circuitry For 1/4 Of An LM339
Comparator Equivalent
Single Supply Comparator Equivalent schematic
An integrated circuit "Voltage Comparator" is equivalent to an
Operational Amplifier, Such as the LM358 or LM324, with two NPN transistors
added to the output of each amplifier. (Refer to the above schematic.) This
arrangement produces an "Open Collector" output for each of the four comparators
in an LM339 chip. Each output can sink 15 Milliamps and can withstand voltages
of up to 50 Volts.
The output is switched ON or OFF depending on the relative voltages at
the PLUS and MINUS inputs of the comparator, see the rules below. The inputs are
quite sensitive and a difference of only a few millivolts between the two will
cause the output to turn on or off.
The LM339, LM393 and LM311 comparator chips can operate from a single
or dual power supply of up to 32 volts maximum.
When operated from Dual or Split power supplies the basic operation of
comparator chips is unchanged except that for most devices the emitter of the
output transistor is connected to the negative supply rail and not the circuit
common. An exception to this is the LM311 which has a separate emitter terminal
that can be connected to either.
Dual Supply Comparator Equivalent schematic
When operated from Dual or Split power supplies the input voltages can
be above or below the common or zero voltage of the supply. If needed, one of
the inputs can be connected to the common so that a 'Zero Crossing' detector is
created.
Comparator Operation
The following drawing show the two simplest configurations for voltage
comparators. The diagrams below the circuits give the output results in a
graphical form.
For these circuits the REFERENCE voltage is fixed at one-half
of the supply voltage while the INPUT voltage is variable from zero to the
supply voltage.
In theory the REFERENCE and INPUT voltages can be anywhere between
zero and the supply voltage but there are practical limitations on the actual
range depending on the particular device used.
Basic Comparator Operation
Input Vs. Output Results
- Current WILL flow through the open collector when the voltage at
the PLUS input is lower than the voltage at the MINUS input.
- Current WILL NOT flow through the open collector when the voltage
at the PLUS input is higher than the voltage at the MINUS input.
Input Vs. Output Results
Input Offset Voltage
Voltage comparators are not perfect devices and in some instances may
suffer from the effects of a parameter known as the Input Offset Voltage. This
problem normally occurs when the Input voltage changes very slowly. The Input
Offset Voltage for many comparators is only a few millivolts and in most
circuits it can be ignored.
The net result of the Input Offset Voltage is that the output
transistor does not fully turn on or off when the input voltage is close to the
reference voltage.
The following diagram attempts to illustrate the effect of the input
offset voltage with a slowly changing input voltage. This effect increases as
the output transistor current increases so keeping the value of RL high will
help reduce the problem.
Effect Of Input Offset Voltage
Input Offset Voltage And Hysterysis
The effects of the input offset voltage can be countered by adding
hysterysis to the circuit. This causes the reference voltage to change when the
comparators output goes high or low.
The effect is that as the input voltage slowly changes the reference
voltage will quickly change in the opposite direction. This gives the comparator
a "snap" action. See the following paragraphs for more information.
An Explanation of Input Hysterysis Voltage
Hysteresis is the difference between the input signal levels at which
a comparator turns off and turns on. A small amount of hysteresis can be useful
in a comparator circuit because it reduces the circuit's sensitivity to noise,
and helps reduce multiple transitions at the output when changing state.
Sometimes, in a discrete design, there is a need to add an external
resistor between the comparator's output and the positive input, creating a weak
positive feedback loop. When the output makes a transition, the positive
feedback slightly changes the positive input so as to reinforce the output
change.
A mechanical analog of this effect can be found in many electrical
switches. As you move the handle just past the center point, a spring in the
switch will try to pull the handle all the way over, ensuring that the switch
ends up in a definite ON or OFF state.
Input Hysterysis
The diagram above shows a hysteresis 'loop' that describes how a
comparator functions. The horizontal 'X' axis is the input, and represents the
difference of the two input voltages. The vertical "Y" axis represents the
comparator's output state.
If the comparator is initially 'OFF', the MINUS input voltage has to
become slightly above the PLUS input voltage before the comparator output turns
'ON'. This is represented by moving right along the bottom part of the loop.
Once the comparator is 'ON', the MINUS input voltage needs to drop
slightly below the PLUS input voltage before it turns 'OFF' again (moving left
along the top of the loop).
The width of the loop outlined by an off-on-off cycle is the input
hysteresis voltage.
The hysteresis voltage for most comparators is in the millivolt range
and usually only affects circuits where the input voltage rises or falls very
slowly or has voltage spikes knoown as "noise".
The Hysterysis voltage range can be increased if needed to help when
the input voltage is noisey so the the output does not change states
unnecessarily. The FLIP-FLOP circuits shown later on this page make use of an
exaggerated hysterysis to create the memory effect.
Increasing The Input Hysterysis Range
Voltage Window Detector Circuit
Comparators with Open Collector outputs such as the LM339 or LM393
must be configured so the both outputs are HIGH when the voltage is within the
desired limits. The LM311 comparator can have other output arrangements as it
has both an open collector and open emitter on the output transistor.
Window Comparator
Comparator Oscillator Circuit
Comparators can also be used as oscillators but are not well suited
for this type of application.
Oscillator Made From A Comparator
Using An OPAMP As A Comparator
If there are Operational Amplifiers left over in a circuit and a
Comparator is needed one can be created by adding a diode or transistor to the
amplifiers output depending on the current capacity required.
Comparator Made From An Operational Amplifier
Basic Comparator Circuits
The following diagrams are of some basic comparator circuits. Most
have a Cadmium Sulfide photocell input but could just as easily use a
phototransistor or a voltage signal from another circuit as an input. The
resistance values are not critical but should be used as a guide. In most
comparator circuits the ratio of the resistances is more important than their
actual values.
Photocell Circuits
Photocell Circuits Schematic
If higher current loads are to be driven a PNP transistor can be added
to the comparators output this will allow loads of up to 300Ma. to be
controlled.
Relay Driver Output Schematic
Time Delay Circuits
Short timing functions such as a pulsed outputs or time delays can
also be created with one or two comparator sections.
Comparator Time Delay Schematic 1
Notice that the second comparator section in the time delay circuit
shares the same reference voltage input as the first. In most cases any number
of comparators can have the same voltage source at one input, this can make
circuits much less complicated.
More Delay circuits.
Comparator Timer Delay Schematic 2
Basic Memory Functions
Comparators can be made to perform a basic memory function by wiring
them as a 'SET / RESET' type of FLIP/FLOP. This type of circuit can be used in
unplugable walk around throttles to remember the direction of the train when the
controller is disconnected. In the next diagram the comparator will remember
which switch was pushed last. If the 'SET' button is pushed the LED will be on,
the 'RESET' button will turn the LED off. A higher current version is also
shown.
Comparator FLIP/FLOP Schematic
How the Flip Flop works. A very basic description.
- When the output of the comparator is off the voltage at the PLUS
input will be the same as the supply voltage. With the PLUS input voltage
higher than the MINUS input voltage the output will remain off.
- When the SET button is pushed the voltage at the PLUS input will go
to zero and the output will turn on.
- When the SET button is release the voltage at the PLUS input will
rise to 1/2 of the supply voltage and the output will remain turned on because
the voltage at the PLUS input is remains below the voltage at the MINUS input.
- When the RESET button is pressed voltage at the MINUS input will go
to zero from its normal level of 3/4 of the supply voltage. The output will
turn off because the voltage at the MINUS input is below the voltage at the
PLUS input. When the output turns off the voltage at the PLUS input will rise
to the supply voltage level.
- When the RESET button is released the voltage at the MINUS input
will rise to 3/4 of the supply voltage. The PLUS input voltage will stay above
the voltage at the MINUS input and the output will stay turned off.
Open Collector Output Transistors
Because the output transistor of the comparator has an open collector
the supply and load voltages do not have to be the same. This means that the
comparator could use a 12 Volt power supply while the load could be a 24 Volt
relay or 5 Volt LED circuit.
The following three diagrams are some examples of Dual voltage
circuits. In the first two the voltage at the output of the comparators could
even be full wave direct current.
Dual Voltage Output Schematic
Dual Voltage Relay Driver Output Schematic
Dual Voltage FLIP/FLOP Schematic
4 Level - voltage Detector
This circuit can sequentially indicate 4 separate voltage levels. The
voltages are determined by the values of resistors R1 through R5 arranged in a
simple voltage divider circuit.
4 Level Detector Schematic
Credit to: http://www.bristolwatch.com/
