The L298 Motor Driver

The L298 Motor Driver Kit allows the user to safely interface two DC
motors to a host microcontroller using only 4 control lines. The motor
driver isolates the host and controls continuous currents of up to 2
Amps per motor (4 Amps total). Short peaks (spikes) up to 3 Amps per
motor can be tolerated without damage.
While ideally suited for use with a microcontroller, the kit may also be
used with just about any form of 0-5 Volt signal (i.e. manual switches,
TTL logic gates, relays etc).
Assembling the Board
Assembly of the kit is straight forward; the silkscreen (white printing on
the circuit board) shows where each component belongs and it’s
orientation. Make sure you install each diode so that the white ring on
the diode is aligned with the white bar on the diode symbol on the
silkscreen. The diodes are sometimes a tight fit in their holes and you
might find a pair of small pliers useful in installing them.
This kit now comes with a 78L05 +5 Volt voltage regulator. The
regulator takes power from the Motor Battery terminals and steps it
down to +5 Volts for the logic circuits of the L298N chip (this assumes
that your Motor Battery voltage is at least 6 Volts). If you are using a
Motor Battery that is less than 6 Volts, the 78L05 will not be able to
supply the necessary +5 volts to the L298. In this case, you will need to
supply this voltage to the Logic Battery terminals yourself (if you do
this, do not install the 78L05 and remove it if it is already installed). If
you have doubts, measure the voltage across the Logic Battery
terminals while both motors are running -there should be a minimum of
+4.5 Volts. If there is less, the L298N may behave erratically.
The 0.01uF tantalum capacitors (marked “103”) are used to suppress
transients (spikes).
The kit includes 2 black terminal blocks. These terminal blocks provide
an easy and conveneint way to connect to the L298 kit; simply ‘poke’ a
solid wire into the terminal holes to make a connection. If you want a
more permanent connection, you can choose not to install the terminal
blocks and solder your control lines directly to the board.
A Bit About Motor Current
Motors are inductive devices; they draw much more current at startup
than when they are running at a steady speed. Before connecting any
motor to the L298 you should know a few things about the motor:
- What voltage it is designed to work at
- How much current it draws when running (unloaded)
- How much curent it draws at stall.
The “stall current” is the current the motor draws when you stop (stall)
the output shaft (if you can). Stalling a motor is very hard on the motor
and can burn open the motor windings and ruin the motor. If you want
to test for stall current, grab the output shaft with your hand while
measuring the current drawn. As the motor approaches stall, the
current will climb.
The L298N can safely handle 2 Amps of continuous current for each
motor. Short surges up to 3 Amps as the motor starts can be tolerated.
A heatsink would be a good idea in situations that see the current surge
above 2 Amps.
Connecting the Motors
A DC motor has 2 terminals on it. If you take the positive and negative
leads from a power source (battery, power supply etc.) and connect
them to the terminals of the motor, the motor will spin in one direction. If
you swap the connections, the motor will spin in the opposite direction.
You will want to wire your motors to the L298 board in such a way that
the motor spins in the direction you call ‘forward’ when the Fwd line is
activated and ‘reverse’ when the Back line activated.
When connecting the motors to the circuit board, use as thick a wire as
is practical. The thicker the wire, the less the voltage drop and the more
power is delivered to the motor. We recommend a minimum of 18
gauge stranded wire. Solid wire is fine, but will break if flexed too often.
Solder the Positive and negative motor leads for the ‘left’ motor to
Motor Left + and - solder pads. Repeat for the ‘right’ motor.
Connect the + and - leads from your motor battery to the Motor
Battery + and - solder pads -again, use as thick a wire as it practical.
A Note on ‘Motor Battery’ Voltages: There is a 1.4V drop associated
with the L298. This means that if your motor runs on 12 VDC, you
should use a 13.4 VDC supply in order to get full power to the motor.
Operation
Control is accomplished by grounding the control pin(s) for the desired
function/motor. This is usually done by putting a logic LOW (i.e. 0V or
ground) on an output pin of your host microcontroller, which is in turn
connected to the appropriate control connection (Fwd, Back, Enable).
Fwd
To make a motor move forward, ground the ‘Fwd’ connection on the
appropriate connector (left / right). To stop, un-ground the connection.
The photo below shows how both motors would be made to go
‘forward’.
Back (Reverse)
To drive the motor in reverse, ground the appropriate Back connection.
To stop, unground the connection.
Gnd
Gnd is a ground connection. The L298 Controller board and your control
circuit must share the same gound.
Enable
The Enable connection is an active LOW connection that is pulled
HIGH for you on the circuit board. Ground this connection to disable a
motor. Note: While disabled, commands from the host microcontroller
(grounding/un-grounding the control connections) will have no effect on
that motor. Disabling a motor shuts-down the L298’s internal circuits,
putting it into a low current consumption mode (for that channel). When
both channels are disabled, the motor controller will consume approx.
10 mA.
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Pulse-Width Modulation (PWM)
Pulse-Width Modulation is a method of controlling the speed of a motor
by turning the power on and off at varying speeds. If you have a 12 Volt
motor and turn it on 50% of the time and turn it off 50% of the time
(switching it at several KHz) then the effective voltage you are applying
to the motor is 6 Volts. As the voltage to the motor varies, so does its’
speed.
It should be noted that the L298 Kit was not intended for high-speed
PWM operation. The diodes in the kits are general purpose rectifier
diodes intended for 60 Hz operation. If you want to experiment with
PWM and you don’t get the kind of results you want, try exchanging the
diodes for ‘fast recovery Schottky’ models.
Heat Sinks
The L298 has internal thermal protection circuitry that shuts down the
chip if it becomes too hot (when you try to draw too much current). If
you find this happening, you should add some kind of heat sink to the
L298. A simple heat sink can be made from a piece of scrap aluminum
by cutting as big a piece as you have room for and drilling a 1/8” hole
into it (for a bolt to hold it to the L298). If available, a thin smear of
thermal compound (white, greasy stuff, available at Radio Shack) on
the back of the L298’s metal tab (i.e. between it and the heat sink) will
maximize heat dissipation. Of course, you should always make sure
that air can circulate freely around the L298 and it’s heatsink.
NOTE: If a motor behaves erratically i.e. turning on and off rapidly, it is
likely the L298 senses that it is being overloaded. This usually means
that you are drawing too much current. Either add a heat sink to the
L298 and/or reduce the current being drawn.
Caution: The L298’s heat sink (the metal tab) is at ground potential. Do
not allow any ground-referenced voltage source to touch it or any heat
sink connected to it, or you will cause a short.
H-Bridge Theory
Figure 1 shows the basic schematic for a typical H-Bridge along with it’s
truth table. In order to make a motor turn, we need to apply a voltage to
it. We do this by turning certain NPN transistors on. By looking at the
truth table, we can see that in order to make a motor go forward
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(NOTE: ‘Forward and ‘Reverse’ are arbitrary directions for purposes of
illustration. In your application, forward and reverse will be determined
by how the motors are mounted with respect to each other and the
polarity of the voltage) we must turn on Q1 and Q4. This puts the Motor
Battery Positive on the left side of the motor (through Q1) and grounds
the other side of the motor (through Q4).
To go in the opposite direction, we must turn off these transistors and
turn on Q2 and Q3. Now, the Motor Battery Positive will be on the right
side of the motor (through Q3) and ground is on the left (through Q2).
You have now reversed the polarity of the motor’s supply voltage and
the motor will spin in the opposite direction.
Figure 1.
You will notice that each time a motor is turned on, current passes
through 2 NPN transistors. Each transistor has (approximately) 0.7 Volt
drop across it, so the motor will see about 1.4 Volts LESS than the
Motor Battery Voltage across it’s terminals. This means that if you have
a 12 Volt motor, and you want it to receive maximum power, you should
use a 13.4 Volt battery.
Also notice that if transistors Q1 and Q2 (or Q3 and Q4) were turned
on, that you would make a short circuit across the battery. For this
reason, the L298N has internal logic that prevents this from happening.
Further Information
A complete datasheet for the L298N can be found on the HVW
Technologies web site, on the L298 product page
FWD REV STOP
Q1 1 0 0
Q2 0 1 0
Q3 0 1 0
Q4 1 0 0
Truth Table
Motor Battery +
( Motor Batte ry - )
Motor
Q4
Q3
Q2
Q1
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Specifications
Max supply voltage: 46 V
Max current (per channel): 2 A (DC); Non-repetitive (t=100 uS): 3A;
Repetitive (80% on, 20% off, ton=10 ms): 2.5 A
Total Power Dissipation: 25W