Kit 166V2.
Bidirectional DC Motor Speed Controller
Documentation date: September 19, 2008 and reduces torque. The current drawn by the motor
increases as the load on the motor increases. More
This kit controls the speed of a DC motor in both the
current means a larger voltage drop across the series
forward and reverse direction. The range of control is
resistor and therefore less voltage to the motor. The
from fully OFF to fully ON in both directions.
motor now tries to draw even more current, resulting
in the motor "stalling".
Normally, switches are used to change the direction of
rotation of a DC motor. Change the polarity of the applied 3. By applying the full supply voltage to the motor in
voltage and the motor spins the other way! However this bursts or pulses, eliminating the series dropping
has the disadvantage that a DPDT switch has to be added effect. This is called pulse width modulation
to change the polarity of the applied voltage. Now you (PWM) and is the method used in this kit. Short
have two things to control the motor – a direction switch pulses means the motor runs slowly; longer pulses
as well as the speed control. make the motor run faster.
Also, it is not a good idea to suddenly reverse the voltage KIT ASSEMBLY
on a DC motor while it is spinning. It can cause a current Check the components supplied in the kit against the
surge that can burn out the speed controller. Not to parts list. In particular identify the IRFZ44 and IRF4905
mention any mechanical stress it can cause as well. MOSFETs. They look the same so do not get them mixed.
This kit overcomes both these problems. The direction NOTE: Provision is made on the PCB for some zener
and speed is controlled using a single potentiometer. diodes, labelled Z1-4. These are no longer required
Turning the pot in one direction causes the motor to start and have NOT been supplied.
spinning. Turning the pot in the other direction causes the
motor to spin in the opposite direction. The center Before mounting any components to the PCB we need to
position on the pot is OFF, forcing the motor to slow and assemble the MOSFETs to the heatsinks. Take an IRFZ44
stop before changing direction. and IRF4905 MOSFET and fit to either side of a heatsink.
Loosely secure them together using the supplied 3mm
SPECIFICATIONS screw and nut.
Voltage: The kit and motor use the same power supply.
Since the maximum operating voltage of the LM324 is The MOSFETs need to be perfectly in line with the
32VDC then this is also the maximum voltage available heatsink. The easiest way to do this is to mount the whole
to run the motor. assembly onto the PCB, making sure that the heatsink
Current: The IRFZ44 MOSFET can handle 49A; the pins and MOSFET leads fit into their respective holes.
IRF4905 can handle 74A. However the PCB tracks that Don’t solder anything.
run from the MOSFET pins to the screw terminal block
can only handle around 5A. This can be increased by Make sure the heatsink is sitting right down onto the PCB
soldering wire links across the PCB tracks. If you do then then tighten the screw and nut. Repeat for the other
check that the MOSFETs don’t get too hot – if so then assembly then put them aside. They will be the last items
bigger heatsinks will be required. fitted to the PCB.
However the real limiting factor is how fast the It is recommended that components be inserted and
MOSFETs are switched. Most of the power dissipation in soldered in the following order:
a MOSFET occurs when in its linear region. Therefore 1. All the resistors and diodes.
the transistion from ON to OFF (or OFF to ON) should be
Note: The PCB has provision for some zener diodes
as fast as possible. MOSFETs have high gate capacitance
labeled Z1-4. These are no longer required and have
so, to overcome this, they should be driven by a low
not been supplied.
impedance source. The gate drive circuitry used in this kit
Note: There are two (2) pads on the PCB that need to
does not have a low enough impedance to do this.
be filled with solder to improve the through
connection. One is marked ‘FILL WITH SOLDER’.
The overall effect is that the maximum current this kit
The second one is marked ‘SOLDER BOTH SIDES’.
can handle is around 10 amps.
This is one end of zener diode Z2 which is no longer
required. Make sure these two pads are filled with
SPEED CONTROL OF DC MOTORS
solder as required.
Basically, there are three ways to vary the speed of DC
motors: 2. The 14 pin IC socket
1. With the use of mechanical gears to achieve the 3. Capacitor C3. This fits inside the IC socket. Make
desired speed. This method is generally beyond the sure it doesn’t poke up too high before soldering
capability of most hobbyist home workshops. otherwise it will interfere with inserting the IC into
the socket.
2. Reducing the motor voltage with a series resistor.
However this is inefficient (energy wasted in resistor) 4. Transistors Q1 and Q2 and capacitor C1.
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Kit 166V2. Bidirectional DC Motor Speed Controller
5. The electrolytic capacitor C2. the DC position of the triangle wave accordingly. Shifting
6. The 2-way screw terminal blocks. These should be the triangle wave up causes comparator IC1:D to trigger;
joined together to make a 4-way block before lowering it causes comparator IC1:C to trigger. When the
inserting into the PCB. voltage level of the triangle wave is between the two
voltage references then neither comparator is triggered.
7. Potentiometer P1
8. The previously assembled heatsink/MOSFET The DC offset voltage is controlled by the potentiometer
modules. Make sure they are fitted the right way P1 via IC1:A, which is configured as a voltage follower.
around. The IRFZ44 should be facing towards the This provides a low output impedance voltage source,
screw terminals. Remember to solder the heatsink making the DC offset voltage less susceptible to the
pins to the PCB – this is necessary for mechanical loading effect of IC1:B. As the ‘pot’ is turned the DC
strength. offset voltage changes, either up or down depending on
9. Fit the LM324 to the IC socket. the direction the pot is turned.
There is one last thing to do. There is one via (pin Diode D3 provides reverse polarity protection for the
through) on the PCB that needs to be filled with solder. It controller. Resistor R15 and capacitor C2 are a simple
is just above Q6. It is marked with the words “FILL low pass filter. This is designed to filter out any voltage
WITH SOLDER”. This must be done so that the via can spikes caused by the MOSFETs as they switch to supply
handle the current. power to the motor.
HOW IT WORKS (refer to schematic) HOW TO CONNECT
The circuit can be broken down in four parts: The motor connects to the M1 and M2 terminals.
1. Motor control – IC1:A The power supply connects to the V+ and GND
2. Triangle wave generator – IC1:B terminals.
3. Voltage comparators – IC1:C and D
4. Motor drive – Q3-6 TROUBLESHOOTING
Most faults are due to assembly or soldering errors. Verify
Let’s start with the motor drive section, based around that you have the right components in the right place.
MOSFETs Q3-6. Only two of these MOSFETs are on at
any one time. When Q3 and Q6 are ON then current Inspect your work carefully under a bright light. The
flows through the motor and it spins in one direction. solder joints should have a ‘shiny’ look about them.
When Q4 and Q5 are ON the current flow is reversed and Check that there are no solder bridges between adjacent
the motor spins in the opposite direction. IC1:C and pads.
IC1:D control which MOSFETs are turned on.
Check that no IC pins are bent up under the body of the
Opamps IC1:C and IC1:D are configured as voltage IC. This can sometimes happen when inserting ICs into
comparators. The reference voltage that each triggers at is sockets.
derived from the resistor voltage divider of R6, R7 and
R8. Note that the reference voltage for IC1:D is connected
to the ‘+’ input but for IC1:C it is connected to the ‘-‘
input. Therefore IC1:D is triggered by a voltage greater
than its reference whereas IC1:C is triggered by a voltage
less than its reference.
Opamp IC1:B is set up as a triangle wave generator and
provides the trigger signal for the voltage comparators.
The frequency is approximately the inverse of the time
constant of R5 and C1 – 270Hz for the values used.
Reducing R5 or C1 will increase the frequency;
increasing either will decrease the frequency.
The peak-to-peak output level of the triangle wave is less
than the difference between the two voltage references.
Therefore it is impossible for both comparators to be
triggered simultaneously. Otherwise all four MOSFETs
would conduct, causing a short circuit that would destroy
them.
The triangle waveform is centered around a DC offset
voltage. Raising or lowering the offset voltage changes
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Kit 166V2. Bidirectional DC Motor Speed Controller
PARTS LIST – K166V2
Resistors (0.25W carbon film unless specified)
100R................................R15 .................................... 1
4K7..................................R9,14 ................................. 2
10K..................................R2,7,10,11,12,13................ 6
12K..................................R8...................................... 1
33K..................................R6...................................... 1
47K..................................R3...................................... 1
100K................................R1...................................... 1
220K................................R4...................................... 1
470K................................R5...................................... 1
100K potentiometer..........P1 ...................................... 1
PCB mounting
Capacitors
10nF 63V box poly...........C1...................................... 1
100nF mono, 0.1” ............C3...................................... 1
100uF 63V electrolytic .....C2...................................... 1
Semiconductors
1N4004............................D3...................................... 1
1N4148............................D1,2................................... 2
BC547 transistor ..............Q1,2................................... 2
IRF4905 ..........................Q3,5................................... 2
P-channel Power MOSFET
IRFZ44 ............................Q4,6................................... 2
N-channel Power MOSFET
LM324.............................IC1..................................... 1
Quad opamp
Miscellaneous
IC socket, 14 pin, for IC1............................................ 1
Screw terminal block, 2 way........................................ 2
(joined to make a 4-way block)
Heatsinks for MOSFETs Q1-4..................................... 2
Screw, 3 x 8mm .......................................................... 2
Nuts, 3mm .................................................................. 2
K166V2 PCB.............................................................. 1
DATASHEETS
IRFZ44, IRF4905 MOSFETs - www.irf.com
LM324 quad opamp - www.national.com
CONTACT DETAILS
For our full range of kits see our website at NOTE:
http://www.kitsrus.com PCB has provision for some zener diodes labelled
Z1,2,3,4. These are not required any longer and have
Kit developer: http://www.ozitronics.com/ not been supplied.
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Kit 166V2. Bidirectional DC Motor Speed Controller
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