k166v2.pdf

Kit 166V2. Bidirectional DC Motor Speed Controller

Documentation date: September 19, 2008

This kit controls the speed of a DC motor in both the

forward and reverse direction. The range of control is

from fully OFF to fully ON in both directions.

and reduces torque. The current drawn by the motor

increases as the load on the motor increases. More

current means a larger voltage drop across the series

resistor and therefore less voltage to the motor. The

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

voltage and the motor spins the other way! However this

has the disadvantage that a DPDT switch has to be added

to change the polarity of the applied voltage. Now you

have two things to control the motor – a direction switch

as well as the speed control.

By applying the full supply voltage to the motor in

bursts or pulses, eliminating the series dropping

effect. This is called pulse width modulation

(PWM) and is the method used in this kit. Short

pulses means the motor runs slowly; longer pulses

make the motor run faster.

KIT ASSEMBLY

Check the components supplied in the kit against the

parts list. In particular identify the IRFZ44 and IRF4905

MOSFETs. They look the same so do not get them mixed.

Also, it is not a good idea to suddenly reverse the voltage

on a DC motor while it is spinning. It can cause a current

surge that can burn out the speed controller. Not to

mention any mechanical stress it can cause as well.

NOTE: Provision is made on the PCB for some zener

diodes, labelled Z1-4. These are no longer required

and have NOT been supplied.

This kit overcomes both these problems. The direction

and speed is controlled using a single potentiometer.

Turning the pot in one direction causes the motor to start

spinning. Turning the pot in the other direction causes the

motor to spin in the opposite direction. The center

position on the pot is OFF, forcing the motor to slow and

stop before changing direction.

Before mounting any components to the PCB we need to

assemble the MOSFETs to the heatsinks. Take an IRFZ44

and IRF4905 MOSFET and fit to either side of a heatsink.

Loosely secure them together using the supplied 3mm

screw and nut.

SPECIFICATIONS

Voltage: The kit and motor use the same power supply.

Since the maximum operating voltage of the LM324 is

32VDC then this is also the maximum voltage available

to run the motor.

Current: The IRFZ44 MOSFET can handle 49A; the

IRF4905 can handle 74A. However the PCB tracks that

run from the MOSFET pins to the screw terminal block

can only handle around 5A. This can be increased by

soldering wire links across the PCB tracks. If you do then

check that the MOSFETs don’t get too hot – if so then

bigger heatsinks will be required.

The MOSFETs need to be perfectly in line with the

heatsink. The easiest way to do this is to mount the whole

assembly onto the PCB, making sure that the heatsink

pins and MOSFET leads fit into their respective holes.

Don’t solder anything.

Make sure the heatsink is sitting right down onto the PCB

then tighten the screw and nut. Repeat for the other

assembly then put them aside. They will be the last items

fitted to the PCB.

It is recommended that components be inserted and

soldered in the following order:

However the real limiting factor is how fast the

MOSFETs are switched. Most of the power dissipation in

a MOSFET occurs when in its linear region. Therefore

the transistion from ON to OFF (or OFF to ON) should be

as fast as possible. MOSFETs have high gate capacitance

so, to overcome this, they should be driven by a low

impedance source. The gate drive circuitry used in this kit

does not have a low enough impedance to do this.

All the resistors and diodes.

Note: The PCB has provision for some zener diodes

labeled Z1-4. These are no longer required and have

not been supplied.

Note : There are two (2) pads on the PCB that need to

be filled with solder to improve the through

connection. One is marked ‘ FILL WITH SOLDER ’.

The second one is marked ‘ SOLDER BOTH SIDES ’.

This is one end of zener diode Z2 which is no longer

required. Make sure these two pads are filled with

solder as required.

The overall effect is that the maximum current this kit

can handle is around 10 amps.

SPEED CONTROL OF DC MOTORS

Basically, there are three ways to vary the speed of DC

motors:

The 14 pin IC socket

Capacitor C3. This fits inside the IC socket. Make

sure it doesn’t poke up too high before soldering

otherwise it will interfere with inserting the IC into

the socket.

With the use of mechanical gears to achieve the

desired speed. This method is generally beyond the

capability of most hobbyist home workshops.

Reducing the motor voltage with a series resistor.

However this is inefficient (energy wasted in resistor)

Transistors Q1 and Q2 and capacitor C1.

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Kit 166V2. Bidirectional DC Motor Speed Controller

The electrolytic capacitor C2.

the DC position of the triangle wave accordingly. Shifting

the triangle wave up causes comparator IC1:D to trigger;

lowering it causes comparator IC1:C to trigger. When the

voltage level of the triangle wave is between the two

voltage references then neither comparator is triggered.

The 2-way screw terminal blocks. These should be

joined together to make a 4-way block before

inserting into the PCB.

Potentiometer P1

The previously assembled heatsink/MOSFET

modules. Make sure they are fitted the right way

around. The IRFZ44 should be facing towards the

screw terminals. Remember to solder the heatsink

pins to the PCB – this is necessary for mechanical

strength.

The DC offset voltage is controlled by the potentiometer

P1 via IC1:A, which is configured as a voltage follower.

This provides a low output impedance voltage source,

making the DC offset voltage less susceptible to the

loading effect of IC1:B. As the ‘pot’ is turned the DC

offset voltage changes, either up or down depending on

the direction the pot is turned.

Fit the LM324 to the IC socket.

Diode D3 provides reverse polarity protection for the

controller. Resistor R15 and capacitor C2 are a simple

low pass filter. This is designed to filter out any voltage

spikes caused by the MOSFETs as they switch to supply

power to the motor.

There is one last thing to do. There is one via (pin

through) on the PCB that needs to be filled with solder. It

is just above Q6. It is marked with the words “FILL

WITH SOLDER”. This must be done so that the via can

handle the current.

HOW TO CONNECT

The motor connects to the M1 and M2 terminals.

The power supply connects to the V+ and GND

terminals.

HOW IT WORKS (refer to schematic)

The circuit can be broken down in four parts:

Motor control – IC1:A

Triangle wave generator – IC1:B

Voltage comparators – IC1:C and D

TROUBLESHOOTING

Most faults are due to assembly or soldering errors. Verify

that you have the right components in the right place.

Motor drive – Q3-6

Let’s start with the motor drive section, based around

MOSFETs Q3-6. Only two of these MOSFETs are on at

any one time. When Q3 and Q6 are ON then current

flows through the motor and it spins in one direction.

When Q4 and Q5 are ON the current flow is reversed and

the motor spins in the opposite direction. IC1:C and

IC1:D control which MOSFETs are turned on.

Inspect your work carefully under a bright light. The

solder joints should have a ‘shiny’ look about them.

Check that there are no solder bridges between adjacent

pads.

Check that no IC pins are bent up under the body of the

IC. This can sometimes happen when inserting ICs into

sockets.

Opamps IC1:C and IC1:D are configured as voltage

comparators. The reference voltage that each triggers at is

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

Page 2 of 4

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

http://www.kitsrus.com

NOTE:

PCB has provision for some zener diodes labelled

Z1,2,3,4. These are not required any longer and have

not been supplied.

Kit developer: http://www.ozitronics.com/

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Kit 166V2. Bidirectional DC Motor Speed Controller

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