SC_12V_24V_Speed_Controller_Pt1.pdf

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Want to control a really big DC motor? This circuit can

handle 12V or 24V DC motors at currents up to 40A.

This 12V or 24V high-current DC Motor Speed Controller is rated

at up to 40A (continuous) and is suitable for heavy-duty motor

applications. All control tasks are monitored by a microcontroller

and as a result, the list of features is extensive.

.I.

controller is based on a PIC16F88

microcontroller. This micro provides

all the fancy features such as bat

tery monitoring, soft-start and speed

regulation.

also monitors the speed

setting potentiometer and drives a

4-digit display board which includes

two pushbuttons.

The 4-digit display board is optional

but we strongly recommend that you

build it, even if you only use it for

the initial set-up.

unlocks the full

features of the speed controller and

allows all settings to be adjusted. The

microcontroller will detect whether

SILICON CHIP

THIS

COMPLETELY NEW speed

the display board is connected and if

not, the speed controller will support

only the basic functions. In this sim

ple mode, it will function as a simple

speed regulated controller with auto

matic soft-start and with the speed be

ing directly controlled by a pot (VR1).

All the other settings will be the initial

defaults or as last set (with the display

board connected).

When connected, the 4-digit display

allows you to monitor the speed and

the input voltage (useful when running

from a battery).

also enables you to

navigate through the various menus

to adjust the settings.

The circuit can run from 12V or 24V

batteries and can drive motors (or re

sistive loads) up to 40A. Furthermore,

this is our first DC speed controller

(except for out train controllers) incor

porating speed regulation under load.

In other words, a given motor speed is

maintained, regardless of whether the

motor is driving a heavy load or not.

Monitoring the back-EMF

In speed controllers which do not

have good speed regulation (ie, the vast

majority of designs), the more a motor

is loaded, the more it slows down. In

order to provide speed regulation, the

siliconchip. com.au

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circuit must monitor the back-EMF

of the motor, since this parameter is

directly proportional to its speed.

As a result, our new speed controller

monitors the back-EMF of the motor.

"Back-EMF" is the voltage generated

by any motor to oppose the current

through the windings. EMF stands for

"electromotive force" and is an obso

lete term for voltage. Back-EMF is di

rectI y proportional to the motor speed

and so by monitoring this parameter,

we have a means of controlling and

maintaining the motor speed.

In practice, the main control loop

of the microcontroller tries to match

the speed of the motor (back-EMF) to

the speed set by the pot or recalled

from a preset memory.

the measured

speed is lower than the set speed, the

duty cycle of the pulse width modu

lation (PWM) signal used to drive the

power Mosfets that control the motor

is gradually increased. In other words,

siliconchip.com.au

if the speed tends to drop, more power

is fed to the motor and vice versa.

The frequency of the pulse width

modulation can be set from 488Hz to

7812Hz. This is a useful feature since

different motors will have differ

ent frequency responses, as well

as different resonant frequencies.

This is important to reduce the

audible buzzing from the pulse

width modulation, as these fre

quencies are well within the

range of hearing.

By now you're probably won

dering how the microcontroller

monitors the back-EMF of the mo

tor, considering that the motor is

continuously driven with pulse-width

modulated DC. The answer is that the

micro periodically turns off the PWM

signal to the motor for just enough time

for the back-EMF to stabilise. This

"window" needs to be wide enough

to ensure that we are measuring back

EMF and not the spike generated by the

last PWM pulse. On the other hand, we

don't want the window so wide that

the maximum power to the motor is

significantly reduced or that the motor

noticeably slows.

The compromise value is that the

motor is monitored for 200llS every

7.4ms (ie, about 135 times a second),

as shown in the scope diagrams in

this article. As a result, the fact that

we do monitor the back-EMF around

135 times a second means that the

power applied to the motor is slightly

less than the theoretical maximum,

although this effect is negligible.

A low-battery alarm is also incorpo

rated to warn when the battery level

drops below a preset value. This is

especially useful for applications like

electric wheelchairs.

There are also eight memory speed

settings. All settings are persistent,

meaning they are retained in non

volatile memory.

• Good speed regulation under

load

• Automatic soft-start and fast

switch-off

• Eight memory settings

• 4-digit 7-segment display

• Variable frequency for pulse

width modulation (PWM)

• Battery level meter

• Low-battery alarm

• Persistent settings

defaults

• Rated up to 40A continuous

current

• 12-24V

input voltage

dition and sets the duty cycle of the

PWM to 0%. This ensures that when

the motor is sWitc;hed in, its speed will

increase gradually from the stationary

state to the desired speed setting.

Turn-on currents for motors can be

very high and it is desirable to reduce

these surge currents as much as pos

sible. That is why the automatic soft

start feature has been incorporated into

the firmware.

will ensure that the

motor is brought up to the set speed

gradually.

Fast

switch-off feature

Another feature that has been in

corporated into the firmware is the

so-called "Fast-off" feature. This

means that the duty cycle of the PW

modulation is set to 0% (turning off the

motor) whenever the selected speed

setting of the pot goes to 0%. Rather

than decreasing the speed gradually,

setting the pot to its lowest setting

turns the motor off immediately.

This design also incorporates our

extensive experience with previous

speed controllers featured in SILICON

CHIP. As a result, it uses four high

current Mosfets to do the switching

(pulse width modulation), uses very

wide tracks on the PC board and heavy

duty (40A) terminal blocks to carry the

heavy currents.

Soft start

When the motor is switched off,

perhaps by an external switch in series

with one of its terminals, the voltage

at the drain of the Mosfets will be

(this is due to the voltage divider

used to scale the back-EMF voltage

to within the operating range of the

microcontroller). The microcontroller

converts this analog value to a digital

value using an on-board ADC (analog

to-digital converter).

The firmware detects this

con

User interface

Two pushbuttons on the display

board are used to navigate through

the menus, while the pot is used both

to vary the speed and to vary certain

settings.

MARCH 2008

1 PC board, code 09103081,

1'24mm x 118mm

2 heavy-duty PC-mount terminal

blocks (3-way) (Altronics

P2053)

8-pin DIP IC socket

1 18-pin DIP IC socket

1 SPOT toggle switch (S1)

1 50A 5AG fuse (F1) (Jaycar

SF1976)

1 60A 5AG fuse holder (Jaycar

SZ2065)

1 12-way pin header (Altronics

P-5502)

1 200mm length 16-way rainbow

cable

1 PC-mount mini piezo beeper

(Jaycar AB3459 or equivalent)

1 220J.lH inductor (L

Jaycar

LF1276 or equivalent)

1 10kQ 16mm PC-mount linear

single-gang pot (VR1)

1 500Q horizontal trimpot (VR2)

Semiconductors

1 PIC16F88-I/P microcontroller

programmed with 091 0308A.

hex (IC1)

1 MC34063 switch mode DC-DC

converter (IC2)

1 BC327 PNP transistor (01)

3 BC337 NPN transistors (02-04)

4 IRF1405 N-channel Mosfets

(05-08) (Jaycar ZT2468)

1 1N4004 diode (01)

1 1N5819 Schottky diode (02)

2 MBR201 OOCT 20A diodes

(Jaycar ZR1 039) OR

1 40EPF06PBF 40A ultra-fast

diode (Farnell 910-1560) (03)

5 1N4745 16V 1W zener diodes

(ZD1-ZD5)

2 1N5364BG 33V 5W zener

diodes (ZD6-ZD7) (Farnell

955-8217)

1 3mm red LED (LED1)

Capacitors

1 2200J.lF 50V low-ESR

electrolytic (Altronics R-6207)

1 470J.lF 16V electrolytic

1 100J.lF 63V electrolytic

1 1OOJ.lF 25V electrolytic

1 10J.lF 25V electrolytic

3 4.7J.lF 16V electrolytic

1 220nF 100V MKT polyester

1 1OOn F 100V MKT polyester

3 1OOnF monolithic

1 470pF ceramic

Resistors (O.25W,

233kQ

1 100Q

24.7kQ

156Q

1 3.6kQ

1 22Q 1W

61kQ

415Q

2470Q

31Q

Display Board

1 PC board, code 09103082,

73mm

58mm

1 SPST PC-mount momentary

contact switch, yellow (Jaycar

SP0722; Altronics S-1 097) (S2)

1 SPST PC-mount momentary

contact switch, red (Jaycar

SP0720; Altronics S-1095) (S3)

1 16-pin DIP IC socket (optional)

1 100nF monolithic capacitor

Semiconductors

1 74HC595 shift register (IC3)

4 BC337 NPN transistors

(09-012)

4 7-segment common cathode

red LED displays (Jaycar

ZD1855; Altronics Z-0190 )

Resistors (O.25W, 1%)

4470Q

839Q

The two pushbuttons are sensitive

to two types of presses, short and long.

A short press is of the order of half a

second or less while a long press is

one around one second.

To change a setting, a long press

is usually needed. This prevents

unwanted changes to the settings,

which are stored in EEPROM and thus

recalled at the next switch on.

Because of the capabilities offer

ed by the PIC microcontroller, we

SILICON CHIP

have been able to incorporate a large

number of features into the firmware,

as described in the separate panel later

in this article.

Circuit description

The circuit for the speed controller

is shown in Fig.l. As noted previously,

it can work with 12V or 24V batteries

but has been optimised for operation

at 24

Within the circuit itself, there

are two separate voltage rails: +sV for

the microcontroller and +16V for driv

ing the gates of the Mosfets. Both are

derived from the +24V input supply.

The main input supply is filtered by

a 2200J.lF low ESR capacitor, to mini

mise high-voltage transients which

can be produced by the inductance

of the battery connecting leads. This

capacitor is absolutely vital to the

proper operation of the speed control

ler at high currents.

Sl is the power switch and diode

Dl protects the low-power part of

the circuit (TCl

IC2) from reverse

polarity. A 22Q 1W resistor, a 33V SW

zener diode (ZD7) and a 100J.lF capaci

tor also protect the MC34063 IC from

transients on the supply rail.

The filtered supply is then fed to

the MC34063 (TC2) which operates

in a standard step-down converter

configuration to provide the +sV rail.

Three lQ resistors between pins 6

are used to set the maximum switch

ing current. The output voltage is set

by the voltage divider associated with

trimpot VR2.

Only about 200mA is ever drawn

from this supply and most of this is

used to drive the display.

ICl is the heart of the circuit and is

the popular PIC16F88 microcontrol

leI' which incorporates a number of

peripheral functions. Of these, the

timers, hardware PWM (pulse width

modulation) and three ADC inputs

are used.

The three ADC inputs used are at

pins 1, 2

18. As these need to be

within the O-SV range, voltage dividers

consisting of 33kQ and 4. 7kQ resistors

are used to scale both the input voltage

rail (which could be as high as 29V)

and the back-EMF from the motor, to

be fed to the ADC inputs at pins 1

18. The ADCs convert the monitored

voltages to lO-bit values.

The +16V rail is used as the gate

drive supply for the Mosfets and is

derived from the 24V supply via a

lkQ resistor and a 16V 1W zener

diode (ZD1). Bypassing of this rail

is particularly important and is ac

complished using 100J.lF and 100nF

capacitors near ZDl and adjacent to

the transistors Ql

Q2.

the battery supply is to be 12V,

the lkQ resistor feeding ZDl should

be reduced to 100Q. In this case, the

supply will actually be between 12V

and 14V (depending on the actual bat

tery voltage); still enough to provide

adequate gate drive for the Mosfets and

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Fig.I: the circuit uses PIC16F88 microcontroller ICI to provide PWM drive to power-Mosfets Q5-Q8 which in turn control the motor. The microcontroller also

monitors the back-EMF from the motor, to provide speed regulation. IC2 is a DC-DC switchmode converter and this provides a +5V rail to power ICI.

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DISPLAY BOARD

Fig.2: the display circuit interfaces to the microcontroller

uses a 74HC595 shift register (IC3)

transistors Q9

.Q12 to drive four 7-segment LED displays. Switches S2

S3 are used to control the display

for software set-up.

ensure minimum heat dissipation (low

on-resistance).

The PWM output of the PIC16F88

(adjusted by firmware) appears at pin

6 and drives transistor Q3 which then

drives complementary transistors Ql

Q2. Ql, Q2

Q3 thus provide buff

ering and voltage level translation for

ICl 's PWM output to drive the gates of

Mosfets Q5-Q8 via 15n resistors.

Note that these resistors need to be

relatively low in value (ie, 15n) in

order to ensure quick charging and

discharging of the gate capacitances.

That's because the gate capacitance

of these Mosfets can be quite high, of

the order of5000-10,000pF each.

the

gate charging time is too long, the Mos

fets will spend too much time between

the on and off states and this will lead

to much higher heat dissipation.

In fact, the gate voltage transitions

need to be very fast, of the order of 1

or less. This has been accomplished,

as shown by the oscilloscope screen

grab of Fig.4.

The specified Mosfets are from

International Rectifier, type IRF1405.

This is a 55V 169A N-channel Hexfet

with an exceptionally low on-resist

ance (Rds) of 5.3 milliohms (5.3mn)

typical. Their pulse current rating is

a stupendous 680A.

The IRF1405is specifically intended

for automotive use, in applications

such as electric power steering, anti

SILICON CHIP

lock braking systems (ABS), power

windows and so on and is therefore

ideal for this speed control applica

tion.

Why

four Mosfets?

In fact, since the ratings of this Mos

fet are so high, you might think that

just one device on its own would be

enough to handle the 40A rating of this

speed controller project. So why are

we using four Mosfets in parallel?

As always, real world use brings us

down to earth. For a start, we are using

these Mosfets without heatsinks, apart

from the vestigial heatsink effect of

their being bolted to and connected to

the copper side of the PC board - not

much heatsink benefit there. Their

thermal characteristic is 62°C per watt

(junction to ambient), assuming that

are mounted in free air (which they

are not).

Assuming an ambient temperature

of 25°C and an on-resistance of 10mn

(conservative), we can approximate

the temperature of the Mosfets at

their highest operating current (lOA

per Mosfet for a total of 40A). At lOA

and 10mn on-resistance, the power

dissipated is: 10

x .01

This means that the temperature of

the case will be approximately: 25

62 x 1

87°C

This means that at full current, the

Mosfets will be very hot to the touch.

Careful: they will burn you. Our meas

urements produced a top temperature

of around

noc

after a test period of

half an hour.

In practice, even with much higher

ambient temperatures, the Mosfets

should not get quite this hot because

in "real world" operation, the speed

control is not likely to be providing

full power to the motor on a continu

ous basis. At 24V and 40A, the motor

would have 960W applied (ie, more

than IHP) and this equates to relatively

high power operation.

Protection

Zener diodes ZD2-ZD5 are included

to protect the Mosfets from excessive

gate voltages. In normal circuit opera

tion, these zener diodes do nothing.

Additional protection for the drains

of the paralleled Mosfets is provided

by 33V 5W zener diode ZD6, in paral

lel with a 100nF capacitor. The zener

is there to clip any residual voltage

transients which get past the

2200~F

low-ESR input filter capacitor.

The Mosfets are further protected by

fast-recovery diode D3 and its parallel

220nF capacitor. These parts are wired

across the motor terminals and are

used to suppress the high back-EMF

spikes caused by the armature induct

ance when the motor is switched off

by the Mosfets.

These components are crucial to

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