designideas
Edited By Martin Rowe
and Fran Granville
readerS SOLVE DESIGN PROBLEMS
Platinum-RTD-based circuit
provides high performance
with few components
Equations for DI4264 (5 − 29)
D Is Inside
72 Proportional-ac-power
controller
Equation
1 doles out whole cycles
of ac line
78 Extend monolithic program-
Jordan Dimitrov, Toronto, ON, Canada
mable-resistor-adjustment
range
Y1
VOwith
= VREF
× negative
×
active
resistance
The standard way of using an additional current to the sensor that
Y2
RTD (resistance-temperature- relates to the temperature you are mea- R (Y78+1-YWire
�
Y
�Y
network
Equations for DI4264 (5 − 29) Θ 0
2
3 Y4 )�1 controls remote
,
detector) sensor is to include it in a suring.
With proper
Equations
for DIdesign,
4264 (the
5 − circuit
29) R [ Y + SPI
peripherals
bridge followed by a differential am- can provide good linearity and stabil-Θ 1 Y3�R 2Y4 (Y0 + Y1)] + 1
EWhat are your design problems
plifier. The problem is that two non- ity over a wide range of input tempera2
1
and solutions?
Publish them here
linearities—one from the sensor and tures. TheEquation
output voltage,
VO, depends Equation
Equation
1
and receive $150! Send your
another from the bridge—affect the on circuit components in the followDesign Ideas to edndesignideas@
transfer function. Some approaches ing way:
are available that attempt to avoid the
Y1
R Θreedbusiness.com.
= R 0 (1 + α × Θ + β × Θ2),
VO = VREF
×
problem, but they tend to be bulky and
Y1 ×
ETo see all of EDN's Design
VO = VREF ×
× Y2
expensive (references 1, 2, and 3).
Y2
Ideas, visit www.edn.com/design
R Θ (Y0 + Y2�Y3�Y
Y4 )�1
An alternative circuit proposes add,
ideas.
R Θ (Y0 + Y2�Y3�Y
Y4 )�1
ing only one extra resistor to the difR Θ [ Y1 + Y3�R 2Y4 (Y0 + Y1)]
, +1
R
[
Y
+
Y
�
R
Y
(
Y
+
Y
)]
+
1
Equation 3
ferential amplifier but provides neither Θ 1
3
2 4 0
1
design guidelines nor results (Refer- where YI51/R
and
I=0
to
4.
in
the first and doing some rearrangeEquation
2
I
Equation
ence 4). This Design Idea fills the gap.
For
positive 2temperatures, a second- ments, you get:
Although circuit analysis is somewhat degree polynomial of the following form
Θ�B
V0 = 2
× K × Θ = f (Θ)KΘ,
complex, performance is good, and the can approximate RTD characteristics:2
Θ
�
B
Θ
�
C
R
=
R
(
1
+
α
×
Θ
+
β
×
Θ
),
Θ
0
circuit uses few components.
R Θ = R 0 (1 + α × Θ + β × Θ2),
Besides the platinum RTD, RU, the
where B, C, and K are constants and
circuit features only six precision resis- where R0 is sensor resistance at 08C, f(U) is a function of temperature. Figtors, an op amp, and a voltage refer- a and b are coefficients, and U is the ure 2 shows the general shape of f(U).
ence (Figure 1). R4, the extra resistor measuredEtemperature.
The output voltage depends linearly
quation 3
for the differential amplifier, delivers
After
replacing
Equation
3 the second equation on temperature when f(U) is as close
as possible to a conVREF
stant. This situation
R2
Θ�B
is most true around
V0 =Θ�B
×
K
×
Θ
=
f
(
Θ
)
K
Θ
,
V0 = 2 Θ2�BΘ×�KC× Θ = f (Θ)KΘ,
R1
R1
the minimum point
Θ �Bf(�)
Θ�C
of f(U).
�
Some additional
VO
�/�
relations
provide
�
that
the
output
volt�, �C
age is 0V at temperature 08C, the conR4
R0
R�
R3
version coefficient
is 10 mV/8C, the
minimum of function f(U) is in the
Figure 1 This generic RTD circuit needs few
Figure 2 The general shape of function f(U)
middle of the meacomponents.
varies with temperature.
surement span, and

edn080529di42641
DIANE
edn080529di42642
DIANE
september 4, 2008 | EDN 71
designideas
the current through RU causes
adjustment at 5508C to match
TABLE 1 experimental results
negligible self-heating of the
the magnitudes of the posiMeasurement range
1100 to 1600°C
sensor.
tive and the negative errors.
Figure 3 shows the circuit Nominal sensitivity
You can also extend the tem10 mV/°C
that meets these requirements. Basic accuracy (nonlinearity)
perature range to start from
Well below 61°C
The sensor is a DIN-IEC 751
21008C instead of 08C withAmbient-temperature effect
0.05°C/10°C
platinum RTD. Microsoft (www.
out exceeding the basic non0.1°C/V
microsoft.com) Excel software Power-supply effect
linearity. The three-lead confits 13 points of 0 to 6008C in Cable effect (three-lead connection)
nection to the sensor signifi0.7°C/V
steps of 508 from the RTD’s cal- Power-supply range
cantly reduces the influence of
612 to 618V
ibration table. The spreadsheet
connection-cable resistance,
9 and 13 mA
software determined R0 to have Consumption (600°C input)
RC, on accuracy.
140 to 185°C
a value of 100V, a to have a Operating temperature
Table 1 shows the results of
value of 3.908310238C21, and
evaluating this circuit’s perb to have a value of 25.801310278C22 coefficient is 50 ppm/8C. You can use formance with a calibrated, precisionwith an R2 factor of one.
two trimming potentiometers, VR1 and decade resistance and a calibrated,
All the circuit’s resistors have toler- VR2, to independently adjust zero and 4.5-digit multimeter with readings
ances of 0.02%, and the temperature span readings. You should perform span at ambient temperatures of 24 and
688C; power supplies of 612, 615,
R2
VR1
and 618V; and cable resistances of 0
2.46k
100
and 5V.EDN
15V
2
REF01H
R1
10k
6
5
VR2
100k
�
OP07C
R0
100
4
RC
15V
R1
10k
R e fe r e nce s
VO
�
RC
RC
R�
R3
2.67k
�15V
R4
28.4k
Figure 3 The full circuit needs trimming potentiometers VR1 and VR2 to adjust
zero and span, respectively, and a three-lead cable for sensor connection.
RC is the cable’s resistance.
EDN080529DI42643
DIANE
Proportional-ac-power controller
doles out whole cycles of ac line
Richard Rice, Oconomowoc, WI

In industrial and process control, it is often necessary to accurately control the temperature of a
process. You control most heating elements using the “bang-bang” method—turning the power to them on and
off at a predetermined setpoint. The
temperature of the heated substance
constantly hunts back and forth around
the setpoint. You can achieve much
72 EDN | september 4, 2008
Bryant, James, Walt Jung, and
Walter Kester, Op Amp Applications,
Analog Devices, 2002.
2 Villanucci, Robert S, “Design an
RTD interface with a spreadsheet,”
EDN, Feb 7, 2008, pg 57, www.edn.
com/article/CA6526816.
3 Moghimi, Riza, “Low-error platinum
RTD circuit has shutdown capability,”
EDN, Sept 14, 2000, pg 186, www.
edn.com/article/CA47186.
4 Gutnikov, VS, Integrated Electronics in Measuring Devices, Leningrad,
1980.
1
greater temperature precision using
proportional power control. With this
method, the controller monitors the
temperature, proportionally varying the
heater power to keep the temperature
as close as possible to the setpoint. A
PID (proportional-integral-derivative)
control loop usually accomplishes this
function. Varying the ac power to the
heating element in a linear-proportion-
al manner is neither easy nor simple.
This Design Idea borrows from the
delta-sigma-modulator concept. The
controller sends cycles of the ac line
to the load as the delta-sigma modulator determines. For example, when the
input-control voltage is 15% of fullscale, only 15 of 100 ac cycles arrive at
the load. Likewise, at 85%, 85 of 100
arrive (Figure 1). The control-voltageinput stage, IC1A, is an inverting amplifier with a gain of negative one. This
stage makes the control-voltage range
over the positive side of 0V. In this example, the control-voltage input ranges from 0 to 2V full-scale. The control
designideas
R2
100k
VIN
R1
100k
�
CONTROL VOLTAGE
0 TO 2V
R3
49.9k
VCC
C1
0.22 �F
IC1A
½TL072
�
OP AMP
R4
100k
� IC
R5
71.5k
VCC
R14
1k
R8
100k
½TL072
�
IC2A
½LM319W
VCC
OP AMP
4
R18
1k
�
2
2
1
COMPARATOR
VCC
D1
�
7
IC3A Q1
74HCT74
D FLIP-FLOP
1
R1
4
S1 C1
D3
120V AC
+
8V AC
4
IC4
MOC3011
Q1
2N3906
SCR1
C2
0.1 �F
250V
LINE 2
VCC
5V
IN
D1
2
COMPARATOR
C3
470 �F
D2
6
R10
100
R13
180
60-Hz CLOCK
6
+
16V AC
CT
500 mA
T1 8V AC
Q1
5
R12
180
6
3
IC2B
½LM319W
9
R11
180
1
LED1
3 �
8 �
R16
4.7k
LINE 1
R23
390
R9
1k
R7
100
1B
R17
100k
R15
100k
LINE 1
TO AC LOAD
(HEATER)
VCC
R6
249k
D4
LINE 2
C6
220 �F
IC4 OUT
LM317
ADJ
R19
750
R20
243
R21
750
ADJ
IC5 OUT
IN
LM337
+
C4
33 �F
+
R22
243
C7
33 �F
C5
0.1 �F
C8
0.1 �F
VEE
�5V
Figure 1 This ac controller borrows from a sigma-delta converter to output a number of whole cycles of ac-line power according
to an input-control voltage.
voltage’s input impedance is 100 kV.
The next stage, IC1B, is an integrator.
The integrator output ramps either up
or down depending on the polarity of
the input current. The speed at which
it ramps depends on the magnitude of
the input current. The integrator is the
heart of the delta-sigma modulator. It
forces a balance, on the average, between the control-voltage current in
R4 and the feedback current in R6. In
other words, the duty cycle of the output of IC3A, a CMOS D-type flip-flop,
must match the control-voltage percentage of full-scale.
Comparator IC2A detects whether
the integrator’s output is positive, thus
requiring more feedback current, or
negative, thus requiring less feedback
to maintain the balance. The output
of the comparator switches between 0
and 5V. The flip-flop latches the comparator’s decision on the next rising
edge of the 60-Hz clock.
76 EDN | september 4, 2008
PNP transistor Q1 and optoisolated
SCR (silicon-controlled rectifier) IC4
drive load-switching SCR1 into conduction whenever the flip-flop provides
feedback current to the integrator. Indicator LED1 lights when the load SCR
is on. The secondary of transformer
T1 detects the zero crossings of the acpower line; these crossings provide the
60-Hz clock. The output of comparator
If you turn off
the SCR too late,
its self-latching
nature may cause
it to stay on for
an extra half-cycle
when it should
have been off.
IC2B switches high during the positive
half-cycles of the ac line and low during
the negative half-cycles. Resistor R15
provides a small positive bias, causing
the edges of the 60-Hz clock to occur
slightly early—which is better than late
in this case. If you turn off the SCR too
late, its self-latching nature may cause it
to stay on for an extra half-cycle when
it should have been off.
Both comparators IC2A and IC2B use
a small amount of hysteresis to promote
fast, clean switching. The remaining
components generate the regulated 5
and 25V power supplies. Transformer
T1 and optoisolator IC4 provide isolation from the ac-power line.
This Design Idea works well for an
application such as a spa-heater control
but does not work for light-dimming or
motor-speed control because the output power is pulsating in nature. You
can easily adapt the design for 240V-ac
or 50-Hz operation.EDN
designideas
Extend monolithic programmableresistor-adjustment range
with active negative resistance
W Stephen Woodward, Chapel Hill, NC

A variety of solid-state, in-circuit-programmable replacements exist for the traditional electromechanical trimmer potentiomHGND
eter. These replacements have many
obvious advantages, such as automatic adjustability, miniaturization,
and immunity to vibration. But these
R1ADJ
e
REFFECTIVE =
R�RC =R�RM.
R
R1
R1
V
RM=MINIMUM
PROGRAMMED
RESISTANCE.
I
�I
VC
RF
�
VM�VC=0,
IF RC=RM.
�
RF
VC=IC�RC.
RC
�IC
VREF
Figure 1 This circuit uses an op amp in a negative-resistance topology that,
in effect, subtracts the minimum programmable resistance from the total
programmed resistance.
edn080515di42591
DIANE
1-Wire network controls
remote SPI peripherals
Michael Petersen, Maxim Integrated Products, Colorado Springs, CO

Many 1-Wire-compatible peripherals are available, but, for
those that lack the 1-Wire capability,
the circuit in Figure 1, pg 80, illustrates one way to implement it. The
example controls a remote LED display
by the 1-Wire network through an SPI
(serial-peripheral-interface)-compatible display controller.
To produce the three-wire SPI that
a MAX7221 display controller requires for the CS (chip-select), DIN
(serial-data), and CLK (clock) signals,
the 1-Wire network serially addresses
three DS2405 1-Wire switches. The
first switch directly creates CS; the
second switch directly creates DIN;
and the third switch, aided by three
78 EDN | september 4, 2008
devices, unlike humble mechanical
potentiometers, have relatively large
minimum programmable resistance.
Although you can adjust a typical
trimming potentiometer down to a
fraction of 1V, solid-state-potentiometer substitutes usually bottom out at
10s, 100s, or even 1000s of ohms. This
limitation can sometimes be problematic and frequently precludes use of
the solid-state option in some design
applications.
The Rejustor family of devices, which
Microbridge (www.mbridgetech.com)
recently introduced, provides an extreme example of this effect. You can
program a typical Rejustor over only
a narrow span of 30%. For example,
you can program a 10‑kV Rejustor to
no lower than 7 kV, imposing a serious and obvious obstacle to generalpurpose application of these devices.
Figure 1 suggests a generally applicable workaround that works not only
with Rejustors, but also with all adjustable resistances. It uses an op amp in
a negative-resistance topology that, in
effect, subtracts RMIN (minimum programmable resistance) from the total
programmed resistance.EDN
exclusive-OR gates, creates CLK.
The edge detector and one-shot
IC4A, IC4B, and IC4C combine the outputs of IC2 and IC3—Data 1 and Data 0—to create a clock signal for the
SPI. This one-shot clock-generation
circuit improves the data rate by requiring only a single 1-Wire transaction per SPI bit, instead of the three
transactions—data, clock low, and
clock high—that would be necessary
if you directly use the IC3 output as a
clock signal.
To transmit data to the SPI inputs,
first set the output of IC1 low. Then,
transmit the data bits using the following rules: If the current data bit differs
from the previous bit, set IC2’s Data 1
output accordingly. If the current data bit is the same as the previous bit,
toggle IC3’s Data 0 output. The circuit
automatically generates a clock pulse
each time and requires only one 1-Wire
command for each data bit sent. When
data transmission is complete, send a
final 1-Wire command to set the IC1
output high.
This circuit allows a 1-Wire network
to control a remote temperature display, but similar techniques can provide an interface to I2C (inter-integrated-circuit)-compatible devices and
to other SPI peripherals, such as ADCs
and DACs. You can also produce a bidirectional-data capability by adding
a fourth DS2405. Note that the SPI
data rate and updates to the peripheral
are relatively slow, but speed is not an
issue for many remote-monitoring applications.EDN
designideas
5V
R1
4.7k
R2
4.7k
PIO
IC2
DS2405
DATA
GND
IC4B
IC4A
DATA 0
DATA 1
CS
PIO
IC1
DS2405
DATA
GND
R3
4.7k
R4
100
74HCT86
PIO
IC3
DS2405
DATA
GND
74HCT86
IC4C
74HCT86
C1
0.01 �F
1-WIRE
5V
DIGIT ZERO
CS DIN CLK
DIGITS
ONE TO SIX
DIGIT SEVEN
SEG A
V+
SEG B
R5
SEG C
ISET
SEG D
IC5
MAX7221
SEG E
SEG F
SEG G
SEG DP
GND
GND
DIG 7
DIG 6 . . . DIG 1
DIG 0
Figure 1 Three 1-Wire switches—IC1, IC2, IC3; three XOR gates, IC4; and the associated components enable a 1-Wire
network to control this display through the SPI peripheral IC5.
edn080710di42871
DIANE
(placed in the 8-21 folder)
80 EDN | september 4, 2008