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design
ideas
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Edited by Brad Thompson
Impedance transformer flags failed fuse
Kevin Ackerley, Future Electronics, Vancouver, BC, Canada
F
IGURE 1
DEPICTS a circuit that detects
V
CC
C
2
5-TO-1
CONNECTOR
TOROIDAL
TRANSFORMER
the opening of a miniature circuit
breaker or high-rupture-capability
fuse in a high-reliability telecommunica-
tions power supply. The circuit generates
an alarm when a failure changes the im-
pedance of an electromagnetic sensor.
Traditional fault-detection circuits sense
the voltage difference developed across
an open fuse, leakage current flowing
through a fused circuit, or closure of an
auxiliary (volts-free) contact by an actu-
ator fuse. All three methods suffer from
disadvantages: Voltage-difference circuits
can introduce unacceptable delays as long
as 30 minutes because the system’s bat-
teries sustain the bus voltage. Leakage-
current sensors rely on the presence
of a load that may not be present un-
der certain conditions. Adding auxiliary
miniature-circuit-breaker support cir-
cuits or special high-rupture-capability
indicator fuses and their connectors can
significantly increase system cost.
Capacitor C
4
and the secondary in-
ductance, L
2
, of transformer T
1
resonate
at approximately 42 kHz, a frequency that
minimizes noise production in the audio,
RF, and psophometric noise bands. Op-
erational amplifier IC
1
and associated
components form an ac-coupled posi-
tive-feedback amplifier with a gain of 20.
10 nF
R
1
100k
TO LOAD
CIRCUIT
R
5
3.3k
+
IC
1
AD8606
L
2
L
1
_
C
4
470 nF
T
1
R
2
100k
R
4
10k
V
CC
LOGIC
OUT
D
1
1N4148
C
3
100 nF
R
3
470
+
IC
2
AD8606
D
3
1N4148
R
9
47k
D
2
1N4148
R
6
100k
Q
1
2N3904
_
C
1
100 nF
R
7
4.7k
R
8
10k
C
5
100 nF
Figure 1
This sensor circuit operates from a single 5V power supply.
Under normal operation, an intact fuse
or closed circuit breaker completes a low-
impedance path through T
1
’s single-turn
primary (sense) winding. Transformer
action presents a low impedance at the
junction of C
2
,C
4
,and R
5
and reduces the
loop gain around IC
1
to an amount
which in turn drives a peak detector
formed by D
3
and C
5
. Transistor Q
1
satu-
rates and provides a logic-low signal to an
external alarm.
Figure 2
shows a typical
application for sensing backup-battery-
circuit failure.
To design transformer T
1
, you calcu-
insufficient to sustain
oscillation.
When a fault occurs
and interrupts current
through T
1
’s primary
winding, its secondary
impedance increases,
allowing full loop gain
and permitting IC
1
to
oscillate at 42 kHz,
which L
2
and C
4
deter-
mine. Under fault con-
ditions, T
1
’s turns ratio
injects less than 10 mV
of wideband conduct-
ed noise into the dc
bus. Capacitor C
3
couples the oscil-
lating signal to IC
2
,a
gain-of-3 amplifier,
TOROIDAL
TRANSFORMER
SYSTEM EARTH
+
BATTERY
STRING 1
CUSTOMER'S
L0AD
+
Impedance transformer flags
failed fuse ........................................................
67
Digital waveform generator provides
flexible frequency tuning for
sensor measurement ....................................
68
Battery-operated remote-temperature
sensor drives 4- to 20-mA current loop ....
70
Precision current source
is software-programmable ..........................
72
Publish your Design Idea in
EDN
. See the
What’s Up section at www.edn.com.
–
RECTIFIER 1
LVD
1
BACKPLANE FUSE
LOAD FUSE
BATTERY SHUNT
LOAD SHUNT
LVD
2
The system wiring diagram shows transformer
T
1
’s primary winding. Low-voltage-disconnect
units LVD
1
and LVD
2
isolate the 48V battery or the customer’s
load for maintenance.
www.edn.com
DECEMBER 17, 2004
|
EDN
67
Figure 2
design
ideas
TOROIDAL CORE
late the required impedance and turns
ratio.
Equation 1
describes the basic
transformer relationship:
PLAN
CONNECTOR
(1)
where Z
1
is the impedance of the primary
winding, Z
2
is the impedance of the sec-
ondary winding N
1
is the number of pri-
mary turns, and N
2
is the number of sec-
ondary turns.
Under normal operation with
current flowing in the primary winding,
the secondary impedance comprises the
low primary-side impedance plus T
1
’s
leakage reactance. When no current
flows in the primary winding, the num-
ber of turns in the secondary and the
toroidal core A
L
(inductance per turn)
determine the secondary winding L
2
’s
inductance and number of turns per
Equation 2
:
ELEVATION
Figure 3
The primary winding (battery cable) passes through transformer T
1
’s center.
teration of T
1
’s design, but if that data is
unavailable, you can use
Equation 3
to
calculate the inductance.
Also, select a core material that doesn’t
saturate at full primary current.
Note that the core’s central area must
provide clearance for the battery cable
(primary winding) and secondary wind-
ing. This application uses a Philips 3C85
toroidal ferrite core (part no. TN 16/9.6/
6.3-3C85) with a secondary winding com-
prising five turns of 0.2-mm
2
insulated
copper wire. (Philips, however, has dis-
continued the 3C85 ferrite core. Ferrox-
cube’s type 3C90 ferrite may serve as a re-
placement. Specifications are available at
www.ferroxcube.com.)
Figure 3
shows the
completed transformer.
(3)
e, the effective permeability, equals
the magnetic constant, 4
10
7
Hm
1
,I
is the path length, and A is the cross-sec-
tional area in millimeters squared.
Select a core that presents a high val-
ue of inductance to ensure that the dif-
ference between an open and a closed
primary circuit causes a large change in
relative secondary-winding impedance.
(2)
where N
2
is the number of turns around
the toroidal core.
Ferrite-core manufacturers publish in-
ductance-per-turn data that simplifies al-
Digital waveform generator provides flexible
frequency tuning for sensor measurement
Colm Slattery, Analog Devices, Limerick, Ireland
V
ARIABLE-RESISTANCE SEN-
SENSOR ASSEMBLY
MOVING OBJECT
frequency and measuring
changes in the phase or ampli-
tude of output voltage V
2
with
respect to excitation voltage
V
1
. However, this approach
limits the sensor’s dynamic
range and resolution.
As an alternative, you can
drive the sensor with a swept-
frequency ac source that tracks
the sensor’s resonant-frequen-
cy variation.
Figure 2
shows
one approach in which IC
1
,a
DDS (direct-digital synthesis)
device, produces a sine-wave
excitation voltage. Lowpass filter IC
2
re-
moves clock artifacts and harmonics, and
amplifier IC
3
drives the sensor. Amplifi-
SORS convert a fixed dc ex-
citation voltage or current
into a current or voltage that’s a
straightforward func-
tion of the quantity un-
dergoing measurement. In an-
other class of sensors, moving
objects or fluids produce a sen-
sor signal by altering an LC cir-
cuit’s inductance or capacitance.
Figure 1
shows a basic ac-driven
tuned-circuit proximity sensor,
L and C, and sampling resistor,
R. Under static conditions, L and
C resonate and provide maximum im-
pedance at one frequency. As an object
approaches the sensor, the value of L or
Figure 1
C
L
V
1
REFERENCE
VOLTAGE
EXCITATION
VOLTAGE
R
V
2
SENSOR
OUTPUT
The amplitude and phase of the resonant-circuit sensor’s output
voltage, V
2
, vary with moving object’s position.
C varies and alters the circuit’s resonant
frequency. You can derive the object’s po-
sition by exciting the sensor with a fixed
68
EDN
|
DECEMBER 17, 2004
www.edn.com
where
design
ideas
er IC
4
boosts the sensor’s output voltage,
V
2
, and drives IC
5
, a dual-channel, 12-bit
ADC, which simultaneously samples and
digitizes reference voltage V
1
and IC
4
’s
output. IC
5
, a DSP-capable microcon-
troller, analyzes the sensor output’s am-
plitude and phase, setting the frequency
of IC
1
via alternate programming of ei-
ther of IC
1
’s dual frequency-control reg-
isters. One of IC
6
’s serial ports delivers
position data to an external controller.
Using a DDS/DSP combination offers
considerable flexibility when using var-
ious types of sensors. For example, cer-
tain sensors require a relatively narrow
but high-resolution range of
excitation frequencies, and
others may work best with broadly swept
excitation.
V
DD
AMPLIFIER
CLOCK
IC
1
V
OUT
IC
2
IC
3
AD8XX
AD9833
SCLK FSYNC SDI
LOWPASS
FILTER
V
DD
V
D
D
V
1
REFERENCE
VOLTAGE
VDRIVE
SENSOR
SPI
IC
5
DR0
DR1
SCLKO
TFSO
DOA
DOB
SCLK
CS
VA
1
VA
2
SERIAL
DATA
OUT
IC
6
ADSP-218X
IC
4
AD8XX
V
2
SERIAL PORT 0
SIGNAL
VOLTAGE
AD7866
AMPLIFIER
Figure 2
A swept-frequency source and a DSP controller combine to offer a versatile sensor-
excitation system.
Battery-operated remote-temperature sensor
drives 4- to 20-mA current loop
Scot Lester, Texas Instruments, Dallas, TX
Y
OU CAN REMOTELY measure temper-
TPS62300 series of ICs, for example, con-
verts a battery voltage of 2.7 to 6.5V into
a constant current, which you program
using an external resistor and voltage on
its I
SET
pin. The current that normally
drives the LED instead powers the loop
(
Figure 1
).
In the sample circuit, which occupies
50 mm
2
, the LED driver drives the 4- to
20-mA current loop proportionate to a
sensed temperature of
10
C at 4 mA
C at 20 mA. The driver applies
0.6V to the I
SET
pin and monitors current
flow from the pin. This current is multi-
V
+
V+
R
1
51.1k
2
IC
2
3
V
REF
V+
IN
OUT
0.1
F
R
2
29.4k
0.1 F
IC
4
TPS60230RGT
1 F
3
+
5
R
ISET
6.49k
1
1
2
3
4
5
IC
3
I
SET
V
ISET
16
15
14
13
4
_
EN2
EN1
GND
VIN
V+
2
4
5
2
1
R
3
150k
R
4
100k
IC
1
6
D1
C2
C1
12
11
C
8
0.015
7
F
PGND
0.47
F
0.47
F
10
9
8
V
OUT
C1+
C2+
GND
17
Figure 1
1
F
TWISTED PAIR
In this circuit, the LED driver drives the 4- to 20-mA current loop propor-
tionate to a sensed temperature of
10
C at 4 mA and 50
C at 20 mA.
RECEIVER
100
70
EDN
|
DECEMBER 17, 2004
www.edn.com
ature using a 4- to 20-mA current
loop as long as 4000 feet and a bat-
tery-powered, white-light LED driver.
You usually configure this equipment to
provide a programmable, constant cur-
rent to an LED from a battery source. The
and 50
design
ideas
plied by 260 and mirrored to the LED
drive output:
REF2912 voltage reference, IC
2
, with the
OPA374 op amp to scale the output of
the TMP36 to the required voltage for the
LED driver, IC
4
. In general terms, the cur-
rent in the current loop for the circuit is:
of resistance
with battery voltages as low as 2.7V.
Therefore, the LED driver can drive more
than 1500 feet of 24 AWG or 4000 feet of
20 AWG twisted-pair wire with a 100
load resistor at the receiver. You can
achieve much longer distances with high-
er battery voltages. Because this circuit
powers the current loop, the battery life
for these circuits depends on the meas-
ured temperature. For the circuit shown,
a loop current of 13.3 mA corresponds
to a measured temperature of 25
re-
sistor for R
ISET
means that V
ISET
needs to
be 0.1V to provide 20 mA of loop current
and 0.5V to provide 4 mA.
The TMP36 temperature sensor, IC
1
,
provides 750 mV of output at 25
C.
Therefore, using two AA alkaline batter-
ies in series should provide more than 120
hours of remote-temperature monitoring
at room temperature. The accuracy for
the circuit is about 2.5% of full scale with-
out any calibration. For tighter accuracy,
reduce the range of the measured tem-
perature or calibrate the output.
C and
varies its output voltage by 10 mV/
Substituting for the component values
shown in the
figure
yields:
C.
The output of the TMP36 is 0.4V at
C. Because these
voltages do not directly match the volt-
age requirements of V
ISET
, you use a
C and 1V at 50
The output of the LED driver can drive
Precision current source is
software-programmable
Joe Neubauer, Maxim Integrated Products Inc, Sunnyvale, CA
W
ITH THE ADDITION OF a few inex-
to these devices, besides the hard-wired
type, can be one, two, or three wires. IC
1
,
for example, has a three-wire SPI inter-
face, and provides an end-to-end resist-
ance of 50 k
(V
CC
- V
IN
)/R
SENSE
.
The circuit can provide any current lev-
el for which the external components,
R
SENSE
and the pass transistor, can handle
the associated power dissipation (P
with 256 incremental set-
tings. Thus, each increment of the digital
potentiometer changes V
IN
by:
IV).
Because the ratio setting of digital poten-
tiometers is good, with a typical ratio-
metric resistor temperature coefficient of
5 ppm/
C), precision and stability for the
current source depend primarily on the
precision and stability of IC
3
and R
SENSE
combined.
Op amp IC
2
regulates current through
the pass transistor, and the digital poten-
tiometer sets current through the R
SENSE
V
CC
R
SENSE
Figure 1
SHUNT
MAX6138
IC
3
GND
I
SET
_
IC
2
MAX4165
MAX5400
IC
1
DIGITAL
POTENTIOMETER
V
IN+
P
This software-
programmable
current source
applies current to
the load in 256
equal increments.
+
LOAD
Many types of digital potentiometer
are currently available, and the interface
R
S
I
SHUNT
72
EDN
|
DECEMBER 17, 2004
www.edn.com
loops with as much as 180
Because resistor R
ISET
, which is tied to
the I
SET
pin, is fixed in the example, the
output current is proportional to the
voltage, V
ISET
, which the output of op
amp IC
3
determines. Using a 6.49-k
10
pensive miniature components,
the hard-wired, voltage-controlled
current source of yesterday becomes a
software-programmable voltage-con-
trolled current source (
Figure 1
). A digi-
tal potentiometer, IC
1
in conjunction
with a precision op amp, IC
2
, sets current
through a pass transistor, I
SET
, and a shunt
regulator, IC
3
, provides a constant refer-
ence voltage across the digital poten-
tiometer. By operating in its linear region,
the transistor controls load current in re-
sponse to the applied gate voltage. Each
incremental step of the digital poten-
tiometer increases or decreases the wiper
voltage, V
IN
, at the op amp’s
noninverting input. Thus, V
IN
varies with respect to the reference volt-
age, which in turn remains stable with re-
spect to the supply rail:
resistor. The voltage across R
SENSE
deter-
mines current through the pass transis-
tor, I
SET
:I
SET
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