121704di.pdf

design

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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

V 1 REFERENCE

VOLTAGE

EXCITATION

VOLTAGE

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

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

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

IC 2

V REF

V+

OUT

0.1

R 2

29.4k

0.1 F

IC 4

TPS60230RGT

1 F

R ISET

6.49k

IC 3

I SET

V ISET

EN2

EN1

GND

VIN

V+

R 3

150k

R 4

100k

IC 1

C 8

0.015

PGND

0.47

V OUT

C1+

C2+

GND

Figure 1

TWISTED PAIR

In this circuit, the LED driver drives the 4- to 20-mA current loop propor-

tionate to a sensed temperature of

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

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:

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+

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

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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