Transducers in Electrical
Systems
Overview
What is an electrical
transducer?

An electrical t/d changes electrical input
to a mechanical or electrical output.
• Mechanical
• Electrical
– analog scale
- numbers on a digital scale
What is a moving-coil
transducer?


A device that changes
an input electrical signal
into a mechanical
output signal.
Components
•
•
•
•
•
Permanent magnets
Moving coil around an
iron core
Spring
Pointer
Scale
Moving Coil Transducer

How does it work?
•
•
Current flows through the
coil around the iron core,
creating a magnetic field
that works with/against the
permanent magnets. Like
poles repel and unlike
poles attract.
The needle position
represents a balance
between magnetic field
strength and torsional force
in the spring.
Moving Coil Transducer

What does it
measure?
• It can be calibrated to
•
measure amps, volts,
ohms, pounds,
pressure, etc.
The meter reading is
always proportional to
the amount of current
in the moving coil.
How does an ammeter measure
current?


Meter in series with the circuit.
Inside the meter, the current
splits into 2 paths:
•
•


The moving coil of the
transducer
Low ohm “shunt” resistor.
Most current flows through the
resistor as it is less resistive
than the transducer.
Maximum current through the
coil is often less than 50 A (50
x 10-6 amps)
How does a voltmeter measure
voltage?



Meter in parallel with the
circuit – the voltmeter is
connected across 2 points
(load) of the circuit.
Inside the meter, a high-value
resistor is connected in series
with the transducer.
Most circuit current flows
through the circuit, with only a
small amount “shunted” off to
drive the moving coil
transducer.
Voltmeter (more)


Scale is calibrated in volts, despite
measuring current.
Ohm’s Law is used to calculate the voltage,
knowing the current (I) and resistance (R)
using V = IR, where I is the current through
the moving coil and R is the high-resistance
value. V is the voltage drop across the load
resistor which is the same as the voltage
drop across the load.
What is the transducer for a
digital readout meter?

Analog to digital converter
• Electronic device that changes the current in
the circuits of the meter to a digital display on
the readout.
Electrostrictive Transducers



Some crystals change shape when they are
exposed to an electric field – electrostrictive
effect.
When the crystals change shape their
resistance changes, and thus the current
changes.
These are commonly used to sense and
measure the strength of electrical fields around
transformers and power lines.
Photoconductive Transducer


The electrical resistance of some materials
changes when irradiated – a p/c transducer is
made from this material.
Silicon is a p/c material and it has the property
that is has high resistance in the dark, but
lower resistance when irradiated with light.
•

Silicon acts as a transducer, changing input light
energy to output electrical energy.
Also, cadmium sulphide (CdS) and cadmium
selenide (CdSe) display p/c properties.
Transducers in Sonar Systems.
Transducers in Sonar Systems.


SONAR = Sound Navigation Ranging
2 subsystems
• Transmitter of sound energy
• uses a vibrating crystal transducer (electrostrictive effect)
• Receiver of sound energy
•


uses a microphone (piezoelectric effect)
Sound is generated and the waves pass through the
water at sound-in-water speed.
Sound waves echo/bounce of objects and return to
the receiver where the piezoelectric transducer
converts the echoes into electrical signals that are
fed into a computer to compute distance to the object
(d = v * t) or (d = v * t/2)
Practice 1


If sound travels @ 1460 m/s in water,
how long will it take for a signal to travel
from a sonar buoy to a sub 2000 m
away?
Dist = speed x time
• 2000 = 1460 x t
• T = 2000/1460 = 1.37 seconds
Practice 2


If the same sub transmits a signal and
2.5 seconds later receives a response
from an iceberg, how far away is the
iceberg, given sound travels at 1460 m/s
in water?
Dist = speed x time
• D = 1460 x 2.5 = 3650 m
• Note: this is the distance the sound wave has
•
traveled (or echoed out & back), thus
Distance to iceberg = ½ x 3650 = 1825 m
Where are electrical
transducers used?
• Multimeters
• ECG for heart monitoring
• EEG for brain monitoring
• Cameras and lighting systems