Transducer Characteristics

Posted on Sep 5, 2024 in Electronics

Variable Resistor

Non-linear, not suitable for positional transducer

Resistance along the track is not a linear relationship, increasing with the square of the spindle’s rotation. (R α S2)

Linear Variable Resistor

Can be used to provide a variable voltage (wiper picking up variable voltage potentiometer)

Characteristics

  • Voltage vs. Resistor Setting: Linear
  • Resolution: Has been defined as the largest change in the input which does not cause a change in output.

Slider Carbon Potentiometer

Effect of Loading on Potentiometer Output

As resistance increases, voltage increases.

Wire Wound Track (Variable Resistor)

Characteristics

  • Higher current ratings
  • Durability (Reliability)

Carbon Track (VR) (Linear Track)

Characteristics

  • Cheap
  • Good resolution
  • Can be made miniature

Slider (VR)

Linear

Servo Potentiometer

Wheatstone Bridge

Applications

  • Measure Resistance
  • Measure Voltage

Unknown Voltage = (Total resistance of variable resistor Rt) / (Voltage across R section) * Standard Voltage

R4 = (R2 / R1) * R3

IC Temperature Sensor

It provides an output of 10mV/degree K.

Characteristics

Measurement of output voltage indicates the temperature directly in Kelvin.

Platinum Resistance Temperature Detector RTD (Unit Connected to DC Supply)

Platinum film strip trimmed with a laser beam to cut a spiral for 100 Ohm at 0° Celsius.

Characteristics

  • Resistance of film increases as temperature increases (R α T) – Linear
  • Rt = Ro + 0.385 * t
  • R = Resistance at temperature t° Celsius
  • Ro = Resistance at 0°C

Negative Temperature Coefficient Thermistor (NTC)

As the temperature of the element increases, its resistance falls; the resistance/temperature characteristics being non-linear.

Characteristics

Not suitable for an application where accurate temperature is required. As transducers, they are more suitable for applications in protection and alarm circuits where an indication of temperature threshold is required.

Thermistors are used in many electronic circuit applications for the control of currents and voltage as equipment temperature varies.

Two Thermistor Bridge Circuits

As temperature rises, the resistance of both thermistors will decrease.

NTC Bridge Circuits

Output with 2 active thermistors is greater than that with only one thermistor.

Type “K” Thermocouple (Made of Alumel and Chromel)

Fairly linear

Characteristics

The magnitude of the output voltage depends on the temperature difference between the hot and cold junctions and the material used.

Output voltage is fairly linear over a 0-100°C range and a magnitude of 48 μV/°C difference between junctions.

Filament Lamp

Not that precise because of spectral response.

Characteristics

  • Temperature of filament is proportional to input power.
  • Power varies with the square of voltage.
  • Resistance increases as the temperature of the filament increases.

Photovoltaic Cell

The output short circuit current being directly proportional to the luminous flux received.

Characteristics

  • The device can be used as a voltage source or as a current source and is inherently a linear device.
  • Series: Increases voltage
  • Parallel: Allows greater current to be drawn.

Phototransistor

Vout = V – Iceo * R

V = Supply voltage

Iceo = Collector leakage current

R = Collector load resistance

Characteristics

Light falls, leakage current increases.

Photoconductive Cell, LDR

Consists of a semiconductor disc base with a gold overlay pattern.

Characteristics

  • Resistance of semiconductor material between the gold contacts reduces when light falls on it.
  • With no light, material resistance is high.
  • Light falling on the material produces hole-electron pairs of charge and reduces resistance (Light Dependent Resistor).

PIN Photodiode

  • Sensitivity: 0.44 A/W
  • Current characteristic: 2856 Ln A/lx
  • Response time: 3.5 ns
  • Peak spectral response: 850nm (IR)

Characteristics

Used in two ways:

  1. As a photovoltaic cell (measuring voltage output)
  2. By amplifying the output current and converting it into voltage

PIN Output Voltage vs. Lamp Filament Voltage

Linear Variable Differential Transformer

Characteristics

  • A-B
  • Core at left: Va greater than Vb
  • Vout = Va – Vb
  • Core at right: Va will be less than Vb
  • Vout = Va – Vb (Antiphase with input)

Output voltage vs. Core Position graph

Linear Variable Capacitor

Consists of two conducting plates separated by an insulator referred to as a dielectric.

Characteristics

The capacitance of the capacitor is directly proportional to the cross-sectional area that the plates overlap and is inversely proportional to the separation distance between the plates.

Capacitance magnitude depends on the length l of the slug enclosed within the brass sleeve. Capacitance increases with the length.

  • Capacitance (min): 25pF
  • Capacitance (max): 50pF
  • Mechanical travel: 15mm
  • Fairly linear

Strain Gauge Transducer

Consists of a grid of fine wire or semiconductor material bonded to a backing material.

Characteristics

Loading the beam increases the length of the gauge wire and also reduces its cross-sectional area.

Both of these effects will increase the resistance.

  • Load capacity: 100g
  • Max deflection: 0.5mm
  • Sensitivity: 25 μV/g
  • Non-linearity: 0.1%
  • Hysteresis: 0.03%
  • Creep: 0.05%

Air Flow Transducer

Consists of 2 RTD’s in a plastic case (RTD’s have a positive temperature coefficient).

Characteristics

When air flows over the RTD’s, the temperature of the heated unit will fall more than that of the unheated unit.

The temperature will be related to the air flow rate, which will, in turn, affect the resistance of the RTD’s.

  • Heater power: 1W
  • Output impedance: 1.7 kOhm
  • Voltage output (-) Pump off: 2.1V
  • Voltage (+) Pump off: 1.7V
  • Voltage change (airflow): 0.06V

Air Pressure Transducer

Outer plastic case which is open to the atmosphere via two parts.

Can be calibrated and called an absolute pressure transducer.

Characteristics

  • Sensitivity: 200 μV/kPa
  • Temperature coefficient: 1350 ppm/°C
  • Output voltage (-) Pump off: 2.48V
  • Output voltage (+) Pump on: 2.51V
  • Voltage difference pump off: 35mV
  • Voltage difference pump on: 39mV
  • Output impedance: 1.6 kOhm

Humidity Transducer

Thin disc of material whose properties vary with humidity.

Characteristics

Variation of humidity around alters the permittivity or thickness of the dielectric material, changing the value of the capacitor.

Output voltage varies with humidity due to the variation of the capacitance of the transducer.

  • Capacitance (25°C, 45% R/H): 122pF ± 15%
  • Humidity range: 10-90% RH
  • The device is slow to respond fully to humidity changes, but of no consequence in practice as natural humidity changes are slow.

Slotted Opto-Transducer

  • Beam broken voltage: 0.1V
  • Beam taken voltage: 4.9V

Characteristics

  • Gallium arsenide LED and silicon phototransistor
  • As voltage increases, motor speed increases.
  • Collector current is low when the infrared beam is broken and increases when the beam is admitted.
  • Positive voltage pulses are obtained from the emitter circuit each time the beam is taken in, and hence the device generates pulses suitable for counting rotations.

Reflective Opto-Transducer

  • Beam broken voltage: 0.5V
  • Beam taken voltage: 5V

Characteristics

  • Similar to a slotted opto-transistor
  • Beam not reflected, emitter output is low
  • When reflected, it’s high