Advantages of Thermocouple Type Instruments

  • The thermocouple type of instruments accurately indicates the root mean square value of current and voltages irrespective of the waveform. There is a wide variety of thermocouple instruments available in the market.
  • Thermocouple types of instruments give very accurate reading even at high frequency, thus these types of instruments are completely free from frequency errors.

  • The measurement of quantity under these instruments is not affected by stray magnetic fields.

  • These instruments are known for their high sensitivity.

  • Usually for measuring the low value of current bridge type of arrangement is used i.e. ranging from 0.5 Amperes to 20 Amperes while for measuring the higher value of current heater element is required to retain accuracy.

Disadvantages of Thermocouple Type Instruments:

  • Instead of many advantages these types of instruments possess one disadvantage, The over load capacity of thermocouple type of instrument is small, even fuse is not able to the heater wire because heater wire may burn out before the fuse blows out.

Synchro Position Transducer Working Principle:

  • We know that Syncro is an inductive device which works on the principle of rotating transformers. Here the term rotating transformer means the primary to secondary coupling can be changed by physically changing the relative orientation of the winding. So based on this working principle of synchro we can use it as a position transducer.

Construction Of Position Transducer:

  • Position transducer is one of the basic applications of the Synchro. It uses dumb-bell shaped rotor. Single phase ac supply is given to the rotor of the Synchro. This rotor is mechanically coupled with the shaft of rotating element whose angular position is to be determined.

Construction Of Position Transducer

Position Transducer Working Principle:

We know that the stator of the synchro has three windings. These three winding of the stator are connected in star connection. Remaining ends of each winding are taken out to connect them with the voltmeter as shown in the figure. When the angle of the rotor changes the output voltage i.e. the stator voltages of each winding is given by,

E1 = Eom cosθ sin wt = instantaneous voltage for stator windings S1.

E1 = Eom cos(θ+120) sin wt = instantaneous voltage for stator windings S2.

E1 = Eom cos(θ+240) sin wt = instantaneous voltage for stator windings S3.


θ= angular position of the rotor

Eom = peak value of voltage of each winding 

w= 2πf

f= frequency of the rotor 

t = time in seconds.

All instantaneous voltages are sinusoidal in nature. But they give different values of voltages at different positions of the rotor.Thus using these three values of stator voltages we can easily measure the position of the rotor. Hence Synchro can be used as a position transducer.

Applications Of Position Transducer:

  • For measuring the angle of the rotating machine like antenna platform.

  • Position transducer can be used as rotary position sensor for aircraft control surfaces

Piezoelectric transducer:

Piezoelectric transducer

  • A piezoelectric quartz crystal is a hexagonal prism shaped crystal, which has pyramids Jt both ends. This is shown in the Fig. (a). The markings of coordinate axes are fixed for such crystals. The axis passing through the end points of pyramids is called the optic axis or z axis. The axis passing through corners is called electrical axis or x axis while the aXIs passing through midpoints of opposite sides is called mechanical axis or y axis. The axes are shown in the 

Photovoltaic cell:

  • Fig shows the structure of photovoltaic cell. It shows that cell is actually a PN-junction diode with appropriately doped semiconductors. When photons strike on the thin p-doped upper layer, they are absorbed by the electrons in the n-layer; which causes formation of conduction electrons and holes. These conduction electrons and holes are separated by depletion region potential of the pn junction. When load is connected across the cell, the depletion region potential causes the photocurrent to flow through the load N.