PRINCIPLE AND DEFLECTING TORQUE
The position of the pointer depends on the three types of torques :
1. Deflecting Torque: This torque provides the deflecting or operating force proportional to the quantity to be measured and moves the pointer from its zero position.
2. Controlling Torque: The controlling torque is equal and opposite to the deflecting torque, in order to make the deflection of the pointer proportional to the magnitude of the quantity to be measured. The controlling torque also brings the pointer back to zero position when the force that causes the movement of the pointer is removed.
3. Damping Torque: The damping torque provides the damping force so that the pointer quickly comes to the final steady state position without any oscillations.
When current flows in the coil, the developed torque causes coil to the rotate. The electromagnetic (EM) torque is counterbalanced by the mechanical torque of control springs attached to the movable coil. The balanced torques and therefore the angular positions of the movable coil are indicated by a pointer against a fixed reference called a scale.
LAW:
T_{d} = BAIN
B = Fhx density in the air gap. (wb / (m ^ 2))
A = Effective area (m ^ 2)
l = Current in movable coil (A)
N = n*alpha of turns in the coil.
The equation shows that the developed torque is proportional to the flux density of the field in which the coil rotates, and the current coil constants (area and number of turns). Since both reflux density and coil constants are fixed for a given instrument, the developed torque is a direct indication of the current in the coil. The pointer deflection is used for measure.
• This torque causes the pointer to deflect to a steady state position where it is balanced by the opposing control spring torque (T_{c}). The controlling torque is the springs and is proportional to the angular deflection of the pointer.
Controller torque T_{c} Angular deflection (0).
T_{r} = k*theta
Where
T. Controlling torque (N - m)
K = Spring constant (N - m /rad)
0 = Angular deflection (rad or deg).
• For the final deflected position, T_{d} = T_{c} and θα1
The pointer deflection can therefore can therefore be used to measure current.
WORKING OF FET INPUT VOLTMETER
FET input voltmeter decreases the amount of power drawn from a circuit under test by increasing the input impedance using an amplifier with unity gain. A source follower drives an emitter follower. This combination is capable of a thousand-fold or more increases in impedance while maintaining a voltage gain of very nearly one. The input impedance of this meter is 10 MQ, which would require 0.025 µW of power for a 0.5 V deflection, as compared to 25 µW for an unamplified meter.
Construction: The FET input voltmeter circuit can be divided into three sections;
1. Ranging Attenuator section.
2. FET source follower section.
3. Bridge circuit.
The ranging attenuator section is nothing but a potential divider circuit with the three resistors connected in series. The section is used to select the desired range of voltage in 500 mV, 5 V, and 50 V ranges. A 1-pole 3-way switch is used to select the required range of voltage. The rotary switch is connected to the gate of FET by means of a 1 ΜΩ resistor which is used to limit the gate current thereby preventing any damage to the FET. The FET Q₁ is connected in source follower configuration and the transistor Q₂ acts as its load. The transistor T₂, its emitter resistance 10 K, the 2.5 K potentiometer, and the 2.2 K resistor act as four arms of the bridge circuit. The switch 'S' applies the power supply to the circuit. A 10 K calibration potentiometer and microammeter are connected to the circuit by means of a switch 'S'.
Here, the FET is connected in source follower configuration so as to provide impedance matching, thus delivering maximum power to the load and thereby providing high accuracy
Working: Initially apply the power supply to the circus when switch 'S' is closed and the test terminals T₁ and T₂ assorted. The unknown voltage to be measured is applied between the terminals T_{1} and T_{2} The working of FET input voltmeters is divided into 3 parts.
1. Zero adjustment.
2. Calibration.
3. Actual measurement.
1. Zero Adjustment: when switch 'S' is closed for power supply to the circuit and the test terminals T_{1} and T_{2} are shorted. Two test terminals T_{1} and T_{2} are shortly circulated there is no drain current in FET, thus there is no emitter current following in the transistor T_{2} therefore the microammeter, which connected between the points of the bridge should show rereading. This is a zero adjustment part.
2. Calibration: Now, the short circuit between the two test terminals is replaced by a known voltage source of 500 mVThe 500 mV voltage range is selected by means of the range switch (rotary switch). The known voltage source causes current flow in the drain of the FET. This causes an emitter currents flow in the transistor T_{2} This results in a voltage drop across the emitter resistance R_{E} This causes a deflection in the micro ammeter. The 10 K calibration potentiometer is adjusted until the meter shows full-scale deflection. Now, the meter is ready for actual measurement.
3. Actual measurement: Apply the unknown voltage between the two test terminals T₁ and T2. The microammeter then deflects in proportion to the unknown voltage. If the rotary switch is in the 500 mV range, the reading can be had directly. If it is in the 5 V range, the reading should be multiplied by 10 and if it is in the 50 V range the reading should be multiplied by 100.
Advantages:
1. High accuracy and High sensitivity
2. High input impedance.
3. No loading effect.
4. Power consumption is very low.
Disadvantages:
1. The balance of the bridge is affected by any changes in the parameters of the transistors.
2. It can measure only DC voltages.
