SERIES TYPE OHMMETER

 

CONSTRUCTON:

The series type ohmmeter essentially consists of a D’Arsonval movement connected with a resistance R₁ and A battery to a pair of terminals to which the unknown resistance is connected. Figure shows the circuit of a series type ohm meter.

 

Figure (a) Series Type Ohmmeter

 The current flowing through the movement then depends on the magnitude of the unknown resistance. Therefore, the meter deflection is directly proportional to the worth of the unknown resistance.

 R₁= Current limiting resistor

 R₂ = Zero adjust resistor

 V = Internal battery

 R_m = Internal resistance of the D’Arsonval movement

 R_x = Unknown resistor

                                      Figure (b) Dial Of Series Ohmmeter

Condition 1:

When the unknown resistance R_x = 0 (terminals A and B shorted), maximum current flows within the circuit. Under this condition, shunt resistor R­­­­­­₂ is adjusted until the movement indicates full scale current (I_fsd). The complete scale current position of the pointer is marked “0Ω” on the size.

When, R_x = 0, current is maximum

Meter indicates I_fsd---- = full scale deflection

Meter pointer indicates = 0Ω

Condition 2:

When R_x = ∞ (terminals A and B open). The present within the circuit drops to zero and therefore the movement indicates zero current, which is then marked ‘∞’ on the size. When R_x = ∞, current is zero

 Meter indicates zero current

 Meter pointer indicates = ∞ Ω.

Condition 3:

For intermediate markings on the size, different known values of R_x are connected between terminals A and B, the accuracy of those scale marking depends on the repeating accuracy of the movement and therefore the tolerance of the calibrating resistor.

 A major drawback within the series ohmmeter is that the decrease in voltage of the interior battery with time and age. thanks to this, the complete scale deflection current drops and therefore the meter doesn't read “O” ohms when A and B are shorted.

The variable shunt resistor, R₂ across the movement is adjusted to counter act the drop by battery voltage, there by bringing the pointer back to “0” ohms on the size.­­

It is also possible to regulate the complete scale deflection current without the shunt resistor R₂ within the circuit, by varying the worth of  R₁ to catch up on the drop . Since this affects the calibration of the size, varying by R₂ is far better solution. the interior resistance of the coil R_m is extremely low compared to R₁ when R₂ is varied, the present through the movement is increased and therefore the current through R₂ is reduced. They by bringing the pointer to the complete scale deflection position.

The series type ohmmeter may be a simple and popular design, and is employed extensively for general service work. Therefore, during a series type ohmmeter, the size marking on the dial has “0Ω” on the proper side, like full scale deflection current and “∞” on the lift side like no current flow as given in figure (b).

Design:

A convenient quantity to use within the design of a series type ohmmeter is that the value of R_x which causes half- scale deflection of the meter.

 At this position, the resistance across terminals A and B is defined because the half – scale position resistance R_h.

Value of R₁ and R₂  are often determined from the worth of R_x which provides half the complete scale deflection.

The design are often approached by recognizing that, if introducing R_h­ reduces the meter current to ½ I_fsd. The unknown resistance must adequate to the entire internal resistance of the ohmmeter.

 R_h = R₁ + R₂ R_m/ R₂ + R_m

The total resistance presented to the battery then equals 2 R_h and therefore the battery. Current needed to provide half scale deflection is

 I_h = V/ 2R_h

To produce full scale current, the battery current must be doubled. The total current of the circuit,

I_t  = 2 I_h = 2 * V/ 2R_h  = V/ R_h

The shunt current through R₂  is

I₂ = I_t – I_fsd

The voltage across the shunt (V_sh) is equal to the voltage across the meter.

V_sh = V_m

I₂ R₂ = I_fsd R_m

R₂ = I _fsd  R_m / I₂

But       I₂  = I_t - I_fsd

R₂ = I_fsd R_m / I_t – I _fsd                  …........................                (1)

But        I_t = V / R_h

R_s = I_fsd R_m/ V/R_h- I_fsd    = I_fsd  R_m  R_h/ V- I_fsd R_h

As   R_h  = R₁ +  R₂ R_m/ R₂ + R_m

R_t  = R_h – R₂ R_m/ R₂ + R_m

=  R_h - I_fsd R_m R_h /V- I_fsd R_h  * R_m   /  I_fsd  R_m R_h /V - I_fsd R_h  + R_m

=  I_fsd  R_m R_h  /V                      ..........................                        (2)

Hence R₁ and R₂ can be determined.