Ratiometer Type Frequency Meter Construction Operation


 Construction of Ratiometer Type Frequency Meter

A ratiometer type frequency meter consists of a ratio meter. This ratio meter gives a relationship between the current ratio and the deflection. There are two coils in the ratio meter. We fit them perpendicular to each other. This frequency meter also consists of two bridge rectifiers. These two rectifiers feed the ratio meter coils.

ratiometer type frequency meter

We connect the input side of one bridge rectifier across the supply mains with a capacitor (C) in series. Also, we connect the input side of another rectifier across the same supply main with a pure resistance (R) in series.

 

Working Principal of Ratiometer Type Frequency Meter

Let us consider V is the supply voltage. Also, we consider f is the supply frequency. Again we consider I1 is the output current of the first rectifier and Ic is the input current of that rectifier. As the input path of the first rectifier is capacitive.

ratio meter

Again, the output current of a rectifier is proportional to its input current. So, we can write,

On the other hand, in the second rectifier, the input current is proportional to V/R. Therefore, the output current is also proportional to V/R.

ratio meter

In a ratiometer type instrument, the deflection angle (θ) is proportional to the ratio of the current of two coils. Therefore,

Again, here both C and R are constant. Therefore,

ratio meter

Advantages of Ratiometer Type Frequency Meter

Thus, the ratiometer type frequency meter has a linear relationship between the deflection of the pointer and frequency to be measured. As a result, this instrument has a linear scale.

The expression of the deflection angle does not contain voltage as its term. So we can apply this instrument to any voltage level. But for very low voltage level the meter may give erroneous readings due to waveform distortion.

A ratiometer type frequency meter can measure the frequency correctly up to 5 KHz.

 

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