Pmmc Meter



Definition: The instrument which uses the rectifying element for measuring the voltage and current is known as the rectifying instruments. The rectifying element converts the alternating current to the direct current which indicates by the DC responsive meter. The PMMC uses as an indicating instrument.

The sensitivity of the rectifying instruments is high as compared to the moving coil and the Electrodynamometer instrument. Thereby, it uses for measuring the current and voltage. The circuit arrangement of the rectifier instrument shown in the figure below. The device uses the four diodes which act as a rectifying element.

DC voltmeter is a measuring instrument, which is used to measure the DC voltage across any two points of electric circuit. If we place a resistor in series with the Permanent Magnet Moving Coil (PMMC). PMMC stands for ‘Permanent magnet moving coil’. It is one of the simple and most used instruments on board with sophisticated names. They are used on board to assist in regular maintenance of electrical equipment or when precise measurements are required. Permanent Magnet Moving Coil (PMMC) Instrument The permanent magnet moving coil instrument is the most accurate type of DC measurement. It is one of the two types of moving coil instruments used to measure dc work only. The other moving coil instrument is a dynamometer type which can measure AC and DC work.

The multiplier resistance Rs uses for limiting the value of current so that their value does not extend more than the rating of the PMMC instrument.

Rectifying Element

The rectifier element is used for the conversion of the AC to DC so that the unidirectional current flows through the PMMC instrument. The copper oxide, selenium cell, germanium and silicon diode are uses for making rectifying element.

The rectifying element offers the zero resistance when it is in forwarding bias and infinite resistance when it is in reverse biased. This property of the rectifying element use for rectification purpose.

Characteristic Curve of Rectifying Element

The characteristic curve of the rectifying circuit shown in the figure below. Ideally, the rectifying instrument does not have any voltage drops in the forward direction and no current flows in the reverse direction.But practically, this is not possible. The real characteristic curve of the rectifying element shown in the figure below.

Half Wave Rectifier Circuit

The figure below shows the half-wave rectifying circuit. The rectifying element connects in series with the voltage source, resistance multiplier and the permanent moving coil instrument. The forward resistance of the diode is neglected.

When the DC voltage source applies to the circuit, the Im current flows through it. The magnitude of the current is equal to the V/(Rm+RS). The current shows the full-scale deflection to the instrument.

The AC voltage applies to the same circuit. The rectifying element converts the AC voltage into unidirectional DC voltage. Thus, the rectified output voltage obtains through the rectify instrument. The PMMC instrument deflects through the average value of current which depends on the average voltage of the apparatus.

Average Value of Voltage The above calculation shows that the sensitivity of the instrument through AC is 0.45 times the current through the sensitivity of DC.

Full Wave Rectifier Instrument

The circuit of full wave rectifier shown in the figure below.The DC voltage applied to the circuit causes the full-scale deflection of the PMMC meter. The sinusoidal voltage applies to the meter express as
The average calculation of AC is 0.9 times with that of the DC for the same value of voltage. Or we can say that the sensitivity of the instrument along with AC is 90% with that of the DC.The sensitivity of the full wave rectifier is double to that of the half wave rectifier instrument.

Sensitivity of Rectifier Instrument

The sensitivity of the instrument shows the variation of the measured quantity from input to output. The DC sensitivity of the rectifier instrument

The sensitivity of the AC rectifier type instrument depends on the type of the rectifying element use in the circuit.

Factors Affecting the Performance of Rectifier Type Instruments

The following are the factors that affect the performance of the instrument when it is used on AC.

  1. Effects of Waveform – The calibration of the rectifier instrument can be done regarding the RMS value of voltage and current. The form factor of the half wave and the full wave rectifier type instrument fixes on the calibrated scale. And if the waveform of the other form factors applies to the device, the waveform error occurs in the reading.
  2. Effect of Temperature Change – The resistance of the rectifying element varies with the change in temperature. And this property of the rectifying component causes the error in the instruments.
  3. Effect of Rectifying Instrument – The rectifier instrument has the property of the imperfect capacitance. It allows the high-frequency current to pass through it.
  4. Decreases in Sensitivity – The sensitivity of the rectifier type instrument for AC operation is lower than that of the DC operation.

Advantages of Rectifying Instrument

The following are the advantages of the rectifiers instruments.

  1. The frequency range of the instruments increases from 20HZ to high-frequency range.
  2. The current operating range for such type of instrument is much lower for voltmeter as compared to the other AC instrument.
  3. The instrument has uniform scales for the large range.
  4. The accuracy of the instrument is ±5 percent when it is in normal operating condition.

Applications of Rectifying Instrument

The following are the applications of the rectifying instrument.

  1. The instrument uses for measuring the voltage whose range lies between 50 – 250 V.
  2. It use as a milliammeter or micro-ammeter.
  3. The rectifying instrument use in the communication circuit for measurement.

The sensitivity of the rectifying type voltmeter is less than that the sensitivity of the DC voltmeter. Thus, the loading effect of the ac rectifier voltmeter is higher than the dc voltmeter.

Related terms:

  • Electronic Measuring Instruments
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DC voltmeter is a measuring instrument, which is used to measure the DC voltage across any two points of electric circuit. If we place a resistor in series with the Permanent Magnet Moving Coil (PMMC) galvanometer, then the entire combination together acts as DC voltmeter.

The series resistance, which is used in DC voltmeter is also called series multiplier resistance or simply, multiplier. It basically limits the amount of current that flows through galvanometer in order to prevent the meter current from exceeding the full scale deflection value. The circuit diagram of DC voltmeter is shown in below figure.

We have to place this DC voltmeter across the two points of an electric circuit, where the DC voltage is to be measured.

Apply KVL around the loop of above circuit.

$V-I_{m}R_{se}-I_{m}R_{m}=0$ (Equation 1)

$$Rightarrow V-I_{m}R_{m}=I_{m}R_{se}$$

$$Rightarrow R_{se}=frac{V-I_{m}R_{m}}{I_{m}}$$

$Rightarrow R_{se}=frac{V}{I_{m}}-R_{m}$ (Equation 2)

Where,

$R_{se}$ is the series multiplier resistance

$V$ is the full range DC voltage that is to be measured

$I_{m}$ is the full scale deflection current

Pmmc

$R_{m}$ is the internal resistance of galvanometer

The ratio of full range DC voltage that is to be measured, $V$ and the DC voltage drop across the galvanometer, $V_{m}$ is known as multiplying factor, m. Mathematically, it can be represented as

$m=frac{V}{V_{m}}$ (Equation 3)

From Equation 1, we will get the following equation for full range DC voltage that is to be measured, $V$.

$V=I_{m}R_{se}+I_{m}R_{m}$ (Equation 4)

The DC voltage drop across the galvanometer, $V_{m}$ is the product of full scale deflection current, $I_{m}$ and internal resistance of galvanometer, $R_{m}$. Mathematically, it can be written as

$V_{m}=I_{m}R_{m}$ (Equation 5)

Substitute, Equation 4 and Equation 5 in Equation 3.

$$m=frac{I_{m}R_{se}+I_{m}R_{m}}{I_{m}R_{m}}$$

$Rightarrow m=frac{R_{se}}{R_{m}}+1$

$Rightarrow m-1=frac{R_{se}}{R_{m}}$

Pmmc Meter

$R_{se}=R_{m}left (m-1 right )$(Equation 6)

Pmmc Meter Working Principle

We can find the value of series multiplier resistance by using either Equation 2 or Equation 6 based on the available data.

Multi Range DC Voltmeter

In previous section, we had discussed DC voltmeter, which is obtained by placing a multiplier resistor in series with the PMMC galvanometer. This DC voltmeter can be used to measure a particular range of DC voltages.

If we want to use the DC voltmeter for measuring the DC voltages of multiple ranges, then we have to use multiple parallel multiplier resistors instead of single multiplier resistor and this entire combination of resistors is in series with the PMMC galvanometer. The circuit diagram of multi range DC voltmeter is shown in below figure.

We have to place this multi range DC voltmeter across the two points of an electric circuit, where the DC voltage of required range is to be measured. We can choose the desired range of voltages by connecting the switch s to the respective multiplier resistor.

Let, $m_{1},m_{2}, m_{2} $ and $m_{4}$ are the multiplying factors of DC voltmeter when we consider the full range DC voltages to be measured as, $V_{1} , V_{2}, V_{3}$ and $V_{4}$ respectively. Following are the formulae corresponding to each multiplying factor.

$$m_{1}=frac{V_{1}}{V_{m}}$$

$$m_{2}=frac{V_{2}}{V_{m}}$$

$$m_{3}=frac{V_{3}}{V_{m}}$$

$$m_{4}=frac{V_{4}}{V_{m}}$$

In above circuit, there are four series multiplier resistors, $R_{se1}, R_{se2}, R_{se3}$ and $R_{se4}$. Following are the formulae corresponding to these four resistors.

$$R_{se1}=R_{m}left (m_{1}-1 right )$$

Pmmc Volt Meter

$$R_{se2}=R_{m}left (m_{2}-1 right )$$

$$R_{se3}=R_{m}left (m_{3}-1 right )$$

$$R_{se4}=R_{m}left (m_{4}-1 right )$$

So, we can find the resistance values of each series multiplier resistor by using above formulae.