USING THE METER FOR HIGHER RANGES OF ELECTRICAL PARAMETERS
CURRENT METER/AMMETER
Some current meter designs incorporate a moving coil in which as little as 10 microamperes of current causes full-scale deflection; other coils may require 50 microamperes of current and still others may be designed for 1 milliampere or higher. How can these meters be used to measure more current than can safely flow through the moving coil? The problem is solved by using a parallel current path, or shunt, around the coil, either, inside or outside the meter. This
arrangement is explained below: The general philosophy of the meter shunt is as follows: if a meter is designed to indicate the flow of 1 mtlliampere (ma) of current at full-scale deflection and it is desired to measure 10 ma, the ohmic value of the shunt must be such (relative to the ohmic value of the moving coil in the meter) that 9/10 of the total current (9 ma) passes through the shunt and 1/10 of the current (1 ma) passes through the meter.
Assume that we wish to use a 0-I-ma meter (milliameter) having a moving coil resistance of 50 ohms in a circuit that may have as much as 10 ma flowing through it. To use the 1-ma meter to measure up to 10 ma, we must add a shunt to the meter movement, connecting the shunt directly to the outside terminals of the meter. To calculate-the shunt, the following formula is used:
where Rs is the resistance of the shunt in ohms, Rm is the resistance of the meter movement in ohms, Im is the full-scale current rating of the meter movement, and It is the total current in. amperes to be carried by the meter and shunt.
Substituting the values, we get:
The construction of accurate shunts can be done using precise resistance measuring devices. To calculate other shunts, substitute the new values in the above formula.
OR, Rs = Rm / ( Is/ Im) as V SHUNT= V COIL, IsRs=ImRm
Rs=ImRm /Is . AS Is=It-Im, Rs = ImRm /It - Im
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VOLTMETER
A DC current meter can be used as a DC voltmeter in the following way. Assume that a current meter has a moving coil rated at 100 micro-ampere full--scale current and a DC resistance of 1000 ohms. Full-scale coil current means, therefore, a voltage drop (E = IR) of .0001 x 1000 = 0.1 volt across the moving coil of this instrument. In fact, every current meter bears a fixed, internal voltage-drop rating equal to the product of the full-scale coil current and the DC resistance of the coil. Thus, although the ability to measure voltage is inherent in every current meter, its usefulness is limited because its maximum range and resistance are both very low.
How is a current meter used as a voltmeter to measure voltages in excess of the internal voltage drop across the moving coil? By making the current meter part of a circuit in which a multiplier resistance is placed in series with the meter coil. The ohmic value of the multiplier resistance is such that when added to the meter coil resistance, the total resistance limits the circuit current to the full-scale current rating of the meter for any given applied voltage. In this way, the applied voltage divides between the
multiplier-resistance and the meter multiplier-resistance in direct proportion to their respective multiplier-resistances; the voltage drop across the meter coil never exceeds the internal voltage rating for full-scale deflection. The function of the multiplier resistance is to develop a voltage drop equal to the excess between the applied voltage (the voltage being measured) and the internal voltage drop across the meter coil. This proportioning of voltage drops becomes automatic when the multiplier resistance limits the circuit current as described.
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CALCULATING THE MULTIPLIER RESISTANCE VALUE
Let us assume that we have a 100-microampere ammeter (microammeter) having a DC resistance of 1000 ohms. We want to
convert this meter movement into a voltmeter that reads 25 volts full scale. When 100 microampere of full-scale current flows through the meter, there is a voltage drop of 0.1 volt across the meter coil. Our task then is to calculate what resistance must be placed in series with the meter movement so that when a current of 100 microamperes flows through the meter and resistance, there will be a 25volt drop across the meter resistance and multiplier combination, We can find the total resistance of the combination at 25 volts by using Ohm's law as below:
Of course, we must not lose sight of the internal meter resistance of 1000 ohms. Thus, the series multiplier resistance is equal to 250,000 ohms less the meter resistance of 1000 ohms, or 249,000 ohms
100 MICRO AMP
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DC VOLTMETER: OHMS-PER-VOLT RATING
An important electrical characteristic of voltmeters is their ohms-per-volt rating. This term is the basis for the total resistance of the voltmeter on each of its voltage ranges (including the multiplier resistance used on each range). The ohms-per-volt rating is based on the current requirement of the meter coil for full-scale deflection. If the meter coil in a voltmeter .requires 50 µa (.00005 ampere) for full-scale deflection, the multiplier resistance required for each volt indication is R = E/I, or 1/0.00005 = 20, 000 ohms. The instrument therefore has a rating of 20,000 ohms-per-volt on each range.
The total resistance of the instrument on each range, then, is the ohms-per-volt rating multiplied by the full-scale limit on each range.
For example, if the range selected is 5 volts full scale, the total resistance presented by the meter is 20,000 x 5, or 100,000 ohms. A meter that requires 1 ma (.001 ampere) for full-scale deflection has an ohms-per-volt rating of I/0.001 or 1000. If the voltmeter movement uses 10 microampere moving coil, the instrument has a rating of 1/0.00001 or 100, 000 ohms-per-volt.
Voltmeters are connected across circuits; hence, the resistance of the voltmeter is in parallel with the resistance of the circuit being measured. To minimize the shunting effect of the voltmeter resistance across the circuit resistance, it is essential that the voltmeter resistance be as high as possible; It is therefore best to use the highest range possible consistent with readability
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MORE ACCURATE READING IF THE VOLTMETER OHMS/ VOLT RATING IS MUCH HIGHER THAN THE LOAD
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SERIES OHMMETER
THIS IS GENERALLY USED TO MEASURE HIGH RESISTANCE.
The same moving-coil meter that we used in our ammeter and voltmeter can be used to make an ohmmeter, an instrument for measuring resistance. In building a basic ohmmeter circuit, we start once again with a 0-1 rna meter movement connected in series with a fixed resistor (4K), a variable resistor (lK), and a 4.5-volt battery. This entire circuit ends in a pair of terminals across which the unknown resistance (Rx) is to be placed: When the unknown or external resistance is zero, or the output terminals are short
circuited, we want enough current to flow for full-scale meter deflection. By Ohm's law, 1 rna will flow when the total series circuit resistance is: R = 4. 5/.001, or 4500 ohms. Since it is possible for the voltage of a new battery to be slightly higher than 4. 5 volts, the total series resistance should be about 5000 ohms to be safe. Having a fixed resistance of 4000 ohms and a variable resistance of 1000 ohms permits "zero-adjust" to be made on the ohmmeter. As the battery voltage lowers with age, the potentiometer is varied to give full-scale deflection when Rx is zero.
From this, we see that in the series ohmmeter, full-scale deflection is equal to zero ohm at the input terminals. With Rx "open”, this is equivalent to an infinitely high resistance. Thus, the left side of the meter scale reads "infinity" or some very high value of resistance.
We see then that the ohmmeter scale reads opposite in direction to the ammeter and voltmeter scales. In addition, it is not a linear scale. If Rx is 1500 ohms, circuit current drops by 25 rna, to 75 rna (I = 4. 5/(6000). However, the addition of another 1500 ohms across Rx produces a current of I = 4. 5/7500, or 0.60 rna, a drop of an additional 0.15 rna. This produces the nonlinear scale shown.
THIS IS IN ADDITION TO MECHANICAL ZERO ADJUSTMENT
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SHUNT OHMMETER
For reading very low values of resistance, the shunt-type ohmmeter is better suited than the series type. In the shunt unit, the unknown resistance Rx is now shunted or placed in parallel with the meter, instead of in series with it. With the unknown resistance connected in this manner, some of the Current in the ohmmeter circuit now takes the path through Rx. The current
through the meter movement is reduced accordingly, and the amount of deflection drops in proportion to the reduction in current. The amount of current through the meter depends upon the ratio of the shunt resistance of Rx to the internal resistance of the meter.
The zero-adjust in the shunt ohmmeter operates oppositely from that in the series ohmmeter. In the shunt instrument, the variable resistor is adjusted so that full-scale current flows through the moving coil when there is an open circuit across the Rx terminals.
Thus, maximum or infinite resistance is indicated on the right-hand side of the meter scale. A short circuit across terminals Rx would bypass all current around the meter and produce no pointer deflection. Thus, zero ohms would appear at the left-hand side of the meter scale. Any resistance connected across the Rx terminals will provide a path for current, and will cause the current through the meter to be less than full scale. Low resistances will bypass considerable current and produce little deflection; high resistances will bypass little current and produce large deflection. By using a selector switch and various-sized shunts, a multi-range ohmmeter can be made, with each range having a different multiplying factor.
UNLIKE THE SERIES
OHMMETER, THE INFINITY IS ON THE RIGHT AND ZERO , ON THE LEFT (LIKE VOLTS AND AMPS)
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THE WATTMETER
We have discussed thus far the various ways and means of measuring voltage, current, and resistance. Another important
measurement is that of power. Since power is equal to the product of voltage and current (E x I), we can obtain this measurement by voltmeter and ammeter readings in a circuit. There is, however, a direct means for measuring power in watts--the
electrodynamometer wattmeter. This wattmeter consists of two coils--one stationary and one movable. There are two ways of connecting a wattmeter. In one method, the stationary coil is the voltage coil and consists of many turns of small wire having a high resistance; the movable coil is the current coil and consists of a few turns of large wire having low resistance. The voltage (or high-resistance) coil is connected across the voltage source, or across the device whose power consumption is being measured. The current (or low resistance) coil is connected in series with the load, and current through the load passes through the current coil.
The second method of wattmeter connection has the high-resistance voltage coil as the movable coil, with the low-resistance current coil being stationary. This second method is superior for large currents because it removes the difficulty in conducting large currents into and out of the spring suspension of the moving coil. Current through the voltage coil is proportional to the voltage across the load.
The interaction of the magnetic fields from the fixed and movable coils causes the movable coil to rotate. The effect is almost the same as if the voltage applied across the load and the current through the load were multiplied together. The torque on the movable coil is proportional to the current and also to the voltage; it is therefore proportional to their product. The meter pointer thus registers according to the power consumption E x I. Wattmeters are used more in AC circuits than in DC circuits.
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