AMMETER SENSITIVITY

Ammeter sensitivity is the amount of current necessary to cause full scale deflection (maximum reading) of the ammeter. The smaller the amount of current, the more "sensitive" the ammeter. For example, an ammeter with a maximum current reading of 1 milliampere would have a sensitivity of 1 milliampere, and be more sensitive than an ammeter with a maximum reading of 1 ampere and a sensitivity of 1 ampere. Sensitivity can be given for a meter movement, but the term "ammeter sensitivity" usually refers to the entire ammeter and not just the meter movement. An ammeter consists of more than just the meter movement.

AMMETER RANGES

If you have a meter movement with a sensitivity of 1 milliampere, you can connect it in series with a circuit and measure currents up to 1 milliampere.

But what do you do to measure currents over 1 milliampere?

To answer this question, look at figure 1-21.

In figure 1-21(A), 10 volts are applied to two resistors in parallel. R₁ is a 10-ohm resistor and R₂ is a 1.11-ohm resistor. Since voltage in parallel branches is equal-

Figure 1-21. - Current in a parallel circuit.

In figure 1-21(B), the voltage is increased to 100 volts. Now,

In figure 1-21(C), the voltage is reduced from 100 volts to 50 volts. In this case,

Notice that the relationship (ratio) of I_R1 and I_R2 remains the same. I_R2 is nine times greater than I_R1 and I_R1 has one-tenth of the total current.

If R₁ is replaced by a meter movement that has 10 ohms of resistance and a sensitivity of 10 amperes, the reading of the meter will represent one-tenth of the current in the circuit and R₂ will carry nine-tenths of the current. R₂ is a SHUNT resistor because it diverts, or shunts, a portion of the current from the meter movement (R₁ ). By this method, a 10-ampere meter movement will measure current up to 100 amperes. By adding a second scale to the face of the meter, the current can be read directly.

By adding several shunt resistors in the meter case, with a switch to select the desired resistor, the ammeter will be capable of measuring several different maximum current readings or ranges.

Most meter movements in use today have sensitivities of from 5 microamperes to 1 milliampere. Figure 1-22 shows the circuit of meter switched to higher ranges, the shunt an ammeter that uses a meter movement with a sensitivity of 100 microamperes and shunt resistors.

This ammeter has five ranges (100 microamperes; 1, 10, and 100 milliamperes; 1 ampere) selected by a switch.

Figure 1-22. - An ammeter with internal shunt resistors.

By adding several shunt resistors in the meter case, with a switch to select the desired resistor, the ammeter will be capable of measuring several different maximum current readings or ranges.

Most meter movements in use today have sensitivities of from 5 microamperes to 1 milliampere. Figure 1-22 shows the circuit of meter switched to higher ranges, the shunt an ammeter that uses a meter movement with a sensitivity of 100 microamperes and shunt resistors.

This ammeter has five ranges (100 microamperes; 1, 10, and 100 milliamperes; 1 ampere) selected by a switch.

With the switch in the 100 microampere position, all the current being measured will go through the meter movement. None of the current will go through any of the shunt resistors. If the ammeter is switched to the 1 milliampere position, the current being measured will have parallel paths of the meter movement and all the shunt resistors (R₁, R₂, R₃, and R₄).

Now, only a portion of the current will go through the meter movement and the rest of the current will go through the shunt resistors. When the meter is switched to the 10-milliampere position (as shown in fig. 1-22), only resistors R₁ , R₂, and R₃ shunt the meter. Since the resistance of the shunting resistance is less than with R₄ in the circuit (as was the case in the 1-milliampere position), more current will go through the shunt resistors and less current will go through the meter movement. As the resistance decreases and more current goes through the shunt resistors. As long as the current to be measured does not exceed the range selected, the meter movement will never have more than 100 microamperes of current through it.

Shunt resistors are made with close tolerances. That means if a shunt resistor is selected with a resistance of .01 ohms (as R₁ in fig. 1-22), the actual resistance of that shunt resistor will not vary from that value by more than 1 percent.

Since a shunt resistor is used to protect a meter movement and to allow accurate measurement, it is important that the resistance of the shunt resistor is known very accurately.

Figure 1-22 represents an ammeter with internal shunts. The shunt resistors are inside the meter case and selected by a switch. For limited current ranges (below 50 amperes), internal shunts are most often employed.

For higher current ranges (above 50 amperes) ammeters that use external shunts are used. The external shunt resistor serves the same purpose as the internal shunt resistor. The external shunt is connected in series with the circuit to be measured and in parallel with the ammeter. This shunts (bypasses) the ammeter so only a portion of the current goes through the meter.

Each external shunt will be marked with the maximum current value that the ammeter will measure when that shunt is used. Figure 1-23 shows an ammeter that is designed to use external shunts and a d'Arsonval meter movement. Figure 1-23(A) shows the internal construction of the meter and the way in which the external shunt is connected to the meter and to the circuit being measured. Figure 1-23(C) shows some typical external shunts.

Figure 1-23. - An ammeter employing the d'Arsonval principle and external shunts.

A shunt resistor is nothing more than a resistor in parallel with the meter movement. To measure high currents, very small resistance shunts are used so the majority of the current will go through the shunt. Since the total resistance of a parallel circuit (the meter movement and shunt resistor) is always less than the resistance of the smallest resistor, as an ammeter's range is increased, its resistance decreases. This is important because the load resistance of high-current circuits is smaller than the load resistance of low-current circuits. To obtain accurate measurements, it is necessary that the ammeter resistance be much less than the load resistance, since the ammeter is connected in series with the load.