4.2.3 Experimental principles
The ammeter(gauge head)generally can only measure small current and voltage because of its structure.The common magneto electric ammeter is mainly composed of a rotatable coil made of fine enameled wire placed in a permanent magnetic field,a hairspring for generating a mechanical counter torque,a pointer for indication and a permanent magnet.When the current passes through the coil,the current-carrying coil generates a magnetic torque MM in the magnetic field.The same torque makes the coil to rotate and drive the pointer to deflect.The deflection angle of the coil is proportional to the current passing through the coil.So the current value can be directly indicated by the deflection angle of the pointer.
1)Measurement of the range Ig and the internal resistance Rg of the gauge head
The maximum current allowed by the ammeter is called the range of the ammeter which is represented by Ig.The internal resistance in the coil of the ammeter is represented by Rg.Ig and Rg are two important parameters representing the characteristics of the ammeter.
(1)Half-deviation or median method for measuring Rg.The measurement schematic diagram is shown in Figure 4.2.1.First,turn off the switch S leaving the measured ammeter connected to the circuit only and make the ammeter fully deflective.Then,turn on the switch S,and the resistance box and the ammeter in parallel are used as the shunt resistance.When the ammeter pointer points to the middle value and the total current intensity remains unchanged,the shunt resistance value is equal to the internal resistance of the ammeter.
Figure 4.2.1 Schematic diagram of the median method for measuring the internal resistance of the gauge head
Figure 4.2.2 Schematic diagram of substitution method to measure the internal resistance of the gauge head
(2)Substitution method for measuring Rg.The measurement schematic diagram of the substitution method is shown in Figure 4.2.2.First,make the measured ammeter connected to the circuit and the pointer fully deflective,and write down the current value of the standard ammeter in the circuit.Then replace the ammeter with a resistance box,and only change the resistance of the resistance box.When the current value of the standard ammeter in the circuit becomes the same as the value of the measured ammeter,the resistance value of the resistance box is the internal resistance of the measured ammeter.
2)Refitting the gauge head to an ammeter
According to the resistor parallel rule,if a resistor R2 with appropriate resistance value is paralleled at both terminals of the gauge head as shown in Figure 4.2.3,the part of the current that the gauge head cannot withstand can be shunted through R2.The unit consisting of the gauge head and the parallel resistor R2 is the refitted ammeter,which is enclosed by the dotted line in Figure 4.2.3.If the range of the gauge head is in need of expanding to n times,it is likely to obtain:
Figure 4.2.3 is the schematic diagram of the ammeter after range expansion.A multi-range ammeter can be made by paralleling shunt resistors with different resistance values on the gauge head.
Figure 4.2.3 Schematic diagram of the ammeter refitted by the gauge head and the calibration circuit
Figure 4.2.4 Schematic diagram of the voltmeter refitted by the gauge head and the calibration circuit
3)Refitting the gauge head to a voltmeter
According to the resistor series rule,if a resistor RM(RM=R1+R2)is connected in series with the gauge head as shown in Figure 4.2.4,the part of the voltage that the gauge head cannot withstand can drop in RM.The unit consisting of the gauge head and the series resistor RM is the voltmeter.The series resistance RM is called the extending resistor.From Figure 4.2.4,the extending resistance value can be obtained(https://www.daowen.com)
The actual schematic dliagram of the voltmeter after extending range is shown in Figure 4.2.4.Voltmeters with different ranges can be obtained by selecting different values of RM.
4)Refitting the gauge head to an ohmmeter
According to the different zero adjustment methods,the ohmmeter can be divided into two types:series divided-voltage type and parallel shunt type.The principle circuit is shown in Figure 4.2.5.
Figure 4.2.5 Schematic diagrams of ohmmeters refitted by gauge heads
(a)Series divided-voltage type;(b)Parallel shunt type
In Figure 4.2.5,E stands for the power supply,R3 stands for the current-limiting resistor,RW stands for the“zero”potentiometer,RX stands for the measured resistance,and Rg stands for the equivalent internal resistance of the gauge head.In Figure 4.2.5(b),a shunt resistor is formed by RG and RW.
It is necessary to adjust the“zero”point before using the ohmmeter.That is to say short-circuit a and b to adjust the resistance of RW so that the pointer of the gauge head just deflects to the full scale.It can be seen that the zero point of the ohmmeter is at full scale(the measurement limit)of the gauge head,which is exactly the opposite to the zero point of the ammeter and the voltmeter.
In Figure 4.2.5(a),when the measured resistance RX is connected to terminals a and b,it can be known from Ohms Law of the whole circuit that the current in the circuit is:
Where Rg,RW and R3 are constants for the given circuit and given gauge head.It can be seen that there is a one-to-one correspondence between the measured resistance and the current value when the power supply voltage E remains unchanged.That is to connect different resistors.The gauge head will have different readings.The larger the RX,the smaller the current I.When terminals a and b are short-circuited(RX=0),the pointer is fully deflective,and the current is:
When RX=Rg+RW+R3,
At this time,the pointer is in the middle of the gauge head,and the corresponding resistance is the median resistance.That is to say
When terminals a and b are open(RX=∞),I=0.That means the pointer is at the mechanical zero position of the gauge head.So the scale of the ohmmeter is reversed,and the scale is uneven.The larger the resistance R,the closer the scale interval.If the scale of the gauge head is scaled according to the known resistance value in advance,the ammeter can be used to directly measure resistors.
The terminal voltage of the battery will change during the use of the ohmmeter,but the internal resistance Rg and the current-limiting resistance R3 of the gauge head are constant.So RW is required to change with the change of E to meet the requirement of“zero”adjustment.When designing,use an adjustable power supply to simulate the change of battery voltage,and its range should fall within 1.35～1.6 V.
