An ammeter is a meter used to measure current in AC and DC circuits. In the circuit diagram, the symbol of the ammeter is "Circle A". The current value is in "A" or "A" as the standard unit. This article mainly introduces the size of the internal resistance of the ammeter, together with several methods for measuring the internal resistance of the ammeter.
What is the internal resistance of the ammeter?1, micro-ampere ------ internal resistance of about a few hundred Ω, the bigger the more sensitive and more expensive, such as the indicator on the socket - only 2-3 yuan
2, mA meter ------ internal resistance is about a few Ω. The bigger the more sensitive, the more expensive
3, ammeter----- internal resistance is about 0. a few Ω need to see the range
Some problems sometimes abstract it into an ideal ammeter and assume that the internal resistance is zero.
The current meter full bias voltage Ug is measured according to the circuit shown in Figure 1. The current meter G to be measured is connected in parallel with the millivolt meter mv, R is the protection resistor, and R0 is the sliding rheostat. When measuring, r first sets the maximum value. After closing switch K, adjust R0 and r to make the pointer of ammeter G reach full deviation. At this time, the reading on the millivoltmeter is the full bias voltage Ug of the ammeter, and the internal resistance Rg of the ammeter is:
Where Ig is the full scale value of the ammeter.
The experimental circuit diagram uses a voltage divider circuit. If the resistance of the protection resistor r is large enough, it can be simplified to the circuit measurement as shown in FIG. Among them, R can be used as a potentiometer (resistance is about 10KΩ). The measurement method is the same as above.
1. Current equivalent substitution method
As shown in Figure 3, G is the current meter to be tested, G0 is the auxiliary current meter, the range is the same as G or slightly larger, r is the protection resistor, R0 is the sliding rheostat, K1 is the single-pole switch, and K2 is the single-pole double-throw switch. When measuring, r is set first, the switch K1 is closed, K2 is pulled to the 1st terminal to turn on the current meter G, and R0 and r are adjusted, so that the pointer of the auxiliary current meter G0 reaches a certain scale value close to the full offset range (note that I cannot be larger than the current meter G) Range). Then, the resistance value of the resistor box R is set to the maximum value, K2 is pulled to the 2 terminal to turn on the resistance box R, and the resistance value of the resistance box is gradually decreased. When the pointer adjusted to the auxiliary current meter G0 still refers to the original scale value I. The resistance value R indicated by the resistance box is equal to the internal resistance Rg of the current meter G, that is,
2, voltage equivalent replacement method: As shown in Figure 4, mv is a millivoltmeter for the auxiliary meter, the range is the same or slightly larger than the head pressure drop of the current meter G to be tested, the current meter G to be tested, the resistance box R is connected in parallel with the millivoltmeter mv by a single pole double throw switch K2. When measuring, first close K1, K2 to 1 end, adjust the sliding rheostat R0 and r, so that the current meter pointer to be measured is close to the full offset value, and the millivolt meter pointer refers to a certain scale value U. Then disconnect K1, the resistance value of the resistance box is set to zero value, K2 is pulled to the 2nd end, then K1 is closed, and the resistance value of the resistance box is gradually adjusted, so that the millivoltmeter pointer still refers to the original scale value U. The resistance value R indicated by the resistance box is equal to the internal resistance Rg of the ammeter, namely:
1. Constant current semi-bias method:
As shown in Figure 5, the current meter G to be tested is connected in parallel with the resistor box R, and then connected to the monitoring current meter G0 and the protection resistor r. The range of the G0 meter is the same as or slightly larger than the range of the G meter. When measuring, disconnect K2, close K1, adjust sliding varistor R0 and protection resistor r, make the current meter G to be fully biased, and record the reading I on the G0 meter. When the accuracy of the current meter G and G0 is different, the reading Ig and I will be different. Then, the switch K2 is closed, and the resistance box R and the varistor R0 are alternately adjusted so that the pointer of the ammeter G is half-biased, and the reading of G0 remains unchanged. At this time, the current intensity through the resistance box R and the G meter is equal, both are Ig/2, and the internal resistance Rg of the ammeter is equal to the reading R of the resistance box, that is:
If the experiment does not use a voltage divider circuit, the potentiometer R for protection resistors (resistance is about 10KΩ), and the auxiliary meter G0 is not used, as shown in Figure 5. During measurement, the potentiometer R is first set to the maximum value, K2 is opened, K1 is closed, and R is adjusted to bring the current meter G to full deviation and keep R constant. Then close K2, adjust the resistance of R0, so that the current meter G pointer is half-biased. At this time, the resistance value of the resistance box R0, when R»R0 is satisfied, the internal resistance of the current meter is equal to R0, namely:
This method is used to measure the internal resistance of the ammeter.
2. Constant pressure semi-bias method (1):
As shown in the circuit shown in Fig. 7, a voltage dividing circuit is used, and the current meter G to be tested is connected in series with the resistor box R and then connected in parallel with the millivolt meter. r is the protection resistor and R0 is the sliding rheostat. When measuring, r is set to the maximum value, and the resistance of the resistor box R is adjusted to a small value R1. Close K, adjust R0 and r, so that the ammeter pointer is fully biased, that is, the current intensity through the ammeter is Ig, and note the UAV reading at this time. According to the Ohm's law of some circuits:
Then adjust the resistance of the resistor box R and the sliding contact of the sliding varistor R0, so that the ammeter pointer reaches a half-bias, and the reading UAB on the millivoltmeter remains unchanged (ie, the voltage between the two points A and B does not change). At this time, the resistance indicated on the resistance box is R2, then:
Available from (1) and (2):
If the resistance of the resistor box is set to zero when the adjustment is started, that is, R1=0, adjust R0 and r to make the pointer of the ammeter reach full deviation, and the millivolt meter reads UAB. Then adjust the resistance of the resistor box R and the sliding contact of the sliding rheostat, so that the pointer of the ammeter is half-biased, and the reading of the millivolt meter is still UAB. Then, the resistance value R2 of the resistance box is equal to the internal resistance Rg of the ammeter, that is, Rg=R2. The error of the internal resistance Rg thus measured is smaller than that measured when R1 is not zero.
2. Constant pressure semi-bias method (2):
As shown in the circuit of Figure 8, the current meter G to be tested is connected in series with the resistor box R. The battery for the power supply can be ignored. When measuring, the resistance box resistance is placed at the maximum value, the switch K is closed, and the resistance value of the resistance box is gradually reduced, so that the current meter G pointer is half-biased. At this time, the resistance box reading is R1, which is obtained by Ohm's law:
Reduce the resistance of the resistance box again, so that the current meter pointer reaches the full Ig, (be careful not to exceed the full bias current Ig, so as not to burn the current meter). At this time, the resistance of the resistance box is R2, then:
Available from (3), (4):
This experiment requires that the resistance of the resistor box varies widely.
The above experimental design methods can also be used to measure the internal resistance of a large range of ammeters and voltmeters.
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