Resistance measurement circuit in pointer multimeter
According to Ohm's law, the current flowing through the meter head is:
In the formula:
RC - internal resistance of the meter head;
RP - Zero potentiometer resistor;
E - Battery terminal voltage in the table;
RX - measured resistance;
When the battery voltage E in the meter, the internal resistance RC of the meter head, and the resistance RP of the zero adjustment potentiometer are timed, there is a certain correspondence between the meter head current I and the measured resistor RX, that is, different measured resistors will have different meter head currents I. If the scale line on the meter head is directly pressed against the resistance value scale, the magnitude of the resistance value can be directly read, so measuring the resistance value is actually still measuring the current value.
① When Rx=0, the current I in the meter head is maximum. Adjust the zero potentiometer RP to Rp so that the current I flowing through the meter head is equal to the full bias current. That is to say, after adjusting the zero potentiometer, the resistance is adjusted from the original Qin to RP. Therefore, when the meter head current is measured, the pointer reaches the full scale position of the current, which corresponds to the "0" scale position of the resistance.
② When Rx=∞, the meter current I=0, so the mechanical zero position of the meter is positive.
③ When Rx is any value between 0 and ∞, the meter pointer will indicate the corresponding position between the current full scale (resistance 0 scale) and the mechanical zero position (resistance ∞ scale). Due to the imbalance between the current I flowing through the meter and the measured resistor RX, the division of the resistance scale line is uneven, as shown in the figure.
Resistance meter scale line
④ When RX=RC+Rp (Rc+RP is the total internal resistance of the resistance meter)
At this point, the pointer on the meter head is at the geometric center of the scale line, and the value indicated is called the Ohm center value. It can be seen that the resistance value marked at the geometric center of the resistance scale line (ohm center value) should be equal to the total internal resistance value of the resistance meter.
The Ohmic center value of an ohmmeter determines the effective measurement range of the ohmmeter. Although the scale of the resistance meter has a range from 0 to ∞, due to the uneven scale of the resistance meter scale, the practical measurement range is only between 0.1 and 10 times the center value of the ohm. If the measured resistance exceeds this range too much, accurate measurement results cannot be obtained. To enable the multimeter to accurately measure resistance values over a large range, the multimeter resistance measurement circuit uses a multi range resistance meter. In order to share a resistance scale line, the center value of the ohm can be increased by a factor of 10 to expand the range of resistance measurement. For example, if the ohm center value of RX1 gear is 50 Ω, then the ohm center values of other gears are taken as 500 Ω, 5k Ω, etc., thus forming a resistance meter for multiple gears such as RXl, RX10, RX100, etc.
