Why do you need to adjust the resistance to zero when the ohmmeter shifts gears?
1. Principle
Connect the battery pack, ammeter and rheostat in series to form the internal circuit of the ohmmeter.
1) Measurement state
Connect the resistance to be measured between the two test leads of the ohmmeter, then the battery pack, ammeter, rheostat and the resistance to be measured form a closed circuit, the current in the circuit changes with the change of the resistance to be measured, and the current scale value of the ammeter is changed to the corresponding The scale value of the external resistance can be directly read from the ohmmeter to measure the resistance value of the resistance.
Rx=εI-(r+Rg+R)
Example A sensitive ammeter with a full bias current of IG=100μA and an internal resistance of Rg=100(Ω), a battery pack with an electromotive force of ε=1.5V, an internal resistance of r=0.1(Ω), and a rheostat with a total resistance of R=I8KΩ Connect them in series and adjust the rheostat to R=14.9 (KΩ), that is, assemble into an ohmmeter. The resistance value to be measured corresponding to each current value is calculated from the above formula as shown in the table:
Mark the corresponding resistance value to be measured at each current scale on the dial, and then read the resistance value to be measured directly.
2) Zero adjustment state
①Mechanical zero adjustment
When the two test leads are separated, that is, when the resistance to be measured is infinite, the current intensity at this time is zero according to Ohm's law. That is, when the two test leads are separated, the state indicated by the meter pointer should be zero current and infinite ohms. However, due to various reasons, when the two test leads are separated, the pointer of the ammeter sometimes does not point to the zero current scale, which requires mechanical zero adjustment. Turn the mechanical zero adjustment screw with a screwdriver to drive the pointer to rotate, so that the pointer points to the infinite ohm scale.
② Ohm zero adjustment
When the two test leads are short-circuited, according to Ohm’s law, the ammeter can be fully biased by adjusting the sliding rheostat, that is, the pointer points to the full-bias current scale of the ammeter, that is, the zero-ohm scale. That is, when the two test leads are short-circuited, the state indicated by the pointer of the ammeter should be full-bias current and zero-ohm resistance. Otherwise, adjust the rheostat so that the pointer of the ammeter points to the full-bias current scale, that is, the zero-ohm scale, and the zero-ohm adjustment is completed.
2. Internal resistance
1) Design value
Short-circuit the two test leads of the ohmmeter, that is, the ohmmeter is in the zero-adjustment state. According to Ohm's law, the internal resistance of the ohmmeter is equal to the ratio of the electromotive force of the power supply in the ohmmeter to the full-bias current of the ammeter in the ohmmeter RΩ=ε /IG. So after the sensitive ammeter and battery used to assemble the ohmmeter are selected, the internal resistance of the assembled ohmmeter is determined.
2) Actual value
The actual internal resistance of the ohmmeter is composed of the internal resistance of the power supply, the internal resistance of the ammeter and the resistance of the zero-adjusting rheostat in series, and its total resistance value should be equal to the design value. RΩ=r+RG+R. We should choose the total resistance of the sliding rheostat reasonably to meet the requirements of the design value of the internal resistance of the ohmmeter.
3) scale value
When the resistance value of the measured resistance is exactly equal to the internal resistance RΩ of the ohmmeter, the total resistance of the entire measurement circuit is equal to twice the internal resistance of the ohmmeter, and the measured current is half of the full bias current of the ammeter, that is, the pointer points on the scale plate. Median R? stains. That is, the median scale of the ohmmeter indicates the internal resistance value of the ohmmeter R? Stain = RΩ.
3. Error
1) Power supply error
After the ohmmeter is used for a long time, the electromotive force of the battery decreases and the internal resistance increases. Although the ammeter is fully biased when performing ohm zero adjustment, this change makes the read resistance value greater than the real value of the measured resistance.
The design standard value of the internal resistance of the ohmmeter is determined by the electromotive force of the new battery and the full bias current of the ammeter: RΩ=ε/IG; the corresponding relationship between the resistance scale and the current is determined by the standard value of the electromotive force of the new battery and the internal resistance of the ohmmeter: RX *=ε/I-RΩ; when the old battery is installed, the actual internal resistance of the ohmmeter is less than the standard internal resistance after ohm zero adjustment: RΩ*=ε`/IG; when the old battery is used, the electromotive force of the power supply and the internal resistance of the ohmmeter and the The actual value of the measured resistance determines the measured current I=ε`/(RΩ+ RX) in the table, and the above four formulas are solved simultaneously
RX=εε'RX
It can be seen that as the electromotive force of the power supply gradually decreases, the measured value of the resistance gradually increases in inverse proportion.
Example The electromotive force of an ohmmeter battery is 1.5v. After long-term use, the electromotive force drops to 1.2v. Use it to measure a resistance. The measured value is 500Ω. What is the actual value of the resistance?
Solution: Rx=(ε`/ε) RX*=1.2÷1.5×500=400Ω
2) Reading error
Due to the limited observation ability of human beings, there are always geometric errors in the readings. Let the current scale at the actual position of the pointer be I, and the corresponding ohm scale be RΩ, and the current scale at the observed pointer position be I`, and the corresponding ohm scale be RΩ`. Then by
RX=εI-RΩ and R'X=εI'-RΩ
Get ΔRx=εI-εI'=-I-I'I·I'-ε=εI2·ΔI
That is, δ=ΔRxRx=εI2·ΔIεI-εIG=IGI(IG-I)·ΔI
That is, δ=Θθ (Θ-θ) Δθ
It can be seen that the sum of the two factors of the denominator is a certain number, that is, the maximum deflection angle, so when the two factors of the denominator are equal, the maximum product reading error is the smallest.
That is, when θ=Θ2, δ=δmin=4·ΔθΘ
Therefore, at the geometric midpoint of the scale arc, the ohmic error caused by geometric parallax is the smallest.
The appropriate gear should be selected so that the indicated value of the pointer is as close as possible to the median value on the panel, so that the reading error is minimized.
