①Select the range of the instrument
When using an analog voltmeter or analog ammeter, the most accurate reading should be near the middle of the scale. For example, if the measured voltage is around 30V, then you should choose a full-scale range of 60V instead of 300V. An analog ohmmeter usually has only one resistance scale.
Many digital meters have automatic transmission, which means that the meter can automatically select the range. Another advantage of the digital meter is that it does not require the user to estimate the reading between the scale lines.
The instrument should be selected according to the measurement needs, taking into account the requirements of range and accuracy. The analog voltmeter may include the following DC ranges: DC0~10V, DC0~50V, DC0~250V and DC0~1000V. The range of AC voltage is similar. If you are not sure about the measured voltage value, you should choose to start the measurement from the highest voltage file, so as to protect the meter from damage.
The analog ammeter also has multiple ranges, such as 0~50 microampere (μA), 0~1 milliampere (mA), 0~100mA, 0~500mA and 0~10A. When measuring current, the ammeter should be connected in series with the circuit under test. Most multimeters can measure AC and DC voltage and current, but some analog meters cannot measure AC current.
Some digital multimeters can automatically shift gears, but only within the selected voltage and current ranges, such as AC/DC current 0~2mA, 0~200mA, 0~500mA and 0~10A. The multimeter will automatically change gears within the selected range, so the range closest to the measured value should be selected.
If working under high voltage, be careful when the test leads touch the measuring point or make circuit connections. If possible, disconnect the power supply before connecting the meter.
② Ohm’s law
Ohm’s law describes the proportional relationship between voltage, current and resistance in a circuit. This relationship was first discovered by the German scientist George Simon Ohm. Ohm’s law states that a voltage of 1V can drive a current of 1A through a resistance of 1Ω. Ohm also uses a formula to express this proportional relationship:
In this formula, if the voltage (V) and resistance (R) are known, the current (I) can be calculated. It can be seen that the current is proportional to the voltage and inversely proportional to the resistance. If the voltage does not change, the increase in resistance will cause the current to decrease, and the decrease in resistance will cause the current to increase. If the resistance does not change, the increase in voltage will increase the current, and the decrease in voltage will decrease the current.
Because Ohm’s law uses algebraic expressions, two other formulas can be derived:
When applying Ohm’s law, you can choose the formula that suits your specific problem. If V and R are known, then the formula I=V/R can be selected; if I and R are known, then V=I×R; if V and I are known, then R=V/I should be selected. In some applications, a combination of formulas can be used to solve the problem.
a. Knowing that V=12V, R=3Ω, what is the current (I)?
b. Knowing that 1=10A, R=8Ω, what is the voltage (V)?
V=I×R = 10 ×8V = 80V
c. Knowing that V=120V, I=6A, what is the resistance (R)?
R=V/I= 120/60 =20Ω
d. Knowing that there is an electric toaster (toaster) connected to a 120V power supply, and the current consumption is 2A, what is the power (watts) of the toaster? What is the resistance of the heating element of the toaster?
The unit of measurement for work (P) is W, which can be calculated using the formula P=V×I; resistance (R) can be calculated using Ohm’s law.
P=V×I=120 ×2W = 240W
R = V/I = 120/2Ω = 60Ω
Although the formula for calculating power does not belong to Ohm’s law, the two are still closely related. Like Ohm’s law, the power formula shows the relationship between voltage, current, and resistance, and how they affect power. The graph shown in Figure 3 can be used to calculate current, voltage, resistance, and power. It is divided into four parts, which are used to calculate current (I), voltage (E), power (P) and resistance (R). Each part has three formulas. By choosing one part, for example, I (ampere), you get three formulas, corresponding to different measured values (known quantities). If E=24V, R=2Ω, to calculate the current (I) of the circuit, you can choose the formula
If P and E or P and R are known, you can choose the other two formulas to calculate the current (I)
a. If the power (P) is 200W and the voltage (E) is 48V, calculate the current (I). Choose the formula I=P/E from the chart in Figure 3, there is
I=P/E = 200/48A=4.16A
b. If the power (P) is 100W and the circuit resistance (R) is 10Ω, calculate the current (I). Choose the formula I= √P/R from the chart in Figure 3, there is
I= √P/R= √100/10A=3.16A
When applying Ohm’s law, two variables must be known in order to calculate another unknown quantity.