What is a power supply, voltage and voltmeter?
Electrical measurement and Ohm's law

What is a power supply, voltage and voltmeter?

There are many kinds of commonly used power supplies, here are 6 kinds:
(1) A battery is a direct current (DC) power source that converts chemical energy into electrical energy.
(2) A rotary generator is a direct current (DC) or alternating current (AC) power source that converts mechanical energy into electrical energy.
(3) A photovoltaic cell is a direct current (DC) power source that converts solar energy into electrical energy.
(4) An electronic power supply device is a power supply device that converts one form of electrical energy into another form of electrical energy, such as an AC120V/DC12V power adapter.
(5) The fuel cell is a direct current (DC) power source that converts the energy of fuel (such as hydrogen) and oxidant (such as oxygen) into electrical energy. Voltage.
(6) The lithium ion battery ups power supply is an uninterruptible power supply. If the main power supply fails, the UPS will provide emergency power. Under different conditions, AC or DC power will be supplied .
The power supply can provide current and voltage at the same time. You can use an ammeter to measure current and a voltmeter to measure voltage.


The voltage produces the power that pushes the current through the conductor, and its unit of measurement is volts (V). Figure 1 shows a 120V voltage source whose “pressure” can push current through a light bulb. Voltage is like the pressure of a water pump in a hydraulic system. In a hydraulic system, the pressure generated by the pump drives the flow of water, while in an electrical system, the voltage drives electrons. The voltmeter is in the water pipe, although water does not necessarily flow, but the water pressure still exists: in the circuit, although there is no current, there is still voltage at both ends of the power supply, and the current is not equal to the voltage. Please note that the switch in Figure 1 is off and the current is zero, so no voltage can be measured at both ends of the negative teaching, but 120V can still be measured at both ends of the power supply.
Voltage is also called electromotive force (EMF or E). The capital letter V or E is commonly used to indicate the voltage of the circuit or power supply, and the capital letter I is used to indicate the current.

What is a power supply, voltage and voltmeter?
Figure 1 Measuring voltage in a disconnected circuit

The magnitude of the voltage from one point to another is equal to the work done by a unit positive charge moving from one point to another due to the force of an electric field. The direction of the voltage is defined as the direction from a high potential to a low potential. The charge q moves from point A to point B in the electric field. The ratio of the work done by the electric field force to the amount of charge is called the potential difference (voltage difference) between the two points AB, and this voltage difference is called the voltage drop. Voltage drop is the value of voltage required to drive current through the resistor. The method of calculating the voltage drop is to multiply the current (I) by the resistance (R), that is, V=I×R, which is consistent with the formula of Ohm’s law. When the circuit has two loads, the voltage (E) must be high enough to drive current through all loads. An important circuit law is that the sum of the voltage across load 1 and load 2 is equal to the power supply voltage E.

A voltmeter (also called a voltmeter) is used to measure voltage. As shown in Figure 2, when measuring voltage, you need to connect the meter pen of the voltmeter to both ends of the load, that is, connect the voltmeter in parallel with load 1 or load 2. This is completely different from the measurement method of series ammeter and load in series.

What is a power supply, voltage and voltmeter?
Figure 2 Two loads are connected in series, and a voltmeter is connected across each load.

Please note that the voltage reading V1 on R1 is 10V, the voltage reading V2 on R2 is 18V, the power supply voltage E is 28V, and the sum of V1 and V2 is equal to 28V. The power supply in Figure 2 is a photovoltaic (PV) array, which is a DC power supply.

When the voltmeter is connected across the load, a load effect will occur, that is, a small part of the current that should pass through the load under test will be shunted by the voltmeter. At this time, the voltmeter becomes another parallel to the original load. load. The small amount of current shunted to the voltmeter will not affect the normal operation of the circuit, but it is still very important to choose a voltmeter with a large internal resistance (ohm per volt, Q/V) specifications. The higher the ohm per volt, the less current the voltmeter “draws” from the circuit, and the more accurate the measurement will be (the ohm per volt is usually indicated on the voltmeter).

If the specification of a voltmeter is 20000Ω/V and the range is 30V, the internal resistance of the meter is 600000Ω(20000Ωx30). The voltmeter can be an analog meter with a scale and pointer, or a digital meter with numerical display. A typical DC analog voltmeter is about 20000Ω/V, and an AC analog voltmeter is about 10000Ω/V. The components used in the digital voltmeter enable extremely high input resistance between the test leads (up to the order of megaohms), which greatly increases the ohm per volt. The ohm per volt of a digital voltmeter is higher than that of an analog voltmeter.

What is a power supply, voltage and voltmeter?
Figure 3 Load effect of voltmeter

In Figure 3, the voltmeter shunts a small amount of current (Im) from the power supply current (Is) that should flow through the load. As a result, the load current (IL) is reduced by these shunt currents. If the ohm per volt of the voltmeter is larger, the IM will be reduced, and the load effect that affects the work of the load will also be reduced. In Figure 3, if Is=1.000A, IM=0.005A, the load current will be reduced to 0.995A (IL=1.000-0.005=0.995A). Figure 4 shows a digital multimeter with a digital display, and Figure 5 shows an analog multimeter.

What is a power supply, voltage and voltmeter?
Figure 4 Digital multimeter with digital display
What is a power supply, voltage and voltmeter?
Figure 5 Analog multimeter

In Figure 6, 300, 60, and 12 are marked on the right side of the horizontal scale line (AC-DC) in the middle position. The voltage reading on this scale line should be multiplied by a multiple of 10 or divided by a multiple of 10. This multimeter has a selector switch to select the range. Assuming that the measured voltage is between 0~3V, if the selector switch is set to the full scale 3V range, the 300V scale should be divided by 100V (300V +3=100V). As shown in Figure 6, the 250V reading should be divided by 100, that is, the measured value is 2.5V. If the selector switch is set to the full scale 0 ~ 30V range, the 250V reading should be divided by 10, that is, the measured value is 25V. If the selector switch is set to the full-scale 60V range, the measured value is 50V. If the measured voltage range is unknown, select the highest range first, and then gradually reduce the range until the reading is roughly in the middle of the scale. Using this method can reduce the risk of meter head damage and obtain the best measurement accuracy.

What is a power supply, voltage and voltmeter?
Figure 6 Scale of analog multimeter