How is the current measured in a DC loop? What is the principle?

The probe is put on the cable, and the signal processing is carried out, and the output is 4-20mA, RS485, wireless, and used by other testing equipment. The non-contact detection method eliminates the hidden danger of short circuit that may be caused by the contact measurement voltage method. It has two installation methods: closed-loop and open-ended, with a variety of shapes and mounting structures (PCB boards, guide rails, screws); High isolation between the input terminal and the line under test, and high withstand voltage.

Because the measured current is connected in series in the loop, the principle of the ammeter is to keep the internal resistance as small as possible, so that the current can be measured without affecting the operation of the circuit.

DC ammeter and AC ammeter are very different, can not exchange measurements, and there is no way to exchange measurements, for large current ammeters, DC ammeters are actually just a 75 millivolt DC voltmeter (measuring the millivolt voltage on the shunt), and the internal resistance of the AC ammeter is very small (measuring the current of the transformer secondary short circuit), the circuit is short-circuited, and the current can allow AC 5 amperes to flow through.

For small current meters, although the shunt or transformer cannot be seen from the outside, the basic internal structure is the same as that of high current.

The above refers to ordinary mechanical watches, and other special, special watches are a different matter.

In the AC circuit, the ammeter is connected to the secondary side of the current transformer, which uses the current transformer to change the AC current flowing through the equipment into the secondary current according to a certain ratio, and the secondary current is transferred into the ammeter, so that the ammeter has an indication. Since the direction of the current in the DC circuit does not change with time, the magnetic field generated by the current is balanced, so the current transformer cannot be used.

There are generally two ways to measure current in a DC loop:

1. Connect the ammeter coil directly to the DC circuit, and the current in the loop is the current in the ammeter coil, but the ammeter can only be used in the circuit with a small current.

2. The method of parallel connection between the shunt and the ammeter is adopted, and the ammeter is connected to the loop and its principle is as follows

The shunt is actually a resistor, and the greater the current through the shunt, the higher the voltage between the two ends of the shunt, which causes current to flow through the ammeter between the two ends of the shunt. Therefore, it is very important for the meter to have the coordination of the indicated shunt and the ammeter, and the same proportion should be selected. The selection of the inappropriate side makes the meter indication incorrect, such as the shunt

1500A, 75MV, that is, when the current flow through the shunt is 1500A, the voltage at both ends of the shunt is 75MV, and the ammeter used with the shunt is 75MV, 1500A. That is, when the voltage at both ends of the shunt is 75mV, the ammeter indicates 1500A, and its principle is to use Ohm's law u=i*r, and the wiring between the shunt and the ammeter is as follows:

Two points on the shunt are directly connected to the circuit and two points are connected to the ammeter.

The voltage measured by a digital meter is calculated by comparing the detected voltage with an internal reference. Here's how it works:

The current measurement connection method is as follows (pay attention to the range matching, small current can be connected to the MA gear) <>

The digital multimeter measures all signals to be measured, capacitance, resistance, current, inductance, DC, AC signal, unified into DC voltage signal, after the internal attenuator attenuation to, 1 100 1 1000 1 10000 and then sent to a D converter to process the display, of course, the external connection must have a voltage divider resistor, the working principle of the digital multimeter is to use an ICL7106 as the core A D converter and an LCD liquid crystal display form a full-scale range of 200mv digital voltmeter head based on the expansion of the range, through this basis, external voltage divider resistor, shunt resistor, rectifier op amp, composition, DC voltage range, AC voltage gear, resistor, current block, digital multimeter, DC voltage gear, AC voltage gear, electrical resistance is generally a common voltage divider resistor, DC current block and AC current file are common shunt resistors, AC voltage gear and AC current gear common rectifier op amp.

Direct current (DC) and alternating current (AC) are two common forms of current, which are distinguished by Jane in the following aspects:

1.Directions:

Direct current: The current flows in a fixed direction in a circuit, and the flow of electrons moves in only one direction, such as the current provided by a battery.

Alternating current: An electric current periodically changes direction in a circuit, and the flow of electrons alternates back and forth, such as the current provided by a home power supply.

2.Voltage waveform:

Direct current: The voltage remains stable and does not change with time, presenting a smooth straight line.

Alternating current: The voltage periodically changes with time and appears as a sinusoidal waveform.

3.Transmission Loss:

Direct current: Since the direction of the current does not change, the energy loss of direct current during transmission is small.

Alternating current: Due to the change in the direction of the blocking current, there will be energy loss caused by factors such as resistance and inductance in the transmission process of alternating current.

4.Voltage regulation:

Direct current: The voltage of direct current can be adjusted and controlled relatively easily, for example through the use of resistors, transformers, or voltage regulators.

AC Coolwood: The voltage of AC can be regulated by a transformer, but regulation and control are relatively complex.

5.Applications:

Direct current: Direct current is often used in electronic devices, battery-powered mobile devices, etc.

Alternating current: Alternating current is commonly used in household power supplies, power transmission networks, industrial equipment, etc.

It is important to note that although direct current and alternating current differ in characteristics, they can be converted to each other, for example, by using an inverter, direct current can be converted to alternating current, while using a rectifier can convert alternating current to direct current.

(1) Expansion of DC ammeter and range: The instrument used to measure DC current is called DC ammeter. According to the different current ranges measured, it can be divided into kiloampere meter, ampere meter, and microampere meter.

The most common ones are ampere meters and milliampere meters, which are marked with the words A and MA on the surface, respectively. There are two types of DC ammeters: fixed and portable, and there are square and round fixed ammeters.

A DC ammeter consists of a meter head and a shunt, which are housed inside a sturdy case. The meter has two binding posts with + and - symbols next to the binding posts.

The meter head is made according to the measuring principle of a magnetoelectric instrument. This type of meter is designed to have a range of 50 uA to 5 mA, and can be used directly to measure DC currents below a few milliamps. The DC ammeter that measures the larger current is connected in parallel at both ends of the meter head, and this parallel resistor is called a shunt, and the shunt resistor is generally installed inside the meter.

Figure 5-1 shows a DC ammeter circuit with a shunt.

2) Expansion of measuring range: The use of shunt can expand the measurement range of the ammeter. This is because the current IA passing through the meter is only a part of the total current I, and the vast majority of the current IM passes through the resistance RM, that is, most of the measured current i is shunted by the resistor RM, and the parallel resistor RM is named a shunt.

Obviously, the smaller the shunt resistor RM, the stronger the shunt effect and the larger the range of the meter. For the same ammeter, changing different shunt resistors can make ammeters of different ranges. Usually the transfer switch changes the size of the shunt resistor and selects a different range.

There are two ways to connect the shunt resistor of a multi-range ammeter:

Independent shunt table: see Figure 5-2a. Different shunt resistors are used in different processes. The advantage of this kind of circuit is that the circuit is simple, and the blocks do not affect each other; The disadvantage is that the contact resistance of the switch is connected in series with the shunt resistance, and the contact resistance affects the measurement accuracy.

The closed-circuit tap shunt method is shown in Figure 5-2b. There is no switching contact resistance, and the measurement accuracy is high, but there is an influence between each block, and one resistance is damaged, and each block cannot be used.

3) Measurement of DC current: When measuring DC current, the DC ammeter must be connected in series in the circuit. Here are a few things to keep in mind when connecting:

The polarity should be correct, the + termination of the ammeter is connected to the high potential end of the circuit, and the - termination is to the low potential end, and the current flows from the + pole to the - pole of the ammeter.

The range should be appropriate, and the range should not be too small, so as to prevent damage to the ammeter due to excessive current; Do not choose too large the range, to avoid the pointer deflection angle is too small, the reading is not accurate, the pointer is in the center. The optional range is larger, and then the transfer switch is adjusted to increase the deflection angle of the pointer.

To measure a high-voltage circuit, the meter is connected to the end that is close to zero potential.

Digital multi-purpose means that digital measuring instruments have become the mainstream, and there is a tendency to replace analog instruments. Compared with analog meters, digital meters have high sensitivity, high accuracy, clear display, strong overload capacity, easy to carry, and simpler to use. The following takes the VC9802 digital multimeter as an example to briefly introduce its use and precautions.

1) How to use.

Before use, you should carefully read the relevant instruction manuals and be familiar with the functions of power switches, range switches, jacks, and special sockets.

bPut the power switch in the ON position.

c. Measurement of AC and DC voltage: turn the range switch to the appropriate range of DCV (DC) or ACV (AC) according to the need, insert the red pen into the V hole, insert the black pen into the COM hole, and connect the pen in parallel with the line to be measured, and the reading will be displayed.

dMeasurement of AC and DC current: Dial the range switch to the appropriate range of DCA (DC) or ACA (AC), insert the red pen into the MA hole (200 mA) or 10A hole (200 mA), insert the black watch pen into the COM hole, and connect the multimeter in series in the circuit to be measured. When measuring straight flow, the DMM automatically displays the polarity.

Measurement of e-resistance: Dial the range switch to the appropriate range, insert the red pen into the V hole, and insert the black watch pen into the COM hole. If the measured resistance value exceeds the maximum value of the selected range, the multimeter will display "1", in which case a higher range should be selected.

When measuring resistance, the red pen is positive and the black pen is negative, which is the opposite of an analog multimeter. Therefore, when measuring polarized components such as transistors and electrolytic capacitors, it is necessary to pay attention to the polarity of the stylus.

2).Precautions for use.

aIf the size of the voltage or current to be measured cannot be estimated in advance, it should be measured once in the highest range, and then gradually reduce the range to a suitable position according to the situation. After the measurement, the range switch should be turned to the highest voltage and the power should be turned off.

When the full range is b, the meter only displays the number "1" in the highest position, and the other digits disappear, so you should choose a higher range.

cWhen measuring voltage, the digital multimeter should be connected in parallel with the circuit under test. When measuring the current, it should be connected in series with the circuit under test, and the positive and negative polarity do not need to be considered when measuring the straight flow.

dWhen the AC voltage is misused to measure the DC voltage, or the DC voltage is misused to measure the AC voltage, the display will show "000", or the number on the low position will jump.

It is forbidden to change the range when measuring high voltage (above 220V) or high current (above ) to prevent arcing and burning of the switch contact.

fWhen ""batt" or "low bat" is displayed, it means that the battery voltage is lower than the working voltage.

Whether you measure DC or AC voltage, if you use a multimeter, you should talk about the circuit disconnection, and then select the corresponding gear, and connect the meter pen in series in the circuit, which is in series. When the side voltage, as long as there is a potential difference between the two points of the circuit, the watch pen can be inserted directly into these two points after selecting the gear.

To determine the current flow direction of each circuit, you can use the hypothetical method, first assume the direction of the current, and in the calculation, if the result is a positive return, the original hypothesis answer.

Correct, if the result is negative, it means that the original assumption is reversed.

The basic method of calculating a circuit is to make use of Kirchhoff's first and second laws.

For the circuit shown in the figure, it can be seen that according to the analysis of the constant current source, the direction of the current passing through the resistance should be from left to right, and the current is 2A, but according to the analysis of the constant voltage source, the current direction through the resistance is from right to left, and the current is 3A, so the final result should be the current from right to left, and the current is 1A

The current direction of the current source is the marked current direction, the current direction of the voltage source is from the positive pole, and the 5 ohm is from c upwards.

It can be done in steps.

First of all, there is an ammeter or voltmeter in the circuit, you can go to the meter first, at this time, change the current to a wire, and remove the voltmeter and the wires at both ends. Like the third one, you can remove the ammeter and voltmeter first and replace it with a simpler circuit.

Don't listen to the waste above, there is only one positive electrode out of the current direction, and the negative electrode enters, flowing from high voltage to low voltage.