Purpose of an ammeter.
An ammeter is an electrical measuring device that is designed to measure the strength of electric current in some part of an electrical circuit. This value is given in units called amperes, hence the name of the device - “Ammeter”. In practice, electric current values are measured in various ranges - from microamperes (µA) to kiloamperes (kA).
An ammeter is the same galvanometer, only adapted to measure current strength; its scale is graduated in amperes.
In the diagrams, the ammeter is depicted as a circle with the letter A in the center.
You can also use a multimeter to measure current. Before taking measurements, you must read the instructions for the specific multimeter model in order to properly configure it and connect it to the electrical circuit.
How does an ammeter work?
There are two types of ammeters: analogue, which indicate the value by deflecting the needle of a mechanical device, and the increasingly used digital instruments, equipped with complex electronic circuits.
When making analog ammeters, it is necessary to use effects that depend on the magnitude of the electric current. Most often they are associated with the creation of a magnetic field by a conductor in which an electric current flows. The higher the current, the greater the effect produced by this phenomenon.
Each analog ammeter has a moving and a stationary part. An arrow is attached to the moving part, which moves along the scale and allows you to read the readings of the device. To avoid reading errors that are caused by the parallax effect, you should look at the needle at a right angle to the scale, which is facilitated by a mirror located next to the scale (see Figure 1).
Rice. 1. Indicating microammeter with a mirror installed to reduce the parallax effect when taking readings
Physical quantity
An ammeter measures the physical quantity Ampere.
Ampere determines the strength of current. Current strength is the speed of passage of charged particles with a volume of 1 Coulomb through 2 parallel conductors 1 meter long. This law was discovered by the French physicist Andre-Marie Ampère.
The first functional device for measuring current was invented by Johan Schweiger in 1820. At that time it was called a galvanometer.
Ammeter
An ammeter is an instrument that is used to measure current. It is a device with a scale and arrow. Inside the device there is a metal or magnetic frame. A coil is installed inside the frame. The operating principle of the ammeter is as follows:
- An electric current passes through the coil.
- The current creates a magnetic field that moves the needle on the instrument.
The measurement has almost zero error, due to the fact that the resistance of the ammeter is very small. It cannot influence or change the passing voltage parameter.
The display of each mechanical ammeter has a scale. The scale shows the ammeter's measurement limit. Such devices can measure from a high value of 10 Amperes, to the lowest in units of up to 200 microamps. When working, it is necessary to take into account the measurement limit. It can be expanded by connecting a transformer or a shunt element.
When working, you also need to know what current the ammeter is designed to measure. They are: constant and variable. On the scale of each device there is a designation for what type of voltage the device is intended for.
A DC ammeter is used to measure current in devices that operate through a step-down transformer and diode bridge. Consumers are often equipped with ammeters to monitor the load. It is very easy to find an ammeter in the circuit diagram - it is indicated by the letter “A” enclosed in a circle.
AC ammeters are used to measure the load of household or high-voltage networks. They operate by connecting a transformer or shunt resistor in parallel with a high resistance value. In this way, it is possible to reduce the load on the measuring device itself. Next, the main types of ammeters will be discussed.
Effect of temperature on current measurement
The ammeter is a sensitive device that is significantly affected by ambient temperature. Changing the temperature causes an error in the readings. You can use additional resistance (ballast resistance). Resistance with zero temperature coefficient is called additional resistance (swamping resistance). It is connected in series with the coil of the electrical measuring instrument. Ballast resistance reduces the influence of temperature on the instrument readings.
The ammeter has a built-in fuse that protects it from current surges (incorrect connection). If significant current flows through the ammeter, the fuse will blow, thereby breaking the electrical circuit and preserving the instrument's measuring system. Accordingly, the device cannot be used until the fuse link is replaced.
Varieties
The described measuring device has come a long way and many upgrades. Today, there are analog and digital types of these devices. There are also: magnetoelectric, electromagnetic, electrodynamic, ferrodynamic types of ammeters. Each type has its own characteristics of the device and its operation. Each type will be described in more detail below.
Magnetoelectric device
A special feature of these devices is a magnetic coil, which moves when exposed to electrical voltage.
All such devices are used to measure direct current. The advantage is very high sensitivity and measurement accuracy.
Electromagnetic
The device does not have a rotating coil in its design.
The change in the angle of the hand on the dial occurs due to the magnetic field acting on the cores of the coils. Similar ammeters are of a universal type. With their help you can measure the strength of direct and alternating current. The main disadvantage is the presence of error.
Electrodynamic
The device is similar in design to a magnetoelectric one. The main difference is the presence of moving and fixed coils.
When connected, the magnetic fields of the two elements influence each other, which leads to a change in the position of the arrow. The device is quite accurate. The only drawback is that its operation can be affected by extraneous magnetic fields.
Ferrodynamic
This measuring device is considered the most accurate. The ammeter device includes a ferrite wire, a metal core and a coil.
The device operates on the principle of coil rotation due to the formation of a magnetic field. The main feature is complete independence from the influence of extraneous magnetic fields. Has high sensitivity.
Electronic
With the development of electronics, ammeters began to be produced in digital variations. The two most famous are a simple household multimeter and a clamp meter tester.
The main advantage of such devices is the simplicity and versatility of current measurement. They are not susceptible to external magnetic fields, are not afraid of shocks, minor damage and shaking. Close to an ideal ammeter.
For information! An ideal ammeter is an ammeter with zero self-resistance.
All the devices described are used to this day in instrument making, laboratories, industry and by individual enthusiasts.
Safe work rules
When using the device, the following safety precautions must be observed:
- The device must not be shaken or dropped.
- If the arrow on the device goes off scale, you must immediately open the circuit.
Diagram of correct connection of the device
Connection rules:
- Connect the positive terminal of the device to the positive terminal of the current source. If the circuit consists only of a current source, the device cannot be connected to it!
- The ammeter is connected in series. The connection occurs with the element whose current strength needs to be measured.
- The device must be in a horizontal position.
Knowing the connection rules and types of devices, you can choose the most suitable ammeter for measurement.
Examination
The accuracy of any measuring instrument depends on its calibration. A stamp was placed on analog devices, which confirmed testing in the laboratory. Modern digital multimeters with current measurement mode do not have such stamps, and they have not been tested. Many novice electricians do not know how to test an ammeter. To check the measurement accuracy you must:
- Connect the device under test in parallel with the reference one. It is advisable that this be a mechanical ammeter.
- Connect a variable resistor to the circuit.
- Connect a resistor with high resistance to the variable resistor.
- Connect the circuit to a power source.
- Using the variable resistor “R2”, set the voltage threshold so that the needle of the mechanical device is exactly in the middle of its scale.
- Turn on the multimeter to measure current.
- Compare the results.
- Use a variable resistor to regulate the deflection of the needle of a mechanical device towards the maximum.
- If there is a deviation for each subsequent value, compare the test results.
This way you can check the accuracy of the digital multimeter when measuring current. To increase the load, you can add several incandescent lamps to the circuit. The following will give a detailed description of how to use an ammeter.
How to choose
The most convenient and accurate ammeters are digital. Recently, they have been replaced by more universal devices - multimeters, which, among other functions, can measure current.
Expert opinion
Torsunov Pavel Maksimovich
The ammeter should not be connected to a circuit with a higher voltage than it is designed for - this could cause the resistor to burn out. The voltage range in which the ammeter can operate is usually marked on the device body.
When choosing a device, you need to pay attention to the following criteria:
- Coating of contact terminals. Clamps with an anti-corrosion layer will last much longer.
- A device with a resistance of up to 0.5 Ohm will have more accurate readings.
- It is desirable that the case be sealed - this will protect its functional parts from moisture.
- When taking measurements, do not touch the non-insulated elements of the device - they can conduct current.
Connection
Next, we will consider 2 options for measuring current strength: for a circuit with alternating and direct voltage. Before connecting the measuring device, you need to remember that any ammeter has a very low intrinsic resistance. It is impossible to measure the current strength without a load from a third-party element. This is especially important when working with alternating voltage. All instructions will be given using the example of a digital multimeter in current measurement mode.
Alternating current
In order to measure the AC current you need to:
- Switch the multimeter switch to AC current measurement mode.
- Select the largest value.
- Connect the red measuring probe to the “10–20 A” socket, depending on the type of device.
- Insert the black probe into the “COM” socket.
- Connect the power cord to the transformer (it is forbidden to plug it into an outlet).
- Connect one contact of the test lamp to the “+” terminal from the transformer.
- Connect the second contact from the lamp to the red measuring probe of the tester.
- Connect the black test lead to the second terminal of the transformer.
- Apply voltage to the transformer.
The ammeter will show the consumption value of the test lamp in amperes. Connecting test leads without a lamp is strictly prohibited.
Alternating current can also be measured using a clamp meter. To do this you need:
- Remove the control probes from their sockets.
- Switch the tester to current measurement mode.
- Grasp the wire core with pliers.
The ammeter will display the consumption value.
D.C
To measure direct current, a parallel connection of the tester is also used. Next you need:
- Switch the device to DC voltage measurement mode.
- Insert the red test lead into the “mA” socket.
- Leave the black one in the “COM” socket.
- Select the largest measurement parameter in milliamps.
- Connect the minus input of the device being measured to the minus terminal of the battery.
- Connect the “+” input of the device to the black measuring probe.
- Connect the red test lead to the “+” terminal of the battery.
In this way, you can find out the threshold consumption of an appliance or device operating on constant voltage.
Replacing the battery
The device is equipped with a lithium battery ER14250 or CR2032. Possible replacement options are given in the user manual.
To replace you will need:
- Remove the terminal compartment cover secured with screws on the front side.
- Dismantle the electronic unit in the same way.
- Remove the used element by soldering a new one.
- Reassemble the device in reverse order.
After completing the work, the organization that changed the battery indicates this fact in the corresponding section of the meter passport.
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