Long gone are the days when measuring instruments could only be found in physics class at school or from electrical specialists. These were mainly voltmeters - rather bulky units with a large percentage of error. Everything changed when semiconductor radio components were created. The market was filled with various devices, the first multimeters appeared. What functions does one of these devices perform - can be seen from the DT 838 instruction.
Content
-
Functions performed by the device
- Constant pressure
- Effective value
- Current measurement
- Determination of resistance
- Other features of the multimeter
Functions performed by the device
The word “multimeter” itself consists of two words: “multi” means “a lot”, and “meter” means “measure”. It turns out that with the help of the device many different measurements can be made. The first devices were pointer devices. The arrow was turned on the scale with the help of an electromagnet, and a spring returned it back. Modern devices have basically completely switched to digital indication. What can they measure? To understand how to use the DT 838 multimeter, it is important to know its features.
Constant pressure
The presence of electric current is difficult to determine without a device. You can, of course, touch it with your hand if you know that the voltage is small, but how do you know what it is? Existing indicators only indicate the presence of life-threatening voltage. It is measured between two points and shows the potential difference if there is no external influence. The circuits in which the measurement is made are divided into two types:
- D.C.
- Alternating current.
Constant is called a current, the magnitude and direction of which do not change over time. An example would be a battery, a rechargeable battery.
A variable is a current that changes its magnitude and (or) direction in time. It includes:
- sinusoidal;
- intermittent;
- rectified.
In practice, alternating voltage means a sinusoidal current that reverses its polarity. It is also called periodic because the polarity changes regularly at regular intervals. Measuring DC voltage is not difficult, since the value remains unchanged over time.
On the panel of the DT 838 multimeter itself, there is a letter V in the upper left corner, next to which are drawn straight and dashed lines. The polygon circled in white contains numbers. This is a scale for measuring DC voltage, where the maximum values of the measured voltage are indicated. If the letter m is next to the number, then millivolts are measured. 1 volt contains 1000 mB. To connect the required value, the marked end of the handle of the multimeter is aligned with the selected number.
Effective value
The DT 838 digital multimeter comes with test leads with different colored wires. Black connects to the bottom jack, red to the middle. These sockets on the panel of the device are graphically connected, and the present inscription shows the limits of the measured current and voltage. The indicators allow you to measure direct and alternating voltage up to 600 V, and current - up to 200 mA.
The alternating (sinusoidal) voltage is constantly changing over time, and this presents a certain difficulty. If we take the average, then it will be equal to zero, obtained by adding the maximum "plus" with the maximum "minus". Therefore, different measurement methods are used:
- instant;
- amplitude;
- acting.
The instantaneous value shows the voltage at a specific time, and the peak value determines the maximum value. These methods are rarely used, since they mainly reveal the acting voltage. To do this, compare the work of alternating and direct current, divide the amplitude value by the root of two (approximately 1.41). Knowing the effective value, you can determine the amplitude. For example, if the network is 220 V (effective value), then the amplitude will be equal to 311 V.
Technically, this happens as follows: two series-connected diodes are connected in parallel with the same other two diodes. An alternating voltage is connected between two series-connected diodes, the positive voltage is removed from the combined cathodes, and the negative voltage is removed from the anodes. Thus, the alternating voltage is converted into direct voltage and then measured. To extinguish its excess, a resistance is connected in series.
By turning the switch lever, one or another resistor is connected, expanding the capabilities of the device. If the measured voltage is unknown, the measurement always starts with a higher value. It is strictly forbidden to find and use a voltage exceeding the maximum permissible for the device.
Current measurement
Unlike voltage measurement, when a voltmeter is connected in parallel with a power source, current is measured differently. The measured electrical circuit is broken, and an ammeter is connected to the break. In this case, the multimeter introduces its resistance. To reduce distortion and expand the measurement limit, shunts are used - resistors with a very precisely selected resistance, which are connected in parallel with the device and lower the total resistance.
In a multimeter, such a shunt allows you to measure significant currents., because its resistance is less than the resistance of the measuring device, and most of the current flows through it. A very large current is dissipated on it, therefore, a warning is put on the panel of some multimeters about how long it is possible to measure large currents. For example, DT 838 C states that a 10 A current measurement should last no more than 10 seconds with 15 minutes rest.
In the DT 838 multimeter, the measured current can reach 10 A. In this case, the probe with the red wire is connected to the upper contact (it serves only for this purpose), and the switch position is set to 10 A. The scale for measuring current is indicated by the letter A with straight and dashed lines. Small currents are measured in milliamperes (the letter "m" stands) or microamperes. 1A = 1000 mA = 1 million microamperes.
It is strictly forbidden to turn on the ammeter according to the voltmeter circuit, i.e. parallel to the power supply. The device is designed to measure only direct or unidirectional current. This is due to the fact that diodes are needed to rectify the current, and they have a very large forward resistance, which is unacceptable for an ammeter. To measure alternating current, special transformers are used.
Determination of resistance
The third main quantity of electric current is resistance. It is measured against a DC current. The device uses a battery for this purpose. You can also use a battery, but this is undesirable, since the power consumption is small and the battery will lose capacity. The readings are given in Ohms, and if after the number there is the letter "K" - in kilo-ohms.
To check the resistance of the resistor, put the switch of the device at the mark that is most suitable for the value of the resistor. This scale is marked on the instrument with the letter "omega". When checking variable resistors, both the common and between the moving contact and one of the extreme ones are measured. Moreover, when the movable contact is turned, the resistance should change smoothly. This measurement shows the quality of the moving contact.
If the resistor is on the board, then one of its terminals must be unsoldered (variable, possibly completely), otherwise the reading may be inaccurate. An ohmmeter is allowed to check not only resistors, but also almost all other radio components. For example, you can check the short circuit (short circuit) of the motor winding on the case. The working condition of semiconductor devices, capacitors and other elements can be checked by knowing how they work.
Other features of the multimeter
In addition to basic measurements, the multimeter makes it easier for an electrician to work in other respects. Different devices have their own characteristics, therefore, before use, you must read the instructions.. As for DT 838, it allows:
- measure the temperature;
- check the performance of bipolar transistors;
- use a sound generator.
A special probe with a thermistor is used to measure the temperature. It can be supplied with the device or purchased separately. The knob is positioned opposite the TEMP mark, and the wires are connected to the lower and middle connectors. The probe is pressed against the measured surface, a digital indication is displayed on the scale. The temperature can be measured without a probe. In this case, the temperature of the ambient air (the device body) will be measured.
The multimeter allows you to check low-power bipolar transistors, since for more, voltages are needed much higher. The sockets for the terminals of the transistor are made in such a way that you can connect any transistor with any order of outputs. To check, set the control knob opposite the hFE mark. No wires are needed, of course.
The last thing left in this device is a sound generator. Its difference from an ohmmeter is that with a low resistance, the multimeter emits a sound signal. It is very convenient to use when the resistance value is not so important, and the main thing is to determine the small resistance, for example, if in a multicore cable the conductors are not separated by color or there are a lot of them (telephone), but it is necessary to find the ends of one wires.
In this case, at one end of the cable, two wires are connected together, short-circuiting them. At the other end, a probe is connected to one wire, and with the other, they alternately touch all the others. If no pair is found, connect another wire and again touch all the others in turn. The procedure is repeated until the desired pair is determined. After that, the cores are disconnected, and a new one is connected to one of the found wires and everything is repeated.
Although the instrument is easy to use, it must be handled with care. It is necessary to be very careful, especially when measurements are taken in different directions. Failure to match the selected scale may result in breakdown or even electric shock.
