Digital multimeter

Digital Multimeter is a multifunctional electronic measuring device. The list of estimated parameters includes values: current, voltage, capacitor capacitance, resistance of resistors.

Chinese Multimeter

What does the digital multimeter

consist of? Historical reference: magnetic needle, inductance coil as a measuring device

Having once opened the old analog tester, it will not be surprised at the multimeter device. There is a sensitive element, flavored with a fair amount of typical resistors. In old testers, capacitors were used to measure capacitors; in today's devices, the principle of operation is different. Let us consider briefly the historical constructions so that the transition to the novelties does not cause futurous.

The tester is based on the principles used in 1820( September 16) by Schweiger for the first design of the galvanometer. In the topics about direct current, magnetic induction, the historical sequence of events was discussed. The first device the author called the multiplier. Translated into Russian - multiplier. The effects of multiple turns of wire evolved. The result was a physical multiplication of the field strength by the number of elementary contours. Speech affects the inductance coil.

Everything happened like that. At the beginning of 1820, Hans Oersted discovered: a wire with a current deflects a compass needle located nearby. Opinions diverge, sometimes it is asserted: the observation was made by an assistant( student, free listener), others hold an opinion - a third-party person who happened to come into the room noticed what was happening. Then it was decided to use visual experiments, luring the audience.

Pedagogy was earned by many people of science due to the scarcity of state subsidies. As Sir Humphrey Davy put it, instructing young Michael Faraday to avoid giving up immediately: science is a miserly lady, and she is not very generous about people who are passionate about her.

Hans Oersted was going to show students the effect of wire heating, discovered two decades earlier. Those who wish to read more in the section on the bulb. The discovery was made by the aforementioned Sir Humphrey Davy, the founder of the Royal Scientific Society( England) - among other founders. With the closure of the terminals of the voltaic column( a kind of modern battery), the platinum filament quickly glowed red hot( soon it burned in the atmosphere).At the time of 1820, it was not known about the invention of the incandescent bulb( see the contradictions of the historical reference in the section on incandescent bulbs), the consequence of the unresolved Joule-Lenz law was widely known - the thread glowed under the action of electric current.

The magnetic field lines spiral the wire. They have a circular cross section in the transverse plane. During the demonstration by Hans Oersted of the properties of electricity, the wire passed over the compass needle. Due to the interaction of its own and induced by a magnetic field, the latter deviated. The effect was observed in 1802, Giovanni Domenico Romanozi wrote about him, the lonely cry of the science luminary passed unnoticed. Hans Herstdede did not leave an unknown phenomenon, immediately sent the message in Latin - then the generally accepted language in the scientific community - to many scientists. Even made a report.

Later, Amper at a regular meeting demonstrated a new phenomenon. Laplace, who was present, remarked: it is permissible to reinforce the effect by flexing the wire. The first inductor appeared, which Schweiger built into the multiplier. Such a long introduction was made to show how the ammeter appeared, which until recently was the basis of the tester.

DC Ammeter

As used multiplier in mechanical testers

Due to the nature of the digital multimeter measures voltage, mechanical tester - electric current. In the inductance coil, the coil field is enhanced by deflecting the arrow. Reminds experiments Oersted. A simple device will serve for a variety of tasks:

  1. Voltage Measurement.
  2. Estimation of AC and DC.
  3. Measurement of active resistances and capacitances.

Let's describe what is happening:

  • Low current measured directly. Each next limit is passed through the resistor of the desired value. The greater the current is weakened, the small is fed almost without changes to the multiplier( ammeter).To switch the limits, there is a control knob that flips the contactor to the desired position. Alternating current before estimating the value required to straighten. A full or half diode bridge is used. The rectified current is passed through the desired value of the resistor for attenuation, the limit is regulated by the control knob, the result is fed to the multiplier.
  • Voltage is measured in a similar way. A constant forms a resistive divider with additional resistance, the active part of the impedance of the multiplier coil. There is a pre-calculated current, taking into account which the instrument scale has been graduated. Similarly, there are a number of limits switched by the handle. Each nominal has an individual resistor, the scales may coincide( see the inscriptions on the dial of the tester).AC voltage is rectified by a diode bridge.

    Multimeter Case

  • A capacitor block is used to measure capacitances. The measured element is connected in parallel to the bundle, branches off a part of the current( network 220 volt 50 Hz).The multiplier estimates losses; the deviation of the arrow indicates the graduation of the scale in units of farad fractions( named after Michael Faraday).It should be noted here: the readings of the device will strongly depend on the frequency of the network, the amplitude of the outlet voltage.
  • Resistor ratings are measured using a built-in battery( Krone).The principle is the same: a constant voltage forms a current, the value is known in advance. Rejects the arrow at a certain angle, the scale is graduated by the appropriate units( Ohms).

How the digital multimeter

works The digital multimeter is based on a controller with an analog-to-digital converter module. A block analyzing the size of the incoming voltage is included in the microcircuit( in the photo filled with a drop of compound).The difference from the construction described above: allows you to perform the operations already mentioned plus:

Multimeter IC

  1. Paging resistances, resistors. In the jargon of electronics engineers, the operation denotes a procedure for evaluating the integrity of conductors or pn-junctions of semiconductor devices. The call reminds a typical buzzer, we will meet in any system unit of a personal computer( see photo).When the circuit is closed, it makes a sharp sound. From here comes the name of the procedure. The buzzer is silent - the estimated electrical circuit element is faulty.
  2. Similarly, we will check transistors, but a modern multimeter has one pleasant surprise: many devices will measure current gain. The parameter is often denoted by the Greek letter beta, or represented by h-parameters, as h21.Sometimes a letter is added here. For example, "e" means: the parameter is measured by a transistor connected according to a common emitter circuit( most commonly found in simple devices).For these purposes on the front panel of the digital multimeter there is a special slot. Two whole - for pnp and npn structures. The parameters of field-effect transistors are estimated differently, the specifics are beyond the scope of the article.
  3. The principle of measuring containers has undergone maximum changes. Now, the terminal where the legs of the element are inserted is briefly energized, then the discharge time is estimated. The voltage of the capacitor decreases exponentially, and after studying, you can give an estimate of the parameter under study. The fact is widely used technique for different purposes. The literature often cites examples with the RC time constant characterizing filter parameters. It is believed for three periods equal to the constant time, the charge is lost almost completely.
  4. An additional bonus of expensive digital multimeters is a temperature meter. The action is based on the thermocouple effect. Once Georg was able to estimate the voltage, the electronics will do all the more. The voltage is estimated by an analog-to-digital converter, displayed as a temperature display.

Using the Multimeter

Differing from the current operating multiplier, the controller evaluates the voltage. The series of chips 7601 occurs more often. Descriptions of typical representatives are given everywhere. Watch the video on YouTube channel Chip & Dip. There are two measuring inputs: one for high voltages. Inside there is a comparator that converts the level of an analog signal into a digital input. The 7601 series has several reference inputs for external resistors, capacitors, which are structurally part of a clock-generating timer. Sometimes the built-in is used; in others, the time constant is set by the values ​​of R and C.

The code received from the comparator is divided into groups in thousands, hundreds, tens, units, fed to the latch( latch).The latter is a memory cell capable of storing information entered. Otherwise, the readings on the display will be unstable. The device updates the numbers so that it does not seem too fast to a person( about three times per second).The screen is controlled by a special driver - a microcircuit that generates signals for the glow of the display segments. A separate line is a minus symbol. There is no backlight, although there may be an option.

The mode switch is considered to be an interesting part of a digital multimeter. Handle, equipped with a set of contacts( see photo), which closes in the right order the maze of contact tracks located on the board. Little differs from a mechanical tester by the principle of action, despite the apparent complexity: passive circuit elements alternate.

Mode Switch

After assembly, the device often needs to be calibrated. For the temperature measurement circuit, do the following:

  1. Place the thermocouple in a mixture of cold water and ice at a temperature of 0 degrees Celsius, and adjust the lower limit potentiometer( low-voltage input) with a corresponding reading on the board.
  2. The sensor heats up to one hundred degrees, the upper limit is regulated. Until the desired value appears on the display.

In the process of a digital multimeter, the heat generation from the chip is minimal. Typical power dissipation is a fraction of a watt. Cooling is not required. It is important to connect the probes correctly. Black is a schematic ground, denoted by COM.Measuring inputs often two, one for large currents. Although fuse protection is present( marked with fused), if the signal is incorrect, the digital multimeter may malfunction. Avoid using the black probe to touch high-voltage circuits, perform actions not provided for by the instructions.

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