What is the advantages and disadvantages of analog and digital instrument?

Digital Instruments:

An instrument indicating the value of measurment is in the form of a decimal number

and is known as Digital Instrument.The digital meters work on the principal of

quantization. The analog quantity to be measured is first quantized into a number of

small intervals upto the many decimal places.

The objective of the digital instrument is to be determine in which portion of the

subdivision or quantum of the measure and can thus be identified an integral multiple of

the smallest unit called the quantum and choose for subdivision.

The reading accuracy can be increased by increasing the number of decimal places.

Advantages of Digital Instruments:

(i) The digital instruments indicate the reading directly in decimal numbers.

(ii) The reading may be carried to any number of significant figures by merely positioning

the decimal point.

(iii) The digital instrument requires smaller power.

(iv) Its output is in digital form,so it is directly fed into the memory devices like the tape

recorder,printers,floppy discs, and digital computer etc.

Advantages of Digital Instruments over Analog Instruments:

(i)In this limit can be made with a resolution of one part in several thousands.(ii) Its accuracy is much higher.(iii)Most Digital instruments are D.C instruments that measure upto 100V to 1kV by means of a range attenuator.(iv)These instruments are free from observational errors, like a parallax and

approximation errors.(v)They are complex & consists of large number of parts which individually react to change in temperature and humidity. 

Here are some of the advantages of the digital instruments over the analogue instruments:1) They are very easy to read.2) Since there are very few moving parts in the electronic instruments, they are usually more accurate than the analogue instruments. Even the human error involved in reading these instruments is very less, which adds to the accuracy of digital instruments.3) The electronic items tend to be cheaper than the mechanical items.4) The data from the instruments can be recorded for future reference.5) The output of the digital devices can be obtained in the computer

Digital Device Components

A simple processor illustrates many of the basic components used in any digital system:

Datapath: The core -- all other components are support units that store either the results of the datapath or determine what happens in the next cycle.

Digital Device Components

Memory:

oA broad range of classes exist determined by the way data is accessed:

Read-Only vs. Read-Write Sequential vs. Random access

Single-ported vs. Multi-ported access

oOr by their data retention characteristics:

Dynamic vs. StaticoStay tuned for a more extensive treatment of memories.

 

Control:oA FSM (sequential circuit) implemented using random logic, PLAs or memories.

 

Interconnect and Input-Output:oParasitic resistance, capacitance and inductance affect performance of wires both on and off the chip.

oGrowing die size increases the length of the on-chip interconnect, increasing the value of the parasitics.

Digital Device Components

Datapath elements include adders, multipliers, shifters, BFUs, etc.

oThe speed of these elements often dominates the overall system performance so optimization techniques are important.

 oHowever, as we will see, the task is non-trivial since there are multiple equivalent logic and circuit topologies to choose from, each with adv./disadv. in terms of speed, power and area.

 oAlso, optimizations focused at one design level, e.g., sizing transistors, leads to inferior designs.

Datapath Operators: Addition/Subtraction

Let's start with addition, since it is very a common datapath element and often a speed-limiting element.

  Optimizations can be applied at the logic or circuit level.

Logic-level optimization try to rearrange the Boolean equations to produce a faster or smaller circuit, e.g. carry look-ahead adder.

Circuit-level optimizations manipulate transistor sizes and circuit topology to optimize speed.

 

Working of basic operational amplifier?Theory

Operational Amplifiers

Operational Amplifiers (OAs) are highly stable, high gain dc difference amplifiers. Since there is no capacitive coupling between their various amplifying stages, they can handle signals from zero frequency (dc signals) up to a few hundred kHz. Their name is derived by the fact that they are used for performing mathematical operations on their input signal(s).

 Figure 1 shows the symbol for an OA. There are two inputs, the inverting input (-) and the non-inverting input (+). These symbols have nothing to do with the polarity of the applied input signals.

 

 Figure 1. Symbol of the operational amplifier. Connections to power supplies are also shown.

 

The output signal (voltage), vo, is given by: 

vo = A(v+ - v-)

v+ and v- are the signals applied to the non-inverting and to the inverting input, respectively. Α represents the open loop gain of the OA. A is infinite for the ideal amplifier, whereas for the various types of real OAs, it is usually within the range of 104 to 106.

OAs require two power supplies to operate, supplying a positive voltage (+V) and a negative voltage (-V) with respect to circuit common. This bipolar power supply allows OAs to generate output signals (results) of either polarity. The output signal (vo) range is not unlimited. The voltages of the power supplies determine its actual range. Thus, a typical OA fed with -15 and +15 V, may yield a vo within the (approximately) -13 to +13 V range, called operational range. Any result expected to be outside this range is clipped to the respective limit, and OA is in a saturation stage.

The connections to the power supplies and to the circuit common symbols, shown in Figure 1, hereafter will be implied, and they will be not shown in the rest of the circuits for simplicity.

Because of their very high open loop gain, OAs are almost exclusively used with some additional circuitry (mostly with resistors and capacitors), required to ensure a negative feedback loop. Through this loop a tiny fraction of the output signal is fed back to the inverting input. The negative feedback stabilizes the output within the operational range and provides a much smaller but precisely controlled gain, the so-calledclosed loop gain.

Circuits of OAs have been used in the past as analog computers, and they are still in use for mathematical operations and modification of the input signals in real time. A large variety of OAs is commercially available in the form of low cost integrated circuits.

There is a plethora of circuits with OAs performing various mathematical operations. Each circuit is characterized by its own transfer function, i.e. the mathematical equation describing the output signal (vo) as a function of the input signal (vi) or signals (v1, v2, …, vn). Generally, transfer functions can be derived by applying Kirchhoff’s rules and the following two simplifying assumptions:

#1. The output signal (vo) acquires a value that (through the feedback circuits)

practically equates the voltages applied to both inputs, i.e. v+ ≈ v-.

#2. The input resistance of both OA inputs is extremely high(usually within the range

106-1012 MΩ, for the ideal OA this is infinite), thus no current flows into them.

 

Inverting Amplifier

The basic circuit of the inverting amplifier is shown in Figure 2.

 

 

Figure 2. Inverting amplifier.

 

The transfer function is derived as follows: Considering the arbitrary current directions we have:

 i1 = (vi - vs)/Ri   and   i2 = (vs - vo)/Rf

 The non-inverting input is connected directly to the circuit common (i.e. v+ = 0 V), therefore (considering simplifying assumption #1) vs = v- = 0 V, therefore:

i1 = vi/Ri   and   i2 = - vo/Rf

 Since there is no current flow to any input (simplifying assumption #2), it is

 i1 = i2

Therefore, the transfer function of the inverting amplifier is

vo = -(Rf/Ri)vi

 Thus, the closed loop gain of the inverting amplifier is equal to the ratio of R f (feedback resistor) over Ri (input resistor). This transfer function describes accurately the output signal as long as the closed loop gain is much smaller than the open loop gain A of the OA used (e.g. it must not exceed 1000), and the expected values of vo are within the operational range of the OA.

 

Summing Amplifier

The summing amplifier  is a logical extension of the previously described circuit, with two or more inputs. Its circuit is shown in Figure 3.

 

 

Figure 3.  Summing amplifier.

 

The transfer function of the summing amplifier (similarly derived) is:

 vo = -(v1/R1  +  v2/R2  +  …  +  vn/Rn)Rf

Thus if all input resistors are equal, the output is a scaled sum of all inputs, whereas, if they are different, the output is a weighted linear sum of all inputs.

The summing amplifier is used for combining several signals. The most common use of a summing amplifier with two inputs is the amplification of a signal combined with a subtraction of a constant amount from it (dc offset).  

 

Difference amplifier

Difference amplifier precisely amplifies the difference of two input signals. Its typical circuit is shown in Figure 4.

 

 

Figure 4. Difference amplifier.

 

If Ri = Ri΄ and Rf = Rf΄, then the transfer function of the difference amplifier is:

vo = (v2 - v1) Rf/Ri

The difference amplifier is useful for handling signals referring not to the circuit common, but to other signals, known as floating signal sources. Its capability to reject a common signal makes it particularly valuable for amplifying small voltage differences contaminated with the same amount of noise (common signal).

In order for the difference amplifier to be able to reject a large common signal and to generate at the same time an output precisely proportional to the two signals difference, the two ratios p = Rf/Ri and q = Rf΄/Ri΄ must be precisely equal, otherwise the signal output will be:

vo = [q(p+1)/(q+1)]v2 - pv1

 

Differentiator

The differentiator generates an output signal proportional to the first derivative of the input with respect to time. Its typical circuit is shown in Figure 5.

 

 

Figure 5. Differentiator.

 

The transfer function of this circuit is

vo = -RC(dvi/dt)

 Obviously, a constant input (regardless of its magnitude) generates a zero output signal. A typical usage of the differentiator in the field of chemical instrumentation is obtaining the first derivative of a potentiometric titration curve for the easier location of the titration final points (points of maximum slope).

 

Integrator

The integrator generates an output signal proportional to the time integral of the input signal. Its typical circuit is shown in Figure 6.

 

Figure 6. Integrator

 

 vo = -(1/RC)∫vi(t)dt

The output remains zero as far as switch S remains closed. The integration starts (t = 0) when S opens. The output is proportional to the charge accumulated in capacitor C, which serves as the integrating device. A typical application of the (analog) integrator in chemical instrumentation is the integration of chromatographic peaks, since its output will be proportional to the peak area.  

If the input signal is stable then the output from the integrator will be given by the equation

vo = -(vi/RC) t

i.e. the output signal will be a voltage ramp. Voltage ramps are commonly used for generating the linear potential sweep required in polarography and many other voltammetric techniques.

 

Applet

This easy to use applet simulates the operation of the aforementioned circuits of operational amplifiers. The actual circuit is selected by the row of 5 “radio buttons” found on the lower part of the applet. In order to make the simulation more realistic, the operational range of all circuits is between -15 to +15 V. Output signals outside this range are “clipped” to the respective limit and the indication “Saturation” appears.

The magnitude of input signals (v1, v2) can be adjusted by the two scrollbars shown on the left side of the screen. The signal range (-2/+2 V,  -20/+20 V) is selected by the corresponding radio buttons.

The values of resistors (in kΩ) and capacitors (in μF) can be freely selected by the user.

The output signal (vo) is monitored by a simulated “digital voltmeter” and an “analog voltmeter”, as well, shown on the right side of the applet screen. The range of the scale of the later (-0,2/+0,2 V, -2,0/+2,0 V, -20/+20 V) can be selected using the corresponding radio buttons. The analog voltmeter is particularly useful for monitoring the time-depended signals of the differentiator and the integrator circuit

1.37 SUMPNER’S TEST

To determine the rise of maximum temperature of a transformer, its load test is of utmost

importance. Using suitable load impedance, small transformers can be put on full load. The full-

load test of large transformers is not possible because considerable wastage of energy occurs and

it is difficult to get a suitable load for absorbing full-load power. Sumpner’s test is used to put large

transformer on full load. This test can also be used to determine the efficiency of a

transformer. Figure 1.45 shows the schematic diagram of Sumpner’s test. This test is also known

as back to back test or load test.

This test requires two identical transformers. The two primaries are connected in parallel and

are energized at rated voltage and rated frequency. The wattmeter W1records the reading of core

loss of both the transformers. Next the two secondaries are connected in series in such a way that

their polarities are in phase opposition and the reading of the voltmeter V2 becomes zero. With the

help of voltage regulator fed from source, a voltage is injected to the secondary of the

transformers, which is adjusted until the rated secondary current flows. The voltmeter reads a

voltage, which is the leakage impedance drop of the two transformers. The reading of W1 remains

unaltered. The wattmeter W2 reads the total copper (Cu) loss of the two transformers. Although the

transformers are not supplying any load current, this test measures the full iron loss as well as

copper loss of the transformers. The net input during this test is W1 + W2. To measure the

temperature rise, the two transformers are kept under rated loss conditions for several hours.

3.1.3 Extraction of the Cut-Off and Maximum Oscillation Frequency

The cut-off frequency   and maximum oscillation frequency   are the most important figures of merit for the frequency characteristics of microwave transistors. They are often used to emphasize the superiority of newly developed semiconductors or technologies. For example, as a rule of thumb, the operating

frequency of a transistor, sometimes referred as  should be ten times smaller

than   [189]. Thus, extraction of these parameters is a commonly performed simulation task usually done by small-signal simulations.

The cut-off frequency   is the frequency at which the gain or amplification is

unity, thus the absolute value of the short circuit current gain   equals unity:

(3.8)

 is defined as the ratio of the small-signal output current to the input current of

a transistor with short-circuited output. For a bipolar junction transistor,   

basically characterizes the ratio between the small-signal collector current   and

the small-signal base current  . For a MOS transistor, a similar ratio regarding the small-signal drain and gate currents can be specified:

bipolar(3.9)

mos(3.10)

The cut-off frequency is normally extracted for various operating points. Thus, the

peak value is the highest frequency for this range of operating points. See the

left side of Figure 3.4 for a typical curve. To extract such a complete curve

or (or or respectively) is stepped. Hence, two stepping variables (see

Appendix B) are necessary to obtain : one for the steady-state operating point and one for the frequency. Whereas the operating point is a matter of ordinary stepping functions, there are several approaches for frequency stepping:

The frequency can be simply increased until is smaller than one. Then,

an interpolation algorithm is used to obtain the frequency for which .

This so-called unity current gain frequency can be calculated by

extrapolation of , because for a conventional transistor drops

with a slope of dB/dec or dB/oct at higher frequencies. This roll-off slope of a one-pole low-pass is depicted on the right side of Figure 3.4.

Thus, the frequency is increased until the dB/dec region of the curve is reached. An extrapolation yields the cut-off frequency. Measurement

equipments often use this approach to obtain , since they are normally not able to measure such high frequencies as required for today's cut-off frequencies. However, this method depends on the frequency chosen, that means which frequency assures the validity of the single-pole approximation [172].

For simulation purposes it is very inconvenient to run a simulator and a post-processing script in the end. For that reason a conditional stepping approach was developed to use a mathematical iteration algorithm to

approximate for a given accuracy (see Section 3.3.3).

Figure 3.4: Complete curve of a bipolar junction transistor (left). Slope of the absolute value of

the short circuit current gain and the cut-off frequency at the unity gain point at mA (right).

The second important RF figure of merit is the maximum oscillation

frequency  , which is related to the frequency at which the device power gain

equals unity. The value of  can be determined in two ways. The first one is based on the unilateral power gain   as defined by Mason

(3.11)

where   is Kurokawa's stability factor [94] defined as

(3.12)

Therefore,   is the maximum frequency at which the transistor still provides a power gain [189]. An ideal oscillator would still be expected to operate at this frequency, hence the name maximum oscillation frequency. Like the short-circuit

current gain  ,   drops with a slope of  dB/dec.

The second way to determine  , which is not entirely correct [189], is based on the maximum available gain (MAG) and the maximum stable gain (MSG). Whereas

MAG shows no definite slope, MSG drops with  dB/dec.

 does not have to be necessarily larger than  . Generally, transistors have

useful power gains up to  , that above they cannot be used as power amplifiers

any more. However, the importance of   and   depends on the specific

application. Thus, there is no general answer whether   should be priorized

over  . Both figures should be as high as possible, and manufactures often strive

for   in order to enter many different markets for their transistors [189].

Astable multivibratorAn astable multivibrator is a regenerative circuit consisting of two amplifying stages

connected in a positive feedback loop by two capacitive-resistive coupling networks. The

amplifying elements may be junction or field-effect transistors, vacuum

tubes, operational amplifiers, or other types of amplifier. The example diagram shows

bipolar junction transistors.

The circuit is usually drawn in a symmetric form as a cross-coupled pair. Two output

terminals can be defined at the active devices, which will have complementary states;

one will have high voltage while the other has low voltage, (except during the brief

transitions from one state to the other).

[edit]Operation

Figure 1: Basic BJT astable multivibrator

The circuit has two stable states that change alternatively with maximum transition rate

because of the "accelerating" positive feedback. It is implemented by the coupling

capacitors that instantly transfer voltage changes because the voltage across a

capacitor cannot suddenly change. In each state, one transistor is switched on and the

other is switched off. Accordingly, one fully charged capacitor discharges (reverse

charges) slowly thus converting the time into an exponentially changing voltage. At the

same time, the other empty capacitor quickly charges thus restoring its charge (the first

capacitor acts as a time-setting capacitor and the second prepares to play this role in the

next state). The circuit operation is based on the fact that the forward-biased base-

emitter junction of the switched-on bipolar transistor can provide a path for the capacitor

restoration.

State 1 (Q1 is switched on, Q2 is switched off):

In the beginning, the capacitor C1 is fully charged (in the previous State 2) to the power

supply voltage V with the polarity shown in Figure 1. Q1 is onand connects the left-hand

positive plate of C1 to ground. As its right-hand negative plate is connected to Q2 base,

a maximum negative voltage (-V) is applied to Q2 base that keeps Q2 firmly off. C1

begins discharging (reverse charging) via the high-resistive base resistor R2, so that the

voltage of its right-hand plate (and at the base of Q2) is rising from below ground (-V)

toward +V. As Q2 base-emitter junction is backward-biased, it does not impact on the

exponential process (R2-C1 integrating network is unloaded). Simultaneously, C2 that is

fully discharged and even slightly charged to 0.6 V (in the previous State 2) quickly

charges via the low-resistive collector resistor R4 and Q1 forward-biased base-emitter

junction (because R4 is less than R2, C2 charges faster than C1). Thus C2 restores its

charge and prepares for the next State 2 when it will act as a time-setting capacitor. Q1

is firmly saturated in the beginning by the "forcing" C2 charging current added to R3

current; in the end, only R3 provides the needed input base current. The resistance R3

is chosen small enough to keep Q1 (not deeply) saturated after C2 is fully charged.

When the voltage of C1 right-hand plate (Q2 base voltage) becomes positive and

reaches 0.6 V, Q2 base-emitter junction begins diverting a part of R2 charging current.

Q2 begins conducting and this starts the avalanche-like positive feedback process as

follows. Q2 collector voltage begins falling; this change transfers through the fully

charged C2 to Q1 base and Q1 begins cutting off. Its collector voltage begins rising; this

change transfers back through the almost empty C1 to Q2 base and makes Q2 conduct

more thus sustaining the initial input impact on Q2 base. Thus the initial input change

circulates along the feedback loop and grows in an avalanche-like manner until finally

Q1 switches off and Q2 switches on. The forward-biased Q2 base-emitter junction fixes

the voltage of C1 right-hand plate at 0.6 V and does not allow it to continue rising toward

+V.

State 2 (Q1 is switched off, Q2 is switched on):

Now, the capacitor C2 is fully charged (in the previous State 1) to the power supply

voltage V with the polarity shown in Figure 1. Q2 is on and connects the right-hand

positive plate of C2 to ground. As its left-hand negative plate is connected to Q1 base, a

maximum negative voltage (-V) is applied to Q1 base that keeps Q1 firmly off. C2 begins

discharging (reverse charging) via the high-resistive base resistor R3, so that the voltage

of its left-hand plate (and at the base of Q1) is rising from below ground (-V) toward +V.

Simultaneously, C1 that is fully discharged and even slightly charged to 0.6 V (in the

previous State 1) quickly charges via the low-resistive collector resistor R1 and Q2

forward-biased base-emitter junction (because R1 is less than R3, C1 charges faster

than C2). Thus C1 restores its charge and prepares for the next State 1 when it will act

again as a time-setting capacitor...and so on... (the next explanations are a mirror copy

of the second part of Step 1).

CROMEASUREMENT OF PHASE DIFFERENCEWe have discussed that when two sinusoidal voltage signals of equal frequency having some phase difference are applied to the deflection plates of CRO, a straight line or an ellipse appears on

the screen. In the case of straight line appearing on the screen, phase angle difference would be zero or 180° but in case of an ellipse we will have to use a formula for determination of phase difference.Let there be two sinusoidal voltage signals given byvh = Vh Sin ωtvv = Vv Sin (ωt+Ф), where Ф is the phase difference.Since deflection is directly proportional to the amplitude of voltageSo dh = Dh Sin ωtdv = Dv Sin (ωt+Ф)At time t=0, values of dh and dv are dh0 = 0 and dv0 = Dv Sin ФSo Sin Ф = dv0/Dv

Graphical meaning of dv0 and Dv are shown in the figure. Thus the phase difference between two sinusoidal voltages of equal frequency can be determined by measuring dv0 and Dv.In the given figure Vv is shown by leading Vh by a phase angle Ф. If the situation is reversed and Vh leads Vvby phase angle Ф, then again same ellipse would appear on the screen. Because of this fact we can determine only the phase angle between two sinusoidal voltages. It does not indicate which one is leading and which one is lagging.MEASUREMENT OF FREQUENCY OF A VOLTAGE SIGNALThe patterns obtained on CRO screen and discussed in previous sections are called the Lissajous patterns. ALissajous pattern is a pattern which is stationary on the screen of a CRO. It means that the spot traces out the same pattern for every cycle of a voltage signal. As we have already studied that the ratio of frequencies of vertical and horizontal voltage signals should be a rational or fractional number to have steady pattern. So the condition for having a Lissajous pattern on the CRO screen isFy / Fx = A/Bwhere A and B are integers.

Lissajous patterns are usually of two types. First one is closed Lissajous pattern and has no free end. Second one is open Lissajous pattern and has free ends. Both types of Lissajous patterns are shown in figure.In a Lissajous pattern ratio of frequency of vertical signal to the frequency of horizontal signal is equal to the ratio of positive Y-peaks to positive X-peaks in that pattern.Fy / Fx = Positive Y – peaks in pattern / Positive X – peaks in patternThus by counting the positive Y-peaks and X-peaks on a Lissajous pattern, ratio of frequencies of two voltage signals can be determined. In case of an open lissajous pattern, free end is treated as half peak. This will be clear from examples.Voltage signal of unknown frequency is applied to the vertical deflection plates and horizontal deflection plates are supplied by an accurately calibrated variable frequency source. Frequency of the variable frequency source is adjusted until a stationary pattern appears on the screen. Now by reading the value of frequency of horizontal signal, with the help of calibrated scale, frequency of voltage signal applied to vertical deflection plates can be known.

Lissajous Patterns

In case single loop stationary pattern is obtained the frequency of the sinusoidal voltage applied to vertical deflection pates is the same as that of the voltage applied to horizontal deflection plates. In case complex Lissajous figure is obtained, the

frequency of alternating voltage may be determined using the relationPoints of tangency to a vertical line / Points of tangency to a horizontal line = ωx/ωy = fx/fy

One point, very interesting, to know is that sometimes we may have different types of patterns on the CRO screen for the voltage signals of the given frequencies, as shown in figure.In both of the figures, the ratio of Y-peaks to X-peaks is equal so in both cases, ratios of frequencies of vertical and horizontal signals are same. But appearance of Lissajous patterns is different owing to different phase difference of voltage signals applied to vertical arid horizontal deflection plates.CRO is not a precision instrument for measuring frequency of an alternating voltage because the accuracy depends directly on the accuracy of calibrated scale of variable frequency source, which is usually a few percent. It is used for rough estimate of fre-quency or when voltage waveform is so complex that a frequency counter would not operate reliably.Measurement of Voltage and CurrentCathode Ray Oscilloscope can be used for the measurement of voltage of any electrical specification as the deflection of the electrostatic beam is directly proportional to the deflection plate voltage.For measurement of the direct voltage, firstly the spot is centered on the screen without applying any voltage signal to the deflection plates. Then direct voltage to be measured is applied between a pair of deflection plates and the deflection of the spot is observed on the screen. The magnitude of the deflection multiplied by the deflection factor gives the value of the direct voltage applied. Usually the screen is calibrated for fixed operating condition, so by reading the scale, voltage can be measured directly by the CRO.In case of measurement of alternating voltage of sinusoidal wave-form, it is applied between a pair of deflection plates and the length of the straight line is measured. Knowing the

deflection sensitivity, the peak to peak value of applied ac voltage can be determined. The rms value of ac voltage applied will be equal to this peak value divided by 2√2 for sinusoidal wave-form.For measurement of current, the current under measurement is passed through a known non-inductive resistance and the voltage drop across it is measured by CRO, as mentioned above. The current can be determined simply by dividing the voltage drop measured by the value of non-inductive resistance. When the current to be measured is of very small magnitude, the voltage drop across non-inductive resistance (small value) is usually amplified by a calibrated amplifier.The current and voltage can be measured simultaneously by using double beam cathode ray oscilloscope.

The principle of single phase energy meter

Counter is an induction system and is integrated over time of electrical measuring instruments. Principle of operation - the interaction of magnetic fields of currents flowing through two coils, the magnetic field current induktiruemogo in an aluminum disc, located between the windings. Measuring the mechanism of induction of 1 - current coil, 2 - coil voltage, 3 - aluminum disc, 4 - permanent magnet. The device is a meter - Connection for power consumers, 2 - clamps to connect to the network, 3 - current winding, 4 - permanent magnet, 5 - worm screw 6 - coil voltage, 7 - shaft, 8 - aluminum disc, 9 - housing. The main units are el.schetchika coil voltage and current, the aluminum disk 3 is fastened to the axle, axle bearings - thrust bearing and bearing, permanent magnet. C-axis associated with the gear counter mechanism (figure below). Electromagnet 1 contains W - shaped magnetic core, the middle rod is located multiturn coil of thin wire that is included in the voltage U parallel load N. For a nominal voltage of 220 V winding coil voltage is typically 8-12 thousand turns of wire with a diameter of 0.1 - 0 , 15 mm. Current coil through which the full load current is usually the number of ampere-turns within 70 - 150, ie, at nominal current 5 A winding includes from 14 to 30 turns. Complex parts, consisting of sequential (current) and parallel (voltage) windings with their magnetic circuits, said counter rotating elements. Counter mechanism. 1 - axis of the measuring mechanism, 2 - Transmission system zupchatyh 3 - clips counting mechanism current flowing in the winding voltage creates a shared variable matnitny flow voltage circuit, a small portion of which (worker thread) and suppresses aluminum disk located in the gap between the two electromagnets. Most of the magnetic flux through the closed circuit voltage shunts and side rods are magnetic (non current), which is divided into two parts and is required to create the desired phase angle between the magnetic fluxes circuit voltage and the load circuit (a circuit). Magnetic flux circuit voltage is directly proportional to the applied voltage (supply voltage). Load current flowing through the current windings, creates a variable magnetic flux, which also crosses the aluminum disk and closes the magnetic shunt of the upper magnetic core and partially through the side rods. A minor part (non current) is closed through protivopolyus on crossing the disk. Since the magnetic circuit winding current has a U-shaped structure, its magnetic flux crosses the disc twice. Currents flowing through the windings of the voltage and current variables create the magnetic fluxes that cross the disk counters. According to the law of electromagnetic induction, alternating magnetic fluxes of both windings at the intersection of the disc, suggest it EMF, under which the disk appear relevant eddy currents. The interaction of magnetic flux coil voltages and eddy current on the magnetic flux current winding and on the other side of the magnetic flux current winding and eddy current from a winding voltage, there is the electromechanical forces that create the torque acting on the disc. This moment is proportional to the product of these magnetic fluxes and the sine of phase angle between them. A permanent magnet is mounted in the counter to create a braking torque in the counter. The magnetic field lines of the magnet, crossing the disc, suggest it additional emf is proportional to the frequency of rotation of the disk. This voltage in turn causes the flow to drive the eddy current, whose interaction with the flow of a

permanent magnet gives rise to an electromechanical force directed against the motion of the disk, ie leads to the creation of brake torque. Adjustment of the braking torque and hence speed disk to produce by moving a permanent magnet in a radial direction. When approaching a magnet to the center of the disk rotational speed decreases.