Transformer Polarity MeasurementObjective: Our project aims to measure Transformer polarity in case of multiphase windings. Indication of in phase and out phase will be given by the device when excited from the primary winding.

IntroductionThe Importance of Polarity An understanding of polarity is essential to correctly construct three-phase transformer banks and to properly parallel single or three-phase transformers with existing electrical systems. Knowledge of polarity is also required to connect potential and current transformers to power metering devices and protective relays. The basic theory of additive and subtractive polarity is the underlying principle used in step voltage regulators wherethe series winding of an autotransformer is connected to either buck or boost the applied line voltage.

Transformer Polarity refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half-cycle when the applied voltage (or current in the case of a current transformer) is from H1 to H2 the secondary induced voltage direction will be from X1 to X2. In practice, Polarity refers to the way the leads are brought out of the transformer.

Testing the winding polarity of transformers, especially those of multi-winding SMPS, audio and power transformers, is a common requirement in the industry. An instrument which could easily and quickly test the winding polarity would be extremely useful for not only the industry but also the electronics hobbyists and enthusiasts.

The circuit of transformer polarity tester given can be easily fabricated using commonly available components.The supply voltage selected is 9V, which suits portable battery-powered operation.To test the relative polarity between any two windings, the user has to connect only two sets of probes (or crocodile clips) to the respective winding terminals,the reference winding and any other winding. The polarity is indicated with the help of two LEDs with respect to the reference winding probe leads marked +(red) and - (black). The green LED when ‘on’ would indicate that the two windings are in phase, that is, the ‘+’ marked and ‘-’ marked probe leads are connected to winding terminals with identical ‘dot’ polarity. On the contrary, red LED when ‘on’ will indicate reverse dot polarity of

the two windings. The circuit is based on the principle that when any winding of a transformer is excited with a voltage pulse, all other coupled windings generate pulses of varying amplitude decided by the turns ratio.The ‘dot’ ends of all the windings generate voltages in phase with the ‘dot’ end of the excited winding. This presents the possibility of performing a simple logic AND operation to determine the winding polarity. In the circuit diagram given in Fig.1, NAND gate N2 does this job in con-junction with rest of the circuit.

Though the simple AWD gate will work in principle, various edge delays and ring-

ing phenomenon commonly encountered in transformer circuits will cause errors,

making the test results invalid. The circuit presented here avoids these errors by sampling the pulses somewhere in the middle part of the exciting pulse and then doing the logic detection of the phase. The first part ofthe circuit is a simple

astable multivibrator with ICI having an ‘on’ period of 50 to 100 us and ‘off’ period of approximately 5 ms. Resistance R2A shown in series with diode D1. which de termines ‘on’ period, should be test selected to be low enough but adequate to trigger the MOSFET properly.

The positive going pulse output from IC1 is used to drive the MOSFET (T3}

which in turn produces the voltage pulse at the excitation probe terminals. Resistance R7 and diode D2 provide the free-wheeling path when

reference winding of transformer ‘under test’ is connected across excitation

probes. Resistor R10 senses the excitation current and, if this is excessive,

Compara-tor lC4(a) gives a positive pulse output to trigger the SCR

(T1) to ‘on’ state. As a result, further drive to the MOSFET is inhibited until the reset switch is been pressed .This limits the current to a safe value of accidental shorting of the probes. This condition is indicated by the glowing of led1.

A sampling pulse is generated by monoshot IC2 which gets a delayed trigger derived from the exciting pulse, Output of the monoshot is given to one of the pins of NAND gate N2 which gives an active LOW output when preconditioned pulsed applied to its other input pin is available simultaneously with the monoshot high state.Resistor R20 through R22 zener diode D4 and IC4(b) do the conditioning of the input pulse by amplitude limiting and comparator action. Care should be taken if the turn ratio is very high to produce excessive voltage at the measuring windings.

555 Timer Theory

Introduction

The 8-pin 555 timer must be one of the most useful ICs ever made and it is used in many projects. With just a few external components it can be used to build many circuits, not all of them involve timing!

A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. The 556 is a dual version of the 555 housed in a 14-pin package, the two timers (A and B) share the same power supply pins. The circuit diagrams on this page show a 555, but they could all be adapted to use one half of a 556.

Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with a 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.

The circuit symbol for a 555 (and 556) is a box with the pins arranged to suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply, 555 pin 3 output on the right. Usually just the pin numbers are used and they are not labelled with their function.

The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).

Standard 555 and 556 ICs create a significant 'glitch' on the supply when their output changes state. This is rarely a problem in simple circuits with no other ICs, but in more complex circuits a smoothing capacitor (eg 100µF) should be connected across the +Vs and 0V supply near the 555 or 556.

The input and output pin functions are described briefly below and there are fuller explanations covering the various circuits:

Astable - producing a square wave

Example circuit symbol (above)

Actual pin arrangements (below)

Aims Interactive

Monostable - producing a single pulse when triggered

Bistable - a simple memory which can be set and reset

Buffer - an inverting buffer (Schmitt trigger)

Inputs of 555/556

Trigger input: when < 1/3 Vs ('active low') this makes the output high (+Vs). It monitors the discharging of the timing capacitor in an astable circuit. It has a high input impedance > 2M .

Threshold input: when > 2/3 Vs ('active high') this makes the output low (0V)*. It monitors the charging of the timing capacitor in astable and monostable circuits. It has a high input impedance > 10M . * providing the trigger input is > 1/3 Vs, otherwise the trigger input will override the threshold input and hold

the output high (+Vs).

Reset input: when less than about 0.7V ('active low') this makes the output low (0V), overriding other inputs. When not required it should be connected to +Vs. It has an input impedance of about 10k .

Control input: this can be used to adjust the threshold voltage which is set internally to be 2/3 Vs. Usually this function is not required and the control input is connected to 0V with a 0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.

The discharge pin is not an input, but it is listed here for convenience. It is connected to 0V when the timer output is low and is used to discharge the timing capacitor in astable and monostable circuits.

Output of 555/556

The output of a standard 555 or 556 can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply many output transducers directly, including LEDs (with a resistor in series), low current lamps, piezo transducers, loudspeakers (with a capacitor in series), relay coils (with diode protection) and some motors (with diode protection). The output voltage does not quite reach 0V and +Vs, especially if a large current is flowing.

To switch larger currents you can connect   a   transistor .

The ability to both sink and source current means that two devices can be connected to the output so that one is on when the output is low and the other is on when the output is high. The top diagram shows two LEDs connected in this way. This arrangement is used in the Level   Crossing project to make the red LEDs flash alternately.

Loudspeakers

A loudspeaker (minimum resistance 64 ) may be connected to the output of a 555 or 556 astable circuit but a capacitor (about 100µF) must be connected in series. The output is equivalent to a steady DC of about ½Vs combined with a square wave AC (audio) signal. The capacitor blocks the DC, but allows the AC to pass as explained in capacitor   coupling .

Piezo transducers may be connected directly to the output and do not require a capacitor in series.

Relay coils and other inductive loads

Like all ICs, the 555 and 556 must be protected from the brief high voltage 'spike' produced when an inductive load such as a relay coil is switched off. The standard protection   diode must be connected 'backwards' across the the relay coil as shown in the diagram.

However, the 555 and 556 require an extra diode connected in series with the coil to ensure that a small 'glitch' cannot be fed back into the IC. Without this extra diode monostable circuits may re-trigger themselves as the coil is switched off! The coil current passes through the extra diode so it must be a 1N4001 or similar rectifier diode capable of passing the current, a signal diode such as a 1N4148 is usually not suitable.

555/556 Astable

An astable circuit produces a 'square wave', this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that the durations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'.

The time   period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (f) which is the number of cycles per second.

T = 0.7 × (R1 + 2R2) × C1   and  f = 

                    1.4                    

(R1 + 2R2) × C1

T   = time period in seconds (s) f    = frequency in hertz (Hz) R1 = resistance in ohms ( ) R2 = resistance in ohms ( ) C1 = capacitance in farads (F)

The time period can be split into two parts: T = Tm + Ts Mark time (output high): Tm = 0.7 × (R1 + R2) × C1 Space time (output low): Ts  = 0.7 × R2 × C1

Many circuits require Tm and Ts to be almost equal; this is achieved if R2 is much larger than R1.

For a standard astable circuit Tm cannot be less than Ts, but this is not too restricting because the output can both sink and source current. For example an LED can be made to flash briefly with long gaps by connecting it (with its resistor) between +Vs and the output. This way the LED is on during Ts, so brief flashes are achieved with R1 larger than R2, making Ts short and Tm long. If Tm must be less than Ts a diode can be added to the circuit as explained under duty   cycle below.

Choosing R1, R2 and C1

555 astable output, a square wave(Tm and Ts may be different)

 

555 astable circuit 

R1 and R2 should be in the range 1k to 1M . It is best to choose C1 first because capacitors are available in just a few values.

Choose C1 to suit the frequency range you require (use the table as a guide).

Choose R2 to give the frequency (f) you require. Assume that R1 is much smaller than R2 (so that Tm and Ts are almost equal), then you can use:

R2 =     0.7    

f × C1

Choose R1 to be about a tenth of R2 (1k min.) unless you want the mark time Tm to be significantly longer than the space time Ts.

If you wish to use a variable resistor it is best to make it R2.

If R1 is variable it must have a fixed resistor of at least 1k in series (this is not required for R2 if it is variable).

Astable operation

With the output high (+Vs) the capacitor C1 is charged by current flowing through R1 and R2. The threshold and trigger inputs monitor the capacitor voltage and when it reaches 2/3Vs (threshold voltage) the output becomes low and the discharge pin is connected to 0V.

The capacitor now discharges with current flowing through R2 into the discharge pin. When the voltage falls to 1/3Vs (trigger voltage) the output becomes high again and the discharge pin is disconnected, allowing the capacitor to start charging again.

This cycle repeats continuously unless the reset input is connected to 0V which forces the output low while reset is 0V.

555 astable frequencies

C1R2 = 10kR1 = 1k

R2 = 100kR1 = 10k

R2 = 1MR1 = 100k

0.001µF 68kHz 6.8kHz 680Hz

0.01µF 6.8kHz 680Hz 68Hz

0.1µF 680Hz 68Hz 6.8Hz

1µF 68Hz 6.8Hz 0.68Hz

10µF 6.8Hz0.68Hz

(41 per min.)0.068Hz

(4 per min.)

An astable can be used to provide the clock signal for circuits such as counters.

A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezo transducer with a low frequency of less than 20Hz will produce a series of 'clicks' (one for each low/high transition) and this can be used to make a simple metronome.

An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps. The natural (resonant) frequency of most piezo transducers is about 3kHz and this will make them produce a particularly loud sound.

Duty cycle

The duty cycle of an astable circuit is the proportion of the complete cycle for which the output is high (the mark time). It is usually given as a percentage.

For a standard 555/556 astable circuit the mark time (Tm) must be greater than the space time (Ts), so the duty cycle must be at least 50%:

Duty cycle  =          Tm        

 =   R1   +   R2  

Tm + Ts R1 + 2R2

To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 as shown in the diagram. This bypasses R2 during the charging (mark) part of the cycle so that Tm depends only on R1 and C1:

555 astable circuit with diode across R2

Tm = 0.7 × R1 × C1   (ignoring 0.7V across diode) Ts  = 0.7 × R2 × C1   (unchanged)

Duty cycle with diode  =  

        Tm        

 = 

    R1    

Tm + Ts

R1 + R2

Use a signal diode such as 1N4148.

555/556 Monostable

A monostable circuit produces a single output pulse when triggered. It is called a monostable because it is stable in just one state: 'output low'. The 'output high' state is temporary.

The duration of the pulse is called the time period (T) and this is determined by resistor R1 and capacitor C1:

time period, T = 1.1 × R1 × C1

T   = time period in seconds (s) R1 = resistance in ohms ( )

555 monostable output, a single pulse 

555 monostable circuit with manual trigger 

C1 = capacitance in farads (F) The maximum reliable time period is about 10 minutes.

Why 1.1? The capacitor charges to 2/3 = 67% so it is a bit longer than the time   constant (R1 × C1) which is the time taken to

charge to 63%.

Choose C1 first (there are relatively few values available). Choose R1 to give the time period you need. R1 should be in the range 1k to 1M , so

use a fixed resistor of at least 1k in series if R1 is variable.

Beware that electrolytic capacitor values are not accurate, errors of at least 20% are common.

Beware that electrolytic capacitors leak charge which substantially increases the time period if you are using a high value resistor - use the formula as only a very rough guide!

For example the Timer   Project should have a maximum time period of 266s (about 4½ minutes), but many electrolytic capacitors extend this to about 10 minutes!

Monostable operation

The timing period is triggered (started) when the trigger input (555 pin 2) is less than 1/3 Vs, this makes the output high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further trigger pulses are ignored.

The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches 2/3 Vs the time period is over and the output becomes low. At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.

The reset input (555 pin 4) overrides all other inputs and the timing may be cancelled at any time by connecting reset to 0V, this instantly makes the output low and discharges the capacitor. If the reset function is not required the reset pin should be connected to +Vs.

Power-on reset or trigger

It may be useful to ensure that a monostable circuit is reset or triggered automatically when the power supply is connected or switched on. This is achieved by using a capacitor instead of (or in addition to) a push switch as shown in the diagram.

The capacitor takes a short time to charge, briefly holding the input close to 0V when the circuit is switched on. A switch may be connected in parallel with the capacitor if manual operation is also required.

This arrangement is used for the trigger in the Timer   Project .

Edge-triggering

If the trigger input is still less than 1/3 Vs at the end of the time period the output will remain high until the trigger is greater than 1/3 Vs. This situation can occur if the input signal is from an on-off switch or sensor.

The monostable can be made edge triggered, responding only to changes of an input signal, by connecting the trigger signal through a capacitor to the trigger input. The capacitor passes sudden changes (AC) but blocks a constant (DC) signal. For further information please see the page on capacitance. The circuit is 'negative edge triggered' because it responds to a sudden fall in the input signal.

The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger is normally high (+Vs).

Power-on reset ortrigger circuit

edge-triggering circuit

555/556 Bistable (flip-flop) - a memory circuit

The circuit is called a bistable because it is stable in two states: output high and output low. It is also known as a 'flip-flop'.

It has two inputs:

Trigger (555 pin 2) makes the output high. Trigger is 'active low', it functions when < 1/3 Vs.

Reset (555 pin 4) makes the output low. Reset is 'active low', it resets when < 0.7V.

The power-on reset, power-on trigger and edge-triggering circuits can all be used as described above for the monostable.

555/556 Inverting Buffer (Schmitt trigger) or NOT gate

The buffer circuit's input has a very high impedance (about 1M ) so it requires only a few µA, but the output can sink or source up to 200mA. This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).

It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state:

Input low (< 1/3 Vs) makes output high, +Vs Input high (> 2/3 Vs) makes output low, 0V

555 bistable circuit

555 inverting buffer circuit(a NOT gate)

 

NOT gate symbol

When the input voltage is between 1/3 and 2/3 Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger.

If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least 1/3 Vs to make the output switch back.

TRANSFORMERS Theory

The information in this part is on the construction, theory, operation, and the various uses of transformers. Safety precautions to be observed by a person working with transformers are also discussed.

A TRANSFORMER is a device that transfers electrical energy from one circuit to another by electromagnetic induction (transformer action). The electrical energy is always transferred without a change in frequency, but may involve changes in magnitudes of voltage and current. Because a transformer works on the principle of electromagnetic induction, it must be used with an input source voltage that varies in amplitude. There are many types of power that fit this description; for ease of explanation and understanding, transformer action will be explained using an ac voltage as the input source.

 

 You learned that alternating current has certain advantages over direct current. One important advantage is that when ac is used, the voltage and current levels can be increased or decreased by means of a transformer.

 

As you know, the amount of power used by the load of an electrical circuit is equal to the current in the load times the voltage across the load, or P = EI. If, for example, the load in an electrical circuit requires an input of 2 amperes at 10 volts (20 watts) and the source is capable of delivering only 1 ampere at 20 volts, the circuit could not normally be used with this particular source. However, if a transformer is connected between the source and the load, the voltage can be decreased (stepped down) to 10 volts and the current increased (stepped up) to 2

amperes. Notice in the above case that the power remains the same. That is, 20 volts times 1 ampere equals the same power as 10 volts times 2 amperes.

 

BASIC OPERATION OF A TRANSFORMER

In its most basic form a transformer consists of:

A primary coil or winding. A secondary coil or winding.

A core that supports the coils or windings.  

Refer to the transformer circuit in figure (1) as you read the following explanation: The primary winding is connected to a 50 hertz ac voltage source. The magnetic field (flux) builds up (expands) and collapses (contracts) about the primary winding. The expanding and contracting magnetic field around the primary winding cuts the secondary winding and induces an alternating voltage into the winding. This voltage causes alternating current to flow through the load. The voltage may be stepped up or down depending on the design of the primary and secondary windings.

 

Figure (1). - Basic transformer action.

 

THE COMPONENTS OF A TRANSFORMER

Two coils of wire (called windings) are wound on some type of core material. In some cases the coils of wire are wound on a cylindrical or rectangular cardboard form. In effect, the core material is air and the transformer is called an AIR-CORE TRANSFORMER. Transformers used at low frequencies, such as 50 hertz and 400 hertz, require a core of low-reluctance magnetic material, usually iron. This type of transformer is called an IRON-CORE TRANSFORMER. Most power transformers are of the iron-core type. The principle parts of a transformer and their functions are:

The CORE, which provides a path for the magnetic lines of flux. The PRIMARY WINDING, which receives energy from the ac

source.

The SECONDARY WINDING, which receives energy from the primary winding and delivers it to the load.

The ENCLOSURE, which protects the above components from dirt, moisture, and mechanical damage.

CORE CHARACTERISTICS

The composition of a transformer core depends on such factors as voltage, current, and frequency. Size limitations and construction costs are also factors to be considered. Commonly used core materials are air, soft iron, and steel. Each of

these materials is suitable for particular applications and unsuitable for others. Generally, air-core transformers are used when the voltage source has a high frequency (above 20 kHz). Iron-core transformers are usually used when the source frequency is low (below 20 kHz). A soft-iron-core transformer is very useful where the transformer must be physically small, yet efficient. The iron-core transformer provides better power transfer than does the air-core transformer. A transformer whose core is constructed of laminated sheets of steel dissipates heat readily; thus it provides for the efficient transfer of power. The majority of transformers you will encounter in Navy equipment contain laminated-steel cores. These steel laminations (see figure 2) are insulated with a non conducting material, such as varnish, and then formed into a core. It takes about 50 such laminations to make a core an inch thick. The purpose of the laminations is to reduce certain losses which will be discussed later in this part. An important point to remember is that the most efficient transformer core is one that offers the best path for the most lines of flux with the least loss in magnetic and electrical energy.

 

Figure (2). - Hollow-core construction.

Core Type Transformers

There are two main shapes of cores used in laminated-steel-core transformers. One is the CORE Type, so named because the core is shaped with a hollow square through the center. Figure 5-2illustrates this shape of core. Notice that the core is made up of many laminations of steel. Figure (3) illustrates how the transformer windings are wrapped around both sides of the core.

Figure (3). - Windings wrapped around laminations.

 

Shell-Core Transformers

The most popular and efficient transformer core is the SHELL CORE, as illustrated in figure (4). As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations. The laminations are insulated from each other and then pressed together to form the core.

 

Figure (4). - Shell-type core construction.

 

TRANSFORMER WINDINGS

As stated above, the transformer consists of two coils called WINDINGS which are wrapped around a core. The transformer operates when a source of ac voltage is connected to one of the windings and a load device is connected to the other. The winding that is connected to the source is called the PRIMARY WINDING. The winding that is connected to the load is called the SECONDARY WINDING. (Note: In this part the terms "primary winding" and "primary" are used interchangeably; the term: "secondary winding" and "secondary" are also used interchangeably.)

Figure (5) shows an exploded view of a shell-type transformer. The primary is wound in layers directly on a rectangular cardboard form.

 

Figure (5). - Exploded view of shell-type transformer construction.

 

In the transformer shown in the cutaway view in figure (6), the primary consists of many turns of relatively small wire. The wire is coated with varnish so that each turn of the winding is insulated from every other turn. In a transformer designed for high-voltage applications, sheets of insulating material, such as paper, are placed between the layers of windings to provide additional insulation.

 

Figure (6). - Cutaway view of shell-type core with windings.

 

When the primary winding is completely wound, it is wrapped in insulating paper or cloth. The secondary winding is then wound on top of the primary winding. After the secondary winding is complete, it too is covered with insulating paper. Next, the E and I sections of the iron core are inserted into and around the windings as shown.

The leads from the windings are normally brought out through a hole in the enclosure of the transformer. Sometimes, terminals may be provided on the enclosure for connections to the windings. The figure shows four leads, two from the primary and two from the secondary. These leads are to be connected to the source and load, respectively.