Manual transformers are good for jobs where the equipment is being attended to personally, such as while viewing a TV. Also an automatic transformer would be unacceptable for viewing a TV in an area where fluctuations are rapid, because every time a relay in the automatic voltage regulator changes steps there would be a flicker in the TV picture. However automatic transformers are good for refrigerators air conditioners, coolers etc. g ln the following paragraphs we shall be discussing the basic design of one such transformer which may be used with one relay type of circuits mentioned in subsequent paragraphs.
The first parameter which must be considered while designing an auto-transformer for voltage regulators is the desired output regulation for a certain input voltage range, specified as a certain percentage of a noininal output voltage. In this simple case we shall take this to be +/-9 percent of 220 volts output, a figure most commonly used commercially. This means that in practice the outputvoltage may be anything between 200 and 240 volts approximately for the given input range over which regulation as specified is valid. A typical set up in India would specify this output over an input voltage range of say from 175 to 250 volts.
This means that with a minimum of 175VAC input(between points l and 3 of Fig. 3), the output from the autotransformer (between points l and 4 of Fig. 3) should be 200V AC. This represents a turns ratio of 7:8 between the input voltage tap and output step-up voltage tap with reference to the common terminal. This point hasfbeen diagrammatically described in Fig. 3.
Now, if we allowed the tappings to remain intact(input between 1-3 and output between l-4)`and gradually in- creased the input voltage from a variac till an output of 240V AC was recorded-the upper limit for our,output voltage range-we shall find that an input of 210V was necessary to achieve this. Any further increase in input voltage, without changing the transformer taps, would result in an output voltage crossing the specified danger mark of 240V AC output. So, at this point we need a step-down’ tap which should reduce 210V input AC to 200V AC (a lower limit for output voltage of our regulator).
If the input remains between 1-3 but the output is taken from l-2 instead of 1-4, this aim could be achieved. This switching could be effected electrically with relays or else manually with switches. Accordingly, a turns ratio of 21 : 20 would be necessary between the input step terminal and output tap with reference to the common. Actual number of turns are, however, left for considera- tion of transformer manufacturer, who will take into account core size,load current required and stamping material used.
Again we find that an output voltage of 240V AC-our upper limit-would be given for an input of 252V AC.So with these basic hints in mind you should be able to evolve the correct ratio of steps and the total number of these steps required for certain specified values of input and output voltages. Fig. 4 shows a representative practical circuit that may be used with a transformer of the above type with the relay designed to trip over at 210V input. Mains input is provided between fixed terminals of the winding of autotransformer. ln addition there are two secondary windings provided for control purposes. A 20V AC output of one secondary is half-wave rectified by D1 and smoothed by Cl to give approximately 24V required to drive a relay through switching transistor T2.
A second half-wave recified DC supply is provided by 30V winding and D2 and C2. This is used as a voltage reference supply for voltage sensing circuitry. Referring to the main circuit diagram we find that the transistors Tl and T2 are connected in such a way that only one transistor out of them can conduct at a time. Let us assume that preset VRI is so adjusted that transis- tor Tl is initially off. Accordingly, transistor T2 would be receiving a positive bias at its base through R7.
Hence T2 would be conducting, thereby energising relay RL1, and collector of T2 would be pulled to the emitter voltage level determined by the voltage divider action of R8 and R3, which is very much nearer to ground potential, The N/O (normally open) contacts of the relay, which are connected to the step- down terminal of the auto-transformer, will make contact.
For the transistorTl to be non-conducting, its base should be at a negative potential as compared to its emitter which is fed with a stabilised voltage given by zener diode D3. However the voltage at the base of Tl is variable, depending upon the input mains voltage as well as the preset position. Therefore for the above condition, the voltage would have to be higher than the trip point voltage of 210V at which point B would have to be more negative as compared to fixed reference voltage at point A. Now, if the input voltage started going below the trip point voltage, the point B would slowly become less and less negative as compared to point A which would remain at more or less fixed reference zener voltage. This condition would continue till a point is reached where the base of Tl actually becomes pgzsitive with respect to its emitter. and when ‘this difference exceeds + 0.6V-the forward conduction voltage of a silicon junction-the base emitter diode of Tl conducts, pulling the collector of Tl to ground. This simultaneously clamps the base of T2 to ground making it non-conducting, hence releasing the relay. Relay changeover contact is now connected to N/ C (normally closed) contact of the relay which is connected to the step-up tap on power transformer. Practical adjustment of this is a mere simplicity and needs a variac for continuously changing input voltage. Output voltage is monitored simultaneously with a good quality multimeter and as soon as it approaches the upper limit of 24OV, the preset VRl is adjusted to switch the relay RLl which starts reducing voltage from then onwards. Diode D4 shorts the reverse transients generated due to back EMF in the relay coil while switching, thus preventing T2 from permanent damage. T , Fig. 5 gives another type of circuit generally preferred by some manufacturers on account of its greater reliability and accuracy in the long run. The basic circuit is a Schmitt trigger’ operating between well-defined limits. As usual, relay voltage is derived from Dl and Cl, and reference voltage is given by D2 and C2. 1 Due to the potential divider action of resistors Rl, R2 and preset VRl, a voltage depending upon input mains voltage appears at point marked A. So long as this voltage is below the breakdown voltage of zener dicide D3, transistor Tl cannot conduct. Its collector receives a positive potential via R5 which in turn biases T2 through its base resistor R6.Thus the relay is energised, making N/O contacts of the relay. If however the input voltage increases to a level where the sample voltage at point A increases above the breakdown voltage of D3, transistor Tl would conduct and bring its collector to the emitter voltage of T2. Since base resistor R6 of T2 is also connected here, transistor T2 does not conduct anymore as there is no sustaining positive voltage difference between its base and emitter.As a result the relay is released and its pole is now connected to the stepdown terminal of the auto~transformer through N/ C contacts of the relay.