Automatic Voltage Stabiliser

Conventional voltage stabilisers are used for feeding controlled voltage to refrigerators, TVs, VCRs etc where AC input varies predominently during peak loads. These stabilisers normally use one or two relays and voltage is available in two steps. In first step the output is 240V AC at 240V AC input while in second step the output is 240V AC at 220V or 210V AC input. If input goes below 220/210V AC, the output state varies but to a lesser extent.

The other type of stabilisers available in the market are the manual autocut kind. In such stabilisers the output voltage is changed manually with a rotary switch to maintain 240V AC output. In case input voltage exceed? 240V AC the output is automatically cut and with the help of manual switch voltage is again brought down to 240V AC. At the time of peak load, i.e. in evening time, this manual operation of stabiliser may have to be done frequently.

This circuit solves the above problems efficiently. With this circuit it is possible to keep the output voltage constant at 230V AC, even if the mains voltage goes as low as 170V AC.

This circuit energises one relay at a time from 170V AC upwards, and all relays are energised when 230V AC input is reached. Similarly, if supply input voltage decreases gradually from 230V, the relays are denergised automatically one by one so that the output voltage remains constant at 230V AC.

Working of the circuit

The power supply to this circuit is given through secondary of transformer X2. The 20V AC across two tapping is changed into DC with diodes and then filtered.

Transformer XI senses the input voltage. The variations in AC input voltage are sensed by this transformer output. This voltage is rectified with the help of diodes D5 through D8, filtered and then fed to base of transistors T1 to T4 through trim pots VRl to VR4. Zener diodes D13 to D16 are used for reference.

Transistor T1 conducts when its base voltage exceeds 2.6V (i.e. zener voltage, 2V + silicon diode conduction voltage, 0.6V), and relay RL1 is energised.

Similarly, transistor T2 will conduct only when the voltage exceeds 4.6V (i.e. zener voltages of diodes D13 and D14 + 0.6V).

Likewise, T3 will conduct when the voltage exceeds 6.6V, and T4 will conduct at voltage exceeding 8.6V. Corresponding relays RL2, RL3, and RL4 will energise when T2,T3 and T4 conduct.

The exact conduction of T1 through T4 can be controlled by pots VR1 to VR4.

Diode D1 to D4 are used for rectification while capacitor C1 is used for filtration. Similarly, diodes D5 to D8 have been used for rectification and capacitor C2 for filtration.

Diode D9 to D12 have been used to protect the transistor from failing due to reverse voltage at the time of dropping of relays.

The circuit can be assembled on a veroboard or a breadboard or a strip-board, but the best would be to use the PCB shown in Fig«2. The PCB lends a lot or reliability by avoiding most external connections and thus eliminates chances of failure due to snapping of wires.

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