Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

Batteries are used in a wide variety of applications. For example:

• For the starting of vehicles like scooters, cars, buses etc.
• In UPS and inverter circuits for illumination purposes or for running other household gadgets.
• In electric vehicles.
• In telephone systems

Fig shows an automatic battery charging circuit using an SCR T1

1. A single phase transformer steps down the utility voltage to 15V across each secondary. Diodes D1, D2 and transformer secondary windings constitute a two pulse mid-point rectifier.

2. A 12V zener diode is connected in series with R1. R2 limits the battery charging current whereas R1 limits zener diode current.

3. Terminal A is always positive with respect to terminal B. During the time voltage across AB , i.e. $v_{AB}$ is more than battery voltage (=12V or less than 12V for a discharged battery), SCR T1 is forward biased.

4. Now when vAB makes zener voltage VZ = 12V, then KVL for the closed circuit of zener diode , D3 , gate-cathode junction of T1 , $R_2$ and discharged battery confirms that net voltage (= 12V – discharged battery voltage) forward biases the gate-cathode circuit and as a result gate-current appears in SCR T1.

5. This gate current, associated with already forward biased thyristor T1, turns it on and thus battery begins to charge through $R_2$.

6. During each half cycle, battery is charged whenever $v_{AB}$ exceeds battery voltage. Battery charging current is, therefore , in the form of pulses. Zener diode the voltage of point C at 12V.

7. When battery is charged to its rated of 12V , then net voltage in zener-battery circuit is zero ; gate-cathode junction now cannot be forward biased.

8. Since no gate pulse can appear; SCR t1, though forward biased, cannot turn-on. This shows that after battery is fully charged, further charging of battery is automatically stopped.