The following article explains an enhanced Automatic Transfer Switch (ATS) Circuit, which includes several customized sequential changeover relay stages making the system truly smart!

Designed and written by: Abu-Hafss.

Main Features

The circuit presented here is an ATS with following features:

a) Battery Voltage Monitor - The system will not operate when the battery drops to a certain preset level.

b) In the event of power failure, the generator engine will be cranked after 5 sec. The cranking cycle will be 2 minutes in which there will be 12 cranks of 5 sec. each with an interval of 5 sec.

c) As soon as the engine is started, the cranking will be stopped.

d) Initially the generator will start on PETROL and will shift to GAS after 10 seconds.

e) When Grid Mains is restored the load will be shifted to Mains immediately but the generator will be switched off after 10 sec.

Circuit Diagram


1) The circuit enclosed in the green box makes up the battery monitor and it working may be understood here. If the generator is equipped with battery charging setup then this circuit might not be required because the battery will remain in good health. In that case, the entire circuit may be omitted and point X may be connected to the +(ve) of the battery.

2) When the Grid Mains goes off, the generator will be supplied with 12V via relay RLY1 for ignition i.e. RLY1 acts as an ignition switch and RLY2 shifts the LOAD to Generator 220V (which is not yet generated). The absence of grid Mains will switch off Q4 and as a result BATT 12V will be supplied to the rest of the circuit.

IC2, which is configured as "Power-on delay Timer" causes a delay of 5 sec and then resets IC3. IC3 is configured as self-triggering monostable having ON period of about 2 minutes. IC3 resets IC4 which is configured as an astable vibrator (about 5 sec ON and 5 sec OFF). During 2 minutes, IC4 cranks the generator (via R20/Q7/RLY3) 12 times for 5 sec with an interval of 5 sec.

If the engine does not starts within 2 minutes, LED2 will glow to indicate engine fault and the entire system will come to halt until the Grid Mains is restored. If required, the cranking procedure can be restarted by pressing the (Push-to-Off) reset button SW1.

3) Now, assuming that the engine has started during cranking, the generator will start producing electricity hence, 12V from generator adapter will be available. This will switch on Q6 hence, IC3 and IC4 will be powered off which ultimately stops the cranking cycle.

4) The 12V from generator will also power on IC5 and IC6. Both are configured as "Power-on Delay Timer" for about 10 sec and 20 sec respectively. For the initial 10 sec Q8 will conduct and the solenoid valve for PETROL will be opened to supply petrol to the generator. After 10 sec Q8 will stop conducting thereby stopping the petrol supply.

The engine will continue to run on petrol which present in the fuel lines. After about 10 sec the output of IC6 will become high and Q9 will start conducting. This will switch on the solenoid valve for GAS hence, the engine will now continue to run on gas.

5) Now, assuming the grid mains is restored, the 12V from mains adapter will switch on relay RLY2 which will switch the load immediately to the grid mains. The mains 12V will also switch on Q4 hence, IC2, IC3 and IC4 will be disconnected from the battery 12V.

The 12V mains will also power on IC7 which is configured as "Power-on Delay Timer". The output of IC7 will become high after about 5 sec which will switch off Q5 and de-energize RLY1, ultimately the 12V for generator will be switched off and the generator will be stopped.

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