ATS circuit

Circuit Description

The ATS circuit or automatic relay changeover for generator/ mains circuit as shown below can be understood as follows:

For so long as home mains is present T1 base receives the rectified low voltage DC and keeps T2 base grounded.

With T2 base grounded REL1 is held switched OFF along with REL2, REL3 and REL4, the whole circuit thus stays switched OFF.

With REL4 deactivated, the DPDT holds the home mains supply with the load and the load gets powered via its N/C contacts.

Now in a situation when home mains fails, T1 is inhibited from its base drive and it instantly stops conducting.

With T1 OFF, T2 now activates, switching ON REL1, which in turn activates the LPG solenoid valve for allowing the fuel to reach generator combustion chamber.

After a few seconds delay T3/REL2 also activate pushing ON the choke solenoid into start position. The delay may be fixed by the tweaking the values of R7, C3.

REL2 activation switches ON the 555 astable which starts counting upto 5 seconds and triggers T4/REL3 so that the generator starter motor begins cranking the gen.

The astable allows this to happen for 5 seconds, if the generator starts, a 12V supply from a 12V adapter connected at the output of the generator feeds T6 base and disables the 555 astable.

The above 12V from the gen also activates the 4060 timer/latch which counts for about 10 seconds after which its pin#3 goes high.

The pin#3 high pulse latches the IC and also feeds T5 which deactivates REL2 so that the choke solenoid is pulled back to "close" position.

The 4060 output also simultaneously activates T7/REL4 making sure that the load now gets connected to the generator AC via N/O contacts of REL4.

Now suppose due to some fault, the cranking of the generator starter fails to initiate the generator, the astable makes three attempts with 5 seconds interval between each try.

Since the above pulses also reach IC4017 counter, after three pulses the IC4017 output sequence reaches its pin#10 which instantly latches itself due to a high at pin#13, and also disables the 555 astable by grounding its reset pin#4 via T6.

REL3 now stops feeding the crank mechanism.

An additional transistor driver/RELAY may be configured with pin#10 of IC 4017. The N/O contacts of this relay then could be wired with an alarm for the required warning in case the cranking attempts fails to start the generator.

When mains AC returns, T1 receievs the atached 12VDC at its base, however due to the presence of R2, D3, C5, T1 is restricted from the base voltage for a few seconds, until C5 charges.

In the meantime T7 is disabled and REL4 reverted to home mains position by T8, this happens as soon as mains returns, so that the generator gets immediately unloaded from the connected appliances.

Parts list for the above automatic transfer switch or ATS circuit

R1, R4, R5, R6, R7, R8, R9, R10, R11 = 10K
R2, R3 = 100K
C4 = 0.1uF
C1----C5 = timing capacitors, can be between 10uF to 100uF
All transistors are BC547
All diodes are 1N4007
REL1---REL3 = 12V/10 amps/400 ohms
REL4 = 12V/40amps or as per load specs