Hi Mosaic,
I am using IN4007 across the Solenoid to prevent High spike back voltages that can damage the MOSFETS... and You are right one of my friend "Mr.Roman" also suggested me to remove the IN4007 across the solenoid..His theory is that the IN4007 creates lag in shutdown of the solenoid valves... I will experiment by removing the IN4007 from the solenoid and will check whether this will effect the response time or just will blew-off the IRF540.. If IRF540 damage i will then use IRF840 .. I can also see replacement of IN4007 with any other fast switching diode ...
As i am very newbie to this electronic stuff i will be happy if you can post a SKETCH of installing Transorb in the circuit to understand... I am using 33 VDC 12 Amp regulated Power supply to control the system considering worst case scenario of opening of all 40 valves.. 40 x 260mAmp=10.4 amp.
Currently i am using only three digital pins of ATMEGA 328P to control 5 SIPO shift registers.. One pin is for CLOCK, the 2nd Pin is for DATA, and the 3rd pin is Latch.. If i use 4 pins of microcontrollers as DATA pins then i can reduce the time/4.
Mosaic, I am thankful for your very valuable post on the subject and i hope with all of your help we will make this complete system very low cost upto Hobbiest level.