switching magnetic contactors

[prasad70]

I am using 12F675 to switch on two magnetic contactors which are connected to two air conditioners. PIC detects the status of two inputs pins connected to remote thermostats (One is MCLR pin configured as i/p) and swich air con accordingly. I have used two NPN transistors to swich on two relays and N/O of those two relays drive the 230V contactors.

When contactors are not connected, circuit performs well but when I connect two contactors, PIC stuck when one of contcators is energized. Can some please explain possible causes for this ?


thk

 

[barry]

Do you have flyback diodes to protect your transistors?

 

[prasad70]

yes. I have put protecting diods to supress back emf of coils.

 

[xxtigerxx]

you can also use tranzil at the voltage of the mcu and one coil for filtering

 

[prasad70]

sorry. can you pls explain it little bit.
Is there a way to switch pic to programming mode due to a voltage spike on MCLR pin and to any change of its program to happen ?

Can some please explain?

 

[Wafeeq]

when i switch OFF magnetic contactors my LCD got noisy? why?

 

[barry]

Everything is pointing to an inductive spike causing your problem. Have you looked at your processor inputs with a scope?

 

[xxtigerxx]

Check also the PCB also, if some noisy tracks goes to close to the mcu.

 

[prasad70]

Another observation I could make was that temperature of micro controller went up during it's operation was stuck. Once I gave a power reset, its operation became normal.
When I disconnected the magnetic contactor coils, whole operation performed well.

Can PIC microcontrllar swicth to unknown state due to a magentic field effect or due to a voltage spike on it's pins.

 

[barry]

I think something bad happened to this server, because my last post didn't make it.

Anyway, the fact that your uP is heating up is yet more evidence that you are getting an inductive spike. When CMOS circuits get a high-voltage spike, a parasitic SCR can turn on, causing a lot of current to flow, which will generate heat. When the power is removed, the SCR turns off and, hopefully, nothing has been damaged.

---------- Post added at 00:16 ---------- Previous post was at 00:16 ----------

I think something bad happened to this server, because my last post didn't make it.

Anyway, the fact that your uP is heating up is yet more evidence that you are getting an inductive spike. When CMOS circuits get a high-voltage spike, a parasitic SCR can turn on, causing a lot of current to flow, which will generate heat. When the power is removed, the SCR turns off and, hopefully, nothing has been damaged.

 

[prasad70]

I put a 0.1 micrF cap with a 100ohm resister across the relay contacts and problem disspeared. Thanks a lot for your explernations