Voltage Spike?

[hotpointer]

Hello
could somebody look at the circuit diagram attached

the problem i am having is intermittently the Atmega chip resets ...
this only happens when the relays are d energised supplying the two solonoid valves

 i think that maybe its a voltage spike ...  i do have the circuit protected by flyback diodes(1N4004)

5volts to the relay  ...........12volts to the valves

any ideas

thanks
 

[HalfSpace]

Hi hotpointer,

Does the micro reset if you disconnect the solenoid valves and just have the relays switching?

Having the flyback (Back EMF) diodes for the solenoids and relays is good, but their effectiveness is governed by the PCB layout and GND connections. A large current can flow through these diodes during flyback and the PCB tracks going to them can develop a voltage difference along their length. This voltage on the GND connections may be reducing the 5V supply long enough for the brown out detection circuit in the micro to generate a reset.

Can you post a picture of the PCB layout?

HalfSpace 

[hotpointer]

Hi

thanks for your reply  i will take a picture and post

i have just been told i have no decoupling on the atmega would adding caps help?

i will try with the valves disconnected.

rgds john

[kizzap]

i have just been told i have no decoupling on the atmega would adding caps help?

It will help 1000x. No decoupling means that any voltage spike/drop will cause the chip to reset. 100nF(0.1uF) caps on the V+ pins will help immensely.