Hi I made PCB with arduino uno. It has inputs push-buttons, temperature sensor and four outputs (mechanical relay) which switches three contactors.The PCB has input stage transformer 220/12 V. It has voltage regulator 5V with two block capacitors of 100 nF (placed on input and output of voltage regulator). Also has electrolytic capacitor 4700 uF.I put diodes in parallel with the coil of relays. The mechanical relays switch the three contactors with coil voltage of 220 VAC. All works fine. Only sometimes when all contactors switch off in the same time the ATMEGA328 resets by itself. It is because of the collapsing electromagnetic filed of the coil of the contactors.. Also in parallel of the coil of the contactor I must put the capacitor of 0,22 uF , 500V . Also near the uC I will put 100 nF , and one 100 nF capacitor on reset pin. Is this is enough for block the noise which resets the uC or I must do additional things. Regards Riste
i think the problem is different. why sould the arduino reset ? it has enough power reserverd by the 4700uF and i hope (you did not show the circuit) you dig separate galvanic . i mean the transformer separates ground, so the power of atmega is not interested in peak of coil-off-swich. is it possible that your pcb is a antenna for receiving transients over air ?
sometimes it can be problematic to make C parallel to pins because the wires at the pins in combination with C are LC and want to swing.
but firs i would habe a look if there are no antennas, but when you did not separate galvanic do this first
i hope i could help
A circuit diagram is worth a thousand words here....