• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Mcu immunity of relay noise

Thread starter #21
the circuit is working good with relays on when ac power is off the problem happens when ac power is on
relay on or off and ac power off (circuit is good)
relay on and ac power On (circuit is mad)
Last edited:

Les Jones

Well-Known Member
So from what you say in post #21 the problem has nothing to do with the AC. ( The first and the third line of post #21 seem to contradict each other.)
Looking at the etch layout of you board it looks like the relays are powered from the same power rail as the microcontroller and there looks like a lack of decoupling capacitors. The lack of decoupling capacitors could cause the problem or the power supply may sag when one or more of the relays are energised. It would be helpful if you provided a schematic of your board and told us which microcontroller you are using. (I could only guess what pins were the power pins.)



Well-Known Member
Most Helpful Member
Sorry ,i corrected the last line.
The chip is atmega 8. Where should decoupling capacitors be?
Just across the power pins as close to the mcu as possible. 0.1uF is a good value for most things. All your ICs should have these too. This is standard practice and you should always do this to ensure that ICs work reliably.

If that issues still occur then you need more bulk capacitance so you can try 100uF (you only need one of these and it doesn't need to be super close to the IC. The smaller caps are for decouple higher frequencies so need to have short traces to the IC but the larger ones are for lower frequencies so can be farther away.)
Last edited:
Thread starter #25
I desinged the pcb with 1000uf on regulator input and 100 nf on its output, there is also 100nf connected the MCU VCC close to it. the circuit is working fine with dc motor and ac lamp but the problem appeared when i connected small ac motor(grill motor). it disturbed the MCU code and take random actions.

Latest threads

EE World Online Articles