Hello everybody,

I was recently working on a project with several AC relays operating at 24v DC (http://www.sunshinele.com/htm/power-relay-jqx60f.htm), powered by a 24V DC SMPS power supply. The driver circuit for the relay is given in the image.

The relays are to be switched on and off through an ATMEGA MCU with brown-out mode disabled.

The MCU is powered using a buck converter to step down the voltage from 24vDC (SMPS power supply) to 5V dc, the circuit of the converter is given in the following image.

The heart of this circuit is the LM2596, and the circuit and values are derived from the datasheet of the manufacturer. The output voltage is a 5v stable during regular operation.

Problem: Whenever I switch the relay on and off the MCU restarts. After I connected the oscilloscope to the 5v supply of the MCU I saw a sharp voltage drop followed by a voltage raise before the voltage settles down again to 5V. I added capacitors to accommodate for the sudden change of voltage across the coils of the relay but the problem is still there. Oscilloscope image showing the voltage drop on the 5v supply of the MCU

any help or suggestion would be highly appreciated, thank you.

 

Do you get the reset irrespective of whether a load is attached to the relay or not?

Thank you for replying,
When the load is connected only, noting that the relay is switching a transformer on and off which either raise the AC voltage or decrease it, thus achieving a power regulation on the mains electricity. This is happening only when the load is connected to the relay.