I have an Atmega328p based node with RFM69HW radio, controlling a relay. Power is supplied using a cheap 220V to 5V Chinese power supply (see photo). A 30W led bulb is connected through the relay to mains power.

The problem is, when I run test, which turns on and off the relay once per 5 second, sometimes (once per 5-10) the system just reloads. I checked every part with an oscilloscope and found that the power supply has voltage drop to near zero, which causes the system restart. Checked with different power supply, everything works perfect. Also, without the bulb it works.

Can anything be done to fix the problem?

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The schematic: enter image description here

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    \$\begingroup\$ The power supply is only 3.5 W. So I do not see how you can use a 30W led bulb. You seem to overload the circuit. Please add a schematic to prevent misunderstanding \$\endgroup\$ – Decapod Oct 29 '16 at 5:56
  • \$\begingroup\$ A schematic will help us help you. \$\endgroup\$ – Daniel Tork Oct 29 '16 at 6:40
  • \$\begingroup\$ The bulb uses mains power. \$\endgroup\$ – Roman Simonyan Oct 29 '16 at 6:41
  • \$\begingroup\$ Added the schematic \$\endgroup\$ – Roman Simonyan Oct 29 '16 at 6:51
  • \$\begingroup\$ Are you sure the PSU and MCU match? \$\endgroup\$ – Daniel Tork Oct 29 '16 at 7:26

The situation is more clear now.

I expect that the incomming mains supply causes the problem and that switching the bulb creates a voltage drop high enough to make the power supply restart.

Remember that the led bulb has a very high inrush current due to the rectifier on the input of the build in converter. This could be more than 20 A. The connecting cable could be the cause

To verify this.

  1. Try a short and big connection cable.

  2. Separate the two systems and feed the buck converter from a different outlet.

  3. Try a standard bulb.

PS. I use the same power supplies and have no problems.

  • \$\begingroup\$ Thank you, will try these. When I change the PSU to another, no drop and restart occur. That's why I concluded that the PSU is the cause. \$\endgroup\$ – Roman Simonyan Oct 29 '16 at 7:51
  • \$\begingroup\$ Changed the cable, testing now. No restarts so far. Previous cable was really thin and long. But why other supply had no problems? Better design? \$\endgroup\$ – Roman Simonyan Oct 29 '16 at 8:15
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    \$\begingroup\$ The inrush current of the bulb is the problem with a long cable. That made the incomming voltage drop for a very short moment. \$\endgroup\$ – Decapod Oct 29 '16 at 8:16
  • \$\begingroup\$ It does restart sometimes anyways. Is there anything else I can do? \$\endgroup\$ – Roman Simonyan Oct 29 '16 at 8:52
  • \$\begingroup\$ Tried standart bulb. Does the same. \$\endgroup\$ – Roman Simonyan Oct 29 '16 at 9:04

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