Arduino UNO works partially with external power supply

Hi, I’m using Arduino UNO and built an object placement puzzle. It has 4 Rfid sensors which unlocks the relay if puzzle solved properly. Problem is that everything works perfectly if Arduino UNO is connected with USB cable. If I connect it with external power supply it looks like it’s working (lights on Rfid sensors, relay and arduino is lit), but after solving the puzzle signal from first analog output doesn’t drop and relay is not switched. I’m using DC lab power supply. I’ve tried different voltages from 7v up to 12v. It doesn’t change anything.

The only way I managed it to work was connect 5v directly from the other arduino to first arduino 5v pin. If I connect 5v to that pin from other power supply it doesn’t work the way it should.

If anyone could help me with a problem that would be nice. I can upload a code, but I’m assuming problem is not with a code, because it works with USB powered arduino.

JonasTal:
I'm using DC lab power supply. I've tried different voltages from 7v up to 12v. It doesn't change anything.

The voltage regulator on the Uno is fairly limited in how much current it can supply. It's fine for powering the Uno itself, but once you start powering other components from the Uno's 5V pin you can exceed its capacity.

JonasTal:
The only way I managed it to work was connect 5v directly from the other arduino to first arduino 5v pin.

How was the other Arduino powered?

JonasTal:
If I connect 5v to that pin from other power supply it doesn't work the way it should.

What exactly do you mean by "doesn't work the way it should"? Is it the same problem you described when using the 7-12 V power supply?

When you power the Uno directly through the 5 V pin, then the current limit for the 5 V rail is whatever is imposed by the power supply. The 3.3V pin is still powered by a voltage regulator and so can supply a limited amount of current.

Thank you for your answer.

How was the other Arduino powered?

It was connected to computer with USB cable.

What exactly do you mean by “doesn’t work the way it should”? Is it the same problem you described when using the 7-12 V power supply?

Yes, relay doesn’t react when I put Rfid sensors in a right place. Maybe the problem is in RC522 sensors, that they don’t send the signal to Uno because of something with power supply.

I also have attached to this post my wiring scheme and code, if that can help to help me to find a solution.

Multi_RFID.ino (5.06 KB)

It definitely sounds like your power supply is the problem.