8 relay board - Power failure

Hi, i tried to use a 8 relay board and i experience some power problem.
If someone can help me to understand my error, it will be great.

This case describe perfectly my connection :

The first thing to do is ensure you have your relay module connected properly and adequately powered. This example shows how to make use of the optic isolation feature.


  • Remove blue jumper
  • Connect JD-VCC and GND from the Relay Module to a Separate 5VDC Supply (1Amp or greater)
  • Connect VCC from the Relay Module to 5V on your Arduino
  • Connect required control signals as shown
  • Note: DO NOT connect GND from the Relay Module to your Arduino

It works perfectly if i activate only 1 to 4 relay simultaneously. And it use near to 250 mA on the dedicate 5VDC.
But if i activate a new relay, it works but after some second, power consumption start to decrise and all relay start to be unstable (LOW / HIGH very quickly).
More relay i activate simultaneouly more quickly the power unstability arrive.

I have tried with two differents mega.
And i have try a L7805C and a LM1117 for the dedicate 5VDC with same result.

Thank you

And i have try a L7805C and a LM1117 for the dedicate 5VDC with same result.

What input voltage. 9volt, 12volt?

Eight relays could use 880mA = 640mA when active.
If you have used e.g 12volt input for your regulators, then (12-5)
.640 = ~4.5watt must be dissipated in the regulators. That (getting hot) could shut them down.

Try a 5volt/1Amp phone charger to power the relay card.

And i have try a L7805C and a LM1117 for the dedicate 5VDC with same result.

How much voltage are you feeding into the regulator? Is there a heatsink on the regulator? Have you got capacitors across the regulator as recommended by the datasheet?

The more voltage you "drop" across a (linear) regulator the more heat/power you dissipate. When it gets hot it shuts-down (or "folds back") to protect itself and you can't get the full-rated current.

A switching regulator is much more efficient and it won't heat-up nearly as much. But, it requires an inductor so it's a bit more difficult to build.

That's what i thought.
But i was thinking to put relay module in a box on my electric panel where i have some other modules power by a "big" 12VDC.
Now i need to find a solution to create a large 5VDC from 12VDC without heating.

This Pololu stepdown/buck converter converts 12volt to 5volt with high efficiency (no heat).
Cheaper clones on ebay.