I guess the title is pretty descriptive by itself. So I am making a project to replace the old outdated thermostats in my house. I want to control the boiler and the water pump so I bought 2 30A relay modules from ebay. They also have a aled on them for easier troubleshooting. To the point, The relays are driven by a transistor 2N3904 each. I have connected these modules to digital pins and so far they work great. I was looking on my lcd which displays temps,status etc and I noticed that when a relay is activated it kind of dims a bit. Once the second relay gets switched on it dims even more. So far everything is in the bench so no load connected to the relays. I measured the voltage in my +5V and GND pins of my Mega 2560 and without any relay actuated the voltage is 4.99V. One relay ON, 4.77V, two relays ON, 4.56V! So that causes the fade out of the lcd screen.
One more thing it did, was that this voltage drop made my DHT11 temp sensor fluctuate so it was going up and down near the setpoint and 2 times it caused the relay to turn ON-OFF-ON-OFF like a turn signal on a car.
Is it normal to have such voltage drop with the relays ON? What is the best technique to follow?
Thank you for your kind answers. I thought of doing so but I am wondering, how should I connect the arduino output then? The DigitalOut ok at the command input of the relay board and the 5v and gnd at the external source. What should I do with the ground? Should I connect the gnd of the relay board to the external source PLUS the arduino gnd to close the command circuit?
Also how much current do you recommend for powering 3 relays? 1A or more?
All grounds should be connected together.
There is little you can judge from the ratings of a relay contact in regard to how much the coil will take, it could be anything. It is determined mainly by the wire gauge and number of turns. So look at the data sheet or measure it with a meter.
If you aven't got a meter then get one.