I have this relay module. I don't understand what is a role of the jumper on it (for now it's shorting the jd-vcc (what??) and vcc pins).


1 Answer 1


The jd-vcc pin can be used to power the relay coils using a separate 5V power supply. Why would you want to do that? Each relay coil draws about 73mA @ 5VDC when energized. Theres a circuit schematic on this website: http://howtomechatronics.com/tutorials/arduino/control-high-voltage-devices-arduino-relay-tutorial/

  • Hello! Thanks a lot. In this diagram (howtomechatronics.com/wp-content/uploads/2015/09/…) I don't understand why it's not needed to connect in addition GND from arduino to relay, to make the same ground in the system... Can you explain it please?
    – Michael
    Oct 7, 2017 at 14:48
  • @Michael In the tutorial, he's supplying 5V to the VCC pin and grounding the In1 pin using an Arduino digital pin. Have a look at the circuit schematic on the tutorial page and follow the "path" the current is taking. Current flows from VCC, through resistor R1, through the LED inside the optocoupler and finally through a diode DS1 to pin In1 on the relay board. When the Arduino digital pin is commanded LOW, it provides the ground for the optocoupler LED, turning it on. The digital pin is only suppling a ground for an LED.
    – VE7JRO
    Oct 7, 2017 at 19:13
  • @Michael When the digital pin is made HIGH, there is 5V at pin In1. Diode DS1 is reverse biased so current can't flow from In1 through the circuit and current can't flow from VCC through the LED because it needs a ground to complete the circuit. If you wanted to power the relay board using an Arduino, then you would leave the jumper in place and connect both the 5V and ground connections. In1 would still be connected to a digital pin and work the same way. Keep in mind that each relay draws 73mA of current when energized.
    – VE7JRO
    Oct 7, 2017 at 19:14
  • @Michael: both GND pins on the relay module are already connected, check the continuity. that said, the illustration is missing the AVR power connection, and that indeed could be isolated, in which case it's wrong, and you'd need to common all the GNDs
    – dandavis
    Oct 8, 2017 at 6:29

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