I am trying to control a small 3-12v max 1.2A DC pump using a sainsmart DC-DC relay board. Everything works fine when the pump is not connected but as soon as I connect the pump the relay wont stay on. It simply flashes on for a split second and turns back off. It keeps doing this until I disconnect the pump.
I am powering everything from a 5v 2A wall wart style supply. The relay board is being powered through the Arduino and the pump and the Arduino are split off from the source. If I connect the Arduino to my computer and power everything through that, including the pump from the Arduino, it works fine.
What is different about the external power supple compared to the USB connection.