Relay Board wont stay on with load

Hello,
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.

Thanks,
-Lars

Hi, The fact that you have everything powered from the same wallwart is your problem. The 1.2A pump probably draws more than 2A surge when it is switched on, this causes the 5V to drop and so your Arduino resets. You USB may be able to supply a higher current.

Tom... :)

Motors draw a lot more current at startup, think 5 to 20 times more... You may get away with adding a big electrolytic cap and a diode to hold up the relay supply during the motor startup transient, but the fundamental error is to use a motor supply for something other than the motor, expecting it to be a clean voltage rail.