swtich pump off automaticaly

[bigmannie]

I have a micro pump that draws 600ma under load and 300ma with no load. It's installed on my motorbike to transfer the fuel from the reserve tank back into the main tank. it will run for about 1 min. To avoid running the pump dry, I would like to know, if any of you wizards out there can come up with a simple circuit that will sense the drop in current when the tank is empty and turn the pump off, via a relay. The pump has a brushless motor.

 

[tunedwolf]

To perform any measurement on the pump current will always rely on it running dry, even if only for a short period of time.
There are a couple of obvious ways you could do this. First way would be to measure the current using a low value resistor in line with your power lead and take a votage measurement across it. This can then be fed into a comparator circuit that you adjust for the desired cut-off level. A simpler method may be to use an LED and Photodiode on a piece of clear tubing or sight glass in the suction side of the pump and use the diffraction properties of the liquid to tell you if there is fuel in the pipe or not. There are other methods based on flow rate etc that you could also explore.

rgds

 

[MikeMl]

High-side current monitor IC, like a Zetex ZXCT1009, and a low-resistance shunt resistor.