In an earlier post I showed how to wire a small DC pump to a power supply and a switch to turn it on and off. Even with simple manual control, there are still things to be aware of. Let’s say that you are building a small aquaponic or hydroponic system and you need a pump to circulate nutrient fluid to the plant roots. This is a situation where the pump only needs simple on/off control and can be left to run continuously.
There is a danger that over time the liquid will evaporate and its level will drop too low. Many liquid pumps cannot safely be run dry. This is because they are designed to use the liquid flowing through to cool them to a normal operating temperature. Without a continuous fluid flow, the motor will overheat.
This means we have to be aware of the liquid level. A float switch level sensor can be used in the control circuit to make sure that the pump does not run when the liquid is below a certain level. Most liquid level switches can’t control the current needed to run a pump so we use the switch to control a relay that has this needed current capacity.
It’s also important to remember that some pumps also have a maximum on-time rating. The pump may be able to run only (e.g.,) one minute continuously before it needs to be turned off to cool for three minutes. This proportional on/off time is referred to as its duty cycle.