Mini water pump

A thumb-sized submersible pump with an outlet spout and two leads, the pump of every plant-watering build. Aliases: submersible pump, mini pump.

Inside is a small brushed DC motor driving an impeller, sealed so the whole pump sits in the water. Electrically it is a motor and nothing else, so it needs what a motor needs: a transistor, a MOSFET or a relay. Never a bare board pin.

Pins

Pin	What it does
V+	The red lead.
V-	The black lead.

Properties

The numbers come from the listings: 3 to 6 V, 130 to 220 mA, up to 120 L/h.

Property	Default	Meaning
voltage	6	Full speed and full flow at this voltage.
flow	120	The flow at full speed, in liters an hour.
resistance	4	The winding, in ohms. With the back EMF it gives about 220 mA running at 6 V.
inertia	50	How slowly the impeller follows a change, in ms (an assumption).
back_emf	0.85	The back EMF at full speed as a fraction of the supply.

While it runs

Water arcs out of the spout into the pot as thick as the flow, the readout shows the flow in liters an hour, and the pot keeps a tally of the milliliters pumped.

What the model gets right

It is a motor. A stalled pump draws far more than a running one, and on a bare pin it barely turns. The flow follows the speed, so a lower voltage or a PWM duty pumps less.

What it does not model

The head (pumping higher lowers the flow), running dry (which ruins a real one), and the water. Run backwards, a real impeller pump still pushes water out, more weakly; here it pumps the same either way.

Common mistakes

Running it from a board pin. A pin gives about 20 mA; the pump wants ten times that.

No diode across it on a transistor. The simulator forgives that; a real transistor may not.

See it in action

Plant waterer reads a soil moisture probe and runs the pump through a relay when the soil is dry. Open it at /templates.