# Select PSU for 12V DC water pump

So I bought me this cheap 12V DC water pump for a garden project. Then I used a wall-wart type switchable PSU (similar to this one) to do a quick test. Works, but it has problems:

• The pump is rated with 19 watts @ 12V. Somewhere I've read it needs a min. power of 9 volts to operate, but when I switch my PSU to lower voltage, it usually starts pumping from 6~7.5V upwards, with increasing intensity.

• Now, the PSU: The one I've used for testing is not suitable. It gets hot and smelly after a few seconds. The label on it says it's rated with 18W on the input side. And says "current 1000mA max." OK. I guess the pump drew too much current (see calc. below), ~1500mA while this PSU can only deliver ~1000mA.

My math is: 12V @ 19W = 1.583A (from calculating 19 / 12)

OK, so a wall-wart 12V DC with a 2A output rating would deliver 24W and would fit. Right?

The Question:

How do you calculate a properly sized PSU for a DC water pump?

What about "headroom"? Let's assume something blocks the motor. Will a blocked motor increase the power draw and fry my PSU? (okay, my pump is "brushless", but does this make a difference?)

What if I want to drive the pump at, let's say, 75% power. With my switchable PSU, I got a desirable water flow at ~9 Volts - although the Pump is rated for 12V... Is that inefficient, lowering voltage? Should I instead use 12V and a resistor to lower the current that is able to flow to the pump? Also: is the wattage of the pump at 9V still 19W?? If so, amperes go up and I would need to supply 9V @ 19W = 2.1A (from calculating 19 / 9). Or, is it different: on another pump I saw it was rated with 10W at 12V and 17W at 24V. Does that mean these pumps lower their wattage when I drive them with lower voltages?

What about safety? Can I operate a fixed-output wall-wart type PSUs from eBay for hours and hours with a pump? Or are they only for electronic-type things, like you would find in a household... Are DC LED drivers a better choice? If so, I've read on them that they only operate safely inside a very defined current spectrum. What kind of PSU should/can I use?

Bonus question: Someone explain me these switchable cheap wall-wart PSUs: It says "power: 18W" - does that mean over the whole voltage range? Which would mean it could supply higher Amperes at lower voltages, right?

These related questions didn't help much:

What about "headroom"? Let's assume something blocks the motor.

Centrifugal pumps often experience reduced torque when the inlet or output is blocked. so electrically that may not be a problem.

Motors typically want more current to start than to run, often much more, some power supplies (particularly the adjustable ones) can accomodate this by lowering the output voltage instead of trying to provide more current than they are capable of.

What if I want to drive the pump at, let's say, 75% power. With my switchable PSU, I got a desirable water flow at ~9 Volts

9V may not be 75% power (it will be about 75% speed, but possibly closer to half power)

• although the Pump is rated for 12V... Is that inefficient, lowering voltage?

I'm not sure how to measure the efficiency of an ornamental fountain - joules per smile perhaps?. power consumption will be reduced vs 12V.

Should I instead use 12V and a resistor to lower the current that is able to flow to the pump?

That would definately be less efficient than selectng 9v.

Also: is the wattage of the pump at 9V still 19W??

It will be less than at 12V I would expect around 10W to 14W. you will have to measure amperes to find out.

Whatever the water flow is through the pump will also have an effect on the numbers you get.

on another pump I saw it was rated with 10W at 12V and 17W at 24V. Does that mean these pumps lower their wattage when I drive them with lower voltages?

yes.

• Thanks for taking my question with humor! Also, yes, I will next use my (poor, overloaded with this pump) switchable PSU again, test (quickly) which voltage produces a desirable water flow, then measure with my multimeter - to see which voltage is actually output and to measure which current is flowing to the pump. Then I'll get a PSU which is able to supply roughly twice the amperes the pump is drawing at my selected voltage. (that's the rule of thumb I made up from answers so far) Commented Jun 26, 2022 at 12:27
• Measured it and found the pump is drawing ~1.2A @ 12V and less on lower voltages. Decided for 9V where it drew ca. 0.8A and bought a 2A 9V plug PSU. Commented Jun 27, 2022 at 22:44

If you use a power supply to it's maximum rating, it will always reach it's maximum rated temperature. Pumps demand at least 5x the rated power on startup, often 10x so lower voltage is expected means it is underrated.

The prudent choice is any PS rated for twice the power which is easily met with such a low power load. Margins can be reduced with force air cooling and higher loads. Yet reliability or MTBF is always increased with lower operating temperatures, so cost is not a difficult choice here.

Remember that starting current depends on the coil DCRs and running current has back EMF proportional to RPM to reduce the voltage drop and operating current from no-load full speed on the motor. So if running at some ratio of expected RPM, the supply is labouring and unable to keep up.