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After some research and head-scratching, I plan to connect a 3 Pin UK AC/DC 4.5V 500mA 0.5A Power Adapter (5.5mm x 2.1mm) into a 5.5mmx2.1mm 2 Pins DC Power Jack Female Connector Socket and that directly to a Mini Submersible Motor Micro Water Pump which is rated 3W DC 3-4.5V with a current within 0.05A.

In as far as I have understood so far, this should supply the pump with exactly the power it needs. Have I understood correctly? Should I be mindful of anything else? For example, should I use some sort of resistor to force the voltage to max out at 4.5v?

Matthew Brown aka Lord Matt
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    3w/4.5v = 0.66A, more than 0.5A. if only 3v, then it's 1A, way too much. Get a stronger power supply. also use a flyback diode as cheap supplies often don't have one, and pumps are inductive loads. – dandavis Jan 22 '22 at 03:47

2 Answers2

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Concerns:

  1. 3 pin vs 2 pin
  • the 3rd pin is a dummy plastic pin. The supply is double insulated.
  1. 3V to 4.5V
  • their specs are vague but the 0.05A current and power consumption will reduce with voltage but also increase lifespan somewhat. I read they are rated for only 200h continuously. (due to vibrator contact arc erosion, I expect)
    • However if 4.5V* 0.05A is true, Pd= 225 mW and the current & power will increase to 10x on 1st pulse, which is why they specify a 3W rating
  • I expect it will last longer than 200h, if you limit the number of hours per day to 2 and don't let it warm up. But you should buy 2 just in case or consider a 3V 3W supply and it will last longer. (Arrhenius Law effect)

The size of the Cap and quality of low ESR in the supply may affect lifespan of supply and pump. But I can't be sure what you get. The pump ought to have a RC snubber across the vibrating solenoid contacts inside.

These are not high-reliability items but your choice is OK. Perhaps more flow at 4.5V but lower voltage is also OK with some slight life span advantage.

Tony Stewart EE75
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"with a current within 0.05A" doesn't make much sense. Whatever the case, "supply the pump with exactly the power it needs" must take into consideration the current needed for a very brief time to start the motor. That will be quite a bit more than the current needed to run the motor.

Typically what happens is that the motor lurches in an attempt to start. Then the power supply switches off to protect itself. Many power supplies switch back on after a couple of seconds and the motor lurches and stops again. That sequence continues until the motor is disconnected. The question often seen here is "why did it do that?"