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I've got a machine operating a 12V 1A water pump and a 12V 0.6A solenoid valve. On the power path I have a 1000uF capacitor added. I was suggested to use an adaptor with at least double the ampere capacity to the expected load as pumps could draw extra current during starting, so I bought a 12V 5A adaptor.

Now I'm testing another pump which pulls a rated 3A. Would my 5A 12V adaptor still suffice?

JRE
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DribbleNibble
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  • it depends what the pump's 3 A rating means. If that's the maximum it ever draws, then that's fine. If it's the typical running current, and on startup it draws several times that, then it may or may not be OK, depending on how your power supply handles overloads. If your supply shuts down on overload, then the pump won't start. If its 5 A supply rating is long term continuous but it can grunt its way through an overload for a few seconds, then all will be fine. Those 3 A and 5 A figures are meaningless by themselves, you need to know what they mean. – Neil_UK Nov 06 '22 at 10:40
  • @Neil_UK one site selling this pump says needs 2A "atleast", whatever that means. Maybe will just test it and will know in a couple of uses if its sustainable. Does adding more capacitors help? – DribbleNibble Nov 06 '22 at 10:45
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    Adding more capacitors will help an unregulated power supply grunt its way through an overload. Unfortunately, a regulated power supply will be helped little by capacitors until there are a veritable raft of them. The sort of pump you have linked is at its happiest when driven from a car battery, which will deliver 100s of amps when asked to. – Neil_UK Nov 06 '22 at 10:56
  • Please provide data sheet links to all parts mentioned. – Andy aka Nov 06 '22 at 12:34
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    Of course you will be getting one and at most two switching PSUs for your prototyping project now. I would suggest to start with 12V ***20A***. – tlfong01 Nov 06 '22 at 13:31

1 Answers1

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Measure the resistance of the pump motor with a meter. Divide your operating voltage by the resistance and this will give you the surge current of the motor on startup.

Motor current I=(Vs-Vb)/Ra Ignoring field current

Vs is the supply current Vb is the motor back EMF. Ra is the armature resistance.

Vb is proportional to Motor speed x Field Strength.

When the motor speed is 0 Vb is at a minimum (0). So measuring the resistance of the motor at zero speed leads you to the maximum current the motor can take.

RoyC
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  • Aha, ok, an amateur question please, I just need to measure the resistance between the two wires or do I need to open up the pump? – DribbleNibble Nov 06 '22 at 11:08
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    The pump is not a resistive load. It will consume current based on the load the motor has. Can you explain how measuring the pump resistance will determine the current required to start up or run and pump water? – Justme Nov 06 '22 at 11:23
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    @Justme doesn't measuring the pump motor rotor resistance (assuming a brushed DC motor) give you the lowest impedance at start-up and therefore the stall current i.e. the maximum current that the motor will ever draw whilst working from 12 volts? – Andy aka Nov 06 '22 at 13:40
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    Ignoring the field current which is insignificant, the current taken by the motor is determined by the armature resistance and the back emf generated by the motor. The back emf is proportional to the motor speed. By measuring the resistance of the motor at zero speed you get the maximum current the motor can take. – RoyC Nov 06 '22 at 15:55