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# Turning 12V pump to variable speed...

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#### jdepaul

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I have a simple 12V dc marine pump that I use in some liquid pumping. The pump is quite strong - and it runs a bit too hard for my application so I've been thinking about ways to make its speed more adjustable... I don't suppose a light dimmer dial would work, would it?

Any other ideas how to make this 12V dc pump's speed adjustable?

Thanks,
James

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You could use an adjustable 12V lab supply or build an adjustable DC voltage regulator. Need to know the current required by the motor to recommend more specific circuits.

My Harbor Freight pump...

You could use an adjustable 12V lab supply or build an adjustable DC voltage regulator. Need to know the current required by the motor to recommend more specific circuits.

What is an adjustable 12V lab supply?!

Regarding the current requirements, looks like it's 65W; 7.5A start and 5.0A continuous - how do I build an adjustable DC voltage regulator?

Thanks,
James

P.S.
This is the pump I have: - Harbor Freight Tools - Quality Tools at the Lowest Prices

If your Pump is 12Vdc, reduce the 120 to the 12V supply?

If you have a 12V supply for the pump, it's only 65W= (65W÷12V)=5.4Adc on the output side of the supply.

If the supply is around 80% efficient, the power in is 65W÷0.80=82W.
Line voltage of 115Vac, would be required to furnish 82W÷115V=0.7Amps.
A lamp dimmer could handle the power of your existing supply, and if the existing supply is a common wall wart, it has a transformer which can handle a standard triac lamp dimmer. So try a lamp dimmer to reduce ac voltage going to your 12V supply?

Supply...

So try a lamp dimmer to reduce ac voltage going to your 12V supply?

unfortunately, my supply is a simple 12V car battery... I'd like to stay with the simple 12V battery if I can - other options plz?

James

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Since your full load 12Vdc is 5 amp, you could use a resistor.
for a 10.5Vdc pump voltage: the drop'g R is (12V-10.5V)÷5A=0.3 Ω
The R would be placed in series with the battery and the motor.
The power rating should be IxIxR=5Ax5Ax0.3Ω=7.5W
( 20W would run cooler ) use a wirewound.
OR parallel three 1.0 Ohm, 5W-10W resistors if you can twist the leads together tightly and solder. Treat as a single series dropping resistor.

You could add another 1Ω in parallel with the three, to increase speed or series / parallel two sets of three to reduce speed. Very cheap parts.
Xicon from Mouser has them for $0.60 or$0.50/10.

Using a pot...

If I could use a resistor could I take a leap to a potentiometer that would allow me to increase/reduce resistance and thus adjust the speed of the pump but varying the current thorugh it? Guessing more resistance, less current translates to slower speed: so say a power pot variable from .1ohm to 5ohms? not sure about that...

That's a lot of power for a pot to handle. How about a two or three position toggle switch with an equal number of resistors, each a different value?

I think this could work as well. Pulse Width Modulator. More control than a potentiometer and more efficient, well aside from the fan and the led. Ask the seller though.

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A cheap and simple way to reduce the RPM of the motor is to get some old 20 amp or 40 amp bridges from a junk yard and put them in series. You can use one diode from a bridge or two diodes to get either one-diode-drop or two-diodes-drop and see how much this reduces the RPM.
This way you can use damaged bridges that have one damaged diode.

I'd recommend PWM; here's the simplist way of doing it.

Hero's idea is good. But remember to put a freewheeling diode across the load.

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