On 21/06/2019 16:25, Theo wrote:
wrote:
On Friday, 21 June 2019 15:27:57 UTC+1, Graeme wrote:
I have a 12v aquarium pump supplied by a 12v power supply, but, for my
purposes, the pump performs ideally when running at 6v. Can I just put
a resistor between the power supply and pump, to reduce the voltage, and
if so, how do I calculate which resistor I need?

You need to know the current taken by the pump.

...which will almost certainly not be constant. When the pump tries to
start, it'll take a big gulp of current. The resistor will thus drop the
voltage to very low, and it won't start.

A switching converter might help - things like these:
https://www.ebay.co.uk/itm/Durable-6.../113655869520?
https://www.ebay.co.uk/itm/5A-DC-DC-...K/312565786982
https://www.ebay.co.uk/itm/9A-DC-CC-...V/312612183383

but we don't know what the pump's ratings are to make any better
suggestions. You're probably better off with a PWM speed controller, for
example:
https://www.ebay.co.uk/itm/DC6-60V-1...w/401782991554
https://www.ebay.co.uk/itm/DC-PWM-Mo...V/202657320802

Theo

Well maybe.

Or you could put a car light bulb in series, that will have low
resistance for starting and higher once it gets warm. I think I would
probably try a headlamp bulb for a start.

You don't say whether you have a suitable multimeter (I guess perhaps
not). There should be a current rating on the power supply, though,
which will assist us in doing sums. Or possibly a power rating. Is there
any "spec" on the pump?