On 21/06/2019 16:32, Archibald Tarquin Blenkinsopp wrote:
On 21 Jun 2019 16:25:07 +0100 (BST), 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

You are quite correct. The only thing I would suggest though is that
if the pump is happy at starting and running on 6V DC, a three
terminal regulator would be cheap.

From the following line we assume you mean a 5V 3-terminal regulator.

A couple of caps and lift the ground terminal up with two diodes. 6.2V

But 6V regs are just as cheap as 5V ones + 2x diodes.