Calculating the size of a resistor

You need to know the current taken by the pump.

Then Volts / Amps = R of the pump in Ohms, and place the same R in series.

Power rating of resistor Watts = Amps * Volts

Owain

...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:

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.

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

AB

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?

I'm intrigued. Current rating on the PSU?

That's deep!!

Why the hell do people that are totally clueless pop up with their stupid advice?

If you don't have a clue, post nothing, just read and learn.

AB

Thank you all. This is potentially rather more complicated (and expensive!) than envisaged. The pump is eBay item 283477907575, three quid from China, including postage. It works perfectly, but too well for my needs. I just want a fairly gentle jet of air to turn a fan (don't ask), and whilst the pump works, it makes the fan spin rather than gently rotate. I've tried various combinations of angle of jet, moving the jet around etc., but the best results are when the pump is running on 6 - 8v. I could try running the pump at 12v and restricting the flow out of the pipe, but worry that could overload the motor?

In message <Pb3Dr+ snipped-for-privacy@binnsroad.myzen.co.uk>, Graeme snipped-for-privacy@nospam.demon.co.uk> writes

Just found a control valve for 99p which could be the answer. Worth trying at that price, anyway.

Unfortunately thats not so good either as

(a) the resistor will get hot and (b) it limits stall current and starting torque

Get a 6V supply

Or a 5-6V output downconverter.

That is something we use in the RC model world a LOT

Is 3A enough to start the motor

Anyway search for UBEC to get similar products

S*it Ive got doezens of those motors but all 6V 0nes for model planes

It works perfectly, but too well

Mm. RC speed controller + RC servo adjuster BUT you are spending WAY mnopre than you want to

Why not arrange a variable air leak? Put a tee in the output line, near the pump, and a little plastic valve on the side arm of the tee that you can open varying amounts to reduce the amount of air getting to the output end of the pipe. Doesn't load the motor and can be varied at will.

Is it 12V DC or 12V AC?

Diaphragm pumps are usually AC which limits what you can easily do.

OK that does not look like a significant current draw.

How about this approach: get yourself a pack of silicon rectifier diodes[1], and add a bunch of them in series with the power supply. Each will drop around 0.6 to 0.7 volts, and unlike a resistor will not exbibit a voltage drop that changes with load. You can then "tune" the performance by adding diodes until you get the level of performance required.

In message <1o9jhab.14k721j1ewo53iN% snipped-for-privacy@hayter.org, Roger Hayter snipped-for-privacy@hayter.org writes

I like that. Thank you.

In message snipped-for-privacy@brightview.co.uk>, John Rumm snipped-for-privacy@nowhere.null writes

Another simple, straightforward approach (and cheap!). Thanks.

In message <_mcPE.148790$ snipped-for-privacy@usenetxs.com, "dennis@home" snipped-for-privacy@invalid.invalid writes

DC.

Just buy a universal PSU with adjustable outputs like

use a buck converter like

You still need a power brick to get the input.

Looking at the actual pump you have I doubt if it can over aerate even a small tank left on 12V.

I'd imagine there is. What you would need to know is the current used by the pump to do this by resistor, but personally I'd just go out and get a 6v psu, indeed if its not too much a 5v USB double output psu might do it. I'm a little worried though why they would make a pump that runs too fast? Surely most pumps rely on the flow to keep them unclogged to some extent. Slowing one down seems to cause issues. I used to have a drill operated pump for watering the garden from the rainwater but, and if you used a slower speed the pump clogged. Those I think were centrifugal pumps, I don't know what yours is. Brian

Not that long ago, modern pwm controllers started to come in for Scalextric and model railway speed controllers. These did not get very warm , so were quite efficient, but fed the motor with 12v in short bursts. Thus no decrease in power or torque but a slower running speed. I've never tried one but suspect the modules mentioned are based around similar devices. You cold use a normal regulator chip but like the resistive approach they waste power though heat generation, ie dumping the extra voltage by dissipating it as heat. Brian

Not the best idea. Motors generally need a low impedance supply to develop their best starting up torque, etc. PWM is probably a better way - but likely to cost more than the correct PS.