Calculating the size of a resistor

In message <qel5fb$1f8b$ snipped-for-privacy@gioia.aioe.org>, Bill Wright snipped-for-privacy@f2s.com writes

Tried that Bill, but it doesn't make enough difference.

Cheers

Well how about a variable PSU? Too expensive I suppose.

Bill

Model plane ESC and a model plane servo tester will allow variable control of the motor from nothing to full but you are talking over £10

that's what I'd do, but diodes passing power drop nearer a volt each, sometimes more. 0.65v is the knee voltage not the running voltage at highish current.

NT

It would be better to use a regulator. Look here:

You could possibly try a 9V one, or look for an 8V one but they aren't common.

I use versions of these regulators in several applications.

Just get a 6V power supply with adequate current - it will probably waste less energy as heat than any lash-up you're likely to cobble onto the existing one.

It seems to pump air not water, so instead of restricting it, perhaps you could add a hole in the tube to let out unwanted air?

I found this alternative supplier who rates the pump at 12V 250mA:

Indeed, but its hardly an exact science in this case - we don't really know what voltage the motor is going to work at the required speed and still start under load etc. Adding or subtracting cheap diodes should be a fairly quick empirical exercise in finding the optimal number.

By th time you have bought all those resistors and diodes it would be as easy to buy a model plane controller with a servo tester.

In message snipped-for-privacy@4ax.com, Dave W snipped-for-privacy@yahoo.co.uk> writes

It does indeed pump air, and I assume, although haven't looked, it is just a basic 12v DC motor driving an impeller. A cheap inline restriction valve is on the way, and the plan is to cut the tube and insert a three way connector, with the valve connected to one of the outlets, to control the amount of air exiting the business end.

There have been some interesting suggestions (thanks!), and I like John's pack of silicon rectifier diodes, as they will be useful elsewhere.

The pump is picking up power from a 12v bus bar, and I would prefer that option, although could run a dedicated variable supply via a toy train controller or similar, but am trying to avoid that option.

No need for resistors, and the ebay diodes come in a pack of ten for under 2 quid.

better in what way?

indeed. Just don't go with an initial estimate that leaves the motor no power.

NT

Motor starting current is many times run current. You can't overload zeners like that & expect them to last long if at all.

NT

Next time keep some e-waste and you won't need to spend or wait.

NT

They are usually diaphragm pumps. The Brrrrrr sound is the give-away. May be a motor and crank, may be like a door-bell, i.e. a solenoid with a breaker switch on the arm. Screen washers are often impeller, note the higher frequency sound.

Excellent, thank you.