Calculating the size of a resistor

[Bill Wright]

Could you reduce the efficiency of the fan by bending or clipping the blades?

Bill

[Graeme]

In message <qel5fb$1f8b$ [e-mail]>, Bill Wright [e-mail] writes

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

[Clive Arthur]

6V zener diode. I have a shed load of surplus 3.3V 5W zeners, two in series would get you down to 5.4V. I could send you some if I knew where...

Cheers

[Bill Wright]

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

Bill

[The Natural Philosopher]

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

[tabbypurr]

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

[Brian Reay]

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

Select on that gives the more than the current you need at 6V

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.

[Rob Morley]

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.

[Dave W]

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?

[Dave W]

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

assuming it would take 125mA at 6V, the zener would dissipate 750mW so a 1W zener would do. 3V zeners don't have very sharp voltage curves so I wouldn't recommend them.

[John Rumm]

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.

[The Natural Philosopher]

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.

[Graeme]

In message [e-mail], Dave W [e-mail]> 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.

[John Rumm]

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

[tabbypurr]

better in what way?

[tabbypurr]

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

NT

[tabbypurr]

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

NT

[tabbypurr]

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

NT

[newshound]

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.

[Graeme]

Excellent, thank you.