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[Nordlys]

I want to vary the speed of a small motor running on a 9v pp3.
Can I use a variable resistor and what size would that be please
 Edit - or would it be a potentiometer
N

[JimG]

A potentiometer is just basically a variable resistor. What you need to think about first is what sort of current will the motor take, this will allow you to work out what wattage of resistor is needed. It will probably need a wire wound resistor or pot to take the likely current for the motor, a carbon track will quickly burn out. As for the size you might need to try a number of different resistances to get the best one. A good way to try and find a suitable value is to take a range of fixed resistors and put them in series with the motor to see what it does to the motor speed.

Jim

[Nordlys]

I've now tried a 1k potentiometer. The motor speed goes from zero to full in just a fraction of movement of spindle.
I'm going to try a 500 ohm pot guessing that this will give more gradual, variable speeds.
Would this be correct?

[BrianB6]

Probably you need something even lower.  I seem to remember using a 40 ohm pot to regulate a d.c. motor but not in a boat.

[Nordlys]

Thanks Brian  I'm ordering a 100r and 220r potentiometer to experiment with.
This is not for a boat either  -  a model truck for my grandson.
It won't be rc just a simple on/off with speed control, I hope!
N

[chas]

There's 2 options, measure the current and use ohm's law to work out the resistance properly. it's very simple but doing sums isn't for everyone.
Alternatively, buy a simple DC speed controler , it's a tiny thing with a knob to adjust the speed. It'll work better than a pot.
 Available from Amazon UK for around 5 or six quid.

[Nordlys]

OK, thanks Chas I've seen those items on Amazon and look to be very suitable for my needs.
I dont have the means to measure current, gets a bit too complicated all that calculating when Amazon have these items at £6 or so.


Thank you.