# How to work out the Resistors needed to get 25% & 50% speed on a 6v motor using a rotary switch?

Hi there, I have a small motor rated from 1.5 to 9 V DC that operates off a 6 volt lead acid battery.
I need to connect a rotary switch (1 pole 3 way) so that in position 1=off  position 2= 25% speed and position 3 = 50% speed.

How can I work out what Resistor to use to get the motor to operate at roughly 25% & 50% speed?
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EXTRA INFORMATION ON THE MOTOR:
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No Load @ 6V   =   Current = 0.021 A    Speed = 2790 rpm
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At Max. efficiency @ 6V = Current = 0.084 A    Speed = 2234 rpm  Torque  = 12.9g.cm
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Maximum Load @ 6V = Current = 0.180 A Speed = 1395 rpm Torque = 32.5g.cm
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Any help will be greatly appreciated.

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Jul 24, 2012. 8:36 AMsteveastrouk says:
Agree with Rick, you can't realistically control motor speed by adding resistors - it destroys the "regulation" of the motor - small changes in torque make variable changes to the speed of the motor depending on the resistor.

You should ALWAYS use a voltage regulator (or a variable DC power supply)
Jul 24, 2012. 3:19 PMsteveastrouk says:
A 317 and two resistors, is all you need.
Jul 30, 2012. 12:03 AMsteveastrouk says:
Yes, very nicely done. Simple isn't it ?
Jul 24, 2012. 8:03 AMrickharris says:
Not the best or most economical way to regulate speed - The resistors use power and will get warm

You will be better off controlling the voltage LM317 would be better.

However what you need information wise is ohms law.

V=I x R So if you know V and I then you can calculate R - Put similar resistor in series to halve the speed.
Jul 24, 2012. 7:18 AMmpilchfamily says:
Use voltage dividers to give you 1.5V (25%) and 3V (50%). Do an internet search for a voltage divider calc and start playing around with values. But any 2 matching resistors in a voltage divider configuration will half the voltage. Just make sure you use resistors that can handle the amount of power you'll be passing through them. The lower the resistance value the better.

You could also go a little more complex and have the selector switch rout the power through an LM317 with the right resistors values so the LM317 is regulating the voltage to where you need it.
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