# History of Stepper Formulas

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Revision 3 . . May 26, 2008 1:50 am by Fritz
Revision 2 . . (edit) November 3, 2007 1:28 pm by BigJohnT

Difference (from prior major revision) (no other diffs)

### Compute Value and Wattage of Current-Limiting Resistors

:Note: For L/R systems only.

:This is a basic application of Ohms Law for a series circuit.
:Your resistor must drop the difference in voltage between
:the voltage at which your stepper is rated and your supply voltage:

::Resistor voltage drop = Supply voltage - stepper rated voltage

:Applying Ohms Law, divide by the rated current to get the resistance:

::Resistor value = Resistor voltage drop / stepper current

:Finally, and very importantly, you need to know how much wattage
:you will be dissipating as heat, which your resistors must be rated to handle:

::Resistor wattage rating = Resistor voltage drop * stepper current

:For example: Steppers rated at 2.5V @ 5A, with a 26V power supply

::Resistor voltage drop = 26V - 2.5V = 23.5V
::Resistor value = 23.5V / 5A = 4.7 Ohms
::Resistor wattage rating = 23.5V * 5A = 117.5 Watts

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