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View Full Version : imp: why use a current limiting resist. at all in steppers?

DceRobo
08-24-2006, 02:17 PM
pls dont shoot me for this question...

current limiting resistros will limit the current to desired values , yes. but the voltage drop acrosss the winding will remain R(winding)Xcurrent. so effectively if i try increasing the voltage across the motor winding say 4X, i end up increasing the current 4X too!!! does not that huge current harm the motors?? and if i donot increase the current through the coil, why not use the voltage source of rated voltage only, avoiding wastage of power?

ppppking04
08-24-2006, 04:41 PM
Not sure about current limiting resistors but i think you should much rather be searching for current limiting circuit aka chopper drive...

That uses a current sense resistor which gives op voltage proportional to current. So when the current in turn the voltage reaches a preset value the circuit switches off the voltage across stepper winding. So the current falls a bit and when it goes below preset value it is switched on again.

So in turn what you get is a current switching rapidly around the preset value. Thats basically a chopper circuit.

But again i dont know if you meant that....

allbits
08-25-2006, 11:06 PM
hmm...

didnt quite understand your question...
anyway,

does not that huge current harm the motors??

yes, if it is higher than the current rating of the motor. So, simply calculate the current that will flow through your motor, using our good old law put forward by OHM. Current limiting circuits (like choppers or simple resistors or switching regulators) are used in special situations.. not in simple cases where the motor has a lower torque..

check this link.. hope it was what you asked for.

http://www.cs.uiowa.edu/~jones/step/current.html

docel
08-26-2006, 05:17 AM
:wink: A simple , cheap resistor will save expensive motors from reaching the garbage-bin..... :lol:[/b]

docel
08-26-2006, 05:30 AM
Resistors have a voltage drop associated with the current through them.
The more the current , more the voltage drop across the resistor leading to less voltage across the series motor.

The resistor is used in PWM drive condition. You cannot drive a 12v motor in PWM @12V :!: :!:

A design trick for the stupid stepper motor (which cannot pull its own weight :wink: ) is to PWM @ higher voltage through a series resistor.

A simple , cheap resistor will save expensive motors from reaching the garbage-bin..... :lol:

pls dont shoot me for this question...

current limiting resistros will limit the current to desired values , yes. but the voltage drop acrosss the winding will remain R(winding)Xcurrent. so effectively if i try increasing the voltage across the motor winding say 4X, i end up increasing the current 4X too!!! does not that huge current harm the motors?? and if i donot increase the current through the coil, why not use the voltage source of rated voltage only, avoiding wastage of power?
:?: :?: :roll:

1.When you increase the Vmotor, The Vdrop across the Resistor will also increase. The Motors draw huge current at startup....several times more than the stall current, sometimes. The current rise-time does not correspond with Voltage rise time, in any inductor.

2. If the motor stops when powered( at a wall or obstacle ) the huge current will fry the motor. The series resistance will, according to the basic Ohms Law, drop the voltage to a harmless level-for the motor. It will even give up its life trying to protect the motor. This is good as a resistor costs nothing compared to your precious motor.