Light bulb as resistor?

[dazza000]

Hi
I have a 40volt dc psu and I want to connect it to a stepper motor driver. the stepper mortor coils are 2.5 volts and I want 2.5 amps to go through them.
so the resistor needs to be about
37.5 volts / 2.5 amps = 15 ohms
but the wattatges is 97.35 watts. thats a lot of heat to dissapate.
and a relativley expensive resistor/s

Could I use a light bulb or a toaster element(part of) to do the job?
thanks
david

 

[Jaguarjoe]

Incandescent lamps are non linear. Their cold resistance is about 1/3 or 1/4 of their hot resistance so you will not be able to know the lamp's resistance under load until you try it. As the load changes, so will the lamp's resistance. You will still be burning up 94 watts to run a 6 watt motor. That is a phenominal waste of energy. Did you ever look into building a switching regulator that would provide the same output with almost no wasted heat?

I haven't messed with nichrome wire since grammar school.

 

[alec_t]

If I were you (but I'm not ;-) )I would abandon the 40V psu and use a lower voltage one.

 

[Diver300]

The higher voltage power supply allows faster switching of the phases on the stepper motor, which in turn allows faster rotation without loosing torque.

Resistors are not the best thing to use. as Jaguarjoe said, there are current control circuits that will apply the full supply voltage until the correct current is achieved. Once the current is achieved it is held constant by alternating between full supply voltage and freewheeling. When the current needs to be stopped, reverse voltage is applied to the winding so that the current stops quickly.

 

[MrAl]

Hi,

Yes it's common to use a higher voltage supply like 30 or 40vdc to drive a stepper motor. The trick is to use the high voltage to drive the motor but with current regulation. Note i am saying here "current regulation" and not "current limiting" as with a resistor. The current regulation is usually obtained by using pulse width modulation, and many motor controller IC chips are made with this PWM circuit already inside. The driver chip works through a sense resistor, and the sense resistor provides current feedback for the controller chip, which in turn regulates the current.
This kind of control works very nice for steppers because it allows a high voltage to bang the motor to overcome the inertia and quickly get it rotating, but once rotating the current is regulated to some nominal value that the motor winding can handle without burning up. Without the higher voltage the motor would only be able to slowly get up to speed which of course reduces the overall speed of operation substantially.

If you dont care about reaching the highest speed possible, then a lower voltage supply can be used without current regulation or current limiting. No resistor required and less waste heat generated. If you absolutely must use this higher voltage then a switching regulator is pretty much mandatory here unless of course you want to simultaneously build a stepper motor drive AND a heater for winter at the same time

 

[Diver300]

I think that the high voltage is to overcome the inductance of the windings. Even when the motor is rotating steadily, but at high speed, the current needs to be reversed quickly in the windings, so that is what the high voltage is needed for.

At low speeds, low voltages could be used, because it doesn't matter as much if the current takes time to get to maximum.

 

[MrAl]

Hi Diver,

Yes, in a manner of speaking, but what we say is 'overcome the induction' is really overcoming the inductances reluctance to allow current flow. The higher voltage also starts the rotation more quickly in apps that need fast response, and as i pointed out apps that need highest speed need higher voltage.