1. ## Steppers and resistor-question.

Hello you all.

I have tried the steppercalc to find the correct value for my limiting resistors. I just need somebody to confirm that I'm on the right way and that my values are correct. I have a Slo-syn 53oz 200step pr. rev. stepmotor (3 actually) with the following specs: 1.25v & 3,8A. I will drive this with a FET-controller and a current limit resistor i series with the powersupply rated 14,7v. (I've tried to measure the resistance in the windings and found 0.4-0.8-0.4 ohms, but steppercalc doesn't ask for that). According to the programme the limit value is 1,77ohms ca.100 watts. I plan to put 14 resistors rated 27ohms & 9watts i parallell to get a total about 1.9ohms in series with the powersupply. Am I doing the right thing here guys? I will repeat this for each axis and I'm planning to buy the resistors soon. Any feedback is much appreciated. Keep up the good work.

2. I make it 3.54 ohms to drop 13.45 volts at 3.8amps the wattage would be just over 50watts.
Al

3. Can your power supply put out the 23 amps that you will need for 3 axis? Why not use a chopper drive instead? 50 watt resistors might be expensive.

4. First of all I want to thank you both for the replies. I trust Al The Man with his tip on the resistance value and will go with that. Thanks Al! For the next reply from H500 I will consider your suggestions, but at a later time as I already have bought and installed the FET drivers. My powersupply will do about 42 amps and the resistors I can get cheap. I have one question that pops up in my mind: When the label on the stepmotor says 3,8A that means each winding and not the total amount right. So I will have to multiply 3,8A x 2 x 3 motors to get the total current flow from the supply?

• I don't know for sure, but typically the rating is for 2 coils energized at the same time.

• The rating is per phase, so yes, 3.8x2x3 = 22.8a.

If you use a chopper, like a Xylotex or Gecko, they only use at most 2/3 of 1 phase, or 2/3 x 3.8 x 3 = 7.6a. Much more efficient.

• hello,
If i run my motors in full step than i need only half of that current?
when full step, then only one coil is energized at a time right ?

• Originally Posted by extreme
hello,
If i run my motors in full step than i need only half of that current?
when full step, then only one coil is energized at a time right ?
Full stepping is usually done by energizing both coils to maximize the torque.

• if two coils are energized in same polarity then it's a halfstep? if in different polarity then it is a step. Right?
so for 1.5 amp/phase I need atleast 3 amp supply? if i use more voltage than rated and resistors than i need even more current or not?

Thanks
Robert

• In half stepping, you alternate between energizing one and two coils. Full stepping can be done energizing one or two coils.

3 amps is correct. If you use more voltage with resistors, the current remains the same. You need to size the resistors to limit the current to 1.5 amps per phase.

• Thanks for help. Anyone knows a good chopper schematic ? I've made the one from free source and it doesn't work like it should . I'm aiming at 30 volts.