What current set resistors do I need.

[Apples]

I will be hooking my cnc table up for the first time this weekend. I am a bit unsure on working ou the current set resistors. I am running a 270oz-in and a 1200oz-in stepper motor. The motor specs can be found at:

http://www.homeshopcnc.com/page3.html

Can someone please tell me exactaly what resistors I need to buy.

Cheers

Peter

[Mariss Freimanis]

Let's assume you want to wire the motor in parallel. The current rating is 4 amps per phase then. The resistor formula is:

Ohms = 47,000 * I / (7 - I)
Ohms = 47,000 * 4 / (7 -4)
Ohms = 188,000 / 3
Ohms = 62,667

The closest standard 5% value is 62K, the closest standard 10% value is 68K. Either will do.

Mariss

[Xerxes]

Mariss,

Is it possible to control current with external voltage reference? If yes, then what is the current set formula?

It would be cool to adaptively control current with MCU.

[Mariss Freimanis]

Yes, you can. The phase current is directly proportional to the voltage on terminal 11. The transconductance is 2.5V per amp of phase current.

Mariss

[Apples]

Hey Mariss,

With those current set resistor calculations. Do I just use the motors max rated amps? Or do I have to use 2/3 of the motors rated amps to work out the resistors?

Cheers Peter

[Mariss Freimanis]

Use the motor's unipolar rating if you plan to run a 6-wire motor 'half-winding', use 1/2 the unipolar rating if you plan to run it 'full-winding'.

If it's a 4-wire motor, use the motor rating. For 8-wire motors, use the published series or parallel ratings.

Mariss

[Apples]

Mariss,

With the 270 motor that I have. I want to run it in bipolar mode. As bipolar mode has a holding of 270, whilst unipolar has holding of only 190.

Can I just run the calculations by you just to make sure, I don't want to blow anything up!

Now what is half and full windings? Is that just unipolar(half windings) and bipolar(full windings)?


The first motor that I am using is the 270. This motor will be run as bipolar so it can make 270oz-in. So the Rated current per phase (amps DC) for this motor to make 270oz-in is 2.8amps.


So do I use this figure of 2.8amps, then half that to get 1.4amps to use in the formula, Ohms = 47,000 * I / (7 - I)?


So we have:



Ohms = 47,000 * I / (7 - I)
Ohms = 47,000 * 1.4/ (7 -1.4)
Ohms = 65,800 / 5.6
Ohms = 11,750.

If this is all correct, and I have to use this 11,750ohms rated resistor can you please let me know. Also what % do these have to be 5% or 10%. I think that 5% would be better because it is more accurate????

Now what happens if I can not get a resistor that is rated at exactaly 11,750ohms? Do I use the next higher rated one? or do I use the next lower rated resistor?


Now the 1200 motors are 4 wire bipolar, rated at 6amps per phase. I think that these can only be hooked up a bipolar so that means that I will have to use a resistor worked out using 3amps not 6amps? Is this right?

Cheers

Peter

[Apples]

Mariss,

Taht last bit about the 1200 motor. I re-read what you said earlier. This motor is a 4 wire.. So I will use the rated amps to work out this current set resistor, which is the 6 amps.

Peter