limit amps to stepper motor?

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heath1980
 
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limit amps to stepper motor?

Post by heath1980 »

Hi,
I am a beginner to electronics and I am learning how to control a stepper motor using an Arduino. I have the motor running using the stepper_oneRevolution sketch. I am using a YM2754 stepper motor connected to a ULN2003 Darlington transistor array IC. I am using a guide from this link http://forum.arduino.cc/index.php?topic=102592.10;wap2 that mentions I should limit the current to 1.1 amps so that you don't over heat the motor.

I am using a 12v 1.5 amp external power supply and would like to know where I can learn how to limit the amps to the stepper motor. Although I might not need anything with the power supply I am using, I would like to learn how so that when I move to multiply motors I can use one power supply to power them all.

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adafruit_support_mike
 
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Re: limit amps to stepper motor?

Post by adafruit_support_mike »

There are circuits that control current, but making them work in the 1.1A range (and with a stepper motor) is a fairly advanced subject.

Honestly, the suggestion to limit the motor current is kind of a weak design choice. It basically says, "I picked the wrong parts but am trying to make them work anyway". It can be done, but you spend more effort working around the limits of the parts than making the basic circuit work.

The strong design choice is to pick a driver that can handle more current than the motor uses.. usually by a factor of 2 or more. If your motor wants 1.7A, find a driver that can handle 4A. That way nothing the motor can do will hurt the driver.

One easy way to get more power is to wire multiple Darlingtons in parallel. On a circuit board you can piggyback one chip on top of another to double the power capacity, but that causes some problems with heat dissipation. If you're building the circuit on a breadboard or perfboard, it's best to wire multiple Darlingtons in a chip to the same input and output.

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adafruit_support_bill
 
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Re: limit amps to stepper motor?

Post by adafruit_support_bill »

There are some cases where you do want to do current limiting - not necessarily to protect the driver, but to squeeze more performance out of the motor.

Current limiting or "chopper" drives use a higher drive voltage to counteract the inductive lag and increase current to the motor at the start of a step. They then "chop" or limit the current toward the end of the step to prevent overheating the motor coils. This optimization allows for higher torque and faster step-rates than a simple constant-voltage driver.

As Mike says, designing a current limiting drive circuit is not a simple task. And it needs to be tuned to match the characteristics of the motor being driven. Until recently, current limiting drivers were quite expensive. The Synthetos boards are some of the first reasonably priced boards with this capability: https://www.adafruit.com/search?q=synthetos

For more on driving stepper motors, see this guide: https://learn.adafruit.com/all-about-st ... he-stepper

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heath1980
 
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Re: limit amps to stepper motor?

Post by heath1980 »

Thanks for the info.

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heath1980
 
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Re: limit amps to stepper motor?

Post by heath1980 »

If the motor requires 1.1 A per phase, and the uln2003 can supply 500mA per channel with peak at 600mA. Does this mean the ULN2003 Darlington arrays are under powered and will eventually fry? Just wanting to know if I am making sense of what I am reading from the data sheets.

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heath1980
 
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Re: limit amps to stepper motor?

Post by heath1980 »

sorry just read the link you provided and I used the formula current = voltage / resistance, the motor is 12 VDC, Current per phase 1.1 A, Resistance Per Winding 20ohm, 12 / 20 = 0.6

Would 0.6 be 600mA? and would that mean the uln2003 can handle the motor?

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adafruit_support_bill
 
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Re: limit amps to stepper motor?

Post by adafruit_support_bill »

the motor is 12 VDC, Current per phase 1.1 A, Resistance Per Winding 20ohm, 12 / 20 = 0.6

Would 0.6 be 600mA? and would that mean the uln2003 can handle the motor?
If that is the actual resistance measurement, then it should be OK. I am a little curious though. Usually the manufactrer's current per phase rating is based on Ohm's law and the rated voltage. Did you verify the resistance reading? Do you have a link to the motor spec sheet?

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zener
 
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Re: limit amps to stepper motor?

Post by zener »

Maybe the 1.1A is assuming two coils on at the same time.? Since it is about double the calculation for a single coil.

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adafruit_support_bill
 
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Re: limit amps to stepper motor?

Post by adafruit_support_bill »

Maybe the 1.1A is assuming two coils on at the same time.? Since it is about double the calculation for a single coil.
That makes sense - but per phase should mean per coil. It could be a typo. Or it might be an 8-wire motor - in which case it can be wired as bipolar series or bipolar parallel - each of which would give different effective coil resistances.

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heath1980
 
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Re: limit amps to stepper motor?

Post by heath1980 »

Thanks for all of the help.

The data sheet was a small sheet of paper provided with the motor.

http://www.jaycar.com.au/productView.asp?ID=YM2754
This is the place where i bought the motor from as it is the only local supplier i know that has a store i can walk in and buy stuff as i am sometimes too impatient to wait for orders from adafruit.

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adafruit_support_bill
 
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Re: limit amps to stepper motor?

Post by adafruit_support_bill »

Looks like a 6-wire unipolar. No way to wire it bipolar/parallel. The safest thing to do is measure the actual coil resistance and go by that.

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heath1980
 
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Re: limit amps to stepper motor?

Post by heath1980 »

Just watch a youtube video from colin's lab that showed me how to use my cheap multimeter and it was reading 20 for resistance.

thanks again, it has been a good learning experience.

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