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Topic: a4988 stepper driver question (Read 628 times)previous topic - next topic

Zarolis

Jun 04, 2016, 12:52 pm
Hello,

i've got myself some a4988 stepper drivers, and since anything from ebay takes more than a month to ship to me, im being very coutious about using it.
i have a small stepper motor from an old dvdrom. the only info i managed to find on it is that it has
a 4-6V dv/0.5A  rating, im fairly new to all this so im not entirely sure if thats what i need.

i've been watching tutorials and they say that i have to adjust Vref on the pontetiometer of A4988.in this  pdf :

I've found this equation:   ITripMAX = VREF /( 8×RS)     RS is 0.7ohm
so first question is:

is  ITripMax = 0.5A?

and i've got a 9V battery  and judging from that first link, im guessing i need 4-6V for my motor, so i need to connect a resistor, so my second question is:

do i use the 0.5A from the first link to calculate the resistor?

MarkT

#1
Jun 04, 2016, 02:54 pmLast Edit: Jun 04, 2016, 02:56 pm by MarkT
Small 9V battery?  No use at all for motors.   Something like a 12V 1A supply would be a good starting
point, more voltage makes motor work at higher speeds.

Stepper motors are not voltage rated, they are current rated, and the voltage depends on the speed
they turn.

Yes Itrip is the operating current, for 0.5A with 0.7 ohm the Vref should be 2.8V

Don't confuse the motor supply and logic supply, the latter is 5V from the Arduino.
Never power the logic supply without the motor supply being present.  Adding 1k series
resistors on the step and direction pins would be one protection measure if you are
paranoid.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Zarolis

#2
Jun 05, 2016, 10:45 pm
but i still need to know about my Vmot pin.  the stepper rated voltage is 4-6V , so why would i need a 12 V supply? isn't the 9V battery and a resistor, I'm thinking 6 ohm, would be enough?

jremington

#3
Jun 05, 2016, 11:01 pmLast Edit: Jun 05, 2016, 11:03 pm by jremington
Quote
but i still need to know about my Vmot pin.
Start with a 12V, 1 ampere power supply for Vmot power. Do not use a 9V battery and a resistor.

The A4988 driver controls the motor current. It would be a great idea to read the data sheet for the A4988 chip, available here: https://www.pololu.com/product/1182

Zarolis

#4
Jun 06, 2016, 10:20 am
But WHY. the rated voltage of the motor is 4-6 V, wont it fry my motors?

MarkT

#5
Jun 06, 2016, 12:50 pm
but i still need to know about my Vmot pin.  the stepper rated voltage is 4-6V , so why would i need a 12 V supply? isn't the 9V battery and a resistor, I'm thinking 6 ohm, would be enough?
Re-read what I said - bipolar steppers _do not have_ a rated voltage, other than the insulation
breakdown voltage.  Yes you can multiply the winding resistance by the rated current, but that's
not of any use to running it as a motor.  The voltage is dominated by back EMF, not winding
resistance, and the back EMF depends on the motor speed.  It is common to run 5A 0.4 ohm NEMA34
steppers from 80, 100 or 120V supplies in high end applications for instance, to get 3000rpm or
even higher.

The exception to this is high impedance motors (typically unipolar) which struggle above 120rpm and
are run at constant voltage, not constant current.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

polymorph

#6
Jun 06, 2016, 06:26 pm
The A4988 acts like a switch mode current regulator. The inductance of the motor takes the place of the inductor.

This is because the nature of an inductor is that current takes time to build up. So you intentionally use a power supply voltage Vmot that is much higher than the motor rated voltage, so the current builds up much faster.

The A4988 switches on and off quickly after the current reaches the set point, to keep the current at that value.

The voltage rating of a stepper motor is merely the voltage at which the resistance of the wires causes the maximum current to flow.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - tinyurl.com/q7uqnvn

Zarolis

#7
Jun 06, 2016, 10:01 pm
Thank you a;;, that was very helpful, i just have one more question

I'm using three of these motors, the're taken out of a dvd-rom, they already had a linearg gear fixed to them so i barely touched anything, but perhaps some lubricant was wiped off. i tried to connect one of those motors for a second with a 9V batery for a second(ofcource later im gonna use a something of a higher voltage). and it worked, but only barely moved. so the question is:

will it turn more powerful with a higher voltage to a Vmot pin,  or do i just need to add lubricant?

jremington

#8
Jun 06, 2016, 10:08 pm
A 9V battery cannot supply enough current to run most motors.

MarkT

#9
Jun 07, 2016, 01:41 pmLast Edit: Jun 07, 2016, 01:42 pm by MarkT
I'll repeat myself again:
Quote
Small 9V battery?  No use at all for motors.   Something like a 12V 1A supply would be a good starting
point, more voltage makes motor work at higher speeds.
I gave you the answer, but you seemingly ignored it.

You will never have any joy with a PP3 sized 9V battery and motors.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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