DDRV8825 stepstick on megatronix 6,4 V vref

Hi,

On all the posts i can find everybody is telling you have to multiply your Vref x 2 to get the wanted max current. If i measure my Vref on my Megatronics V2.0 with Stepstick DRV8825 stepper drivers i can adjust the Vref from 0 to 6,4 Volt. This would mean 12,8A for the stepper motors!! Do the Stepstick DRV8825 use another Vref? I have been googling for it but can't find any conclusive answers. I thought i read somewhere that with the Polulu stepsticks (4498) you have to divide the Vref by 5, but this is another model.

any help is appreciated,

Pieter

Edited 1 time(s). Last edit at 10/09/2014 02:32AM by lazy_mosquito.

Wrong forum?

you must double the 'Steps' in your firmware.
The Power input is the same. although you must manually calibrate them the same you would with your old drivers,

At lest from pololus from their website:
"way to set the current limit is to measure the voltage on the “ref” pin and to calculate the resulting current limit (the current sense resistors are 0.100Ω). The ref pin voltage is accessible on a via that is circled on the bottom silkscreen of the circuit board. The current limit relates to the reference voltage as follows:
Current Limit = VREF × 2"

From drv8825 datasheet current set is I=Vref/(5*Rsence), so with 0.100Ω you get the same VREF*2, I am not sure is stepstick uses the same value resistors but you can check what values are there resistors on yours:


Other thing to worry about is why you get 6,4V as it should be connected to 3.3V of 5V rail, mega runs on 5V, so you should only get 0-~5V range.

Quote
pushthatbolder
you must double the 'Steps' in your firmware.
The Power input is the same. although you must manually calibrate them the same you would with your old drivers,

I changed the resolution already to 160 steps per mm in the repetier firmware configuration tool on the mechanics tab. Is that what you mean by doubling the steps?

Quote
Endlane
Other thing to worry about is why you get 6,4V as it should be connected to 3.3V of 5V rail, mega runs on 5V, so you should only get 0-~5V range.

Are you sure about that? On the documentation about the Megatronics 2.0 on the reprap wiki they state you have to connect the board with a 12V line. (one for the board and one line for the heated bed)
I get the 6,4 V when the pot-meter is turned to the max. I measure from the top of the potmeter to the ground of the printplate.

As far as i can see the values for the resistors are also 100. So this would mean i should multiply the vref also by 2 to set the max current.

At this moment i have running them with 1,2V and everything seems rather smooth. I noticed when the stepper drivers were running at 6,4V they were shutdown regularly (thermal protection i guess) and they were too hot to touch. They smelled also typical like hot electronics (not the burnt smell though)

Is there a kind of golden rule to find the ideal voltage? lower it untill it stops moving and then add 0,2V or something like that?

Edited 1 time(s). Last edit at 10/11/2014 10:05PM by lazy_mosquito.

Ok I have looked at drv8825 datasheet again and looks like I was a little wrong about voltage.
Yes you connect 12V, but microcontroller works on 5V from voltage regulator, so I also assumed that drivers should be getting voltage for Vref from board, but it looks like drv88825 has internal 3.3V regulator which is used to supply voltage for Vref (at least in pololus driver), so it is even more confusing why you get 6.4V. Also be more careful as dataseet states that max Vref should be 4V.

~Thanks for the feedback, i have found the datasheet of the drv8825 myself and indeed they speak nowhere about a voltage of 6,4V. At this moment i will use the rule of the polulu drivers and multiply the Vref by 2 for the maxCurrent. My stepper motors can handle 2,5A, so i expect 1,2V Vref would be OK.
I am wondering if the wrong voltage could be an effect of the measuring points i am using: I connect the ground of my voltmeter to the ground of the Megatronics 12Vinput and connect my red line of the voltmeter to the screwdriver is use for tuning the potmeter of the drv8825.should i use another ground? I read somewhere on the reprap forum you could measure it that way, and it is much safer then trying to measure from the tiny connectors of the ship itself.

There should be no measurable difference between where you connect to GND, unless board is faulty. You could be measuring between +12V and 3.3V rather than GND(0V) and 3.3V, but still you should get ~12V-3.3V=~8.7V, so it is strange why you are getting 6.4V.

I have read through this one in hopes to see where to measure the Vref at any updates?

Not sure if what is the best way to set these boards up. The ones I have make noise when too high or too low so seems like just having a quiet room and adjusted the pot to a happy spot where it stops making noise. Maybe mine are just cheap ones.

Okay I answered my own question this may help someone else. So I am posting.

Here is a link to the Driver board information. LINK

So here is the info in case you don't want to read through the link. The Vref pin is on the bottom of the board and is not reliable for checking the amperage. So reading current inline one of the coils is a better approach.

"The ref pin voltage is accessible on a via that is circled on the bottom silkscreen of the circuit board."

My method:
First make sure your meter has ability to read Amps and at least 5Amps just in case there is a spike.
I pull one pin out of the 4 pin cable going to the RAMPs I use the red one; seems to be universally used as one of the power wires.
Then I made a new wire to go in that hole and run it to one lead on the multimeter and then hook the original wire to the other lead on the meter. It just goes through the meter for testing. Then cycle the motor I use my laptop to move the axis I have it hooked up to and watch the meter. There may be a better way to do this part but works for me to do a quick test.

I know this is common sense for allot of people, but wanted to post it just in case others need to do this.