AstroEQ Discussion Board

AstroEQ Forum => AstroEQ Support => Topic started by: AustroNoob on October 24, 2015, 18:54:59

Title: Overheating despite low Vref?
Post by: AustroNoob on October 24, 2015, 18:54:59
Hi Tom,

have received my motor brackets today and immediatly started testing. Adjusted the current by trial and error which was no big deal and everything runs smoothly and silent after a couple of minutes. But then, hell broke loose ;)

I use two identical NEMA17 motors. SY42STH47-1206B --> Link to datasheet, page 16:

I've tried to adjust the current by measuring Vref. I set both 8825 to 0,41V. My motor specs are 4V and 1,2A so I thought I'm safe but while one motor (RA) is purring like a kitten at goto speed 512, the other one (DEC) becomes hot es hell... to hot to touch in fact. The DEC motor is also rumbling and chattering and is very loud. DRV8825 chips become hot as well at 0,41V
If I lower Vref to eg 0,20V the motor becomes louder but temp is going down as well as torque. If I raise Vref higher than approx 0,30V the motor becomes to hot.

Wiring: According to my datasheet yellow and white are the "middle wires" so red+blue and blk+grn are the pairs. I have soldered one pair to the middle pins and the other pair to the outer pins. Think thats fine.

Step Angle: Reseller datasheet said 0.9 so I asked uncle google and found the (hopefully) original datasheet which told me 1.8 degree :( I have changed this with the confi utility but it's all the same.

RA runs on 0,21V on speed 512 very silent without getting too hot (you can touch it) --> OK
DEC becomes too hot no matter what I do. If I'm below 0,21 torque is too low for moving the scope and if I raise Vref the motor and the DRV8825 becomes way too hot.

Any idea what I've messed up? Why does the DEC motor becomes so hot though Vref is so low?

Title: Re: Overheating despite low Vref?
Post by: TCWORLD on October 24, 2015, 22:32:06
Hmm, seems like a failure of the current regulation. What happens if you plug the DEC motor into the RA port, and RA motor into the DEC port? Does the opposite motor heat up?
Title: Re: Overheating despite low Vref?
Post by: AustroNoob on October 25, 2015, 13:06:22
Short answer: YES ;)

Normal connection, both axes moved about 45 degree:
RA: 0,216V - remains ice-cold, silent movement with speed 512
DEC: 0,212V - becomes hot, but touchable, loud and rumbling with speed 512

Reversed connection (RA and DEC cable changed at controller box):
RA: 0,216V
DEC: 0,212V
As soon as I started EQMOD RA motor starts hissing and becomes warm and also starts smelling. DEC remains cold. Heard that hissing sound also with normal connection before on the DEC motor. Aborted the test immediatly, no axes moved.

Is it a problem with the 8825? It looks like as both motors get hot on the same 8825. I have a brand new one here. Is it safe to change it for testing purposes? (Yes I know. On my own risk ;) Or do I have fried the DEC motor itself? What would you do?

edit: I use a 12V/3A switching power supply btw.

Title: Re: Overheating despite low Vref?
Post by: TCWORLD on October 25, 2015, 15:13:46
If you have a spare 8825, replace the one that is causing the motor to get hot.
Title: Re: Overheating despite low Vref?
Post by: AustroNoob on October 25, 2015, 17:12:36
Have two spare 8825. Tried both and both show 11,67V :( The little poti turns endless, Vref does not change. It is not like yours. What peace of sh... are my 8825?? Do you have a plan B? ;)

edit: hang on... found out whats going with my 8825 -->
via always gives around 12V on these China boards. You have to measure on pin 12/13 on the chip itself :/ Need some time to test now. I'll be back ;)

edit2: Problem fixed! Both run smoothly on 0.37V. Thx for your help Tom!