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DRV8825 Current Limiting

Posted by Rep_Rap_Newbie 
DRV8825 Current Limiting
November 04, 2019 06:22PM
New to RAMPS and trying out my first personal project. Playing around with a very simplified RAMPS 1.4 setup, 1 Stepper for Extrusion and 2 for XY Movement totaling 3 stepper motors. Each stepper is rated for 1.5A/ Phase and 2 phase, but I'm a bit confused what to limit each of my DRV8825 currents to. Is the current limit on the driver the maximum current that will be provided to the motor, or is it the maximum current that will be provided to each phase of the motor. Additionally, what are reasonable current limits to use for this simple setup? I feel so dumb for asking this, but haven't been able to find the answer anywhere.
Re: DRV8825 Current Limiting
November 04, 2019 07:42PM
hmmm.... I never really looked into the technical details, of the current limiter, so I mainly use it to make sure that nothing gets too hot. And by too hot, I mean; warmer than what I like it to be.

Apparently steppers are traditionally (before simple 3D printers) meant to be able to be operated at 80*C. And this is probably the expected operating temperature at the rated amps .
But I do not like anything on my printer to get hotter than 60*C (apart from hot-end and heat-bed of cause), so I use the current limiter to keep my steppers cooler.

When you go deeper into the technical details, then there will be a coupling between holding torque and max amps.

But I try to use my 3D printer so it can still run at lower torque (and lower temperature) settings, by adjusting other parameters (and/or by adding cooling to the steppers), so the steppers never get be 80*C during normal operation.
Re: DRV8825 Current Limiting
November 26, 2019 03:08PM
Stepper motors usually have a voltage rating / phase. you can use that to calculate a starting place for the approximate max current that they can handle without burning up. For example if your stepper is rated for 2.8V / Phase running them with 12v @ 1.5A will probably smoke it. I usually calculate the Power (Watts) to find out how much current my stepper can take at higher voltages, for a starting place, then I will increase the current a little at a time until I get to the desired temperature of the stepper. Steppers normally are always operated well above their rated Voltage / phase. Example for calculation of current; stepper rated voltage = 2.8V Stepper rated current / phase = 1.5A (Watts = Current X Voltage) = 4.2 Watts. for 12V the current comes out to 350mA. As a starting place limit the current to the stepper to 350 mA. Of course the current will probably have to be increased a little from that point due to motor inefficiency, wire resistance etc..... When choosing steppers I always look for low voltage / phase with as low of an inductance rating as I can find. The lower the inductance the faster the stepper can turn (RPM's). This has always worked for me. My steppers run very warm, but I can continuously hold my hand on them without pulling away. Keep enough current going to your steppers to avoid loosing steps. This is just the way I do it, I am sure that others have a different way. The only time my steppers loose steps is if I increase the Jerk and Acceleration too high or if the extruder hits something.
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