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12:46 AM
Thanks for the confirmation Shalvenay. I'm definitely not smarter than the NEC... codes are in place for a reason. But I might not be breaking any codes by running it.
 
 
8 hours later…
8:25 AM
"The time constant of a wire is equivalent to that of a simple RC circuit that has the same resistance and half the capacitance of the wire"

Is this true?
This question is related to the Elmore delay model. I am not able to understand what the statement is saying.
 
 
8 hours later…
4:45 PM
Are you talking about if a wire were open-ended? i.e. "looking into" the signal end of the wire (one-port) while the other end is open or shorted?
 
 
2 hours later…
6:23 PM
So it seems that the DVR8825 stepper motor driver (1.5 A max/phase) I tried using overheats and the motors stop moving.
When looking for a more robust chip, it was suggested I use the A4989 which has external FETs.
^ datasheet for A4989
The DVR8825 has a continuous motor drive output current max of 2.5 A. For the A4989, is the continuous motor drive output current some value on the FET's datasheet?
For example, if I use the IRFZ44N, would the maximum continuous motor drive output current be 35 A? The datasheet for that FET: infineon.com/dgdl/…
In summary, the DVR8825 driver does not work. I'm wondering if the A4989 is more robust.
 
6:54 PM
I wouldn't say it's more robust, just that it's a different product. Having 8 external FETs will significantly increase the size of your solution, which sometimes matters. Take front-page numbers (e.g. 35A*!) with a grain of salt.
 
Number from the FET datasheet
 
You're still responsible for figuring out how to keep the transistor cool enough at that power level. 35A at 1.6 x 17.5mΩ is like 34W of cooling
 
The motors work at 1ish amps
 
And then you have 4 devices in the conduction path, so it's 34W x 4
 
I was told I should try to find an all-in-one chip versus the A4989 chip and external FETs.
 
7:00 PM
I mean, maybe you should look at cooling your existing part better? Maybe you should use the on-chip current limiting instead of hoping for the best
 
The A4989 looks simple: i.imgur.com/m8d5zEZ.png
The DVR8825 has on-chip current limiting
That's the problem. It's overheating after 10 seconds of normal use.
 
then you have the current limit set too high
 
If I restrict the current with the pot on the DVR8825, the torque drastically reduces to be practically useless
I'm guessing I have to use a chip with external FETs just to dissipate heat better
 
Either that, or you find a 4A version.
(if they exist)
 
I'm having trouble just finding a chip. It's usually an entire module/unit beyond the Pololu dev boards:
And there are 8 external FETs on the back, which I'm guessing makes the bridge. So it seems external FETs are...
needed due to thermal physics
 
 
1 hour later…
8:14 PM
1-ish amps? Sounds like much more current than that is going to the motor. Is the motor voltage right for what it is being supplied?
 
The motor voltage was tried at 12 V and 24 V.
 
8:51 PM
But is it a 12V motor, or a 5V motor?
 
Could try measuring the current going to the driver board. Something sounds wrong though... the DRV8825 can run a (small 3D printer) stepper all day and night and barely get warm.
 
The supply's current?
1.5 A limited for one thing
I measured the voltage difference between poles:
24 V → 5-7 V
12 V → 1-2 V
18 mA and turning the pot more, the motor starts slipping maybe at 20 mA
It gets HOT
I measured the power supply's current.
 
9:39 PM
pololu.com/product/2982/specs states it should do 1.5A per phase continuously.
Could the motor wiring be wrong?
 
The A1 A2 B1 B2?
 
Ahh four-wire, not a 6-wire motor
 
It moves as expected, taking the steps told from the MCU
 
Scope the power, make sure there are no strange oscillations? Running out of ideas.
Could be a defective driver... rip a small stepper out of something scrap and try that. :)
 
I have many DVR8825s
I'll scope it tomorrow. Thanks for the help
 

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