## potentiometer as Trim wheel

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Ralph
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### Re: potentiometer as Trim wheel

It's a bit hidden so I can understand.

jph
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### Re: potentiometer as Trim wheel

ChuckK wrote:
Tue Nov 16, 2021 12:27 pm

And I'll throw my 2 cents in on the stepper voltage. Joe is correct, in that there is not really a voltage rating for steppers. What determines the voltage is the required torque and holding current. Since the stepper isn't really doing serious any work, just spinning a wheel pretty slowly, 5V would actually work fine. I will say make sure the current limit is set in the Easy Driver to avoid damaging the driver if the mechanism were to get jammed.

Chuck
Hi Chuck,
I know you are aware of all this but for the sake of others looking at steppers and are new to the concepts of stated 'ratings' then the following should be useful.
The 'rated' voltage is simply calculated as a DC steady state V calculated from the winding resistance and the current rating - in this case - 0.33A and 34 ohms (from the datasheet) so DC non rotating V = 0.33 x 34 = 11.22V they rounded it up to 12 due to the tolerances of the winding resistance.
The stepper though will never (hopefully) be in a static position with full rating applied (unless mechanically stalled) so due to the drive voltage being pulsed the current in the windings is related to the 'on' time of the pulse and also is massively and almost totally effected by the inductance - 46mH in this case which is effing huge ! - of the windings as we can now virtually ignore resistance as the inductance alters the rise time of the current in relation to the voltage applied and that is happening rapidly. Therefore to achieve the quoted torque and wattage the motor would have to be run at 10 to 20 times the 'stated' voltage. For info, when the stepper is static by desire then the stepper driver is outputting (depending on driver) up to the max pre-set current but nothing more - regardless of voltage as - as the inductor is no longer really doing much in the way of delaying rise and decay times.
That doesn't mean you cant run it at 12V, you can but it will be very slow and have very low power (which is probably ok in this case) but it would be important to set the drive max current to 0.33A regardless. At 5V it would be interesting to see if this motor runs as it is one of the strangest I have come across as the inductance is HUGE at 46mH ! That would take a very long (relative) pulse to rise to the current which is already very low. I think this particular stepper could probably handle around 200V !!! (well actually 222V). At 12V the step time is huge to reach 0.33A and the power is only around 3.9W and needs a step time of over 3mS to reach that. At 5V over 6mS and power down to around 1.5W.
Again, it will be interesting to see the results because as you say this is a very low power requirement.
I am unsure as to what this specific stepper is designed to do ? It is a weird one I have not come across before with such high inductance and low current.
As a suggestion to the user of this stepper and the Easy Driver - absolutely make sure you set the maximum drive current on the easy driver board to around 0.33A. Also, dont feed faster steps into the easy driver from AM than the stepper can take. For this motor you should never pulse it at more than 1 pulse per 3mS at 12V - 6.5mS at 5V - although please note that the spec for the easy driver is a minimum of 6V - 5V should work.
It appears from looking at the sparkfun site that they chose this stepper simply because the easy driver board was very limited in current handling - max of around 700mA. (0.7A). I am sure it may be ok for certain models of contactless 3d printers though.
I have a spreadsheet I made a couple of years ago for calculating data for various stepper for 3d printers that can calculate the general data given specified current, inductance and voltage and can calculate the optimal parameters. It is a bit complex though as it includes feed rates for various lead screws and also is tailored for my Gecko drive but the stepper data calcs are all there.
sorry in advance for any typos, tldr haha.
Joe
“Everything in moderation, including moderation.”
“Experience is merely the name men gave to their mistakes.”
Joe (CISSP) -IT security consultant

chefpilot
Posts: 60
Joined: Wed Aug 02, 2017 12:51 pm

### Re: potentiometer as Trim wheel

Hi Joe,
thank you ververy much for your detailed thoughts and information about stepper motors in general and the one I intend to use in detail. That's very helpfull and it safes a lot of time for me to get all these informations from other sources.
Marc

jph
Posts: 995
Joined: Fri Apr 10, 2020 12:50 pm
Location: Somewhere over the rainbow..

### Re: potentiometer as Trim wheel

chefpilot wrote:
Thu Nov 18, 2021 1:46 pm
Hi Joe,
thank you ververy much for your detailed thoughts and information about stepper motors in general and the one I intend to use in detail. That's very helpfull and it safes a lot of time for me to get all these informations from other sources.
Marc
Hi Marc,
No Problem at all.
Over the next few days I will cut down the spread sheet I made to just include the parameters of the stepper that you can enter into the spread sheet and give you the results. I will then upload it.

The basic math is - (from my notes on the sheet)

NOTES - Joe H, (JPH 2020) Stepper Calcs XLS by Joe H (JPH).
From data from - https://www.daycounter.com/Calculators/ ... ator.phtml
Calculating revs per second and power and minimum step time
Equations:
Current through the coil is proportional to the time that the voltage has been applied, and inversely proportional to the inductance.
I= V*T/L
T= I*L/V
For one step the current must go from 0 to Imax and back to 0, or alternatively from -Imax to +Imax.
I= 2*Imax
T= L*Imax*2/V (L is in Henrys !)
T is the number of seconds for a single step.
To compute maximum revolutions per second - divide seconds per step by steps per revolution.
Rev/sec = V/(L*2*Imax)/(steps/rev)
Pmax= Imax*V
Pmax occurs not when the motor is going max speed because the current is a triangle wave. Pmax occurs when the slope of the current is small compared to the on holding time of the step pulse.

But, as said, I have this all in a simple spreadsheet - or more correctly , I will make it simple to use. Hope this helps. I modified the math for the spreadsheet to be used to various calculations on gearing of the drive etc.
The basic math of the spreadsheet used was from - https://www.daycounter.com/Calculators/ ... ator.phtml which is also a fantastic on line calculator. This is a super calc but lacks in depth info.
Joe.
“Everything in moderation, including moderation.”
“Experience is merely the name men gave to their mistakes.”
Joe (CISSP) -IT security consultant

jph
Posts: 995
Joined: Fri Apr 10, 2020 12:50 pm
Location: Somewhere over the rainbow..

### Re: potentiometer as Trim wheel

Here is a very stripped down version of the spreadsheet which is basically the same as the online calculator, however all the math is there if you unprotect the sheet using the password 'Password' to keep it simple. You can add or change whatever you want to or just use it as is.

Joe
Stepper calcs mini.xlsx