Pololu repair?

[Drew]

Is there anyway to repair or otherwise use a Pololu where the pot wiper has been turned past it's stop?

Thanks!
Drew

[Henrique Rossi Altero]

yep, just put another pot ... i did it =]

http://www.reprapbr.com.br/2011/09/stepperstein.html

[Drew]

Can you provide any specifics on the type of potentiometer that you replaced the stock one with?

Thanks,
Drew

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[Trevor Lewis]

That pot is is standard 10k ohm through hole pot. Typically less than $0.25 each...

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Failure is just success rounded down. - T-Rex

 

Sent from your iPhone.

[Drew]

Thanks Trevor... will give this a go when I get a chance to order some...

Drew

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[Drew]

So, something like this should fit the bill?  http://www.amazon.com/Amico-103-Trimmer-Potentiometers-Resistors/dp/B008DFV2YM/ref=sr_1_42?ie=UTF8&qid=1352826848&sr=8-42&keywords=10k+potentiometer

As a non-electronics guy, Digikey and Mouser just seem to confuse me more than help out... I'd rather pay a small premium on Amazon to find the right item...

Thanks!
Drew

On Tue, Nov 13, 2012 at 12:02 PM, Trevor Lewis <trevor...@gmail.com> wrote:

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[Trevor Lewis]

Yeah, that should work. Although hopefully you won't need 100 of them. :)

I think you might be able to pick up a smaller qty at radioshack... Don't quote me on that though.

[Drew]

Yeah, unfortunately our local Radioshack is now a cellular phone dealer and not much of anything else... Amazon.com also has a lot of 5 10K pots, but they are $4.50 for the lot... so, that is like dang near $1 each versus $0.11 each for the bigger lot...  also, you never know when you might need a little pot... ;)

Drew

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[Brenton Hewitt]

i seem to remember the pots on the Pololu's being 20K not 10K, but its not documented anywhere

and i have also done this fix too, works fine no problems

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[Ross Shannon]

100K per - lower section of page - the maths is full http://reprap.org/wiki/Pololu_stepper_driver_board

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[Trevor Lewis]

According to http://reprap.org/wiki/Pololu_stepper_driver_board R9 is 100k, which gives a 10.4A ITripMax (max motor current). 10k will give 4.16A. The Stepsticks I have limit ITripMax to ~1.5A. Either way the 10k pot will work just fine as a replacement to the 100k that's normally on the board.

[Brenton Hewitt]

oh i had it backwards, R5 was 20K and the Pot which is not documented R9 is in fact 10K as per this Forum  thread

http://forum.pololu.com/viewtopic.php?f=15&t=5276

Hewball

On 14/11/2012, at 10:18 AM, Trevor Lewis <trevor...@gmail.com> wrote:

According to http://reprap.org/wiki/Pololu_stepper_driver_board R9 is 100k, which gives a 10.4A ITripMax (max motor current). 10k will give 4.16A. The Stepsticks I have limit ITripMax to ~1.5A. Either way the 10k pot will work just fine as a replacement to the 100k that's normally on the board.

On Tue, Nov 13, 2012 at 8:44 PM, Ross Shannon <i.m.hep...@gmail.com> wrote:

100K per - lower section of page - the maths is full http://reprap.org/wiki/Pololu_stepper_driver_board

On Nov 13, 2012 5:42 PM, "Brenton Hewitt" <hew...@gmail.com> wrote:

i seem to remember the pots on the Pololu's being 20K not 10K, but its not documented anywhere

and i have also done this fix too, works fine no problems

<CIMG0128.jpg>

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[bluecoast]

Even if the pot has been turned past its stop (in other words the wiper is broken), you can probably still measure across the outside leads and find the total resistance of the pot. That would avoid all this 10k/20k/100k sleuthing.

[bluecoast]

Or of course measure the resistance of the same pot on one of your other Pololus.

[vlad]

what I don't understand is why they chose such a rediculous pot value.  it makes setting hte current oh so very touchy.  seeing as how you can only use the first 1/3rd or so of hte travel on the pot.

[Jim Morris]

That is easily fixed by replacing R5 with 68k, then you have full travel.

I did this with 3 of mine. (I say "I" but a friend with better SMD skills than I actually did it, thanks Bill :)

[Triffid Hunter]

On Wed, Nov 14, 2012 at 6:42 PM, Jim Morris <wolf...@gmail.com> wrote:

That is easily fixed by replacing R5 with 68k, then you have full travel.

 http://triffid-hunter.no-ip.info/A4988-replace-this-resistor.jpg :)

[vlad]

love the image title, I'll slip a few of those into my next order.

[Triffid Hunter]

On Wed, Nov 14, 2012 at 6:53 PM, vlad <vata...@gmail.com> wrote:

love the image title, I'll slip a few of those into my next order.

it's 0603 size if you're not familiar enough to eyeball it.. I've just gotten into SMD recently, it's awesome and I thoroughly enjoy it.. so much easier than through-hole!

[Duncan]

I got bit by the smd bug recently and it is great, especially when you design and reflow your first PCB. For small rework stuff, if your doing it on any kind of regular basis I've found a pair of thermal tweezers is super handy. I think I got mine as a set with a 40$ or so soldering iron on amazon. Saved my butt when I got 100 boards in with sensors that had to be flipped 180°. 

Sent from my iPad

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[Drew]

Any tips for getting into SMD?  Equipment or tool recommendations?

Thanks!
Drew

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[Alex]

http://learn.adafruit.com/smt-manufacturing

[Alex]

https://www.sparkfun.com/tutorials/category/2

[vlad]

SMD is very new to me, but it is no where near as hard as I expected it to be.  I got into it because I fried one of my sanguino pro boards (SMD sanguino board) and thought maybe I could replace just the 644P chip, and I did! it was much easier than expected.  I also resoldered a few caps and resistors.  I have a couple of bards I have been desigining, soon I will have to etch something, and the more I think about it, the challenges of SMD are lower than drilling thru hole holes in the right places...


now I have a toasted ATMEG2560 that needs replacing as well, a touch more daunting, but I have the parts, I'll let you konw how the 3234133 pin package is to resolder.

On Tuesday, November 13, 2012 9:21:00 AM UTC-5, Drew wrote:

[Triffid Hunter]

On Wed, Nov 14, 2012 at 11:38 PM, Drew <dr00...@gmail.com> wrote:

Any tips for getting into SMD?  Equipment or tool recommendations?

fine solder

solder wick

hot air station

fine-point tweezers

magnification

apart from that it's pretty similar to normal soldering except you don't have to keep flipping the board anymore.

optional extras:

solder paste

reflow oven

[Trevor Lewis]

I still haven't done more SMD than a few FTDI chips and some simple repairs, but I found it relatively easy with a good soldering iron and a fine tip. Good tweezers were helpful as well as one of those cool little vacuum chip holder things. I taped one to one of the arms of an old third hand I had and it made the FTDI chips much easier.

I could see why you would want a hot air station though. Even though I don't do much SMD, I'm now wishing I had spent the extra money and picked up one of those dual soldering/hot air stations when I got my soldering station. 

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[bluecoast]

Use regular solder -- it is much more forgiving for manual smd work than lead-free. You can do 0805 size components pretty easily with tweezers and a regular iron and nothing else (fine wick maybe). And hot plate/oven reflowing is for new boards, not repairs.

[vlad]

I never use lead free, I just avoid breathing. fine tip and some braid, my first 644P had only 1 or 2 bridges that needed cleaning, I was surprised.  I have decent eyes but extreemly unsteady hands. easier than expected.  neat trick on desoldering is to bend some copper so it touches all the pads you want to remove (U shape w/ a handle for caps/resistors, a box w/ handle for MCUs) and then tin the wire.  place it on all the pads, load it w/ solder and it will get the whole thing molten at once, and surface tension will pull smlaler parts into the wire, larger parts ust slide em to one side and voila, the day is won!

[Larry Knopp]

One of my favorite basics tutorials :

http://www.sparkfun.com/tutorials/59

Hot plate reflow!  (who needs a durn oven?)
My variation on the electric skillet...  one of those small teflon sandwich griddles.  Works like a charm, as long as you are very attentive, and very careful about the heat-up cycle.

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[Alastair Seggie]

I use one of the other half's irons todo my reflow worlds charm!

Regards

C. Alastair 

UK Mobile +44 (0) 77 538 28682

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[vlad]

resistors are in the mail, got 10 cause I'll drop a few.

[Larry Knopp]

<----   needs to do this...  must *make* time

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[vlad]

take longer to hunt down the parts and wait for the iron to heat up than to just solder them in.

[Larry Knopp]

Yeah, that is probably perfectly true.
Convinced...
This weekend I shall at least do the "hunting and ordering" piece of the puzzle.

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[vlad]

welp, I just opened my invoice to copy paste the PN for you, and guess who musta forgot to hit "add to cart"... Next time gadget, next time

[Larry Knopp]

HAHAHAHAHA
oops
;)

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[vlad]

and yet somehow a box just showed up w/ 10 smd resistors and some crimp connectors I had wanted.  I love no sleep mania, you get so many cool things in the mail.  PN 311-68kgrct-nd.  that's a digikey part number.  I have yet to verify they are the right ones by installing one, but they are 68k 1/10 watt resistors w/ the footprint 0603.  not gonna have a second to solder them in till later tho,

[Larry Knopp]

Awesome!
(Mebbe Triffid will chime in w/ verification, but it sounds right!)

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[Triffid Hunter]

yeah as long as they're in the ballpark of 68k and fit on the pads they'll work fine. I have 50k and 56k on some of mine, that's what I had lying around at the time.. not as good as 68k in terms of adjustment range but way better than the 20k they come with!

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[vlad]

just a note, based on this thread here I got all the parts and did the mod thismorning.  Yay easy adjustment!  I updated the wiki with the info, and actually a 51k resistor is ideal if you ever wanna dial your driver up to max current, but good damn luck keeping it cool.

http://reprap.org/wiki/Pololu_stepper_driver_board

[Larry Knopp]

I got mine, just haven't done it yet.
Glad to hear you did, and it's schweeet.  ;)
(Mebbe this weekend...)

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[Triffid Hunter]

On Wed, Dec 12, 2012 at 3:13 PM, vlad <vata...@gmail.com> wrote:
> just a note, based on this thread here I got all the parts and did the mod
> thismorning. Yay easy adjustment! I updated the wiki with the info, and
> actually a 51k resistor is ideal if you ever wanna dial your driver up to
> max current, but good damn luck keeping it cool.
>
> http://reprap.org/wiki/Pololu_stepper_driver_board

I've actually changed my mind about the mods again.. I realised that
changing the resistors around simply meant that we're reliant on Vcc
being accurate and stable, which frankly is daft considering how
wildly the +5v rail can swing when running from USB power.

Instead, I've installed 1N4148 signal diodes across the pot to hold
the voltage near 0.6v (1.5A) regardless of Vcc. This keeps Vref far
more stable as the divider is now acting as a shunt regulator. This
requires putting the 20k back in, or even dropping in a 10k otherwise
the diode current is too low and the max Vref drops into our desired
adjust range.

[Alastair Seggie]

TH

Just so I understand this correctly you keep the 20R then put the diode across the pot and that keeps the stepping current at 1.5A irrespective of where the wiper is on the pot?

Al

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Alastair Seggie

[Triffid Hunter]

On Fri, Dec 14, 2012 at 11:01 AM, Alastair Seggie
<alastai...@gmail.com> wrote:
> TH
>
> Just so I understand this correctly you keep the 20R then put the diode
> across the pot and that keeps the stepping current at 1.5A irrespective of
> where the wiper is on the pot?

No. I keep the 20k, put the diode across the two outer terminals of
the pot (NOT the wiper), and this fixes the max current regardless of
supply voltage.

As it is at the moment, if your USB port delivers 4.6v and you set the
current, then your USB port somehow jumps to 5.1v, your current will
increase and you may overheat your drivers for no apparent reason.

The diode fixes the voltage across the pot at ~0.6v so that the
current at maximum travel is 1.5A, and supply variations do not affect
your selected current.

Google "shunt regulator" for more in-depth discussions of what's going on here.

[vlad]

my mid/high end PC deffinatly does not supply the advertised 5v on the rail, just noticed this today, it seems way out of spec tho, something like 5.36 iirc.

if the drivers will handle it tho, what about using a 3.3v mega, so the linear reg is keeping VDD where it is expected to be?  Or of course I could cobble in a ref diode for the pololus, so that extra resistors are needed but I can still tune it.  thanks to the extruder motor, fine control of current is a good thing to keep heat reasonable.

[Triffid Hunter]

On Sat, Dec 15, 2012 at 2:24 AM, vlad <vata...@gmail.com> wrote:
> if the drivers will handle it tho, what about using a 3.3v mega, so the
> linear reg is keeping VDD where it is expected to be? Or of course I could
> cobble in a ref diode for the pololus, so that extra resistors are needed
> but I can still tune it. thanks to the extruder motor, fine control of
> current is a good thing to keep heat reasonable.

The pololus work fine from 3.3v, however the heater mosfets will not.
At 3.3v, they will run either in or close to their linear region and
overheat, most likely damaging themselves and failing short circuit.

No, I think a diode shunt regulator on the pot is the best way. This
protects the Vref from variations in supply caused by the pololus
themselves, let alone anything else :)