old reprapdiscount stepper drivers ("pololus") giving jerky movement and weak motors

[Triffid Hunter]

Hi all,

I have some reprapdiscount stepper drivers here. When installed on my
mendelmax, they gave rather low currents, very jerky movement and weak
motors.

I had a close look, and the current sense resistors have "000" written
on them- probably a 0 ohm resistor. These should be 0.05 ohm resistors
for proper operation.

I contacted reprapdiscount who said that they *have already discovered
and rectified the problem and have been selling drivers with correct
sense resistors for some time now*.

SO, if you're having problems with setting currents and jerky
movement, check your pololus! if they have '000' on the sense
resistors, find someone at your local hackerspace with SMD rework
capability and switch them for some 0.05 ohm (0603 package) ones.
Alternatively, contact reprapdiscount and see what you can wrangle.
Keep in mind that they'll probably disappear if they have a flood of
demands for flat out replacements, and I for one don't want that.


While you're at it, switch out R5 (the resistor directly below the
trimpot, closest to the vdd pin) for a 68k resistor. This will give
you the entire travel of the trimpot for setting current rather than
just the first 1/4. The maximum current will be 1.6A, which is still
more than these drivers can supply without overheating.

The max current selectable with the trimpot can be calculated as 125 /
[ R5 (in kohms) + 10 ]... 125 / (68 + 10) = 1.6A

In my experience, the absolute maximum that these drivers can supply
(with forced-air under the boards) is around 1.5A.

In normal usage, 1.25A is a better target to aim for. This corresponds
to 0.50v measured between the face of the trimpot and ground.

The standard resistor is 20kohms, which gives a current of 4.17A with
the trimpot set to maximum! Definitely far beyond the capability of
these drivers, and now you know why the current setting is so
sensitive and right down in the first 1/4 turn!

-Triffid Hunter

[Larry Knopp]

This isn't the case w/ "Genuine" Pololu's?

(The R5 resistor part...)

-Triffid Hunter

--

[Triffid Hunter]

On Tue, Oct 23, 2012 at 12:53 AM, Larry Knopp <lwk...@gmail.com> wrote:
> This isn't the case w/ "Genuine" Pololu's?
>
> (The R5 resistor part...)

I believe genuine pololus also have a 20k resistor there

[verohomie]

Great detailed info... Thanks - Brad

[Larry Knopp]

Huh...
I will have to investigate that.  I've got about a dozen nand brew Pololu's sitting here waiting to be done up.  Have to see what's on there and swap 'em if they're 20's.

--
 
 

[Triffid Hunter]

On Tue, Oct 23, 2012 at 12:21 PM, Larry Knopp <lwk...@gmail.com> wrote:
> I will have to investigate that. I've got about a dozen nand brew Pololu's
> sitting here waiting to be done up. Have to see what's on there and swap
> 'em if they're 20's.

The 20k resistors I pulled off mine have "30C" written on them.. how
30C translates to 20k I have no idea

[Larry Knopp]

lol
Yeah, I've never understood that stuff.

--

[Triffid Hunter]

I replaced them with genuine pololus while I wait for my 0R05
resistors to arrive, here's the before/after shot, showing the jerky
movement caused by the chip being unable to properly regulate current:

http://picasaweb.google.com/lh/photo/xdH325uyT2vIzSk-HJg6ttMTjNZETYmyPJy0liipFm0

[Trevor Ajamil]

Wow i just stumbled upon this topic after experiencing smooth movement in some axis yet that same jerky movement that caused a rattling noise from my PBC rail. It wasnt the pot screw. I plugged it into the x axis and it ran smooth. The y axis still worked but would shift during a print and made a significant amount of noise. I have been through a few of the reprapdiscount drivers and I have a printrboard on my prusa that has never experienced these same issues. 

[Trevor Ajamil]

o yeah  the driver that was bad was purchased in august 2012

[Charles Steinkuehler]

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There isn't enough room for the full 4 digits of a 1% resistor, so
it's a code.

The 30 indicates the 30'th value in the E-96 series (or 200), and the
C is the multiplier. The codes aren't 100% standardized, but most of
the parts I've used match the details listed here:

http://www.marsport.org.uk/smd/res.htm

- --
Charles Steinkuehler
cha...@steinkuehler.net
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[vlad]

folks say you need a re-work station, you really don't.  I do not have steady hands, but I can swap out a 0603 size resistor no problem. the trick is have a spare or 3 for when you open the packaging like a bag of chips and dispense them over your workspace
http://www.reprap.org/wiki/Pololu_stepper_driver_board#Upgrades