Gen6 5V power supply is oscillating.

I'm repeating this here from ErikZalm's original post on the mendel-parts forum here: www.mendel-parts.com, so it reaches a wider audience. While I don't have a massive problem with the Gen6 board, the temperature does fluctuate a lot, and motors whine when on hold. Other people seem to have had problems with temperature not being reported after putting the sprinter software on the Gen6, though I don't know if this will help.

Erik says:
"The 5V on the Gen6 board is oscillating at approximately 23kHz. This oscillation is caused by a much to low phase margin. The PWM controller is not designed for a ceramic output capacitor. The ESR of the ceramic capacitor is to low. On my board the oscillation amplitude was approximately 300mVpp. This oscillation causes problems with the temperature measurement. Also the current through the stepper motors is influenced by this oscillation.
I did an openloop simulation with LTSpice to see how we can fix this oscillation
By adding a 1nF capacitor parallel to R12 the circuit is stable. The phase margin is much higher. (Not optimal but this should include more changes)
I soldered a 1nF/0805 capacitor on top of R12. The oscillation is completely gone.
The temperature measurement is now rock stable. (Even with klimentkip firmware without filtering)
The motors are now silent (Unless they move of course)"

I later PM'd Erik to try and get some more details, he replied:
"All 1nF capacitors will work. A 0805 smd housing is the simplest. It can be soldered directly on R12.
Simply. Place the capacitor on top of R12. And place a little solder on both metallic sides to connect it to the resistor.
It a simple job if you have the right tools and a capacitor.
On the reprap wiki you can find the schematics and component layout."

A couple of questions for you good people:
1. Does this sound like a sensible mod to do?
2. I asked Erik if this is likely to be the same across all Gen6 boards, I guess it is?
3. If I were to carry out this mod, do I solder the 1nf resistor on top of the existing resistor, or replace the existing resistor? I have no idea of the value of the existing resistor.
4. I'm willing to give it a go, but the smallest quantity of 1nf resistors I can buy is 100, which is a bit of an overkill. Anyone got a couple of 1nF 0805 capacitors in an smd housing (approx 1mm wide by 2mm long) they can send me?

1. Yes if you have the symptoms.
2. I heard about this problem a while ago so you would think Mendel parts would have fixed it by now, but maybe not.
3. You put it on top and must keep the original resistor.
4. I can send you a couple, where are you located? Seems odd you can't buy less than 100. I can buy 10 for 40p: [uk.farnell.com]

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[www.hydraraptor.blogspot.com]

I think I was looking at RS-online, perhaps the specific resistor I was looking at could only be bought in 100s. So thanks for the quick response and kind offer, nophead. I'll take you up on that, if only because Farnell have a minimum £20 order, and I can't think of anything else I need at present. PM sent.
I was hoping to get other Gen6 users' experience, if they had fluctuating temps and noisy motors.
I'll give it a go and report back, I can always remove the resistor if there is no change.

Finally got around to doing this mod. It's very fiddly to mount one SMD resistor on top of another, but the result is rock solid temperatures. The reported temperature fluctuating a lot, even at room temperature, and seemed unable to get up to temperatures that I set. Now it is much more reliable and consistent. Many thanks to Nophead for the resistors, sent promptly all those months ago!

the datascheet of the L5970 has a 100µF at the uutput rail...

have you test that?

I think changing the loop i not a good solution... (when the changes are not in the datasheet)

droftarts Wrote:
-------------------------------------------------------
> Finally got around to doing this mod. It's very
> fiddly to mount one SMD resistor on top of
> another, but the result is rock solid
> temperatures. The reported temperature fluctuating
> a lot, even at room temperature, and seemed unable
> to get up to temperatures that I set. Now it is
> much more reliable and consistent. Many thanks to
> Nophead for the resistors, sent promptly all those
> months ago!


This is a new mod I'm interested in. Can you take a pic so we can update the users? Has this mod been fixed in new motherboards?

All the power supply oscillating problems are gone in the newer batches of Mendel-Parts electronics.
(Together with other improvements).

This in all units shipped after ~july/august 2011.

The newest electronics have even more improvements like opto-isolated and regulated heated bed.

Hello!
If one would happen to solder R12 to R13 when soldering the capasitor on top of R12 and power up the board after soldering, what would happen? Would something fry?

So I think I need to perform this mod to a gen6 board that came with my printer. I don't see a manufacture date on it but I've got all the symptoms (bad temp readings, noisy motors).

Does anyone in this thread still have any extra caps? I can't find them at Radio Shack and don't need 10+ of them if I order online.

Does anyone in this thread have any 1nf capacitors laying around? Those little buggers seem unusually hard to come across at a reasonable quantity/price. I either have to buy 100 of them, buy more than I need to get a fair price, or spend way too much on a single one. I assume someone here has bought a bunch of them at some point whether they wanted to or not.

I can PayPal your fee +shipping right away.