MakerFarm 8" i3v Prusa build by Printbus

[printbus]

Kevin - I tried the usb 5V isolation thing and got erratic operation of my ABL servo. the failure went away after removing the kapton from the connector. Does it make sense that removing that source of 5V might interfere with somethign in the RC servo section of the board?

Interesting. It could be that there IS a connection. Here's my theory -

5V power for the MEGA2560 board, RAMPS (mainly any servos connected to it) and the LCD panel can normally come from either the USB port or a fixed voltage linear regulator on the MEGA2560 board. With no 5v in the USB cable, 5v power will have to come from the fixed regulator on the MEGA2560 board. To provide maximum torque throughout the desired movement, RC servos typically operate through application of a series of short pulses of power to the servo motor. Each one of these pulses is going to draw a surge of current from the regulator, with the amount of surge dependent on the actual servo being used and the amount of mechanical load on the servo. It could be that these surges are more than the fixed regulator can handle. The other possibility is that there's a power dissipation issue with the voltage regulator supplying 5V for the servo(s) connected to the RAMPS board. Either could cause dropouts in the 5V output that could lead to erratic operation of the MEGA2560, the servos, or the LCD panel.

It could be that there are multiple factors involved. For example, perhaps there's only a small drop in the regulator voltage, but that is combined with voltage drop in the lengthy wiring required to get 5V to the ABL servo.

I'm not sure having a servo that moves too far as mentioned in your original thread would be a symptom I would expect, but who knows. I'll add a cautionary note regarding ABL to the post suggesting the USB cable mod.

FOLLOWUP COMMENT: The Power Supply section of http://reprap.org/wiki/RAMPS_1.4 does imply that there are concerns powering servos from the Arduino (MEGA2560) unless you're running the Arduino directly from a 5V supply or USB power. This would mean you have to also be wary about using a printer with ABL standalone, when no computer is connected to provide the additional 5V power needed for the ABL servo:

"The 5V pin in that connector on RAMPS only supplies the 5V to the auxiliary servo connectors. It is designed so that you can jumper it to the VCC pin and use the Arduino's power supply to supply 5V for extra servos if you are only powered from USB or 5V. Since there is not a lot of extra power from the Arduino's power supply you can connect it directly to your 5V power supply if you have one."

FOLLOWUP COMMENT #2: For the technically curious... The vague "...not a lot of extra power from the Arduino's power supply" statement in the wiki caused me to look at that some more. Per the Arduino baseline, the fixed regulator on the MEGA2560 is an ON Semi NCP1117ST50T3 part. Calculating the maximum power dissipation allowed in a linear regulator is always tricky, since you're having to estimate the design's ability to dissipate heat away from the part. For the SOT-223 regulator package being used here, the datasheet does have Figure 21 that shows how much power dissipation is acceptable for a given square of 2-ounce copper plane around the part. The graph tops out at just over 1.4 watts; let's take that as a maximum amount of power we would want to see in the printer application even though the copper on the board isn't exactly a square, we don't know the copper thickness, the RAMPS board is going to limit airflow, etc. With the printer not doing anything, I measure 140mA of 12V current going into the MEGA2560 board, and this should equate to the current flowing through the 5V regulator. So, without an ABL servo the power dissipation in the 5V regulator is already just about a watt. The math is P=V*I or (12-5)*0.14=0.98 watts. This is already well into our maximum. Using our notional 1.4W limit on the power dissipation, the maximum amount of current we could draw is I=P/V, or 1.4/(12-5)= 0.2 amps. This is only 60 mA above what I've already measured. Whatever margin the MEGA2560 board had in 5V regulator capacity is likely considered used up by the LCD smartpanel. This again emphasizes that those with an ABL servo may not want to operate the printer, or at least may not want to do a lot of bed leveling servo movement, without power being applied by USB or they risk overheating the 5V regulator. The saving grace is that bed leveling isn't a forever thing. The trick would be to get extra load from moving the ABL servo over quick enough.