What could cause a drop in current?

3D Printing
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1

What could cause a drop in current?

15 minutes into a print my Prusa I3 + RAMPS dies. The motors slow way down and the temperatures start dropping. In pronterface, I can either heat the bed or move the motors.

The effect appears to be immediate rather than gradual. It also appears to recover after some time. Aside from a prolonged inactivity, the only change has been a fan on the hotend. Perhaps the fan is making the heater work too hard and stressing the PSU.

I could put a meter between the PSU and RAMPS, but that would only tell me if current dropped, not why. Any thoughts?

2

Getting hot? I’ve had it cause missed steps but never a shutdown.

3

So you’re saying you can either run the heaters or move the motors, but not both at the same time?

4

This sounds like the polyfuses on the RAMPS overheating and cutting out. These are the flat yellow components near the power supply connector. Most likely the 11A heated bed polyfuse (the larger one ) is getting too hot and causing the 5 A smaller one to cut out as well. These fuses recover once they have cooled down.
The question is why are they overheating?
What temperature are you running your heated bed?
Do you have a fan cooling your RAMPS?

See my post here https://plus.google.com/113303711648624409944/posts/am89Pas8xNV

5

cracks knuckles … its possible you have a bad PSU. remove PSU, manually power it to the heater bed and see if it can hold the load.

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@Kit_Adams , that makes a lot of sense. Everything on the ramps is hot, (even including the 11a connector). I don’t have a fan on the ramps yet (I am trying unsuccessfully to print a fan mount). Maybe tomorrow I will try printing with a fan propped up to the ramps and see if this helps.

If this is the cause, then I must have been running close to the threshold for some time.

7

ooo polyfuses

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Definitely the polyfuses. I’ve had the exact same problem on a printer I built for someone and shorting out the polyfuses did the trick.
It is possible for the fuses to kick in even when there is nothing wrong with your electronics, but have your drivers set fairly high or are using a powerful hotend.

9

@Kit_Adams , @Thomas_Sanladerer , I was able to finish the print with a 40mm fan pointed at the RAMPS. Thanks for your help.

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would someone enlighten me on polyfuse? am i right to assume these are a variant of PTC (positive temp thermistor?).

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@3roomlab the polyfuses are the large yellow things on your RAMPS, simply speaking, they contain a polymer that turns non-conductive when it heats up.