Easily troubleshooting MOSFETs!

With the RAMPS 1.4 connected to the power supply (presuming 12 volts attached to the RAMPS inputs 1 and 2), and the wires removed from the D9 and D10 terminals, probe the following with a DC voltmeter, set to 20 volts if not auto-ranging.

1 - Black (negative) probe on 1 and Red (positive) probe on 2 This should read around 12 volts. If not, make sure the power supply is on.

2 - Black probe on 1 and Red probe on 3 This should read around 12 volts.

3 - Black probe on 1 and Red probe on 5 This should also read around 12 volts.

If step 2 AND step 3 resulted in readings of 0 volts, the 6A (smaller) fuse is blown. If only one of them read around 12 volts and the other one 0 volts, then there is a bad trace on the board, but the fuse is still okay.

If step 2 AND step 3 resulted in readings around 12 volts, this proves that the positive 12 volt path is good from the power supply, through the fuse, to the D9 and D10 positive outputs.

4 - Red (positive) probe on 2 and Black (negative) probe on 4 This should read 0 volts and the LED on the far left, over the D should be off.

5 - Red probe on 2 and Black probe on 6 This should also read 0 volts and the second LED from the left (over the 1) should be off.

If either of the readings result in a positive voltage, the MOSFET is probably damaged. If no voltage or LEDs, continue...

Activate the fan by sending M106 S255

6 - Red (positive) probe on 2 and Black (negative) probe on 4 This should read around 12 volts and the LED on the far left, over the D should be on.

Activate the extruder heater by sending M109 S40

7 - Red probe on 2 and Black probe on 6 This should also read around 12 volts and the second LED from the left (over the 1) should be on.

If either step (6 or 7) results in a reading of less than 10-12 volts, and the corresponding LED is lit, then the MOSFET for that connector is bad.

The LEDs are lit (or flickering) according to the PWM output of the Arduino, which sends the same +5 volt pulsed signal to the MOSFET, which in turn amplifies the potential to ground at D10 and D9, thus increasing the output voltage (as referenced from the constant 12 volts and the MOSFET ground-ish output).

Sounds confusing, sure. But follow this quick test and you will determine if the RAMPS components are good.

If you need instructions for the D8 MOSFET (bed heater), just let me know

(Thank you cdru for the suggested corrections!)

Edited 4 time(s). Last edit at 01/14/2015 11:36AM by MindRealm.

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-David

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Edited - Most of the issues that were here were fixed in the above post.

Edited 3 time(s). Last edit at 01/14/2015 02:10PM by cdru.

crdu - Thank you so much for proofreading my instructions.

I have made several changes that you recommended. I didn't originally suggest using the two pin locations for reference points due to them being small and somewhat difficult to identify to folks. Also, they may be otherwise occupied. Mine has a 2-pin header installed that would be easily shorted while trying to hold a probe on just one of the pins. Also, the instructions begin with removing the wiring from D9 and D10, thus removing any load (with exception of the pins I just mentioned) from the polyfuse. If D9 and D10 show 0 volts and the power supply is on, this would indicate a permanent fault in the polyfuse.

Please consider re-editing your response to remove references to the sections I changed, so readers will not be further confused after my edit...

Thanks again for your recommendations! Advice is only good advice if it is correct

Edited 1 time(s). Last edit at 01/14/2015 11:37AM by MindRealm.

---

-David

Find me online at:
Thingiverse
Instructables.com
LinkedIn
Facebook

Check out my FolgerTech Prusa i3 (plexi) at MindRealm.net

Quote
MindRealm
I have made several changes that you recommended. I didn't originally suggest using the two pin locations for reference points due to them being small and somewhat difficult to identify to folks.

Fair enough.

Quote

Also, the instructions begin with removing the wiring from D9 and D10, thus removing any load (with exception of the pins I just mentioned) from the polyfuse. If D9 and D10 show 0 volts and the power supply is on, this would indicate a permanent fault in the polyfuse.

Incorrect. It would only show a fault if the polyfuse is cold. If the load was removed that caused a tripped fuse, until the polymer cools to the point where it conducts again, it still would remain tripped. I'm not sure what the reset time is on the polymer. Also if there was a low resistance short due to say a bad solder connection that bridged and caused the fuse to trip, D9 and D10 may read 0V but the power supply still be on. This is why I originally said to say the fuse was tripped, not blown.

Edited 1 time(s). Last edit at 01/14/2015 03:28PM by cdru.

Please could you provide instructions for the D8 MOSFET (bed heater)
I'm having no luck with my problem
My heat bed problem
details about the D8 Mosfet could help

Thanks
Tony