Fixing a Cheap 3d Printer Power Supply with a Blown NTC Thermister

My older style Anycubic Kossel Pulley didn’t come with a heated bed or a power supply large enough to run it.  Although the newer Kossels Anycubic is making (as of early 2018) come with both, I had to purchase these separately and piece it together.  I spent all of $20 on eBay for the 360W supply and it worked great, until two months later when I smelled some smoke and the printer shut off mid-print.  Opening up the case, I found a NTC 5D-15 thermistor that was ashy and split in two.  This is how I fixed it.

 

 

Sources:

https://www.electronicspoint.com/threads/ntc-5d-9-thermistor.62355/

http://falconchristmas.com/forum/index.php?topic=3742.0

Uxcell a15060900ux0983 5 Piece 5D-15 NTC MOV Varistor Voltage Dependent Resistors

12v 30a DC Power Supply 360w

Diagnosis

The issue is (obviously) that the NTC burned up.  A little research into NTCs (negative temperature coefficient thermistors) told me that they resist current until they warm up and then they allow more current through.  They prevent inrush current from being more than the power supply can handle and then once they are heated up, let the circuit get the normal amount of current for standard operation.

After the repair I looked inside the power supplies that Anycubic is shipping with their newer printers and they don’t have the NTC but just a jumper wire in its place.  After reading some forums it seems that this part can blow pretty easily and so apparently some manufacturers aren’t including it anymore.

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View of the power supply internals

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Noticed some charring…

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and that it was completely split open!

Disassembly

After removing the wires from the Trigorilla board I unscrewed the six screws holding the two halves together.

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A photo of the wires before opening it up

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The screws closest to the top hold the two halves of the case together

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Same thing on the other side…

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And the top two screws

You don’t need to remove the fan, but it makes it easier and it gives you more freedom inside the case.

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Two screws hold the fan to the case

To get the PCB board out you need to take out the screws holding it to the case and the two screws clamping the mosfets to the aluminium heatsinks on the sides of the case.

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With the case apart you can see the screws holding the board down and the heatsink screws

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A better view of all the screws from above

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Heatsinks on the inside of the case with the board out

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The screws that hold the board to the heatsinks is in the center of the case on the top and one side

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And the other screw hidden by the sticker

Repair and Soldering

With the board out it was very easy to see the problem.  Since the NTC was split open it was obvious that it should be replaced.  I used a soldering iron to melt the solder on each leg and pull it out.  Put the new one in place and added some more solder to attach it.

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NTC split open

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Old NTC next to the new one

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Holes on the backside showing where the wires for the thermistor come through

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New component soldered in place

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After the repair it seems that I’m getting 12v again

Conclusion

Including finding the problem and making the repair I spent about 2 hours and $8.08 on the fix.  Buying a new power supply is around $20 and replacing the NTC with just a wire would be free.  I’m glad I did it, if for no other reason than to learn what a NTC does, but it probably isn’t the most cost-effective choice unless you want to do the repair (if you read to the end of this post, you know you want to).

Posted in Random Technology

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