Hello, I had a question on fuses.

3D Printing
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1

Hello, I had a question on fuses. I want to out a fuse on the plug coming into the printer and I am not sure what fuse I should buy. I was about to go buy a 20 amp one when I realized that my breaker box would trip before the fuse did, that is obviously too large.

What size fuse should I use. I am running a 12V RAMPS with MK3 heated bed from RAMPS. Pretty standard setup.

Thanks!

2

The breaker is what a 10amp 120v? It has its own protection verses faults. What you want is a fuse on the dc side to handle the load that the printer is using between the psu and the input into the controller or equipment. On the ramps you have two power inputs one rated at 5amp and the other at 10-11amps, I have replaced my poly fuses with automotive fuses.

3

3A at 120V is ample. Amps=Volts/Watts

4

AC:

  1. Your breaker ONLY protects your house wiring. It doesn’t protect the printer’s power cord or power supply. 15A at 120v will blow shit up.
  2. In order to protect the power cord and AC side of the PSU, you need to put as small of a fuse as possible in the AC wiring before the PSU. Take your PSU wattage, divide by your mains voltage, add 20% for PSU efficiency losses, and pick the smallest fuse above that. I use 4A fuses for most of my printers with 350w PSUs, in a fuse+inlet+switch unit like this: https://www.amazon.com/dp/B01LYMNQ2L
    DC:
  3. The PSU should have short protection that protects the DC side of the PSU. It doesn’t really protect the controller or loads.
  4. Your controller should have a inlet fuse rated slightly higher than its maximum current rating, which is typically determined by the terminal blocks. They’re often 12A-15A.
  5. You should also have fuses or thermal fuses on the load side of the controller for all heaters large enough to damage whatever they’re heating during a runaway. Roughly speaking, you should consider fusing hot ends over 30w and always thermal-fuse heatbeds over 0.5 watts per cm^2. Don’t rely on the PSU short protection! You can arc-weld with the kind of current a big 12v PSU will put out into a short.