Resistors for power supplies questions

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  • LDSisHere
    Badcaps Veteran
    • May 2012
    • 727
    • U.S.A.
    #1

    Resistors for power supplies questions

    I am want to order some spare ~ 1 ohm current sense resistors from Digi-Key but I am not sure what, if any special properties to specify. For example metal film, flame proof, etc. I have attached a picture of an example of what I am talking about, these resistors have a chalky, powder coated appearance if that helps. I am not sure what power rating(s) I want yet as I may order a variety. Any constructive input on this matter would be very much appreciated.

    Also what is special about the Tan/Beige colored resistors that have their value stamped on them instead of the color code bands?

    I have tried to search for this information but I have not found anything very useful.

    Thanks!
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    • redwire
      Badcaps Legend
      • Dec 2010
      • 3907
      • Canada
      #2
      Re: Resistors for power supplies questions

      Current sense resistors have to be low inductance- so wirewound is out and you would use metal-film types.
      If a mosfet shorts, the resistor must be able to hold out long enough to blow the fuse. If this doesn't happen the resistor can catch fire and arc. That would dictate using a flame-proof or fusible type resistor, and mounting it up off the pcb. Fusible resistors blow around 36x rated power. Example 1ohm, 3W is 1.7A max., blows within 60 seconds at 10A; Yageo FKN series
      I think flame proof resistors don't don't have coloured bands (=flammable paint), just laser-etched text.
      Digikey offers many flame proof, film types at 1 ohm.

        Comment

        • LDSisHere
          Badcaps Veteran
          • May 2012
          • 727
          • U.S.A.
          #3
          Re: Resistors for power supplies questions

          Thanks for your response redwire, I have ordered a few in a couple of different sizes that should work well enough. The one in the picture was the current sense resistor for the 5vsb on an Ultra that had the transistor short when I overloaded it. With some of my experimentation I may need more than I can easily scrounge so I thought it best to have some handy.

            Comment

            • everell
              Badcaps Legend
              • Jan 2009
              • 1514
              • USA
              #4
              Re: Resistors for power supplies questions

              Thanks redwire, your response answered many questions.
              Old proverb say.........If you shoot at nothing, you will hit nothing (George Henry 10-14-11)

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