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  • Shunt resistor wattage?

    Guys,

    I screwed up in my recent PE order. I had meant to order a 2 ohm, 10 watt but instead got a pack of 2 ohm 1/2 watt resistors. I got 10 of them so can use four in series / parallel for a combined 2 ohm, 2 watt resistor bank. You'll see it as R2 in the image below. If this were a DC circuit, I'd know the answer. I'm thinking that 2 watts of resistor rating should be enough, but would rather be safe than sorry. Your help would be very much appreciated.

    Thanks!!!
    Attached Files

  • #2
    Re: Shunt resistor wattage?

    Originally posted by blackie View Post
    Guys,

    I screwed up in my recent PE order. I had meant to order a 2 ohm, 10 watt but instead got a pack of 2 ohm 1/2 watt resistors. I got 10 of them so can use four in series / parallel for a combined 2 ohm, 2 watt resistor bank. You'll see it as R2 in the image below. If this were a DC circuit, I'd know the answer. I'm thinking that 2 watts of resistor rating should be enough, but would rather be safe than sorry. Your help would be very much appreciated.

    Thanks!!!
    You'll want at least 10W ratings on speaker resistors unless the resistance value is above 25-30 ohms. The higher the ohm value, the less heat it has to dissipate due to less current flow.

    Later,
    Wolf
    "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
    "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
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    Comment


    • #3
      Re: Shunt resistor wattage?

      I've used 5-watters before (in low-power designs, YMMV). A 9-pack (3/3) would be close to that (or do you only have 10 units to split between both cabs)?

      I'm guessin' that your R2 could be taking about 3X the power (above maybe, 2k?) that the woofer's using (if it's in the 4n-8n range)?? Many times the resistance in a Zobel (-like filter) is not REAL picky. Have you modeled it?

      It could be very cool for someone to add power consumption (by clicking on a circuit component) to one of the XO sim programs, wouldn't it? (It would be freq. dependent, of course.)

      Comment


      • #4
        Re: Shunt resistor wattage?

        I just did some quick modeling of the crossover, and it looks like the worst case power dissipation in that resistor would be right around 600 Hz. The datasheet for the RS180 shows ~7-8 ohms in that area. The RS180 is rated for 60 watts continuous average power. Driving the speaker at that level would put about 1.5-1.8 watts into the shunt resistor. If you were going to drive that continuously, it would make me nervous having a 2 watt resistor in place, but the reality of driving it at that level continuously is also pretty slim unless you're having a house party with the volume cranked for an extended period.

        EDIT: On second thought, the inductive rise of the woofer could increase dissipation in the resistor another watt or so higher up in the frequency band. I still don't think you would drive the speaker at those levels continuously, especially in the 600 Hz to 2 kHz range, but I'd personally feel more comfortable with a 5 watt or more resistor in there.

        Comment


        • #5
          Re: Shunt resistor wattage?

          Originally posted by Chris Roemer View Post
          It could be very cool for someone to add power consumption (by clicking on a circuit component) to one of the XO sim programs, wouldn't it? (It would be freq. dependent, of course.)
          In the very few XO sims I've done to date, this is something I have been very interested in as well. It would be a great feature.

          Comment


          • #6
            Re: Shunt resistor wattage?

            Originally posted by lowpolyjoe View Post
            In the very few XO sims I've done to date, this is something I have been very interested in as well. It would be a great feature.
            Yes, it sure would be useful! Gents, thanks to all of you for your input. Chris is correct in that if I used 9 of the 2 ohm resistors in a 3 x 3 grid, I'd end up with almost exactly 2 ohms for a 4.5 watt package. Good lord, do I want 9 resistors on my board though? Nah.. I'll order the proper part as you've all suggested, the 2 ohm, 10 watt that I meant to order in the first place. Not to drag this out but I am inclined to believe that the non inductive resistors are going to be very desirable.

            Once again, thank you all so much for your help. I am really humbled to get responses from talent such as yourselves. You guys have helped me over the last year more than you would know as I study your design process' in a most addicted manner.

            Cheers from Colorado...

            Comment


            • #7
              Re: Shunt resistor wattage?

              The cheap (38c 5w & 69c 10w - or whatever they cost now) work just fine. I actually prefer them to the Dayton "Audio Grade" ones.

              Comment


              • #8
                Re: Shunt resistor wattage?

                Originally posted by Chris Roemer View Post
                The cheap (38c 5w & 69c 10w - or whatever they cost now) work just fine. I actually prefer them to the Dayton "Audio Grade" ones.
                Agreed. They both have very low measured inductance. I usually double the value of the needed resistor, and put them in parallel to achieve the target value. I've had a few resistor leads break off before I did this.

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