Resistors help

Discussion in 'The Projects Forum' started by noingwhat, Sep 18, 2010.

  1. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I am sure this is a simple fix, but I am trying to build a simple RC Filter, and I had built one, but I wound up screwing up in many places. One of which, would be that I overpowered the resistors. The store I usually shop at only goes up to .5w resistors, yet I measured, and my amp is pumping more like 66 watts on full volume. I usually don't go that high, but I may get to around 50 every once in a while. So how do I get 50 watts to go through a .5 watt resistor, or is there some place where I can order 50 watt resistors.
     
    Last edited: Sep 18, 2010
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    You can order 50 watt (or larger) resistors.

    Check digikey.com
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Another way is to build a parallel set of resistors.

    Say you need 10 ohms(for easy math) at 50 watts. This is gonna take a lot of resistors. 100 1/2 watt resistors would be needed to handle the power. You want each resistor to share equally in the power dissipation so they must all be the same resistance. One hundred half watt 1000 Ohm resistors will make you a single 10 ohm resistor rated at 50 watts.

    So it might just be cheaper to buy the larger wattage resistor. I think Radio Shack carries the large square ceramic 5 and 10 watt resistors in a small number of low ohm values. Lots of people use them for dummy loads on amplifiers.
     
  4. retched

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  5. Wendy

    Moderator

    Mar 24, 2008
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    I like the parallel resistor option myself. Less heat in one place, so it is safer.

    The link to your store is broken.
     
  6. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    You could also try submersing lower power resistors into some transformer oil. Not as easy to find as it was once but it is still out there. Avoid ones containing PCBs ( Poly something something not Printed Circuit Boards) as they are carcinogenic.
     
  7. retched

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    @windoze
    Poly something something... thats funny. ;)

    @noingwhat
    You can do both. You can get a hold of five 10w resistors and parallel them.
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    At windozkilla:

    You can use refined kerosene(the odorless kind with perfume scent is called Johnsons baby oil, don't use it, unless you like that smell :) ). Kerosene is also known as liquid paraffin across the pond.
     
  9. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Thanks for the FYI on the link being broken, I fixed that now.

    When I had opened up the speaker (I am modding it) it had a 1.5 ohm sand resistor, but it is only a 5 watt. It seemed to work before, so if I use this and just get the caps to specs with a 1.5 ohm resistor, should that work?

    Also I had noticed that in my first model of the filter, the resistors were getting very hot. I had messed up and had to use a 50 ohm resistor, so I think that may have something to do with it, but I want to know if the caps will get really hot too.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    If you dont know for sure, you better wear a face shield.

    Exploding capacitors are nothing to sneeze at.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    No, because the resistor is obviously part of a crossover network and capacitors exhibit reactance related to frequency, not resistance.

    How many ohms does your circuit need? If it isn't much even Radio Shack might have a combination to get you there.
     
  12. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    So how did the speaker company have 100 Watt speakers with only 5 watt resistors?
     
  13. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Not quite sure why you find that funny. I just couldn't be bothered looking it up. But now I have....

    polychlorinated biphenyls

    Very nasty stuff.
     
  14. retched

    AAC Fanatic!

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    I have used the same "something something" technique in my days... It just hit me as funny.
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Probably because that resistor is in the midrange or tweeter part of the crossover where little power is passed. About 90% of the power used to drive a speaker at normal listening levels is going to the woofer.
     
  16. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    When I opened it up, all I found was a 1.5 OHM, 5 watt sand resistor, a 6.6 MFD 70v Cap, and a 3.3 MFD 70v Cap. I have no idea what their circuit was, but somehow they did it.
     
  17. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That would have simply been a crossover circuit for the tweeter. Between the caps and the resistor it will help filter out the lower frequencies from getting to the tweeter and burning it out. It takes very little power to drive a tweeter to full output.
     
  18. Ghar

    Active Member

    Mar 8, 2010
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    Resistors and components in general do not 'pass power'.

    A 50W load does not require 50W components everywhere.

    A series component passes current, a shunt component supports voltage across itself.

    Say you have a speaker, say it's strictly 8 ohms just for simplicity, with that 1.5 ohm resistor.
    With 50W in the speaker the current through it is:

    P = I^2 * R
    50 = I^2 * 8
    I = 2.5A

    Because that's in series with the 1.5 ohm resistor it carries the same current. The power in the 1.5 ohm resistor will be:
    P = I^2 R = 2.5^2 * 1.5 = 9.4 W

    (I say for simplicity because a speaker is not entirely resistive)
     
  19. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    That makes sense, but they are 100 watt speakers (8 Ohm) and the resistor, is only 5 watt, so trusting you math, they should have blown way before the speaker even got up to full capacity. Why would they have done this?
     
  20. Ghar

    Active Member

    Mar 8, 2010
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    This is where marshalf's comments are appropriate.
    How many drivers does this speaker have and which one is the resistor connected to?
     
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