Resistors, Capacitors and Inductors; Oh my!

Discussion in 'Archived Threads 2001-2004' started by Clif Forsyth, Dec 2, 2002.

  1. Clif Forsyth

    Clif Forsyth Extra

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    I have been reading this forum for awhile. The advice here is great. Almost every question I have had I have found the answer for here. I understand that buying a kit for your first project is a great idea. However I have found a design that uses drivers that I thought were pretty good. I can follow everything except the crossover. The wiring part shouldn't be too tough the problem is finding all the parts he is talking about. He lists capacitors of 7.7,9.2,10.0,and 10.7 mfd. All the capacitors I see are rated uF. and none of them have been of the 7.7,9.2 or 10.7 variety. The Resistors are .5,4.0,4.5,9.0,11.0. I have not found a single one at 9 ohms. Can you double up the capacitors and resistors to make the desired capacitance or resistance? Or am I missing something? The good news is I found all the inductors in the proper mh and dcr he requires. Any help would be appreciated. Thanks.
     
  2. For the capacitors, parallel values will add (C=C1 + C2)and for parallel resistors, the inverse adds (1/R= 1/R1 + 1/R2 etc) NOTE: "I" personally never series resistors or caps, but others may.
    Example,for 7.7uf, parallel a 4.3uF and a 3.3uF. For 9ohms, parallel 30 and a 12.5ohm for 8.82ohms (close enough). These resistor/cap values were what I found at www.zalytron.com
     
  3. Saurav

    Saurav Cinematographer

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    If you're building a speaker crossover, make sure you get components which have the correct power rating. Crossover resistors are usually in the 10W range, that's very different from resistors used inside electronic components which are in the 1/2W or 1/4W range. Using a resistor of lower power rating inside a speaker crossover will almost certainly cause it to fry, and in the worst case something may catch fire.
    Where did you search for capacitor values? www.partsexpress.com www.mouser.com and www.digikey.com are some good places to look in. uF stands for micro Farad (the u should be the Greek character 'mu'), and mfd usually also means micro Farad. A micro is one millionth. 'm' should be milli (one thousandth) according to the standard shorthand conventions, but I've never seen people quote capacitance values in milli Farads, so I think mfd probably means the same thing as uF.
    You can hook up resistors in series to get the value you want. Resistors in series add up, capacitors in parallel add up (as Anthony said). So to get 9 ohms, you could put 5 ohms and 4 ohms in series. If resistors are in series, they both need to be of adequate power rating, since the same current flows through both. If you're putting resistors in parallel you can afford to go with a lower power rating (since the current splits up between the two), but I would recommend you stick with the specified power rating.
     
  4. Saurav

    Saurav Cinematographer

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    That's a good point. I've never actually bought speaker XO parts from Mouser or Digikey, I just threw them out as popular component sources. Thanks for correcting that.

    I bought some caps from North Creek once, they seem to have some nice stuff too.
     
  5. Clif Forsyth

    Clif Forsyth Extra

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    Thanks everyone for the replies. Now off to do some shopping.
     
  6. GuyS

    GuyS Auditioning

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    "If resistors are in series, they both need to be of adequate power rating, since the same current flows through both. If you're putting resistors in parallel you can afford to go with a lower power rating (since the current splits up between the two):"
    I thought that the power dissipation would be the same weather the resistors are in series or parallel. The formula for power is P=EI so even if the the same current flows in a parallel circuit the voltage would split between the resistors. Does that sound right? I've seen this discussed on the HT forum before and searched for more info on the net. I found this test on a New Zealand Amateur Radio Organization site. Questions 3 & 4 deal with power dissipation of resistors.
    http://www.nzart.org.nz/nzart/examin...LCULATIONS.htm
    I guess it is not positive proof just because the questions are on the test but most of the ham radio people I know are up on their basic electricity.
     
  7. Patrick Sun

    Patrick Sun Moderator
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    For equal values of parallel resistors, you can effectively double the power handling for the pair. For example, if parallelled two 12 ohm resistors with a 10W power rating, that's just as good as a 6 ohm resistor with a 20W power rating.

    For n-paralleled resistors of equal value, you add the power handling among the resistors together, and do math for the effective resistance:

    Rtotal = 1/(1/R1 + 1/R2 + ... + 1/Rn)

    It gets a little tricker to figure out the power handling for parallelled resistors of different resistor values.
     
  8. Andrew Pratt

    Andrew Pratt Producer

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    www.solen.ca is another good site for high quality but reasonably priced cross over parts
     
  9. Saurav

    Saurav Cinematographer

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  10. Patrick Sun

    Patrick Sun Moderator
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    It probably doesn't come up because there is always a chance of increasing the overall inductance from resistors connected in series (voltage divider), and that's not as desirable as choosing the right resistor value, or by parallelling the resistors to get the right value.
     
  11. Saurav

    Saurav Cinematographer

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    So if one doesn't have the right resistor, one should try to arrive at the value by using resistors in parallel rather than series? I'll remember that.
     

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