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A Basic description of Resistors in Series or Parallel

Discussion in 'General Electronics Discussion' started by SniffTheGlove, Dec 24, 2016.

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  1. SniffTheGlove

    SniffTheGlove

    9
    0
    Dec 22, 2016
    Hello,

    Trying to understand using multiple resistors in series or parallel. In another thread it was suggested I use a resistor in series/parallel but I am starting a new thread to ask the question...

    I did a search using resistor series but there was only 18 replies and not actually helped me to understand.

    So,

    Series Resistor
    R1 + R2 + R3
    Lets make it simple and say each resistor is 1K 0.5W, so from the above the total Resistance is 3K.
    I can understand this but what also happens with...
    1) The wattage, does heat dissipation become 1.5W or stays at 0.5W
    2) The current capacity, does this stay the same or does it change


    Parallel Resistor
    R1
    R2
    R2
    Lets make it simple and say each resistor is 1K 0.5W, so from the above the total Resistance still stays the same at 1K.
    I can understand this but what also happens with...
    1) The wattage, does heat dissipation become 1.5W or stays at 0.5W
    2) The current capacity, does this stay the same or does it change

    If you can explain in basic terms the above questions for series and parallel

    Thanks
     
  2. AnalogKid

    AnalogKid

    2,334
    653
    Jun 10, 2015
    The power rating is a maximum for that device, based on its size, internal construction, the diameter of the leads, etc; the things that affect how fast it can get rid of heat. The actual power developed in the resistor is dependent on the resistance value and the current through it, according to Joule's law: P = I^2 x R. So if you had a power supply big enough to pump 1 A through the series string, each resistor would dissipate 1000 W and quickly burn up. If you pumped in only 10 mA, each resistor dissipates only 0.1 W and is barely warm to the touch.

    Ohm's law sets the relationship between the voltage across a resistor and the current through it. You can play with the numbers to get a feel for how they interact.

    And there is nothing wrong with putting resistors in series with different power ratings. Just know that if all of the resistances are equal, the smaller resistors will warm up more than the larger ones because (Kirchhoff's Theorem plus Ohm's Law), the current through a series string is the current through each individual element in the string.

    ak
     
    Last edited: Dec 24, 2016
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,378
    2,046
    Jun 21, 2012
    Resistors in series add their values. So with three resistors in series the value is R1 + R2 + R3. Resistors in parallel have a value that is the reciprocal of the sum of their reciprocal resistances. So, if all the resistors are equal in value, then three resistors in parallel would each have reciprocal resistances of 1/R, and if you add those up you get 3/R for the sum. You then take the reciprocal of 3/R and you get R/3 or one third the value of any one of the three equal-valued resistors. In general, the resistance of paralleled resistors is always less than the value of the smallest-valued resistor.

    As for power dissipation, each resistor will safely dissipate its rated power whether it is connected in series or parallel or any combination of series and parallel. Maximum power dissipation is a function of resistor construction, not how it is used in a circuit. Thus two 1/2 watt resistors will safely dissipate a total of 1 watt whether in series or in parallel. You have to determine the actual power dissipation using P = I²R, where I is the current through the resistor whose value is R. Use Ohm's Law (V = IR) and Kirchoff's Laws for voltage (voltage drops around a loop must add up to zero) and current (currents entering and leaving a node must add up to zero) to perform a circuit analysis that reveals the current through each resistor in the circuit. Then apply the formula, P = I²R, to find the power dissipated in each resistor.

    I hope you understand the above explanation because it doesn't get any more "basic" than this.
     
  4. AnalogKid

    AnalogKid

    2,334
    653
    Jun 10, 2015
    No, it doesn't. In your case, the total resistance is 1K / 3, or 333 ohms.
    In the general case, the total resistance in the inverse of the sum of the inverses of the values (yeoww):

    Rtot = 1 / (1 / R1 + 1 / R2 + 1 / R3 + ... + 1 / Rn)
    As above, the total max. power dissipation is equal to the sum of all of the resistor's power ratings - IF (all caps) - the total power is divided proportionally to the resistors according to their individual ratings. So the total power possible is 1.5 W *only* if the three resistors are equal in resistance value. Since the resistors are in parallel, the voltage across each one is the same. For resistors of unequal values, you can calculate the individual power dissipations using Watt's Law for each resistor:

    P = E^2 / R

    ak
     
    hevans1944 likes this.
  5. SniffTheGlove

    SniffTheGlove

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    0
    Dec 22, 2016
    Thank you both for the replies and for helping someone who is struggling to learn. I appreciate this.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

    4,378
    2,046
    Jun 21, 2012
    We are here to help each other, whether newbies or "old pros" and everyone in between. It will take you awhile to become comfortable with circuit analysis and the jargon used in electronics. Google is your friend, but don't trust the information from just one site: cross-reference and verify the "information" you get from various websites. Come back here with specific questions as you make progress if find something that "stumps" you.

    Oh, yeah... almost forgot! Welcome to Electronics Point! Visit here often, even if just to "read the mail" without comment.
     
  7. SniffTheGlove

    SniffTheGlove

    9
    0
    Dec 22, 2016
    Cheers.

    I have always wanted to do electronics ever since I built a small flashing LED in electronics club at school in 1980 but I never did get my toes wet as I went straight into the Army and got involed with computer,programming and data communications networks.

    But now I am intrested in starting to learn and build my own simple electronic gear.
     
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