SMD Resistors

Discussion in 'General Electronics Chat' started by Dritech, Sep 13, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    5
    Hi all,

    I have a PCB with SMD components. I noticed that there are many resistors is parallel. For instance, two 300ohms SMD resistors are used as a current limiter for an LED. Why is that? Why they did not use a single resistor?

    Thanks in advance.
     
  2. ISB123

    Well-Known Member

    May 21, 2014
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    You mean voltage divider connection?
     
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    380
    hi Dri,
    Its possible the dissipation in a single SMT of that case size, does not have the required heat dissipation, so two, of double the resistance value are connected in parallel.?
     
  4. ISB123

    Well-Known Member

    May 21, 2014
    1,239
    527
    If they are connected in series then they probably had lots of 300 Ohm R but didn't have any 600 Ohm R so they connected them in series to save on budget?
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the replies. Ericgibbs answer makes sense for the LED current limiter resistor since this PCB is powered using 12V without a regulator.
    But I also noticed the same technique used with buttons. This PCB has surface mount buttons which are resistance coded (for instance when button A is pressed it outputs 300ohms... when button B is pressed it outputs 600ohms etc.). In this case power is not an issue since it will most probably be connected to a voltage divider and to an ADC. So in this case why are they connected in parallel? could it be budget wise or to increase efficiency when placing the SMD components?
     
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