Parallel resistance and a counter intuitive application

Discussion in 'The Projects Forum' started by meld2020, Mar 14, 2016.

  1. meld2020

    Thread Starter New Member

    Mar 14, 2016
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    i have a 2 conductor reed switch device (meter) that suggests adding a series resistor after Vcc to increase the life of the sensor. I hadn't done that before, and now I am wondering if the device not ticking is because i fried the magnetic pickup.

    Anyhow, I want to put a 10k in series with Vcc but in the production model of this test there will be 6 of these meters running via the same 24v source. It seems counter intuitive that the resistors are in parallel when looking at the electrical enclosure and envisioning them separately, but in reality, they share the same ground and source, so will I need to find resistors that equate to 10k net resistance after the parallel calculation? Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
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    A picture or schematic would be worth a thousand words. I can't understand your question.
     
  3. meld2020

    Thread Starter New Member

    Mar 14, 2016
    21
    0
    I am using a discrete input module (sinking) to detect pulses from a meter and counting them. There are many of these meters, thus many of these circuits, but they will all operate off the same non-isolated 12V dc-dc regulator I am using to step down from my main 24V source. As stated, they all share common. I am sinking this 12V pulse into the module. According to the literature, it detects as low as 10VDC. I had been counting pulses before as well, so I think I'm okay there.

    What I'm fixating on is the meter circuits all sharing the same 12V source and the same common. Wouldn't this put them in parallel? Thus, if the spec sheet for the pulser asks for a 10kOhm resistor to drop this to just over 2mA, should I be specing out a much higher resistor since 6 of these circuits will be running, thus a parallel resistance calculation?

    Is this more clear?
    Thanks
    M
     
  4. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    What I see is an input module with a resistor in series with each switch. Each input is independent of every other input. They aren't going to share current. They aren't in parallel with each other.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,788
    945
    You have a resistor that 'sets' a limit on current.
    You have several of these limited circuits in parallel.
    If more than one is active, the current draw becomes additive.
    This increased current draw is equivalent to a single lower value resistor, but that is not affecting the current limit you have set in each circuit.
    It just tells you what resistance would allow the same amount of current to flow if you replaced your device with a single resistance to ground.
    Use it to determine your maximum possible load when sizing a power supply.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,387
    1,605
    Follow the spec sheet as if only one sensor was there.

    Make sure the power supply has enough current to handle them all.

    Your resistors are not in parallel, the resistor and sensor combos are in parallel. Treat each one as if it was the only one out there.
     
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