Calculate Discharge Resistor for StrongLink SL030 RFID Reader

Discussion in 'General Electronics Chat' started by Hurdy, Sep 22, 2014.

  1. Hurdy

    Thread Starter Senior Member

    Feb 27, 2006
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    I'm currently in the process of researching and developing a little RFID based project, with the StrongLink SL030 being my reader of choice.

    Whilst reading the documentation it states that the SL030 should be used in conjunction with a discharge resistor to ensure the device restarts correctly after power loss, which I guess makes perfect sense. The document in question can be found at http://www.stronglink-rfid.com/download/AN110221.pdf . The company is Chinese so you will have to forgive the poor English translation.

    My question is, how would you go about calculating an appropriate discharge resistor value when a capacitor value is unknown? The supply voltage is about 3.3V.

    From what I have read so far, calculating the discharge time can be done as t(in microseconds) = R (ohms) x C (in microfarads). The source of this equation can be found at http://www.tpub.com/neets/book2/3d.htm.

    The SL030 manual can be found at http://www.stronglink-rfid.com/download/SL030-User-Manual.pdf

    Any advice on this would be greatly appreciated! Finally, thank you for taking the time out to read this post.

    Rob
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    T2 ≈ 3 * 10 -7 * R implies there is ~ a .3 Ufd. inside the chip so with a 10k resistor it would reset in 3 ms.
     
  3. Joseph Rafferty

    New Member

    Jul 2, 2015
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    Quick correction: 3 * 10E-7 * 10K is 30 ms (0.03s). A 1K resistor would discharge in 3ms.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    For a capacitor discharging into a resistor, the R-C time constant (R x C) is the time it takes for the cap to discharge to 37% of its initial voltage value. For multiples of R-C, the cap discharges more and more. It is an exponential relationship.

    https://en.wikipedia.org/wiki/Time_constant#Time_constants_in_electrical_circuits

    1 x RC discharges to 37% of the initial value.
    2 x RC discharges to 13.5% of the initial value.
    3 x RC discharges to 5% of the initial value (95% discharged).

    In the app note the goal is to discharge to 0.2 V from 3.3V. This is 94%. I suggest that the capacitor value is 0.1 uF (10^-7 uF) and "3" is the multiplier to get to 95% discharged. Three time constants = 95% is a common rule of thumb in analog electronic circuit design.

    ak
     
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