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

1. ### Hurdy Thread Starter Senior Member

Feb 27, 2006
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0
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.

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
3,404
1,492
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
4,708
1,301
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