the capacitance? Are there other resistances that should have been taken into ac
ID: 776390 • Letter: T
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the capacitance? Are there other resistances that should have been taken into account? How would you expect the capacitance values from the individual resistors and the parallel combination to be affected by such resistances? Question 4 The following warning is found in a car-repair manual: "Disconnect and isolate the battery negative (ground) cable, then wait two minutes for the system capacitor to discharge before performing further diagnosis or service." What is the reason for this warning? How might the suggested waiting time be related to the RC time of the circuit? How would the discharge time change if something (for example a screwdriver) with a substantially smaller resistance were to touch both leads of the capacitor?Explanation / Answer
A. Since the negative terminal is almost always connected very solidly to the chassis, if you try to remove the positive terminal with a (conductive) wrench and the tool touches the chassis hundreds of amperes will flow, causing the wrench to get red hot.
B. The fastest limiting case for discharging the capacitor is when any external resistance is set to zero. Then the only limiting resistance becomes the internal equivalent series resistance (ESR) of the capacitor. The RC time constant of a capacitor is based on the R being the ESR.
C. After adding a small resistance between the leads of the capacitor, the time constant will decrease with respect to the time constant of 0 resistance.
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