This chapter is from the book
This chapter is from the book
This chapter is from the book
Charge Storage
Suppose we charged a capacitor up to 10 volts and then disconnected it from the circuit. How long would the capacitor hold the charge? Conceptually, a perfect capacitor would hold the charge forever. In practice the charge would decay through time, primarily because of what we call leakage resistance. Almost all capacitors will slowly leak charge internally from one plate to the other. The mechanism for how this happens is a function of the materials and the fabrication processes used. The speed with which this happens is determined by the magnitude of the leakage resistance, which is usually part of the specification for the capacitor or capacitor family. Leakage resistance is usually quite high and not an issue in high-speed designs except in very special cases.
If we had a charged, disconnected capacitor sitting on a workbench and wanted to see how fast it was discharging (how fast the charge was leaking off), this would be hard to measure. Most measurement tools (especially multimeters and scopes) absorb charge during measurement, therefore affecting the very effect you are trying to measure.
Certain metal-oxide-silicon (MOS) devices (e.g., some charge coupled devices) use little silicon capacitors as "charge buckets" to store digital information. Some devices can be designed to hold charge for a considerable time, allowing them to function as nonvolatile memories (i.e., memories that don't lose their information when the power supply is turned off).
A more serious consideration involves capacitors in high-voltage circuits normally associated with CRTs, laser printers, and copiers. These capacitors may be charged to many thousands of volts and may still hold this charge when the device is turned off. Good safety practice is to place a very high resistance "bleeder" resistor around such capacitors so that they will discharge safely within a few minutes. For various reasons, these resistors may be missing or ineffective in any particular circuit. Service personnel learn quickly (if they don't already know) to be very careful around such circuits when servicing them. An inadvertent shock from a highly charged capacitor is, at a minimum, uncomfortable, and can very easily be quite dangerous.