Capacitor Charge Drain and Surge Impedance

In summary, if a cable has a characteristic impedance of 84 ohms and a DC resistance of 840 ohms, and a capacitor charged at 12 volts is connected for a short time, it will draw 1/7th of an amp from the capacitor. However, this only applies to the first 10 cm of the cable, as the signal propagation speed is too fast for the DC resistance to have a significant impact on the current.
  • #1
kmarinas86
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1
If I have a cable of 84 ohms of characteristic (or surge) impedance (which is independent of length) and whose DC resistance is far greater than 84 ohms (let's say 840 ohms), would I get, by hooking up a capacitor charged at 12 volts across it for a very short period of time (let's say 1 nanosecond, so not enough time for the capacitor to lose any significant charge nor any significant voltage), draw 1/7th of an amp from that capacitor for that period of time? Does this really mean that charge is removed from the capacitor at the rate of 1/7th of an amp?
 
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  • #2
Yes.
 
  • #3
Signal propagation speed is approx 2*10^5 km/s, or 20 cm/ns. For the DC resistance in a section of the cable to have any influence on the current from the capacitor, the signal must propagate to that section, and back.

This means that only the DC resistance of the first 10 cm of the cable is important, and it is probably much smaller than 84 Ohm.
 

1. What is a capacitor charge drain?

A capacitor charge drain refers to the process of discharging a capacitor's stored electrical energy. This typically occurs when the capacitor is connected to a circuit and releases its stored charge to power devices or components within the circuit.

2. How does a capacitor charge drain affect circuit performance?

A capacitor charge drain can affect circuit performance in several ways. If the capacitor is not properly sized or has a high leakage current, it may drain too quickly and cause the circuit to malfunction or shut down. On the other hand, a well-designed capacitor charge drain can help regulate voltage levels and provide a stable power supply to the circuit.

3. What is surge impedance?

Surge impedance is a measure of the resistance of a transmission line or circuit to a surge or sudden change in voltage. It is determined by the characteristics of the circuit, such as the type of conductor and insulation material, and can affect the transmission and distribution of electrical energy.

4. How does surge impedance impact capacitor charge drain?

Surge impedance can impact capacitor charge drain in a few ways. The surge impedance of a circuit or transmission line can affect the rate at which the capacitor discharges, as well as its ability to withstand sudden changes in voltage. A high surge impedance can also cause voltage spikes, which can damage the capacitor and other components in the circuit.

5. Can surge impedance be controlled or adjusted?

Yes, surge impedance can be controlled and adjusted through various methods. This can include using different materials for the conductors and insulation, changing the layout or configuration of the circuit, or adding protective devices such as surge arresters. Properly controlling surge impedance can help ensure efficient and reliable capacitor charge drain and overall circuit performance.

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