Quick question about make before break switches and capacitors

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Discussion Overview

The discussion revolves around the behavior of capacitors when connected to different types of switches, specifically "make before break" (mbb) and "break before make" (bbm) switches. Participants explore the implications of these switch types on the initial voltage across the capacitor when switching circuits, focusing on theoretical aspects and circuit behavior.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that when a capacitor is switched using a make before break switch, the initial voltage across the capacitor (v(0-)) will equal the voltage of the source it was connected to.
  • Others argue that if a break before make switch is used, the initial voltage across the capacitor would be zero, but this depends on the specifics of the circuit.
  • One participant notes that the behavior of the capacitor during switching is contingent on whether it is connected to another hard voltage source, emphasizing that a capacitor cannot instantaneously change its voltage unless connected to another hard voltage source.
  • Concerns are raised about potential issues when connecting two hard voltage sources simultaneously with an mbb switch, suggesting that this could lead to complications.

Areas of Agreement / Disagreement

Participants express differing views on the effects of the switch types on the capacitor's voltage, indicating that the discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

The discussion highlights the importance of circuit details and assumptions regarding the connections made by the switches, which are not fully specified in the posts.

kl055
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Homework Statement



A capacitor is initially connected to a voltage source in series for a long time. At time t0 the make before break switch switches to another series circuit which includes the capacitor. This means that the capacitor will have an initial voltage v(0-) = voltage source, correct? And if the switch was a break before make switch, the initial voltage v(0-) across the capacitor would be zero, correct?

Homework Equations





The Attempt at a Solution

 
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kl055 said:

Homework Statement



A capacitor is initially connected to a voltage source in series for a long time. At time t0 the make before break switch switches to another series circuit which includes the capacitor. This means that the capacitor will have an initial voltage v(0-) = voltage source, correct? And if the switch was a break before make switch, the initial voltage v(0-) across the capacitor would be zero, correct?
It would depend upon the details of the circuit and what the switch is accomplishing when it commutes. A diagram would help.

It might help you to know that if a charged ideal capacitor is removed from a circuit (so that one or both of its leads are no longer connected so that there's no closed circuit for current to flow into or out of the capacitor), then it will retain the charge it had at the moment of disconnection indefinitely.
 
kl055 said:

Homework Statement



A capacitor is initially connected to a voltage source in series for a long time. At time t0 the make before break switch switches to another series circuit which includes the capacitor. This means that the capacitor will have an initial voltage v(0-) = voltage source, correct? And if the switch was a break before make switch, the initial voltage v(0-) across the capacitor would be zero, correct?

The answer depends on the circuit the capacitor is switched to. Assuming it's not switched to a second hard voltage source, the capacitor voltage would be the same before and after the switching regardless of whether the switch is mbb or bbm: v(0+) = v(0-). That's because a capacitor can't instantly change its voltage unless switched to another hard voltage.

If it's switched to a second hard voltage source a mbb switch would cause instant chaos since you'd be connecting two hard voltage sources to each other. If it's bbm then v(0-) would be the first hard voltage and v(0+) = second hard voltage.
 
Thanks for the replies. I took my electric circuits course a long time ago and I need a refresher. I think I got it now.
 

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