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I was reading a schematic for a timer circuit, and it made use of a capacitor and a transistor. The description below the schematic explained that the current through the base of the transistor was not enough to switch it on until the capacitor was charged. I am quite confused as to the behavior of capacitors during charging. I have read countless descriptions of the device, and I understand that it stores energy in the electromagnetic field surrounding its component plates. I still do not understand their behavior, though. I have come up with two hypothesis as to why the timer circuit is designed like this:

  1. The capacitor conducts electricity only while charging. While it is charging, the current is directed through the capacitor, ignoring the transistor and going back into the voltage source. After it has charged, the current no longer flows through the capacitor and flows through the transistor instead.

  2. The capacitor conducts electricity only while charged. While it is charging, the circuit is open and electricity flows through neither the capacitor nor the transistor, all of it ending up in the electromagnetic field of the capacitor until it is charged, when the capacitor is able to conduct electricity. Then, the current flows through both the capacitor and the transistor.

Of these two hypothesis based on what I have read, I am leaning towards #1. I would like to know if a capacitor conducts electricity while charging, and if one or neither of these are right, and which one if so. Thank you in advance - I just learned how a transistor works, and now I am struggling with capacitors. :)

Additional information: this question is asked with the perspective of someone who only works with direct current.

Here is a link to the diagram (I had to dig through my browser history): http://www.instructables.com/community/Timer-Circuit/

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  • \$\begingroup\$ Post the schematic diagram. It is much easier to discuss a circuit with the diagram. \$\endgroup\$
    – JRE
    Dec 24, 2016 at 22:33
  • \$\begingroup\$ That's one ugly diagram. If I were posting from my PC instead of my phone, I'd be tempted to edit a better diagram into your question. \$\endgroup\$
    – JRE
    Dec 24, 2016 at 22:42
  • \$\begingroup\$ Capacitors "conduct" by displacement current while being charged or discharged. \$\endgroup\$
    – Nix
    May 31, 2020 at 21:21
  • \$\begingroup\$ @UncreativeName your #1 above is correct, #2 is wrong. All capacitors are "paths for current." In Electrical Engineering, we'd say that, whenever a constant current goes through the capacitor, the voltage across the capacitor is constantly changing (constant-I creates a voltage-ramp.) PS if you want to stick with DC concepts only, then stay away from coils and capacitors (and filters and resonators, antennas, transmission lines, impedance, fields&waves ...all the good stuff ! ) \$\endgroup\$
    – wbeaty
    Jun 6, 2020 at 3:40

1 Answer 1

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For DC. It is not correct to say that the capacitor conducts current. Current is the movement of charge. Charge moves through the resistor and is stored in the capacitor like a reservoir, as it fills up the pressure (Voltage) increases. When the capacitor voltage reaches the base turn on level the current flows into the transistor turning it on.

In practice a current does move on the opposite side of the cap. This is to balance the charge that is building on the positive plate of the capacitor. It gives the appearance that the capacitor is "conducting" but the current does not flow through the capacitor because the dielectric between the plates is an insulator.

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  • \$\begingroup\$ So in DC circuits, current never flows through a capacitor? To use the water analogy (which I love btw), I can think of a capacitor as a dead end reservoir that reduces the pressure to almost nothing and as it fills up increases the pressure (voltage), and thus the amount of current flowing through, of the "pipe" leading into it? \$\endgroup\$ Dec 24, 2016 at 22:54
  • \$\begingroup\$ correct and think of the connection to the transistor as an overflow pipe set at the base voltage \$\endgroup\$
    – RoyC
    Dec 24, 2016 at 22:56
  • \$\begingroup\$ Thank you, I've never had an explanation of a capacitor that made sense to me but this one does =) \$\endgroup\$ Dec 24, 2016 at 22:58
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    \$\begingroup\$ This got marked as answered rather too quickly; capacitors do have a current flow of what Maxwell calls displacement current electronics.stackexchange.com/questions/276242/… \$\endgroup\$
    – pjc50
    Dec 24, 2016 at 23:07
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    \$\begingroup\$ @pjc50 The wikipedia entry is wrong. They are teaching grade-school physics misconceptions. They should instead say that displacement current is NOT A FLOW OF CHARGE, instead it's a current (just as Conventional Current is not a flow of any particular polarity of particle.) In the WP capacitor-entry, they should say that no CHARGE flows through the dielectric OF AN IDEAL VACUUM CAPACITOR. In capacitors with dielectric, charges certainly do move. Especially in high-K dielectrics such as PZT ceramic, the electron-current is thousands of times higher than the pure Displacement Current. \$\endgroup\$
    – wbeaty
    Jun 6, 2020 at 3:26

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