why the battery (DC) can charge the capacitor

Discussion in 'Electricians' Talk' started by cizhila, Sep 22, 2021.

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  1. cizhila

    cizhila New Member

    Hey, guys!
    I'm so confused that why the capacitor is DC blocking, but the battery (DC) can indeed charge the capacitor?
    I've never thought about this until I read it in the book recently. I really have no idea about it.
    If anyone knows why this happens, please tell me something about it.
    Thanks!
     
  2. cizhila

    cizhila New Member

    I've read some articles like this, but I still don't know how it works.
     
  3. MGW

    MGW Screwfix Select

    It will only stop DC once the capacitor is charged up. If you have a spring on a cable and handle to pull on it you can use energy to keep on expanding the spring, but if you fitted a ratchet on the cable you could only expand the spring once.
     
  4. sparky steve

    sparky steve Screwfix Select

    As far as a constantly applied DC voltage is concerned, a capacitor is open-circuit; those 2 ‘parallel plates’ aren’t connected so they don’t conduct anything. So, DC is “blocked”.

    When you apply DC, once the capacitor is charged no further current flows.

    But AC changes all the time, and so the electric field causes the charge build-up on the opposite plate to change all the time. So a capacitor will pass current back and forth. So a capacitor doesn’t block AC (although the value of Capacitance and other circuit parameters will determine how much current flows, and at what frequency).
     
    ElecCEng likes this.
  5. Bob Rathbone

    Bob Rathbone Screwfix Select

    Think of a battery and a capacitor as two water tanks, one empty, the other full. They are connected by a pipe at the base of each with a tap to control the water flow. When the tap is first opened, the water flows quickly into the meaty tank, but as the tank fills the water flow slows until when the water in each tank is at the same level, no more water flows, the second tank or capacitor is now fully charged.
     
  6. Lectrician

    Lectrician Screwfix Select

    The best example of this is the ring capacitor in a phone master socket. 50V DC is present on the phone line at all times, blocked via a capacitor from ringing the bell/triggering ringing. When someone calls you, the phone line goes AC, the capacitor then passing current to ring the bell/trigger ringing.
     
  7. ElecCEng

    ElecCEng Screwfix Select

    Until we all go VoIP! ;)

     
  8. Lectrician

    Lectrician Screwfix Select

    And all those old dears without broadband and no need for broadband will be sent little boxes to connect to their "line", which will provide a standard BT phone socket, so their VoIP service will be presented as a standard line ;).

    It is already possible to order a "SogEA" phone lines. Copper "phone" lines with no voice capability, "Single Order Generic Ethernet Access". Gives you your broadband and it's down to you or your supplier to provide VoIP handsets or SIP trunks into a PBX.
     
  9. Bob Rathbone

    Bob Rathbone Screwfix Select

    We are fully VOIP at RLML, (Retro Leisure Midlands Ltd) but the socket that the phone is plugged into still has the decoupling capacitor for ringing. Being a rural enterprise we also receive our internet connection through 'Airband', other providers are available. This is not intended a a 'plug' for the company. :)
     
  10. quasar9

    quasar9 Screwfix Select

    It’s not just the traditional phone line merchants , a very large mobile network outside UK, only supports data. The supplied SIM enables a voice over data app to be loaded and all voice calls are via that app. I learnt the hard way when on hols there, my iPhone SE would not make calls but data worked fine. A quick visit to the retailer revealed the app worked only for Android phones. I could use whats app, but that’s only good if the person on the other end is using the same. No good for calling the airline or them trying to call me !
     

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