capacitors without resistor

Discussion in 'General Electronics Chat' started by Gadersd, May 22, 2013.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    I was wondering, can capacitors be directly connected to a power source without a resistor? I know that they short circuit when they are first charging. Would this damage the capacitor?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Depends. You have to provide specifics like, type, model, value of the capacitor and value of the voltage. If the voltage is high enough AC the cap will explode quite dramatically.

    Once we had an annoying chap who would fiddle with every knob even when given orders not to. So we wired a cap to AC and a toggle switch with a label saying DO NOT TURN ON. It was funny to watch him play with that switch.
     
    Last edited: May 22, 2013
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Capacitors are used to filter power supplies all the time. An additional (smaller) capacitor is suggested across the power supply pins on every logic chip, microcontroller, voltage regulator or just about any piece of silicon with more than three pins.
     
  4. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    The voltage is 5v and the capacitance is 0.1uF. If the data sheet says that the maximum voltage is 50v does that mean it can be directly connected to 50v dc.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Yes. 0.1μF cap across the power rails is very common practice. However it is good engineering design to give the cap some spare room. Hence you would use a cap rated for at least 63V if using across 50V.
     
  6. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    So it is fine to directly connect any capacitor to a power source with a voltage higher than the capacitor's maximum voltage?
     
  7. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    No!! You must use a capacitor rated HIGHER than your power supply. And don't forget, for AC work, your rating should be above the PEAK voltage (as AC is usually measured by the RMS average).

    A higher working voltage is better. Unless they are electrolytic caps, in which case you use 'the next highest rating' over your power supply.

    Using a cap rated LOWER than your power supply will likely result in a big POP and smoke when the cap fails. :D
     
  8. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    Sorry, I meant lower instead of higher.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Geez... did I say that? I don't think so!
     
  10. pbrannon

    New Member

    May 22, 2013
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    The purpose of the resistor on the capacitor is so that when they applied voltage is disconnected it will bring anything or voltage inside the capacitor reducing the risk of electric discharge.
     
  11. #12

    Expert

    Nov 30, 2010
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    Capacitors are connected directly to power sources all the time. I don't mean a lot of people "break the rules", I mean it is the proper practice to connect the capacitors directly to the voltage supply.

    Let's do the math.
    5 volts and .1 uf
    In order to charge the capacitor to 5 volts, it will require 5 times .1 micro amp second. That's point five microamps for one second or five amps for one tenth of a microsecond or any combination like that. Most small regulators will not pass 5 amps so they will not charge the "short" in one microsecond. Your average 7805 regulator will pass about 1.5 amps for a short time before it heats up. That means it can charge your .1uf capacitor in 3&1/3 microseconds.

    Do you see that the regulator is refusing to allow "short circuit" current? A million amps for 5 millionths of a millionth of a second?

    Now, suppose you already have a 100,000 uf capacitor charged to 5 volts and you connect it to the .1uf capacitor with a 10 gauge wire. That size wire has 1 ohm per thousand feet. Suppose you use a half foot of wire. The resistance of the wire is .0005 ohms. 5 volts through .0005 ohms is 10,000 amps, and the time required to charge the little "short circuit" (the .1uf capacitor) is 1/2 of a nanosecond.

    All that, and you still don't have enough energy to light a match.

    Tiny capacitors do not short out entire cities. There is resistance in the wires on the power poles. There is resistance in your circuit breaker box. There is resistance in your 5 volt transformer.

    Starting to get a feel for this?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    When I worked on satellite programs there were certain sized ceramic capacitors that NASA determined could fail do the the high current when a voltage was instantly applied to the cap (such as when a relay applied power from the satellite battery to the input power). So it was required that a small resistor be placed in series with the cap for any high reliability application.
     
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