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Capacitor Voltage question — Parallax Forums

Capacitor Voltage question

ElectronegativityElectronegativity Posts: 311
edited 2005-09-03 17:17 in General Discussion
I have a bunch of capacitors on hand that have voltages ranging from 6.3 to over 100 volts.
Everything I do is 5V so I have been using the lowest one I have available, but what's the difference?
Is it any worse to use a 16V 1uF capacitor than a 10V 1uF capacitor in a 5V circuit?

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I wonder if this wire is hot...

Comments

  • NateNate Posts: 154
    edited 2005-09-02 12:28
    The voltage ratings of capacitors indicate the ability of the dialectric between the capacitor plates to impeed the flow of electrons at different voltage potentials.· This means that it is fine to use caps that are rated above or way above the voltage of your circuit - the Farad rating of the cap is still the same.

    If you are building production circuits, you will want to keep the voltage overrating within reason to keep costs down.

    Nate
  • BeanBean Posts: 8,129
    edited 2005-09-02 13:24
    electro,
    It seem standard in the electronics industry to always use a capacitor with at least twice the voltage rating that it will see.
    For instance I would NOT use a 6.3Volt capacitor in a 5Volt circuit.
    Bean.

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-09-02 15:54
    Bean is correct.· You normally use caps rated for twice the working voltage as the circuit is rated for.· Sometimes you have to make exceptions though.· For example, my supplies in my audio amplifiers use 50V caps, even though the supply rails run upwards of 37 volts.· This is because the most common voltage levels for caps in that range are 35V and 50V.· Going up from there is expensive, and 37V is the highest expected value on the supply rails.

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  • ElectronegativityElectronegativity Posts: 311
    edited 2005-09-02 16:02
    Ugh, I just bought a big pack of 10uF 6.3V tantalum capacitors, and they weren't exactly cheap.

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    I wonder if this wire is hot...
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-09-02 16:25
    If you're using them in a regulated 5V system, then you shouldn't have a problem.· It's just a general guideline to use higher rated caps.

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    csavage@parallax.com
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2005-09-03 13:58
    The electrolitic capacitors age and loose their voltage rating in the process. Tantalums and ceramics are fairly inert and mostly useful as a noise filter.

    Doesn't a 10uf Tantalum equal a 1uf Ceramic in a filtering situation due to internal resistance? {I think I read that in a voltage regulator document}

    I got an adjustible voltage regulator kit that uses a 25volt filter capacitor on the 18 volts pulsating DC coming from the rectifier bridge.

    I got to thinking about this and I fear it just might explode under stress.
    I picked up a 50V with twice the microfarads, but have yet to swap it out.

    One has to be careful of how you count Volts. Pure DC without pulse is always simple. Volts are Volts. But.....

    Volt-RMS [noparse][[/noparse]root mean square] is an average used in AC, not the peak. For AC converted to DC, you might have 18volts DC RMS coming out of a rectifier, but that is 70.7% of the PEAK voltage. The capacitor sees the Peak Voltage as a destructive force. So 18/.71 = 25.35volts peak. So, I have no safety margin and the situation gets even worse in the house current is higher than standard.

    120 VAC/.71= 169 Peak Volts !!! in house current

    This is a good example of where it is a problem.

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  • KenMKenM Posts: 657
    edited 2005-09-03 17:17
    Technically,·RMS is the effective voltage or current, which is·higher than average.
    The RMS value is the amplitude of a sinusoidal ac current to deliver the same power as a particular dc current.
    Or stated another way....the equivalent DC value of a sinusoidal current or voltage is 1/(sqr-rt 2) or 0.707 of its maximum value.
    Kramer said...
    Volt-RMS [noparse][[/noparse]root mean square] is an average used in AC, not the peak.

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    Ken
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