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How far can you stray from capacitance and voltage?

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    How far can you stray from capacitance and voltage?

    I've heard you should stick pretty close to the same farad rating but voltage can go higher. I'm new here and this could be in a sticky somewhere but I figured I'd ask.
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    #2
    Re: How far can you stray from capacitance and voltage?

    Originally posted by jayjr1105 View Post
    I've heard you should stick pretty close to the same farad rating but voltage can go higher. I'm new here and this could be in a sticky somewhere but I figured I'd ask.
    Depends on the use in the circuit.

    Caps that are just used for "bulk decoupling" can usually safely be increased in capacitance -- with the caveat that often the device size and lead spacing may increase beyond what an existing board can tolerate (imagine if the cap is surrounded by other devices and can't be "enlarged"). Voltage can go up but often not down (most caps are spec'd at the lowest working voltage that is acceptable for their role in the circuit -- you won't find a 25V cap on a 5V signal)

    Caps that have a "timing" role aren't as easy to handwave. If the cap is used in an RC timing circuit, then increasing C can cause the associated time to adversely be increased.

    Note, here, that timing also applies to the device's impedance (frequency being 1/time) when dealing with AC signals. Making a cap larger can lower its impedance at frequencies that you were hoping it would ignore. E.g., put a cap in series with a speaker and you will restrict the range of low frequencies that can pass through the cap and be reproduced by that speaker. Put too large a cap and you risk letting through frequencies that can physically damage the speaker!

    Note that some caps are deliberately small to act as high-frequency filters. You will typically encounter "bulk" decoupling capacitors to help stabilize the DC voltages in a circuit. But, there will be smaller caps alongside that are intended to shunt higher frequency "noise" directly to ground -- that the larger cap wouldn't be as effective at suppressing.

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      #3
      Re: How far can you stray from capacitance and voltage?

      Wow, thanks for the thorough explanation. I was really just asking about their use as ripple control and smoothing out voltage but you went above and beyond! I learned something new today I guess.

      So replacing them in power supplies, motherboards (power delivery), it's typically okay to go higher on voltage and farads? Just have to watch lead size and barrel size.
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        #4
        Re: How far can you stray from capacitance and voltage?

        and if it is a switching power supply you need to stick with low ESR caps on the DC side
        i had no idea ESR even existed before joining here a few weeks ago, all i knew was brand name, voltage, and capacitance; now i have a tool for reading esr (and consequently alot of other stuff)
        Last edited by evilkitty; 01-04-2018, 09:25 PM.

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          #5
          Re: How far can you stray from capacitance and voltage?

          Yes, I did hear to stick with low esr ones for motherboards and delicate DC stuff.
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            #6
            Re: How far can you stray from capacitance and voltage?

            Originally posted by jayjr1105 View Post
            I learned something new today I guess.
            Then today was a good day!

            So replacing them in power supplies, motherboards (power delivery), it's typically okay to go higher on voltage and farads? Just have to watch lead size and barrel size.
            Lead SPACING will typically hurt you first. It's so easy to get caught up in finding a vendor, temperature rating, "durability", capacitance and voltage that you often forget to check the physical characteristics -- until you're sitting there with a hot soldering iron and a component that doesn't FIT!

            ("How could I have failed to take this into consideration???")

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              #7
              Re: How far can you stray from capacitance and voltage?

              Wait lead spacing can make it not fit at all? i can see the cap being raised off the board a mm maybe 2 depending on the size difference unless you have a very tight height limit is that really a problem? i know some caps are laid over sideways so the leads are not butted right on the board

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                #8
                Re: How far can you stray from capacitance and voltage?

                Originally posted by evilkitty View Post
                Wait lead spacing can make it not fit at all? i can see the cap being raised off the board a mm maybe 2 depending on the size difference unless you have a very tight height limit is that really a problem? i know some caps are laid over sideways so the leads are not butted right on the board
                It depends on how much vertical space you have to play with. Often, electrolytics are the biggest/tallest devices on a board. Any cover that is present (i.e., the case that the board sits in) will typically not have much room to spare as folks strive for sleeker designs and smaller cases.

                So, if you can't afford the vertical space to let the cap float above the board, you are then stuck with trying to lay it over on its side. And, might find that you can't do that, either, because of adjacent components. Or, have to use an axial lead package and stand it up "japanese style".

                You may also find that some manufacturers will squeeze a cap that is borderline in terms of "volume needed for this voltage and capacitance" into a smaller case. And, you won't find any other vendors who offer that same package option!

                Or, you'll be looking for a 5000Hr component and discover its not available in the "size" you want.

                Its considerably easier to pick parts for repairs than it is to pick for new designs -- the original designer has already made a choice for you! For new designs, there are added degrees of freedom in the process. So, you stare at a list of options and have to juggle tradeoffs -- pick a tall component, here... then, later discover it is the single tallest component on the board and has implications in the design of the enclosure. So, back to the drawing board to see if you can change the design to accommodate a shorter component, etc.

                Moral of story is to verify all aspects of a component in a particular application and not just the "obvious" ones!

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                  #9
                  Re: How far can you stray from capacitance and voltage?

                  forget lead spacing, and just watch the diameter.
                  and in height-restricted cases, the height.

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                    #10
                    Re: How far can you stray from capacitance and voltage?

                    Originally posted by stj View Post
                    forget lead spacing, and just watch the diameter.
                    and in height-restricted cases, the height.
                    You will inevitably encounter a situation where you have space to insert a component with a larger diameter -- dropping a 35V device in as a replacement for a 25V device (cuz you don't have any 25V devices on hand at that rating).

                    THEN, you'll discover that the leads are farther apart than the pads in the PCB so that fitting the device means it is now effectively taller. Then you'll find the cover won't fit (or the EMI shield) and you'll be looking for a way to lay the device on its side (though you'll have to remove the first device that you soldered in place cuz its leads are now to short to bend over) as a tenth of an inch (typ the minimum you need to lift a device to force its radial leads into pads spaced incorrectly) is a lot in most newer kit.

                    IME, it's just easier to do ALL of your homework before plugging in the iron.

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                      #11
                      Re: How far can you stray from capacitance and voltage?

                      Some linear regulators and even some switching regulators require capacitors on the output with some MINIMUM ESR

                      For example, the 1117 series of linear regulators is known to require at least 0.1 ohm ESR on the output ... the circuit may use a small capacitor (10..100 uF) which has a high enough ESR value but if you use a higher capacitance or a capacitor with higher voltage rating, that capacitor may have a much lower ESR potentially causing oscillations and overheating of the regulators.


                      You also want to pay attention when the circuit uses an electrolytic capacitor in combination with some ceramic capacitors or tantalum capacitors on the input and/or output. The value may be chosen to work well with the other capacitors for decoupling and general filtering so you wouldn't want to change that value too much or you'd want to pick an electrolytic capacitor that behaves relatively the same at certain frequencies.

                      See this video, it's short and guy is good at explaining things: https://www.youtube.com/watch?v=9EaTdc2mr34

                      Also see this video about using multiple different ceramic capacitors for better decoupling of ICs : https://www.youtube.com/watch?v=BcJ6UdDx1vg


                      In GENERAL, using a higher voltage rating capacitor should be fine, but keep in mind that bigger can sizes (more volume) typically means higher lifetime rating but also lower ESR.

                      You may also want to avoid using very low ESR capacitors or polymer capacitors in audio amplifier circuits, in the audio path. For a lot of people, more "average" capacitors (like the "somewhat low" or "was very low esr around 10 years ago" Panasonic FC series) sounds better than capacitors which are advertised for use in switching power supplies.

                      On the output of regular switching power supplies, it's usually safe to go up one step in capacitance (where one step is that 20% tolerance), for example from 820uF should be fairly safe to go with 1000uF

                      It can be bad in some cases if you put too much capacitance... for example, an empty capacitor would "suck" a lot of energy all of the sudden for that brief period (milliseconds) and the chip producing the lower voltage could enter into current limit mode... in extreme cases, some switching regulators with integrated mosfets could have the mosfets inside them blow due to excess current draw. But anyway, by too much i mean going from 470uF-1000uF to 3300uF or more, something like that... rare in practice.

                      With old style isolation transformers, toroidal transformers (for example let's say a linear power supply for a class AB audio amplifier) , too much capacitance could cause a high current burst on the primary which could pop the fuse on the primary side, or trip the fuse in your house electrical panel

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