Electronic – ny downside to using a larger than needed smoothing capacitor

capacitornoiseripplevoltage-regulator

I work with low-power DC voltage regulators. I am already aware of the formula to calculate the size of smoothing capacitor(s). This can be an iterative process of testing one size with a scope and then using a larger size or adding more until the scope shows acceptable (very low) levels of ripple and noise.

Besides the cost of the capacitors, is there any tradeoff to rounding up (a lot) and just using a very large capacitor(s) rather than trying to calibrate the sizing to "just enough" but not more than that?

Best Answer

As far as caps go, there are two competing requirements:long-term (ripple) and instantaneous (spike). A big electrolytic can give you the former but not the latter. Generally you parallel your large electrolytic with a smaller 0.1uF capable of supplying that instantaneous spike whilst the electrolytic lumbers into action. Or the 0.1uF may be for local decoupling to stabilise that regulator. If the specified capacitor is actually 0.1uF or smaller, then the intention of the capacitor is to supply small amounts of charge very fast. Do not replace this with a bigger electrolytic - that's definitely a case where larger is worse not better.

Going past that, you'll have to tell us what kind of regulators you're dealing with. If it's just a basic linear regulator then it doesn't really matter. If you have a switching regulator though, the capacitor will affect the resonant frequency of the switcher, so be very careful there.

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Electronic – How to avoid ±voltage on capacitors in astable multivibrator

Have you considered bipolar electrolytics? Normal electrolytics only have one plate doped (if that's the right word), so they can only withstand a unipolar signal (in reverse they act as a short, if I recall correctly, which is what causes them to bulge and eventually vent/explode.) Bipolar electrolytics usually internally have two capacitors connected in inverse series, or, in some cases, both plates equally doped. You can create the same effect in a pinch by wiring two electrolytics in inverse series, like this.

  |  / +  + \  |
--| |--------| |--
  |  \      /  |

You can also get axial ceramics in values up to around 1µF, and bigger in surface mount sizes (I use 22µF 6.3V 0805 capacitors.) For example here is a 1µF cap rated for 50V.

Electronic – Low Voltage AC Smoothing Capacitor Calculation

Your calculations are good but if you are using a full bridge rectifier (diode bridge, I think?), your frequency is the double of the input, so here 120Hz.

Moreover, there is the voltage drop of the diodes, so with a full bridge it's roughly 1.2V. Hence Vp = 9-1.2 = 7.8V.

Notice that a ripple of 10% is very good. Usually we add a regulator after the capacitor to have a better constant voltage supply.

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Electronic – How to avoid ±voltage on capacitors in astable multivibrator

Electronic – Low Voltage AC Smoothing Capacitor Calculation

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