How do I decide what capacitor to use in a circuit?

Question

I am using a voltage regulator, and to get cleaner power, the datasheet recommends using a 0.33uF capacitor. However, it doesn't say what type it wants. Stupidly, I went out and bought a 10 pack of 0.33uF 50V Radial Electrolytic Capacitors. After looking up on this site, I found that the symbol means that it is a unpolarized capitator. Will they work because they are polarized? Does this really affect anything in this circuit?

Also, there is a 20% tolerance. Will this affect this circuit?

And so I don't have to ask a similar question again, how did you get that? I know they have different tolerances and ratings depending on the material but does it really matter?

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Transistor datasheet:

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I can get the voltage regulator datasheet if anybody needs it. Thanks in advance.

Answer 1

Relax...

It will work just fine. Make sure to orient the positive terminal towards the input pin of the regulator (pin #1).

To be extra sure, use two in parallel (since you have them anyway!) to reduce the equivalent series resistance (ESR). That's just a precaution since I haven't read your datasheet and electrolytic types can be higher ESR than ceramics.

Also, I'm assuming your maximum input voltage is less than 25V.

Sizing...

The values given are the minimum value needed for stability plus a little margin (usually).

The regulator is a closed-loop system. It watches what happens on the output and adjusts "stuff" internally to make sure the output (really a scaled-down version of the output) always equals a desired value.

Problems occur when it starts chasing its tail. If, as a result of it changing "stuff" internally, the input voltage also starts to change (or the output changes too quickly) then the changes the regulator made will have too much of an effect and it will have to undo the excess.

This corrective change can also overshoot the mark, requiring another corrective change... as you can see, without sufficient "stability" in the system, the regulator can output a continuously fluctuating voltage rather than the flat line you hope for when employing a regulator.

The capacitors slow down voltage changes, thereby helping to ensure overall stability.

Answer 2

In the case of the 7805, which is unconditionally stable, the type and value of the capacitors is not very important- as the note says, you can leave them out entirely and it will 'work' and will not oscillate. It will work better with the minimum capacitances shown and will be happy with much bigger caps (47uF on the input and 1uF-10uF on the output- especially ceramic caps on the output- is always good with 78xx). However, assuming this is a bad habit to get into as most modern regulators are not so forgiving.

The type is not as important as the value, the voltage rating and the ESR. Generally, lower ESR is good, but some older LDO (low drop out) regulators don't tolerate capacitors that are too bad or too good all that well, and can oscillate. Use a value that is too low and they can oscillate. Use a voltage rating that is too low and they can fail early. Usually there is no penalty (other than cost and size) to use a higher than necessary voltage rating, nor to use a somewhat larger than necessary value of capacitance (often tolerances of electrolytic caps are -20/+80% so they might be bigger than marked anyway).

If you have fast logic and that sort of thing you'll want to use a ceramic capacitor (with its low ESR) or a low impedance electrolytic, possibly in parallel with a lower value ceramic capacitor. The input capacitor ESR isn't usually too important on linear regulators, but lower is always better there.

In some cases you may actually have to degrade a ceramic (output) capacitor by adding a resistor of around an ohm or three in series to keep the regulator happy!

In the circuit you show the regulator will be stable and will work with capacitors from 0 to thousands of uF on the input or the output (a 0.33 electrolytic or 0.1 ceramic or greater on the input is advisable if you're far from the input filter cap). If you've got a microcontroller or logic on the output 0.1uF and/or 1uF caps on the output near the loads are required.

The first time you pick up a new regulator datasheet, read it carefully, and search for "capacitor" and "stability" to ensure you've not missed anything. Look at any related graphs very carefully. For example, the 7905 (negative 5V linear non-LDO regulator) does require a minimum capacitance on the output for stability.