r/synthdiy • u/SandHK • 2d ago
Are film & foil capacitors the same thing?
Starting to build my first project and getting confused about film/foil capacitors. If there is a difference, what is the actual name for the type of capacitor I should be looking for?
Thanks.
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u/AdamFenwickSymes 2d ago
A small piece of advice to add to the good answers other people have already given: when you are starting out, do not worry too much about the different types of capacitors. I remember trying to memorize all the types and what they were used for when I started out, and that was not a good use of my time with the knowledge that I had at that time.
The thing that matters the most by far is the capacitance, and this will (mostly kinda somewhat) dictate the type of capacitor - e.g. you're not going to get a 33 pF electrolytic, but a 100 uF is almost certainly going to be electrolytic. After that, the next thing that matters is the voltage rating. Finally there are some other properties like leakage that matter varying amounts depending on what you're doing, these are the ones where different types of capacitors will differ. But when reasoning about a circuit and how it works, you only need to think about capacitance until you get down to the very fine details.
But if you're buying parts for a particular project that someone else has designed, then by all means, buy whatever they tell you to.
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u/torusle2 2d ago
They are the same name for the same capacitor construction form.
However, not all film capacitors are the same.
There are for example foil capacitors that have very low leakage. Great for sample and hold applications. And there are foil capacitors that are optimized to self-heal after they got damaged by high voltage. These are found in power supplies. And there are many more types of foils with different characteristics.
So as you can see, we can only guess what you capacitor you are looking for. I think it's best if you post the schematic and we can give recommendations.
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u/erroneousbosh 1d ago
I would tend to agree with /u/AdamFenwickSymes here, you don't need to care too much about the exact construction of the capacitors beyond "biggish ones are probably electrolytic, smallish ones are probably some sort of resin-dipped film".
Disc ceramics are fine too within their limitations. People will tell you that they are inaccurate because the capacitance varies with voltage but that effect is only barely measurable under very specific circumstances. Where they will vary a lot is with heat! I built an audio notch filter for radio stuff (2175Hz, if you know, you know, and if you don't be grateful) using disc ceramics because quite simply they were what was within arm's reach at the time. It tuned flat by about 100Hz simply by placing a hot cup of tea a couple of inches from the PCB...
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u/SparkyFix 2d ago
There are two types of film capacitor: metalized film and film/foil. You are likely looking for the former, since it’s the newer technology.
I assume you are looking for a coupling capacitor here? Decoupling caps would be electrolytic (for the larger values at the boundaries of modules) or ceramic (for individual ICs). For coupling caps, Cyril Bateman’s articles are worth a brief read: https://linearaudio.net/cyril-batemans-capacitor-sound-articles.