Capacitors are one of the most common components in electronic devices, but not all capacitors are made equal. The capacitor model you were taught were two metal plates are very near to each other and use some dielectric material in the middle might not be as accurate as you think.
Also, the ideal capacitor of your schematics in your ideal world of the non-existence is not an accurate description of the reality, where things (physical devices) have size, age, cost, temperature-dependant behavior, etc.
This post is a short descriptions of the pros, cons and dangers of different capacitor kinds you can use as a hint for your decisions when you work on your designs.
First of all, there’s always a Wikipedia article that explains anything you would like to, but it’s not always done the way you would. The classification image is, in this case, a great resource to start if you want. Find it in this link:
You can read a lot about the internal structure and about how do all those kind of capacitors work in the inside, but this guide is mostly about the pros and the cons and what you should check in the first look.
There are other kinds that are not mentioned here but they are not as common as these. If you want to add something or you see something wrong, please contact (contact info at the bottom of the page) and this list will be updated.
Electrolytic capacitors are mostly crap. But they have some uses though because they have a very high capacitance.
You probably have seen some of this cylinder-looking motherfuckers here and there. They are fairly common. They come also in other shapes but those cylinders are easy to spot.
- Extremely high capacitance. Up to mF or F!
- They are polarized, so you can’t install them in any direction in your circuit. If you install them incorrectly they might blow up1 (around 1V of reverse voltage is enough to have some fun).
- They have a maximum ripple current they can handle, if you exceed it they’ll dry out faster and can, again, explode.
- You might seen in old equipment some electrolytic capacitors with a residue of some liquid that have been dried around them. That’s the liquid they have inside to ensure their operation. When they are old they dry out or they leak and their ESR2 sky rockets.
- Putting them near to a heatsink in your circuit will make them dry faster reducing their (already short) lifespan drastically.
- On top of that, their temperature variability is quite high, meaning that their capacitance varies with the operating temperature.
- WARNING: They can rebuild their charge after being shortened so be careful because you can get a shock.
- Use many of them in parallel to reduce their negative effects.
- Use some of them in series (facepalm) to reduce the voltage they have to handle. If one of them breaks down for a reason the other will explode, so you need to put some resistance in parallel and blah blah blah. Avoid this shit, seriously.
- Use the highest quality ones. The cheap ones are even easier to damage and their lifespan is too short.
- Use low ESR versions if available.
- Best solution: Don’t use them.
- Mostly high-current and low-frequency circuits: Power supplies, large amounts of energy storage, audio amplifiers…
Specific notes about Tantalum capacitors:
- Tantalum capacitors are much more stable than the aluminum ones and they have even more capacitance per unit of volume. As an example: the Panasonic 6THB470M has a 470uF (±20%) capacitance for a 7.8 x 4.3 x 3.8mm SMD device. Crazy high.
- The material to make them (tantalum) is hard to find so there have been shortages that made the capacitors really expensive in the past3.
- They are also flammable, so using them out of the specs is very dangerous.
Ceramic (multi-layer)4 capacitors
Ceramic capacitors are one of the best in business and they are the most popular, but have to be chosen carefully.
- Widely used, valid for almost any application
- As they are made with thin layers of a ceramic material, they are all very brittle so you have to be careful when using them, because they might crack.
They come in two classes depending on their dielectric type: Class I and Class II, that we have to describe separately because they are quite different.
The first class are the best ones, period.
There are different kinds depending on their dielectric material: NPO, COG and more, but they all are similar.
- Very stable on voltage, temperature and even frequency
- Low losses (high Q)
- Almost no aging
- Low values
- Anything that doesn’t require high capacitances
- High Q applications: PLL’s, oscillators, filters…
The class II groups many different dielectrics. They are described by a three character string: X5R, X7R, Y5V…
- The first character indicates the minimum operating temperature.
- The second character (the number) indicates the maximum operating temperature.
- The third character indicates the temperature coefficient.
- Higher values than Class I
- Lower accuracy and stability than class I
- Higher losses than Class I, but still decent
- Aging process (measurable effect)
- Depending on the dielectric you choose, they can be very (VERY!) temperature-dependant (check explanation above).
- They can have some microphonic effect due to the piezoelectric materials they have inside. Like a microphone, they can convert vibration to voltage and the other way around. Depending on the working frequency you might actually hear them ring.
- Any use where class I is an overkill (precision is not needed…).
- As their cost is decreasing thanks to the technology they started to replace Electrolytic Capacitors as bypass capacitors, switching power supply internals and others.
Plastic film capacitors are made using plastic as a dielectric and they come in many flavors, as much as different plastics we have available. The most used ones are Polypropylene (PP) and Polyester (PET) and, interestingly enough, they have opposite characteristics in some cases.
They are mostly made for through-hole technology.
Polyester capacitors are very cheap and they are decent for many applications.
- Low cost
- Relatively good capacitance vs volume ratio
- More temperature dependant than PP, around ±5% over the temperature range
- Not very good for high-frequency, -current, or -pulse applications
- General purpose applications: bypassing, AC coupling, filtering…
Polypropylene capacitors are very stable and have low losses, similar to Class I ceramic capacitors, so they are suitable for the same kind of applications but they can handle higher currents.
- Good for high-frequency or -voltage, and also for fast pulse raise times.
- High breakout voltage
- Low and measurable aging effect
- They have negative temperature coefficient, so they are cool to compensate other components!
- Low capacitance
- High current pulses, high-power, high-frequency…
EXTRA: Line-rated capacitors
Line-rated capacitors or mains-rated capacitors are special capacitors (normally PP or ceramic) for cases where they need to be connected to the power supply mains. They need special safety classification to protect people from danger.
They have a fuse-like capability so if they get burn they lose some capacitance but they are never short-circuited to avoid fires and electric shocks.
They mostly come in two different classes, X and Y, each one for a different application.
If you need to interact with the mains, please, use these and no other.
Nowadays technology is good enough to let you use ceramic capacitors for almost any electronics design or application. For those cases where a very high capacitance is needed, you have no other choice than going for electrolytic.
In the case you are using Class II ceramic capacitors you have to be extremely careful with your selection and check the temperature dependency.
In through-hole technology, Polyester (PET) caps can be a cheap and decent choice for simple applications, while polypropylene plastic film (PP) capacitors can be useful for all other applications where higher capabilities of current or frequency handling are needed.
For extremely high capacitance applications where no other option is valid electrolytic capacitors are the only option you have. If you need them to be small, tantalum ones are your last resort.
If you are working with the mains you MUST go for line-rated versions.
If they are small they are like a firecracker, but if they are large they can be really dangerous. The classic cylinder looking ones have like a cross on the top making the enclosure able to puff up to reduce the chances of explosion, but they blow up anyway if you try hard enough, and even if they don’t they don’t work anymore so your circuit is compromised anyway.↩︎
The ESR is the Equivalent series resistance of the capacitors. Ideal capacitors don’t exist so we have to be satisfied with the crap we have. Non-ideal capacitors are approximated in circuits with a resistor, a capacitor and an inductor in series. The ESR corresponds to that resistor, which represents the ohmic losses in the device at a certain frequency.↩︎
Fun fact: At time of writing, the Panasonic I mention in the point above is out of stock in the distributor where I found it.↩︎
They are called ceramic and multi-layer interchangeably because they are made by multiple layers of ceramic material.↩︎