Caps

Hi,
Can anyone help me with capacitor choice for decoupling, specifically -

  1. Does the voltage matter ?

I often see 0.1uf Capacitors rated upto 100v as the only choice available in local outlets, are these effective in a TTL environment ?

  1. Is there any advantage in paying the extra for tantalum over ceramic ?

  2. I assume that 0.1uf poly film caps are not suitable for decoupling due to internal inductance, right or wrong ?

Thanks

Duane B

rcarduino.blogspot.com

For decoupling you want the lowest ESR, which is going to be ceramic.

DuaneB:

  1. Does the voltage matter ?

Rated Voltage relates to the dielectric's thickness for a given capacitor. Generally the higher the voltage, the less capacitance you can get in a given volume. In the world of hobby-electronics, rarely does a person need to make a tradeoff between size and voltage rating.

There isn't a huge concern with using a 100V rated cap for de-coupling a 5V line. In fact, for ceramic you want a higher rated voltage because ceramics have a negative voltage coefficient. The closer you get to a ceramic's rated voltage, the less actual capacitance you get out of the capacitor.

DuaneB:
2) Is there any advantage in paying the extra for tantalum over ceramic ?

There is to me. I sell caps for a living, so I always like when people pay more than they need to. (Just kidding.) If ceramic will work, there's no benefit to Tantalum. Traditional Tantalum uses MnO2 as their cathode. This cathode system is known for exploding when improperly being used and extremely high ESR. Unless you are talking about bulk de-coupling, there's no need for them (or their expense.)

Today you can get Polymer-Tants which have very low ESR and don't explode under any condition. However, these are extremely expensive for low capacitance (you couldn't get 0.1uF anyway) when compared to a ceramic. They're only used when you want the most cap in the small package.

DuaneB:
3) I assume that 0.1uf poly film caps are not suitable for decoupling due to internal inductance, right or wrong ?

Given their price versus ceramic, I see no benefit in using film-caps for high-frequency decoupling.

Hi,
Thanks for the response, I will get a large bag of these -

http://uk.rs-online.com/web/p/ceramic-multilayer/5381310/

Thanks again,

Duane B

rcarduino.blogspot.com

Male sure you clip the leads when you use them.

DuaneB:
3) I assume that 0.1uf poly film caps are not suitable for decoupling due to internal inductance, right or wrong ?

That rather depends on how they are wound - non-inductively wound ones will fare the best, but ceramic are much more compact which helps control stray inductance and resistance. Datasheets for capacitors will list the ESR and self-resonant frequency so you can check how well a particular brand performs. Ceramic are generally used for decoupling because they are small and cheap and have low ESR.