Passive Component Selection

[FelixK]

Hi,

I'm a newbie who's fresh out of school. I noticed that in certain tear down videos Dave would lament the device used a specific capacitor manufacture. Is there any wisdom that a more experienced designer can pass down in regards to component selection? Are there specific manufactures I should go out of my way to purchase from? Are there specific ones I should absolutely avoid?

On the same subject, does anyone have tips as to how I should I make component choices in regards to the performance ratings? For example if I need a capacitor for a 3V power line, should I chose a capacitor who's rated voltage is exactly 3V or should I be choosing one that has some x*3 V rating where x is some specific fraction that is greater than 1?

Thanks!

[GreyWoolfe]

As far as capacitors go, there are brand names that are worth getting.  Rubycon, Nichicon, Panasonic, Nippon Chemicon and Illinois Capacitor are all good quality.  There are so-so brands like Samwha.  There are brand names to stay away from such as Capxon.  You might want to head over to www.badcaps.com.  The site is useful to learn about the less than stellar manufacturers.

[rdl]

I've always used parts rated for higher voltage and current than they will be exposed to, often by 200-400%. However, one thing I seem to remember reading or hearing long ago was that electrolytic capacitors could lose capacity if used at much less than their wvdc rating. May be true, maybe not, but I've always tried to use electrolytics with wvdc ratings only about 50% higher than the expected maximum voltage they will be used at and have had no problems.

[nanofrog]

Capacitor Types from Radio-Electronics.com might be of some help. Not an end-all source, but certainly enough to get you confused started. 

Regarding brands, GreyWoolfe's list is a good start. There are others, and really, the brands will depend on the type you're looking for. In general however, I'd add United Chemi-Con, Wima, Vishay, and Elna to that list.

[FelixK]

I've always used parts rated for higher voltage and current than they will be exposed to, often by 200-400%. However, one thing I seem to remember reading or hearing long ago was that electrolytic capacitors could lose capacity if used at much less than their wvdc rating

Do you know if there would be any similar concern regarding ceramic capacitors? For most of my projects I've been using SMT passives and thus ceramic capacitors.

As far as capacitors go, there are brand names that are worth getting.  Rubycon, Nichicon, Panasonic, Nippon Chemicon and Illinois Capacitor are all good quality.  There are so-so brands like Samwha.  There are brand names to stay away from such as Capxon.  You might want to head over to www.badcaps.com.  The site is useful to learn about the less than stellar manufacturers.

How about resistors and inductors?

[T3sl4co1l]

Yeah, electrolytics are healthiest near ratings, say 60-90% of ratings.  Less and you're simply wasting money and space; more and you're risking damage*.

*However, it's quite possible to reform a capacitor beyond its rated voltage.  I've heard of 200V parts used at 250, 300VDC.  The problem is, as the oxide layer grows (you're anodizing the aluminum as you increase the voltage slowly), the metal goes away, so the capacitance value and ESR change.  Too much and the foil itself will be oxidized through, significantly reducing capacitance and increasing ESR.  I don't know at what point this normally occurs, if it's 50 or 100% or what beyond rated voltage.

Ceramic capacitors depend on type: C0G (historically, NP0, or erroneously, NPO) and similar type I dielectrics are stable and work up to basically any voltage.  I've operated 50V discs at 500V AC before; they don't even heat up (oh, I was also running an ampere through them at that frequency).  Not that this is recommended operation for reliability, but needless to say they can handle it.

Type II dielectrics have larger values and poor voltage dependency.  Typically an X7R type will fail its tolerance (10 or 20%) within just 30% of rated voltage.  Thus, you typically specify a 35 or 50V part for 12V operation, and etc.  They also age (logarithmically over time), so they aren't suitable for timing or precision applications.

Film capacitors are almost as stable and low loss as C0G ceramics.  They're available in much larger values, especially at higher voltages.  They should be chosen for equal or better voltage ratings, as with electrolytics.  Typically, some nominal overvoltage is acceptable, but an excess will cause incremental damage (self healing) or failure.

Tim

[CatalinaWOW]

There isn't any simple answer to this.  The sources mentioned above will give you things to think about.  If you are doing anything important you should spend some time optimizing your selections for the application.  This may involve price, reliability, precision, temperature range or many other factors, but will be different for different industries and markets.  Something that will be used in a dog collar has very different needs than a heart pacemaker, and both are different than an automotive engine controller.   The selection may be driven by off nominal or abnormal events.  Things like voltage spikes from equipment shutdown or lightning, or failure of some associated piece of equipment.  As hinted at in one post above, both voltage and current matter in all components.    Some types may also be sensitive to dV/dt or dI/dt.

Reputable manufacturers will have data sheets describing the guaranteed operating characteristics of their parts.  It may vary from the typical performance of that family of parts, based on their own proprietary formulations and processes.  If the information you need is not on the data sheets a call to the application engineer may turn up the data.  If it is really important to your application you may need to test the actual parts that will be used in production.  Googling on the performance parameter

All of this complexity is one reason that experienced designers in a field are typically more valuable than fresh new guys.  They have already done the required homework.  Since you are a new guy you will be spending some long hours getting up to speed.  Don't worry, it won't take forever, and looking back it will seem that almost no time has passed since you were the new guy.