Voltage design factor guidelines for caps and semiconductors - Page 1
Forum section: electronics / projects

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 07, 2012, 02:00:56 am
content: I am wonder what design factor ( maybe safety factor ) other people are using when choosing voltage ratings for components in their designs.I have just been using 50 to 100% above rated voltage for most things.To narrow it down, I am mainly interesting in offline stuff at 50v down to 3.3v.The components I am interesting in are Electrolytic Caps, Ceramic Caps, all sort of diodes, fets, bjts.But if people want to give input in other circumstances, say mains, automotive,temperature or current in inductors that's would be welcome too.With a bit of help I would like to have at the end a sort of quick reference that would cover most situations.Looking forward to some input, or links,thanks.

author: Rerouter
author_rank: Super Contributor
timestamp: October 07, 2012, 02:28:23 am
content: in general i go 125% of normal conditions voltage for capacitors, for transistors i generally keep 15V minimum higher than the highest expected voltage under normal conditions, and with diodes usually the cheapest is already grossly over what i would ever need,there are some conditions through, e.g. i commonly have to repair automotive instuments, which have many odd senarios even including reverse biasing standard electros 7V for a clock signal, using 40V diodes on coil based tacho signals, that break down as a cheap zener to protect the other components on the peaks, to a cap on the end of a dropping resistor fed straight off ignitionin those cases its a best fit world. where i can for the clock signal instance i would replace with a non polarised or high capacity ceramic when i have the option (some designs require the high esr to work),for the diode case i would replace it with a proper zener of atleast 2x the wattage but as its protection i would lower the voltage by 5V, (yes that is also related to this subject i believe)and for a dropping resistor cap being how the actual thing pulling load off of it is not garenteed, i would generally triple the voltage rating if feasable, as the electrics on a car are truely horrendous, and a 18V rated cap on what should be 10V with all the spikes and crap will not last even if it was 25V rated, (in this case it was being fed off of 26V truck ignition)

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 08, 2012, 02:33:39 am
content: thanks Rerouter. most usefulI might 'up' my design factor then on caps.Fets and BJTs both 15V?what about cap temperature? Say you have a cap sitting in a box that you wouldn't expect to get above 50C and you guesstimate it would itself generate another 10C while at full load. Is 25C (85C) enough margin, or just use 105C rated caps?

author: Rerouter
author_rank: Super Contributor
timestamp: October 08, 2012, 06:41:00 am
content: for fets and bjt's yes 15V min increase, i have been bitten by them breaking down when taken to near the rated voltage,for capacitor temps yes pretty much 105 everything, even if i am only designing a product for 40 degrees, i like my things to last, (expecially repairs)equally for values under 10uF generally i will push for ceramic over electro or tant, when i know the pcb is held rigid,(sometimes it can even be cheaper!)but that comes down primarily to what your product allows,

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 08, 2012, 01:26:10 pm
content: 50-100% is pretty ideal, above or equal to 100% safety factor is the royal majesty design, good for diy that will last forever. but thats not what i found in everyday circuit. i have here bad gigabyte pc atx psu with a bunch of bad 16V caps after 4 years of usage, i think those operated at 12V, thats only ~30% above operation, business enterpreunership i guess.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 08, 2012, 10:09:38 pm
content: Thanks very useful to hear others thoughts.that link seems to work if you drop the www.electrochem.cwru.edu/encycl/misc/c04-appguide.pdf

author: ptricks
author_rank: Frequent Contributor
timestamp: October 09, 2012, 02:16:25 am
content: For electrolytic caps I use the highest voltage and uf as the project can fit for the cost allowed. It never hurts to put a 50V cap on a 3,3V circuit, but if the budget doesn't allow for then I scale it down to peak-peak + 25% .diodes are really a bit too varied to have one set way of choosing the correct one. It is going to depend on the circuit and what diode is being used, something like a schottky can get expensive when you over size for the need.For cap temperatures I try to locate the caps as far away from heat sources as possible, use 105c if it allows, otherwise 85C, and 85C is really pretty hot for most semiconductors.