SparkFun Forums

[bamboosamurai]

I need a Tantalum 1206 10uF Cap for a PCB microcontroller running at 5V, what voltage rating should I use? Or over voltage rating?
I've heard caps should have a 25% overvoltage rating and no more.
5V system, 5V * 1.25 = 6.25

So would a Tantalum 10uF 1206 CAP with a 6.3 voltage rating be the right one?

Thanks

[Dave Mueller]

I'd probably go with 10or 16V for a 5V system. I'm not so sure that rule of thumb applies in such low voltage circuits.

[n1ist]

I usually derate tantalums by 50-75%, so Dave's recommendations sound good. BTW, do you need a tantalum? Unless you need the higher ESR, a ceramic doesn't need that much derating and is less likely to spontaneously combust. Note, however, that some regulators don't like ceramic caps due to the low ESR
/mike

[bamboosamurai]

Ok great thank you guys, thats exactly what I was looking for.

Now this is my first PCB, self taught in electronics beginning last summer.
I am laying out an Atmega1284p and I have used an Eagle layout of the Arduino Pro Mini sort of as a reference.
A 10uF tantalum is used after the regulator, so I thought to do the same.

[macegr]

I refuse to use tantalums if at all possible. They're expensive and their typical failure mode is to burn up. I'd derate them massively for an unproven application. They're especially bad across things like high amperage power supplies or batteries...the high current spike can cause them to fail after a few cycles. So you need some way of limiting inrush current.

There are very compact electrolytic caps if you need more than 10uF. Otherwise I like the high value ceramic caps, they keep getting cheaper and better.

[bamboosamurai]

I am using batteries, so thank you for the heads up. Ill swap out the tantalum to a ceramic.

Thank you

[bamboosamurai]

Another thing* I am going to be learning/soldering the SMTs using a Hakko/Sparkfuns Special Blend solder. As for flux, is ChipQuik Flux No Clean a good choice?

[bamboosamurai]

Wow your not kidding $0.072! So ill replace the tant with a Ceramic 10uF 16V 1206. Thanks for the advice fellas

[poent]

That flux is fine but if you're getting your board fabricated then you probably wont need it. New smd pads are pretty easy to solder because their already clean, if the pad is grungy that's when flux is really magical.

The real trick is to line up the IC nice and careful, tack down a pin, then be sloppy with getting the rest of it attached. Go back with solder wick to clean up the extra solder and fix the shorts and you'll have a beautiful board.

[bamboosamurai]

Great, Ill go that route.

[bamboosamurai]

Does this still apply concretely to resistors? "deration"
I need 6 10K's for reset and 5 switches, when searching for the right resistor on newark I can only seem to find them with a voltage rating of "75V"

[Grimfox]

resistors are typically rated not by the voltage but by the power they dissipate. I can put 10,000 volts through any 1W resistor I want so long as the current * voltage is less than 1W.

You'll need to do the math to figure out how much power each resistor is going to dissipate in the worst case scenario. I'll assume it is a 5V system. 5V/10k = .5mA ==> .5mA*5V = 2.5mW. Just about any resistor will handle this load easily.

[n1ist]

I can put 10,000 volts through any 1W resistor

Actually, you can't. Resistors, like any device have a maximum voltage rating too, It usually has to do with insulation breakdown and flashover distance.
/mike