I am building a unit with a PIC and various other ICs that needs to be stable at all times when in use. What sort of filtering should I use on each of the power supply pins of the ICs?? Should I use 0.1uF, 1uF or 10uF??? Or all of the above??

If I get gates tripping when they shouldn't it may cause damage to other equipment. What about medical units? They must be failsafe. What do they use?

Also, are there any hobbyists who have released their own designs on to the commercial market before?? What were your experiences with this???

Paul.

 

0.1uF ceramic caps as close to the power pins as physically possible is the norm

 

The type of power supply, how it is wired and the grounding scheme is more important than the size of capacitors used.

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Well, there are plenty of commercial units released on the market full of bugs, so it doesn't seem that big a problem. An important point is to use the watchdog timer, so that if it does lock up the unit wil reset itself and carry on working.

Stable 'at all times' isn't an achievable goal, it's like asking for a part with 0% tolerance - it's not possible. All you can hope for is a high MTBF (Mean Time Between failures).

All you can really do is follow good design practice, then carry out extensive testing.

As for medical applications, as far as I'm aware no microprocessor is quoted as suitable for medical applications - for obvious reasons!.

 

I've run across recommendations by experts and have noticed that multiple capacitors are used in these situations because each responds somewhat differently to transients. Some of the authors offer brief explanations that suggest they understand things. I've seen things like a 10 mf capacitor in parallel with a 10 pf capacitor (when you'd think the extra 10 pf would be insignificant - or the recommendation that a tantalum capacitor be used. One author suggested that A 1 mf tantalum was more effective than a 25 mf standard electrolytic - for this situation. Some of the National Semiconductor datasheets for voltage regulators touch on this.

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stevez

 

Multiple capacitors of different sizes are used because large capacitors have higher ESR (equivalent series resistance). So the small capacitors filter small fast transients and the larger capacitors filter out large slower ripples.

Look at the ESR for capacitors to compare the tantalum to the electrolytic Caps for power supply filtering.

Another safety note: Tantalum caps shoot fire when they fail - not the safest thing.

Brent

 

If you're looking for reliability, remember that one of the #1 components to fail is the electrolytic capacitor. Electrolytic capacitors age, so if you want your device to work consistantly over long periods, I'd use other types over electrolytic.

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Large capcitors actually have lower ESR than smaller ones, and the higher the voltage rating the lower the ESR as well - so a 100uF 63V has a lower ESR than a 100uF 16V.

I use an ESR meter at lot at work :lol:

As you say though, smaller capacitors take out the faster transients and higher frequencies - BTW, ESR tests are done at 100KHz.