what kind of caps to use on PIC

[AndyWatson]

I'm trying to build a circuit that someone else designed and built, and I'm wondering about something. It uses 16C773, and the designer used an electrolytic cap between the Vdd pin (pin 20) and ground. Is it proper technique to use a polarized electrolytic or tantalum cap there, or should I use a standard, non-polarized ceramic cap?

 

[Nigel Goodwin]

I'm trying to build a circuit that someone else designed and built, and I'm wondering about something. It uses 16C773, and the designer used an electrolytic cap between the Vdd pin (pin 20) and ground. Is it proper technique to use a polarized electrolytic or tantalum cap there, or should I use a standard, non-polarized ceramic cap?

If you want to be really sure, use both an electrolytic and a ceramic in parallel - but it's not really required.

Personally I just use an electrolytic, anywhere between 1uF and 100uF, close to the PIC. I wouldn't use a tantalum, in my experience they have been much too unreliable - but others like them!.

However, PIC's are generally very tolerant of their supply rails.

 

[samcheetah]

usually after a voltage regulator it is good to connect two capacitors. one is a big electrolytic and the other is a small ceramic cap. the bigger cap gives the stability. but because electrolytics have a rather high ESR you need to have another ceramic in parallel to take care of the high frequency spikes. but now electrolytics with very low ESR are available everywhere. and one thing more. some voltage regulators dont even need these caps. it all depends on the design of the voltage regulator.

but as Nigel has said, PICs are very tolerant of the variations in the supply voltage so it doesnt matter if you use the smaller ceramic cap or not

 

[Nigel Goodwin]

but because electrolytics have a rather high ESR you need to have another ceramic in parallel to take care of the high frequency spikes.

I measure the ESR of a LOT! of electrolytics, the ESR depends on the value of the capacitor (obviously), but electrolytics are far, far lower than smaller capacitors at the 100KHz frequency used for ESR tests.

The extra ceramic capacitor is for much higher frequencies, something like 20-30MHz? - where the slight inductance of an electrolytic will increase it's ESR

 

[samcheetah]

The extra ceramic capacitor is for much higher frequencies, something like 20-30MHz

:shock: i didnt know that the "high" frequency was that much high. i just read it somewhere that the lower valued ceramic is used to improve the transient response by suppressing high frequency spikes.

mmmm :? i think i should get an ESR meter

 

[AndyWatson]

I don't know what ESR is, but are you all saying that I should stick with using the electrolytic instead of a ceramic?

 

[williB]

I don't know what ESR is, but are you all saying that I should stick with using the electrolytic instead of a ceramic?

i dont know about anyone else but i am using a ceramic cap on my pic , works fine

 

[AndyWatson]

williB: what value ceramic cap do you use?

 

[Nigel Goodwin]

I don't know what ESR is, but are you all saying that I should stick with using the electrolytic instead of a ceramic?

ESR is 'Effective Series Resistance', it's generally measured at 100KHz, and should be fairly low for a good electrolytic - the bigger the electrolytic the lower it will be, ceramic capacitors will obviously be far higher.

ESR has become a VERY common thing to test, modern electrolytics mostly fail by going high ESR, although they will still test fine for capacitance.

I always use electrolytics, other people (like WilliB) use ceramics, both are probably perfectly fine - mostly none at all is perfectly fine!, as I emntioned, PIC's are very tolerant!.

 

[pittuck]

personally i use 100nf polyester (?) caps on the supply of every IC (within reason, some IC's dont need em of course).

I use 47uf electro each side of my Regulator, and if i get some problems i would add a 100nf to the supply rails near the reg.

Not had ne problems yet.

 

[Oznog]

I don't know what ESR is, but are you all saying that I should stick with using the electrolytic instead of a ceramic?

In my experience, ceramic (even a small one) works, tantalum did not. I wouldn't use electrolytic.

Now keep in mind that if your regulator is close enough, a single ceramic cap could go near the reg or the PIC and it won't matter, although I'd prefer to put it near the PIC and just leave a tantalum or electrolytic on the reg output. One ceramic will do both jobs.