Quick one about decouplers

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2camjohn

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I have some 22pf unpolarised capacitors.

Are they suitable for putting across the power pins of a PIC?

How about across the pins of a voltage regulator??



Thanks
John
 
Normally i'd put the small capacitors on the xtal's, hooking each side to ground with a 33pf or 22pf caps.

if it's a sensitive circuit then for my power i use more then one cap and use different sizes... like 1uf, a few 33uf.. etc. connecting power to ground.... all electrolytic with proper voltage ratings... keep that in mind. so use larger caps for putting across the power of the PICs.

that's just how i personally do it... someone might have some more technical insite to decoupling.
 
2camjohn said:
I have some 22pf unpolarised capacitors.

Are they suitable for putting across the power pins of a PIC?

How about across the pins of a voltage regulator??
Click to expand...

Far too small for decouplers at the frequencies used on PIC's, as Plot says, they are about the right sort of value to use in the crystal oscillator circuit.
 
2camjohn said:
OK, thanks.

What value should I get then, is 1nf OK??
Click to expand...

Probably something like 0.1uF is more like it, with electrolytics every few chips as well - but it really depends on the circuit.

If you check my PIC tutorial circuits I just stick a small electrolytic near each PIC - that's all, no smaller caps. PIC's are generally VERY easy about their power requirements - obviously the two capacitors close to the 5V regulator are ESSENTIAL!.
 
Thanks again Nigel,

My circuits are quite simple and run without any capacitors at all.

I will get some 0.1uf ordered.

John
 
My car seems to be running OK even though it's three quarts low on oil!

You may not seem to be having problems now without the caps, John, but glitches can sneak up on you. Although OK now, it may be that if running with the mains a bit low/high, someone throwing on the vacuum cleaner or a nearby electrical storm could cause a glitch in the circuit that would throw it into a tizzy -- a glitch that would otherwise be caught by the bypass caps. So, they're always a great idea to include in a circuit.

Dean
 
Dean,

I intend to use capacitors, I always do.


I was just referring to what Nigel said about it depending on the application.

All my circuits run on batteries so I will have no problem when someone switches on the Vacuum cleaner!!!


That reminds me, I havent checked the oil level in my car for a while...
 
2camjohn said:
All my circuits run on batteries
Click to expand...
Hi John,
If your batteries are disposable, their internal resistance rises as they run down. This results in lousy voltage regulation.
Have you heard of "motorboating" in an amplifier circuit? This occurs when the amplifier draws current to make an output, but the current causes the power supply voltage to sag, which is fed to the biasing at the input of the amp and around and around it goes. I always use at least 100uF, sometimes 1000uF across the battery to keep the battery voltage fairly stable. In addition, for RF circuits I use a few 1000pF ceramic caps marked "102".
 
Audioguru,

Thanks for the tip, I do run some of my circuits from disposable batteries, although an equal number use NiMh.

I also do run very high current loads. I will get some 0.1mf caps for the battery terminals. Cant be too careful...

Thanks
John
 
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