Shorter answer:
The higher the frequency, the more a capacitor looks like a short.
The lower the frequency, the more a capacitor looks like an open.
We use those little 0.1uf ceramic caps next to fast switching IC's
because those IC's insert fast transients onto the power rail.
Capacitors that are placed between VDD and ground have the effect of
making VDD "more DC like". We use the little ceramic caps for this
purpose because they are better at passing fast transients than
electrolytics. Electrolytics work better for longer duration (slower)
voltage changes. You'll see electrolytics after a switching supply to
smooth out the sawtooth, or after a bridge rectifier, etc.
Dealing with capacitors in digital circuits is pretty simple. You
sprinkle them in. You can't go wrong putting a 0.1uf cap next to every
IC and some electrolytics after every powersupply (and some more 0.1uf,
if you like)... I asked my father about this practice once.. "Is every
single one of these capacitors really needed?" He responded with the
following joke:
An engineer, a mathematician, and a physicist are staying for the night
in a hotel. A small fire breaks out in each room.
The mathematician awakes, sees the fire, makes some careful
observations, and on a blackboard installed in the room, does some quick
calculations. He exclaims "A solution exists!", and crawls back into bed
and goes to sleep.
The physicist awakes, sees the fire, makes some careful observations,
and does some quick calculations. Grabbing the fire extinguisher, he
puts out the fire with one, short, well placed burst, and then crawls
back into bed and goes back to sleep.
The engineer awakes, sees the fire, grabs the fire extinguisher, he puts
out the fire by hosing down the entire room several times over, and then
crawls into his bed and goes back to sleep.
-Adam
On 7/21/2011 7:19 AM, Shane Ellis wrote:
Wow, thank you for the headache so early in the morning! I got my one
tube circuit working, but going to a much larger uC, and more tubes,
more noise is bound to come up, so now that I know how to decouple
with caps, I'll be sure to add them. Thank you all.
Such a great group. I hope you all know how sincerely I appreciate
the help. I'm in love with glowing glass, and I'm in love with arcane
technology. Without you guys, I'd still be trying to figure out power
supplies, and binary decoding.
Shane
On Thu, Jul 21, 2011 at 9:14 AM, jb-electronics
<[email protected] <mailto:[email protected]>> wrote:
Hi Shane,
When you say "next to" and IC, do you mean from the +5V, to ground?
Yep. There are some IC sockets who have a 100nF capacitor
connected from the pin on the bottom left to the pin on the top
right, i.e. the most common IC power pins.
Resistors I got, Capacitors frustrate me...
It is not that hard to understand: Capacitor act just like
resistors for AC. The higher the frequency, the more current can
flow through a capacitor. The complex impedance is Z :=
2*Pi*(-i)/f*C, where f is frequency in Hertz, C is the capacity in
Farad and i is the imaginary basis, e.g. i^2 = -1. This just means
that the impedance (the resistance, basically) approaches zero if
the frequency approaches infinity.
In our example: Really fast disturbances can be seen as some very
high frequency (look at the Fourier transform of the signal).
These disturbances will be shorted by the small capacitor of 100nF
due to their high frequency, so that this high frequency does not
corrupt your circuit.
Slow signals are not affected. In the other limes, the frequency
is zero (i.e. DC signals), so the impedance is infinite. We
already know that: Capacitors do not conduct DC current.
Hope this helps.
Best regards,
Jens
--
You received this message because you are subscribed to the Google Groups
"neonixie-l" group.
To post to this group, send an email to [email protected].
To unsubscribe from this group, send email to
[email protected].
For more options, visit this group at
http://groups.google.com/group/neonixie-l?hl=en-GB.
