Smoothing capacitors on power lines

Published by paul at 3:42 PM under 555 | 8-bit computer | General

Well, the best laid plans of mice and men, and all that. Now I've got a long-term project to finally get back into electronics and blogging after an almost 10 year gap.

I'm building an 8-bit computer on breadboards, based on the kit and videos by Ben Eater - see here.

The first thing I want to blog about is something new I learned today. I'd always known how putting small ceramic capacitors across power lines helps to smooth out logic transitions and prevent spurious problems, but I didn't know about using larger capacitors for the same purpose.

Below is a scope plot of the rising edge of the output from a 555 clock circuit, with a 1us timebase, and no smoothing capacitors:

You can see that when the transistors in the 555 switch on, there's a big pull from the power supply and the output voltage spikes momentarily.
This next plot is with a 0.01uF ceramic capacitor across the power lines: 

You can see that the voltage spike isn't so high, but then oscillates down to a steady state over a few microseconds.
Finally, here's a plot with a 10uF electrolytic capacitor across the power lines:
You can see that the rising edge is super-smooth as the capacitor provides all the necessary power without the big pull from the power supply.
You learn something every day!

[KickIt] [Dzone] [Digg] [Reddit] [del.icio.us] [Facebook] [Technorati] [Google] [StumbleUpon]

Be the first to rate this post

  • Currently 0/5 Stars.
  • 1
  • 2
  • 3
  • 4
  • 5

E-mail | Permalink | Trackback | Post RSSRSS comment feed 0 Responses