Subject: Re: how to reduce noise from 555 timer...???
Date: Sun, 22 Dec 2002 23:46:33 -0800
Organization: MindSpring Enterprises
X-Server-Date: 23 Dec 2002 08:00:44 GMT
X-Newsreader: Forte Free Agent 1.92/32.572
On 22 Dec 2002 05:28:59 -0800, Winfield Hill
> It's indeed figure 1 of both the Nov 1996 and Nov 2002 Intersil
> 7555 data sheets, but you're correct, I made a typo - the 555's
> spike is 200ns rather than 200us long on their scope trace. But
> a fast-risetime spike can be more trouble than a long slow one.
> The current spike is 380mA in amplitude with a 7.5A/us risetime
> for 50ns, decaying back to zero after 250ns. Using V = L di/dt
> we can predict that the OP's poor circuit will have a 1.5V 50ns
> pulse on his modest say 0.2uH of wiring. That's enough to cause
> serious trouble with digital circuits, let alone analog stuff.
> To help solve the problem, he can switch to a 7555 as Intersil
> suggests, or he can provide an R-C or L-R-C filter for the 555
> with the C returned directly to the 555's ground pin 1, and
> thence route the 555 ground to the rest of the circuit. While
> it's playing a dangerous game to have such a noisy beast among
> quiet analog circuits, with knowledgeable care it can be done.
> - Win
The old "Rock Band next to the Quire Syndrome". It sounds like you
have nailed it again!
Here is a good place to try a ferrite inductor, ferrite bead or a wire
inductor, as you stated. Its very hard to find caps that are this
Isolating the pathway of power and ground may also help. A good amount
of care and consideration in the construction is needed. But first, he
needs to look into getting the improved '555.