[sdiy] Minimum parts count switch de-bouncer.

[Harry Bissell Jr]

As I mentioned in the screwdriver saga...
you might get 20A the first time but repeated
discharges will be degrading the cap.

I hear your backyard mechanic music... but you need
to become a Harley Davidson engineer

"At Harley-Davidson... if we need something
stronger, we make it bigger. If that makes it ugly,
we chrome it..."

I'd scale the RC values... much bigger resistor and
much smaller capacitor. That will cut the wear
on the switch.  Even if you tossed 100 ohms in series
with the switch, you would never lose sleep over
a cap failure !

(now the heat of the discharge is shared between
the 100 ohms and the .01 ohms or so ESR)

H^) harry

--- Tim Daugard <daugard at sprintmail.com> wrote:

> From: "Harry Bissell Jr" <harrybissell at prodigy.net>
> 
> > One point NOT being made in many of these
> > proposed circuits... is that the capacitor
> > can draw unlimited current when the switch closes.
> 
> In my circuit the problem is not the charge current,
> but the discharge
> current.
> 
> > Bigger the cap value, the higher the current.
> 
> Aggreed
> 
> > Enough current to eventually degrade of destroy
> the
> > switch in many cases...
> 
> Not sure which will go first, the cap or the switch.
> 
> > Kind of the the moral and legal equivalent of
> > discharging the capacitor with a screwdriver
> > (hi Bob ;^)
> 
> Absolutely
> 
> > There should be a small resistor in series with
> the
> > switch, to limit that peak current. Just assume
> the
> > cap is a short, and Ohms' Law will tell you the
> peak
> > current.
> 
> The caps I use are Xicon which I have to assume is
> smoe cheap brand labeled
> specifically for Mouser, a company search turns up a
> distributor in Texas
> and all links for data lead back to Mouser.
> 
> Best I can figure from the Mouser data sheet is an
> internal resistance as a
> current source of about 6/10s ohms. With a 12 Volt
> charging source, a fully
> charged cap will deliever a peak current of about 20
> Amps.
> 
> > No resistor will also rock the power supply rails,
> > bigger the cap, bigger the problem.
> 
> My cap was selected for the RC recharge time.
> 
> > You may want SOME substantial current to keep the
> > switch contact clean, but not enough to wear it
> out.
> 
> I wonder what the current rating is on the cheap
> RatShak switch is.
> 
> > H^) harry
> 
> Who has made me spend 30 minutes researching this to
> come up with the
> conclusions:
> - I'm using the wrong cap
> - I may be stressing the switch
> - I can't find enough data to narrow down the
> failure modes and effects
> further
>    --- However, what I did find says the cap will
> fail open, meaning the
> circuit still works, it just stops being debounced.
> 
> And so my circuit continues to live as a backyard
> mechanic's - "It works
> but who knows for how long." When it fails, I'll
> reengineer. Till then . .
> .
> 
> Tim Daugard
> AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
> http://home.sprintmail.com/~daugard/synth.htm
> 
> My version of the circuit in question:
> http://home.earthlink.net/~synthfred/h_ssssg1.htm
> 
> 
> 
>