RE: Simple question about cleaning up ground line

        It's not bad for the cap (you're dropping the voltage across the
cap to 0,) but you're dumping all of the charge on the cap into the
control panel gnd (through the switch) very quickly. Dunno whether that
matters or not, and/or whether the switch can handle the large current
transient (<shrug> it probably can.) Remember, when dealing with a cap,
I = C*(dv/dt), and you're talking about a large dv/dt by doing that.

-----Original Message-----
From: owner-rasterlist@synthcom.com
[mailto:owner-rasterlist@synthcom.com] On Behalf Of Andrew Wilson
Sent: Saturday, March 30, 2002 9:00 PM
To: rasterlist@synthcom.com
Subject: Re: RASTER: Simple question about cleaning up ground line

>
> Does this happen only when the switch is in a certain position?

Yes, only when the switch is closed, which is how I figured out what was
happening - with the switch open, the circuit is completely isolated
from the control panel.

>
> That won't do anything when the switch is closed, because the switch
> will short the capacitor. (I'm assuming you're saying the cap is in
> parallel with the switch. If it's in series with the switch then it
> will act as a high-pass filter, which is the opposite of what you
> want.)
>

Yeah, I wasn't very clear - I meant to put the cap between the EPROM
address line and a ground line I brought up from the board, as Rodger
suggested. You are correct, putting the cap between the EPROM address
line and the ground line from the control panel (parallel to the switch)
doesn't help me.

My other concern was that when the switch is open, the cap will charge
up to 5V, but when you close the switch the cap will discharge very
rapidly (won't it? There's no resistor in series with the cap). Will a
cap hold up to this?

The circuit will end up looking like this (with the cap connected to X)

+5V ---/\/\/\/\/\/\/\/\--X----[switch]----<control panel gnd>
                         |
                         |
                         +----[Capacitor]------<PCB gnd>

When that switch closes, suddenly the voltage at X is going to very
rapidly drop from 5V to 0V, discharging the capacitor very quickly,
which seems like a bad thing for the cap, but I don't see how else to do
it (at least, without using a relay :). Or will a cap hold up to this
type of use pretty well?

-atw

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