At 10:48 AM 9/29/2009, Rocket J Squirrel wrote...
>I think that David covered that pretty well when he said that the
>blinking
>fails to stop because "internal and external leakage prevent the cap
>from
>ever charging enough to trigger the shutoff."
>
>That's not the same as a 0pf capacitor.

Huh???? I never said it was. I compared removing the cap to replacing
it with a 0 pF one with 0 leakage.

>  It's the same as a resistor, and
>if the capacitor is being charged through another resistor until it
>reaches the shutoff trigger voltage, and that capacitor is paralleled
>with
>a resistor to model the internal leakage (or schmutz on the board, the
>proposed "external" portion of the leakage) then you got a voltage
>divider
>which will prevent the voltage at the top of the cap from ever
>reaching
>blink-o-matic shutoff voltage.

Nothing new here. That's what a leaky cap is.

>I see at least six units per year that need to have
>that cap replaced due to leakage -- they will never charge up enough
>to
>trigger the 555.

Yes. And removing the cap will cause the 555 to trigger immediately on
powerup, instead of being delayed by the RC circuit. (And the flashing
LED in a Vanagon temp gauge to immediately shut off)

David has given a reason for replacing the cap, separate from its
"power-up check" function.

What's your point?