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? |
Please note - During the past 17 years of operation, several gigabytes of
Vanagon mail messages have been archived. Searching the entire collection
will take up to five minutes to complete. Please be patient!
Return to the archives @ gerry.vanagon.com
The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.
Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.