Date:         Tue, 29 Sep 2009 07:48:04 -0700
Reply-To:     Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject:      Re: ARRRGGGHHH Re: Tantalum capacitors to stop blinking temp LED
In-Reply-To:  <20090929115724.75F401165C3@hamburg.alientech.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

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. 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.

The symptoms of the problem match with the proposed failure mechanism. I
have a ton of old products I designed back in the 80's that use (oddly
enough) 10uF or 22uF, aluminum 'lytic or tantalum capacitors in an RC
network. When power is applied to the circuit the capacitor gets charged
through a high value (megohms) resistor and after about a minute, the
voltage triggers a 555 timer to perform a "I'm warmed up" function. Now,
20 to 30 years later, 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. I believe (have not recorded this) that it is the little
cheap aluminum lytics that develop leakage more frequently than the tants,
but we've seen old tants that needed replacing, too.

--
Mike "Rocket J Squirrel" Elliott
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
Bend, OR
KG6RCR



On 9/29/2009 4:56 AM Mike S wrote:

> At 10:34 PM 9/28/2009, David Beierl wrote...
>>> Perhaps simply removing the old cap will allow proper operation,
>>> except for that power-on indication.
>>
>> Actually I believe that removing the cap will force blinking forever
>> -- the reason it fails to stop is that internal and external leakage
>> prevent the cap from ever charging enough to trigger the
>> shutoff.
>
> I don't follow your logic.
>
> If charging the cap turns off the blinking, then changing to a 0 pF cap
> with 0 leakage (i.e. removing the cap) should cause the circuit to
> charge instantly, and stay charged.
>