From: "Phil Allison"
Subject: Re: Low leakage parts
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Date: Sat, 4 Jan 2003 11:02:34 +1100
NNTP-Posting-Date: Sat, 04 Jan 2003 10:52:36 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Tom Bruhns" wrote in message
> Some of you may recall some postings I've made over the past couple of
> years about the self-discharge rate of polyester and polypropylene
> caps. The time constants I saw were on the order of a few years for
> the polyester and over 50 years for the polyprops.
> A month or so ago, someone asked about making a simple toggle circuit
> for turning 12V lights on and off, and I posted a couple solutions.
> One of them was a "this is really simple, but it probably won't work
> very well" circuit using just a capacitor to hold the voltage on the
> gate of a power mosfet. Well, I built that ckt, using an 0.01uF cap
> across the gate-source, and toggled it "on", so the cap was charged to
> about 12V. Then I disconnected the power and went on a holiday trip.
> Just came back, and it's still in the "on" state, three weeks later.
> Sooo...I'd say that modern power mosfets also have pretty low gate
> leakage current. To hold the voltage above the nom. 3V required to
> turn the mosfet on, the average leakage must have been less than 50fA,
> assuming 21 days, 0.01uF and 9V delta (and no arithmetic errors). (It
> was cool, about 18C, and likely wouldn't do quite so well inside a car
> with the windows rolled up in Phoenix in the summer...)
** 21 days is 1.8 exp 6 seconds, divided by 0.01 exp -6 gives 1.8
exp 14 ohms as the resistance in the cap, fet and wiring etc. I don't
believe it - do you ??
I suspect there was a DC leakage path HOLDING the 0.01 uF cap voltage
up that was the real story here.