From: John Larkin
Subject: Re: Low leakage parts
Date: Fri, 03 Jan 2003 17:16:33 -0800
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On 3 Jan 2003 15:46:02 -0800, firstname.lastname@example.org (Tom Bruhns) wrote:
>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...)
I did a post a while back: a 2N7000 with a 9-volt battery and an LED
in its drain, gate hanging open. This can be set on or off and stay in
that state for many days. You can also perch the gate voltage just at
the turnon point and get the LED to stay on dimly, with no perceptable
brightness change over many hours. This is cool... if you wave a
charged object near the gate lead, you can modulate the LED brightness
by inducing voltage into the gate. Maybe I'll repeat this with a power
I figured that the gate leakage was in the 100 electron/second range,
maybe much less... attoamps. After all, eproms and flash store bits as
a few thousand electrons each, for many years.