From: Winfield Hill
Subject: Re: Low leakage parts
Date: 4 Jan 2003 03:35:44 -0800
Organization: Rowland Institute
References: <firstname.lastname@example.org> <email@example.com> <14rR9.16149$jM5.firstname.lastname@example.org>
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Phil Alison wrote...
> John Larkin wrote
>> Tom Bruhns wrote:
>>> 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.
> ** Not quite the same thing as components mounted on a PCB.
> Ever measure resistance between close spaced tracks on a glass or
> phenolic PCB ?
Hey, Phil, those of us who work in the area of very high-Z circuits
always take standard precautions: guard traces for moderate-impedance
wiring, and lifted-leg wiring in air for serious low leakage. This
is my preferred approach, and isn't as painful as it sounds; usually
only one node is involved. If several nodes are necessary, one can
use a teflon standoff. These circuits can work reliably for decades.
We've added a special component to our PCB library, with only one pin.