From: "Phil Allison"
References: <email@example.com> <firstname.lastname@example.org> <14rR9.16149$jM5.email@example.com>
Subject: Re: Low leakage parts
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Date: Sun, 5 Jan 2003 00:06:30 +1100
NNTP-Posting-Date: Sat, 04 Jan 2003 23:56:16 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Winfield Hill" wrote in message
> 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...)
> >> Tom,
> >> 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
> >> fet.
> >> 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.
** That is lovely but of no relevance to the either to OP or my