From: Terry Pinnell
Subject: Re: zero-power toggle circuit; was, how to master electronics
Date: Wed, 20 Nov 2002 19:14:34 +0000
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NNTP-Posting-Date: Wed, 20 Nov 2002 19:14:49 +0000 (UTC)
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Tony Williams wrote:
> Terry, I see that the old Ferranti transistor
> equivalent of this circuit had an extra capacitor
> in it..... across our R3, the g-s of the 2N700x.
> I suspect this was added to avoid triggering-On
> due to noise glitches (as power up). Because
> the gate of the 2N700x has to be reliably pulled
> to 0v by C1, this new cap should be no more than
> 1/100th of C1, say 1nF or less. Try it?
> Note also, this circuit will be reluctant to
> turn Off with light loads (your 100 ohm) and
> possibly with capacitive loads. It should be
> 100% reliable with inductive loads (relays,etc).
> Load-sensitivity is a natural consequence of
> taking any feedback directly off the load.
Thanks Tony. As you know, dodgy CM MOSFET models remains the most
significant factor confusing my simulations. But I've tried
experimenting with an extra C across R3, ranging from 500pF to 5nF,
and reducing load to 10 and then 1 R, but no change.
One change of technique did prove successful. I thought I'd try
ramping up the 10V supply, instead of applying it instantaneously. Now
that I've reduced R4 to 10k, only a few uS ramp proved necessary to
prevent some types switching on at power up. I'll do that as a matter
of course from now on, for sensitive circuits.
Hobbyist, West Sussex, UK