From: Terry Pinnell
Subject: Re: zero-power toggle circuit; was, how to master electronics
Date: Mon, 18 Nov 2002 17:42:17 +0000
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NNTP-Posting-Date: Mon, 18 Nov 2002 17:42:32 +0000 (UTC)
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Fred Bloggs wrote:
>Anyone can see that the circuit has inherently defective time constant
>allocation. The problem arises from the fact that at turn-off the 2N7000
>gate circuit time constant is at most a factor of approximately 10
>greater than the p_channel gate time constant. Turn-on is a done deal
>because the p-channel gate capacitance is charged through the ultra-low
>Rds of the 2N7000, but at turn-off, this capacitance must discharge
>through the 100k R4, and this is happening at the same time as the
>2N7000 gate is re-charging through R2 because the output node still has
>voltage on it. If the thresholds of both mosfets are low then you have
>the situation where the 2N7000 requires only a small fraction of the
>gate circuit time-constant to turn back on, and the p-channel requires
>an integer multiple of its gate circuit time constant to start turning
>off. This is the root cause of failure to toggle off, the 2N7000 turns
>back-on before the p-channel can turn-off enough- very bad circuit- and
>SPICE is not required. Reduce R4 to 10K or increase C1 to 1U for added
Thanks. I'm pleased to hear you reckon it's not down to me!
But I'm still hoping to hear from circuit's sponsor, Win, or original
author, Wafer. Particularly interested to know whether Wafer built it
*only* with that specified low-power MOSFET, BS110?
I had actually tried R1 = 10K, and C1 = 1uF with the IRF9513 and it
still doesn't work. Just tried both combined, and that doesn't
simulate either. Also, with some MOSFET types load is on at power-up,
while with others it's off.
So, whatever inherent flaws in circuit itself, I still suspect my
technique or CM's models may also be implicated.
That's why I'm hoping one of the Spice experts might try it too.
Hobbyist, West Sussex, UK