From: Jonathan Kirwan
Subject: Re: zero-power toggle circuit; was, how to master electronics
References: <3DD39D90.firstname.lastname@example.org> <email@example.com> <3DD90A12.firstname.lastname@example.org>
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NNTP-Posting-Date: Mon, 18 Nov 2002 21:28:57 GMT
Organization: AT&T Broadband
Date: Mon, 18 Nov 2002 21:28:57 GMT
On Mon, 18 Nov 2002 17:42:17 +0000, Terry Pinnell
>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.
I'm not so familiar with all this that I can accept or challenge
Fred's comments until I get a moment to consider them closely.
So I'm anxious to see anyone's informed opinions on these
details, as well. It will only serve to educate a lot, at my