From: Winfield Hill
Subject: Re: zero-power toggle circuit; was, how to master electronics
Date: 14 Nov 2002 15:11:08 -0800
Organization: Rowland Institute
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> Wafer wrote:
>> Ummm...doesn't anybody have any comments on the one I posted?
>> Here it is again, for those who might have missed it. I drew
>> it out as an image rather than doing the ascii thing.
> Hadn't noticed it before. I'd prefer to redraw it, too. For
> example, tying the two 100k resistors over to a leg which is
> only afterwards connected to gnd or V+ kind of makes it look
> like there is some kind of signal intended there, when there
> isn't. My preference is something like this:
>: ^ V+
>: \ R4
>: / 100k ^ V+
>: \ |
>: | |
>: x----||--x Q2
>: | ||>-' BSS110
>: \ R5 |--,
>: / 10k |
>: \ |
>: | |
>: ,-----------------------x |
>: | | |
>: | ,----------- | -------x
>: | | | |
>: | \ R2 | --- D1
>: \ R1 / 220k | \ /
>: / 1M5 \ |--' Q1 ---
>: \ | ||<-, 2N7000 |
>: | S1 x--------||--x o
>: | _|_ | |
>: x---o o----x | LOAD
>: | | ---
>: | C1 \ R3 gnd o
>: === .1uF / 100k |
>: | \ |
>: | | ---
>: --- | gnd
>: gnd ---
> Electron flow is bottom-up, signal is left to right, and no
> busing of power so that there is less chance of confusing signal
> flow where there is supposed to be none. The Tektronix Way. ;)
Nice drawing, Jon. Most servers don't show binary attachments,
so now we can comment. Nice to see wafer's circuit. Thanks.
> The concept is almost the same as Win's corrected entry, yes?
> Except that S1 can be quite properly directly connected without
> any limiting resistor.
Right, that's the benefit of using a FET instead of a BJT.
> Win's circuit included a free-wheeling diode (if I'm using that
> term okay) to allow an inductor a path to wind down its current
> when shut off while yours misses that piece and instead places
> one in series with the load.
I have a few comments.
A series diode doesn't protect from inductive flyback (which
for hi-side switching is negative). Offhand I can't see what
purpose a series diode serves. However, without a shunt diode
one should not drive relays, solenoids, buzzers, motors, etc.
Given that Wafer selected a FET over a BJT his circuit should
have one less part than mine. However with the recommended
shunt diode it has one more. OK, we can eliminate the series
diode. Hmm, I can't see what the 10k resistor R5 is doing.
Without those two parts Wafer beats me by one part. :)
A BSS110 p-channel FET is wimpy, limited to under 100mA, but
Wafer's low resistor values allow using much a bigger FET.
High-side switching is dangerous, unless an effective form of
short-circuit protection is provided, e.g. shutoff, foldback
current limit, a quick fuse, etc. Low-side switching is used
in cars, etc., because it has the nice property that a wiring
short simply turns on the switched device. Recommended.