From: Jonathan Kirwan
Subject: Re: zero-power toggle circuit; was, how to master electronics
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NNTP-Posting-Date: Fri, 15 Nov 2002 01:45:07 GMT
Organization: AT&T Broadband
Date: Fri, 15 Nov 2002 01:45:07 GMT
On 14 Nov 2002 15:11:08 -0800, Winfield Hill
>> 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.
His was kind of hand-drawn, looked like it was done with
Microsoft Paint. But to be honest, I'm following your lead in
ASCII schematic art. The use of , and ' are just one example of
that (I tried to do better and after doing a FET or two and a
few other parts where wires were closely spaced, decided this
actually worked more generally than other methods.)
>> 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.
hehe. Yup, as such a circuit clearly shows. :)
>> 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).
Yes, I understood that, as well. But decided to report, more
than comment, in this case.
> Offhand I can't see what
> purpose a series diode serves.
Good! Then I'm not alone. I didn't see the use of it in this
case, but I also know I'm pretty limited in my viewpoints, too.
I miss things since this isn't my profession, just a hobby.
> 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. :)
I'm glad you pointed out R5, too. I couldn't figure a need for
> 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.
Thanks for all the comments. Here's wafer's new circuit:
: ^ V+
: \ R4
: / 100k ^ V+
: \ |
: | |
: x----||--x Q2
: | ||>-'
: ,-----------------------x |--,
: | | |
: | ,----------- | -------x
: | | | |
: | \ R2 | |
: \ R1 / 220k | |
: / 1M5 \ |--' Q1 |
: \ | ||<-, 2N7000 |
: | S1 x--------||--x o
: | _|_ | |
: x---o o----x | LOAD
: | | ---
: | C1 \ R3 gnd o
: === .1uF / 100k |
: | \ |
: | | ---
: --- | gnd
: gnd ---
Which could be easily reflected so that the power switch is on
the low side, of course.
Oh... 3rd edition???