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Subject: Re: zero-power toggle circuit; was, how to master electronics
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Date: Sat, 16 Nov 2002 11:58:20 GMT
NNTP-Posting-Date: Sat, 16 Nov 2002 06:58:20 EST
Organization: Road Runner
Winfield Hill wrote:
> "Frank wrote...
>>"Winfield Hill" schreef ...
>>>>... The diode is only needed if the load includes a capacitor...
>>>>Now you may think the 10K is not needed... but if the P-ch
>>>>MOSFET ever shorts from source to gate...
>>>A more serious omission is a lack of any load-fault protection.
>>>The 10k resistor becomes useful when adding a current-limit
>>>feature, as I've done below. ...
>>>>: ,----x-----x----------x-------(O) V+
>>>>: | | C2 | Co/100 |
>>>>: | | === Rsc 0.7
>>>>: R4 \ V\| | R6 1k | Rsc = -------
>>>>: 100k / |---x---/\/\---x I-limit
>>>>: \ /| |
>>>>: | | Q3 |
>>>>: '----x------------||--x Q2
>>>>: | ||>-' IRF9520
>>>>: R5 \ |--,
>>>>: 10k / |
>>>>: \ |
>>>>: | |
>>>>: ,--------------x--------, | D1
>>>>: | ,----------- | ------x---|>]--(O) OUT
>>>>: | | | to load
>>>>: | \ R2 |
>>>>: \ R1 / 220k |
>>>>: / 1M5 \ |--' Q1
>>>>: \ | ||<-, 2N7000
>>>>: | S1 x--------||--x
>>>>: | _|_ | |
>>>>: x---o o----x x----------------(O) GND
>>>>: | | |
>>>>: | C1 \ R3 gnd
>>>>: === .1uF / 100k
>>>>: | \
>>>>: | |
>>>>: --- ---
>>>>: gnd gnd
>>>Q3 can be any old small npn. R5 and C6 delay the current limit
>>>long enough to charge load capacitors. C6 can be about C-load/100.
>>I saw you moved D1 ;) The overload protection is nice too.
> Yes, rotating it allows using a bracket ] for the Schottky bar. :)
> I'm not very happy with the above scheme. For one thing, combining
> short-protection with the flip-flop memory leads to awkward issues
> like we saw when charging capacitive loads, etc. Further, momentary
> current-limit conditions could reset the flop. And then there's the
> issue of the series diode, wasting power. The circuit badly needs
> further improvement, and I can picture it all in my mind.
> But right now I'm going to mow the lawn and rake leaves.
> - Win
I would like to see what you have in mind to take care of load
capacitance. After all, your circuit doesn't address this problem at
all. A .2V drop seems a fair price to pay, rather than more components,
and more cost, only to save a tiny amount like that. I'm not happy with
the idea of using a series resistor for current monitoring though, since
THAT can waste MORE power. If you know the load, as in a hand held
device that doesn't power external loads, there's most likely no need
for it. This might also eliminate the need for the diode as well, since
the load capacitance only becomes a problem if it holds the voltage up
too long. However, my design allows the user to hold down the switch for
as long as they want, without side effects, and once they let up on it,
the circuit becomes ready to toggle again more rapidly than yours. The
smaller capacitor also means less cost and requires less board space.
The idea of using a MOSFET capable of 15A seems silly to me. If you can
source that kind of power, why would you need zero power in the off
state? Automotive batteries go dead long before a CMOS chip could drain
them much at all.