From: "Frank Bemelman"
References: <3DC517EB.firstname.lastname@example.org> <3DC57B4E.email@example.com> <firstname.lastname@example.org> <email@example.com> <3DD39D90.firstname.lastname@example.org> <3DD4E13B.email@example.com>
Subject: Re: zero-power toggle circuit; was, how to master electronics
Date: Fri, 15 Nov 2002 15:27:40 +0100
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Organization: EuroNet Internet
NNTP-Posting-Date: 15 Nov 2002 14:28:09 GMT
"Winfield Hill" schreef in bericht
> Wafer wrote...
> > First, the resistors have been selected to work with a 9 volt supply.
> > This design was intended for battery operation, hence the need for no
> > current in the off state. The diode is only needed if the load includes
> > a capacitor to smooth out the supply voltage. As with Win's, enough
> > capacitance can prevent it from operating properly. I would use a
> > Schottky or something, to keep the voltage drop to a minimum.
> You're pointing out that it won't shut off properly, because the user
> will have to hold the pushbutton long enough for a load capacitor to
> discharge, etc.? That's a good point.
> > Now you may think the 10K is not needed, and in the case of the P-ch
> > MOSFET, it *could* do without it, but if the P-ch MOSFET ever shorts
> > from source to gate, the current draw would be big trouble, so it's
> > there just in case.
> In my experience damaged FETs usually fail with a gate to drain short
> (not gate to source). (This means they'll turn on the load without
> being told to do so, but with a 2 to 4V drop across the FET.) Such a
> failure in your circuit wouldn't need the 10k resistor's services.
> 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. I've also edited the drawing specifying a Schottky,
> and changed to a more serious FET for general purpose use.
> >: ,----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.
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