From: Winfield Hill
Subject: Re: zero-power toggle circuit; was, how to master electronics
Date: 16 Nov 2002 08:40:11 -0800
Organization: Rowland Institute
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> <firstname.lastname@example.org> <3DD63323.email@example.com>
X-Newsreader: Direct Read News 4.10
> Winfield Hill wrote:
>>>> Wafer wrote...
>>>>>... The diode is only needed if the load includes a capacitor...
>>>> 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. ...
>>>> [ snip Win's circuit ]
>> 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.
> 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 0.2V drop seems a fair price to pay...
More like a 0.5 to 1V drop at the higher currents a general purpose
design might need. Hmm, your remarks sound faintly like a challenge.
Here's a circuit that's tolerant of load capacitance without a series
diode and includes robust load-fault protection.
: V+ <--+-----(O) V+
: V+ V+ | 6 to 18v
: | Q4 | R6 0.1 5W
: R4 V+ v\| |
: 10k | 2n |------+ 5A limit
: | Q2 | 4403 /| |
: | BSS84 | | |
: +---||--+ +----||--+ Q3 pmos
: | ||>-' | ||>-' IRF9540N
: ,--------------------+ |--, R5 |--, 0.1 ohm
: | R2 | | 22k |
: R1 ,- 220k -- | ------+ | |
: 1M5 S1 | +------ | ----' +-----(O)
: | _|_ | 2n7000 | | _|_ \
: +---o o---+ Q1 |--' R7 /_\ |
: | | ||<-, 1k | LOAD
: | 0.1uF +------||--+ | green LED | |
: === C1 | | '----|>|-------+ /
: | R3 gnd ON indicator +-----(O) -- return
: gnd 100k |
: | gnd <-'
Here I've simply isolated the toggle flip-flop from the power switch.
No ugly time constants, etc. The MOSFET needs a heatsink to handle
serious shorts, but if two resistors are added for a foldback current
limit, the heat sink could be reduced or eliminated. If a fault LED
is desired, R5 could be reduced (R4 too) and an LED placed in series.
> I'm not happy with the idea of using a series resistor for current
> monitoring though, since THAT can waste MORE power.
OK, picky picky, perhaps I can deal with that issue - in a later post.
> 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.
15A maybe, but a low-resistance FET has two benefits: more delivery
to the load and less self heating so a heat sink can be eliminated.
The benefits occur at practical currents like a few amps, which can
be easily drawn from a set of 5-year shelf-life Alkaline batteries.
Also, CMOS chips draw no current; one can use them if they prefer.