Re: op amp controlling a mosfet

kell wrote:
Ban wrote:
Why the opamp, wouldn't this be a *lot* simpler cheaper and have
zero power drain when off?

o-+----+^+-----+-o
| ||| |
| === |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+--| NPN
|>
|
o------+---------o

There could be a glitch when the generator's turning off, and you
would end up with battery current flowing through the windings of the
generator. That would surely keep the voltage at the base of the npn
high enough to keep it and the mosfet turned on.
Maybe this would work, though:


o-+------+^+-----+-o
| ||| |
| === |
|/ | |
/_ 12v | |
| zener| |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+----| NPN
|>
|
o--------+---------o
For the transistor to turn on, the voltage at the generator output has
to be at least 12.7 volts. If there's a glitch while turning off and
for some reason the voltage at the transistor's base doesn't fall far
enough to turn it off, the 12 volt zener gives you a little insurance.
The heavy drain on the battery as it discharges through the low
impedance armature windings will probably drag the voltage below 12.7
volts, turning off the npn transistor and the mosfet.
Adding a common diode in series with the 12 volt zener will bump the
insurance to over 13 volts.
I like it.
That will work better, in fact I had already drawn a similar circuit, but
you were faster. I have used another divider 1k/10k in the gate drive to
increase the voltage drop with a weak input.
But I think a Schottky diode will do the job as well with only 0.4V drop.
Put a transil into the input line in case the switch shuts off very fast to
avoid the spikes. A DC-generator will not be as reliable as a normal
alternator and there will be the problem with overcharging/regulating the
battery voltage as well.
--
ciao Ban
Apricale, Italy


.

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