Re: op amp controlling a mosfet

kell wrote:
John Popelish wrote:

kell wrote:

Imagine you have a vehicle that receives up to ten amps from a
generator, and with the generator off you need a blocking rectifier to
keep the 12 volt battery from discharging into the generator windings,
so you decide to use the body diode of mosfet as the blocking
rectifier; during charging you can have the mosfet turned on to reduce
dissipation. The blocking has to occur in the positive rail because
ground consists of the vehicle chassis.
What is the best way? I was thinking about something along these
lines:

generator
|
,------------+
| |
| _ _|D
'--|-\ | STP80PF55
LF412| >-+-|| p-channel
,--|+/ | |_ mosfet
| | |S
| 100K |
| | |
| | |
'-------+----+
|
|
battery

Somebody told me the LF412 works with common mode inputs at the
positive rail, but if I should be looking at some other op-amp let me
know.
http://www.national.com/ds/LF/LF412.pdf


I doubt this simple circuit will do. The concept is to drive the gate
negative (down to ground) with respect to the source any time the
battery is more negative than the generator. So it gets real
important what powers the opamp, and that it (both the supply pins and
inputs) is protected from voltage spikes. Are you going to power the
opamp at all times from the battery?

I expect to power the op-amp from the generator.

Then you have a problem to solve. How do you keep the gate voltage at the battery positive (to keep the gate to source at zero) when the generator is off and the only voltage the opamp can output is zero?

I didn't indicate power pins, bypass caps, input resistors and such on
the circuit diagram. I was more interested in the concept, the utility
of the LF412 and and the like so I didn't flesh out the circuit like I
guess I should have.
So are you saying that conceptually it's ok, but that actual
implementation is impractical? I get the impresson you don't like the
idea of powering the op-amp from the generator.

That may be practical if you add an open collector or open drain output to the opamp so that it can pull down or let go, only. That way, when the opamp is unpowered, the gate to source resistor can pull the gate voltage up to the source voltage. you may want to lower the gate to source pull up resistor.

.

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