Spehro, maybe you can educate me a little bit. I am hoping that I
could use a mosfet with its gate not energized (connected to drain),
as a diode with low voltage drop, acting, rather, like a resistor. It
would conduct from drain to source.
Is it correct that I can do something like that?
No, it's not that easy. The MOSFET won't really start to conduct until
the gate voltage is several volts, so it can't really be a 2-terminal
device and still be better than a silicon diode. You have to drive the
gate from some other circuitry. What you'd get if you did what you
suggest is just a slow and expensive diode with normal voltage drop.
For measuring applications often we use a "precision rectifier". There
are a few ways of doing it, most of which involve an op-amp and a
diode or two. Rather than drone on (in the Larkinsian sense), I'll
suggest something you might not find-- If you just need to indicate AC
mV on a moving-coil panel meter, you can do something like this:
+---|>|----+----|<|-----+
| + | |
| ,---. |
| | M | |
| '---' |
| - | |
+----|<|---+-----|>|----+
| |
| .-.
| | |
| +V | | Rb
| |\| '-'
+--------|-\ |
| | >-----------+
.-. +---|+/
| | | |/|
Ra | | | -V
'-' |
| o input
|
===
GND
Ra sets the sensitivity, Rb limits the coil current and
causes the op-amp to have to slew faster.
Or maybe this where the extra diodes shunt current away from
overloads and damp the movement.
+---|>|----+------+-----+-- -|<|-----+
| + | | | |
| ,---. | | |
| | M | | | |
| '---' | | |
| - | --- --- |
| | / \ \ / |
| .-. --- --- |
| Rs| | | | |
| | | | | |
| '-' | | |
| | | | |
+----|<|---+------+-----+-----|>|----+
| |
| |
| |
| +V |
| |\| |
+--------|-\ |
| | >------------------------+
.-. +---|+/
| | | |/|
Ra | | | -V
'-' |
| o input
|
===
GND
Best regards,
Spehro Pefhany
