I'm a new poster here. I've posted a couple of times to SED, but always felt
that the general level of SED was _way_ above me. SE_Basics_ is more my
level.
If you aren't designing, it doesn't belong there. You are trying to
interpret an existing schematic.
My questions are about this battery tester (an "electronic load"):
http://home.comcast.net/~bobengelhardt/eLoad.jpg
This is all about loading the battery so you can see what the voltage is
when under load. A battery may show decent voltage when unloaded, but
then drop significantly when under load. This sort of thing means when you
measure the voltage, it's more like when the battery is in the circuit.
You could just load the battery with a big resistor, but that has
problems, so the fancier circuit.
U1 and Q1, the fet, form a constant current sink, meaning no matter what
the voltage the FET sees, it keeps drawing the same current (a resistor
across the battery would draw current varying with the voltage on the
battery).
Since the circuit turns voltage into a current draw, if there was no
voltage regulator, the drain would vary according to whatever is powering
this circuit, the 9v battery. I'm not sure that the voltage regulator
isn't overkill, but it doesn't add much in cost or size to the circuit,
that sort of voltage regulator can be found in plenty of computer
switching supplies, though maybe not that specific device.
1. What is the purpose of R5? My guess is that the ckt would work without
it, but it's in some way better with it.
U1 and the FET form a constant voltage sink.
The resistor is in the feedback circuit to the op-amp. No, it probably
isn't needed, it's there for isolation (note that c1, the .01uf capacitor,
is probably there to limit frequency response, and without R5, it wouldn't
work as well).
2. The 2nd op amp: "The voltage reference, U2, provides a stable 2.5-volt
reference voltage ...". How does it do that? It's not connected, so it must
be internal, but ...??
U2 is the TL531 in the upper left corner, a precision voltage regulator
(kind of like a fancy zener, where you can actually control the point at
which the regulator regulates). It supplies a stable voltage to U1.
The other half of the 358, is simply labelled "U" and is completely out of
the circuit (it's not used, so likely a dual op-amp was specified because
the 358 has certain characteristics that means it works better in a single
voltage power supply)
Michael