Hello again,
The simplest circuit is now here...
Vic:
It doesnt really need a precision op amp, although that would be nice,
but it has to be fast. I'd like even faster than LF347 but that might do.
OP07 will be too slow. OP27 if you feel like spending the money.
Now you see what i've been dreaming about, and how many many options
there are for a circuit like this. So many choices, it's hard to
decide /ubbthreads/images/graemlins/smile.gif
Worst case for 0.1 ohm is that the response to a current overload
step is too slow or rings too much. Im shooting for 1us max with
maybe 10% overload. I'll take 2us, but i think 100us is too slow.
Also as i was saying, a left over section of the op amp can be used
to scale the voltage meter to read directly in Amps with something like
two additional standard resistors.
Transformer doesnt have to be center tapped, but for the advanced
unit some means of supplying a negative bias has to be employed, so
that means either center tapped or an additional wall wart or small
transformer (100ma only).
The main reason for the simpler design is so that people who dont
want to have to connect lots of parts can still build one and get
very good performance and versatility. This means two parts lists
more than anything else, but the connections are a little bit
different too for voltage mode and current mode operation.
See the schematic for a better explanation.
The reason for two types of rectifier diodes is mainly because the
negative supply only has to supply maybe 100ma or less while the
positive supply has to provide maybe 3 amps. This means smaller
diodes can be used for the negative supply, but of course this
isnt mandatory so if you want to use a bridge rect that's rated
for the max positive supply (or more) that's ok too.
Note however that if you use a single winding transformer
(no center tap) you need four 1N5402 diodes (as well as
another transformer).
Take care,
Al
