Looking for simple current sensing circuit... :)

Much more info required. Give us a complete explination of what is involved
e.g. do you want to detect current flowing in a wire, and if so, what is the
'normal' flow? 240v AC, 9v DC, a few microamps from a phono stylus, what?

Without a complete explination (hint: write at least two paragraphs) we
can't help you.

Hi there,
            thanks in advance, I haven't really worked out what current is
going to be flowing yet, but at the moment I'm estimating 12V DC at 40-50W,
but yeah, it'll be monitor current through a wire, in this instance may be
the 12VDC supply line, let me know if you find anything! Thanks!

put finger to keyboard and composed:



The above circuit is a comparator that tells you when a certain
current threshold has been exceeded. Is that all you need, or do you
want a readout of the actual current?

If the former, then a simple circuit such as the following may
suffice:


      PNP transistor
          E     C
 +V o--|---\___/-----> to overload indicator
       |     |
       |     R
       |     |
       |-Rs--|------> to load


Note that you will lose up to 0.6V across sense resistor Rs. If the
max current is to be 5A, then choose Rs = 0.12 ohm, 5W+.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

that'll be all over the place with transistor (expect a 4:1 beta
variation from a single manufacturer), temperature and no doubt sock colour.

"Terry Given"



** Individual transistor beta will have no effect,  the circuit operates on
device Vbe  -  you arrogant sheep shagger.

Temperature will increase the sensitivity by 2mV ( or 16mA in this case )
per degree C  -  likely a trivial amount.

Bet  *YOU*  have socks of different colours as sizes on each foot.




........   Phil

depends on the load really. If you want an astonishingly crude circuit,
then use this one. Beta does count, because the current flowing into the
"sensing circuit" is one beta'th of the load current.... say you use a
grounded resistor, and a comparator. Then the actual trip point will
depend strongly on Beta.

it *wont* provide a nice crisp "switching" action. but often rough
enough is good enough.

google SED, there are plenty of good ways to solve this problem. use a
single supply opamp with an input common-mode range that includes the
+ve rail. Convert the I*Rsense voltage into a current with a pnp
transistor and a resistor. Then feed that current into a grounded
resistor, and voila - high-side current sensing. Nice and accurate,
stable and repeatable.



Nah, barefoot.


I thought your RMS current meter was pretty good, Phil. Its not quite
what I needed though, and I kinda got busy....

Cheers
Terry

"Terry Given"



 **  Bullshit  -  you are a pathetic loser.






**  Really desperate sheep shagger crapology.






**  What   **** utter ****  bullshit.





**  Wot about the gumboots you shove the ewe's hind legs into ?





**  Can I expect to see your clones on the NZ market soon ??




...........  Phil

Thanks for the replies guys, I'll been busy, when I'll have time I'll give
it a try and see how it goes... thanks

finger to keyboard and composed:





With respect, I think you are having a brain fart. The circuit will do
just fine if the OP's requirements aren't too stringent. At the very
least it satisfies his requirement for simplicity.

If OTOH he wants something with all the bells and whistles, then he
could adapt this design by modifying the front end:

 http://www.siliconchip.com.au/cms/A_30551/article.html

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

What about MAX721, MAX722