imranahmed 
Please let me know that signal conditioning need for Ammeter.
I saw in the Ammeter "MAKE:FOTEK" a wire is wound only one round and input signal is pass through that wire.
1)What is the purpose of that wire?
2)What are the basic needs for signal conditioning AC
or DC input?
That wire is a resistor. It has a low value. This is often called a "current shunt".
You measure the voltage across the shunt caused by the current through the shunt. Knowing the resistance of the shunt allows you to use Ohm's Law to determine the current.
Signal conditioning depends. There are now a number of "current sense" ICs that will amplify the small voltage of a current shunt, especially when the voltage level of the current shunt is not close to ground. For example, if you put a shunt in a 12V power supply positive lead, you have such a condition.
Shunts are not often used for AC power. Instead, current transformers will do a similar function AND provide safety isolation.
Hope this helps
Jim
imranahmed Another Ammeter I saw "MAKE:AUTONICS" , they used 40 mili-ohms resistor and current transformer output connects with its leads.
I think it is use for current-to-voltage conversion.
Torby 
You can also measure current by putting a magnet sensor, say a hall effect part, by a coil.
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imranahmed You can also measure current by putting a magnet sensor, say a hall effect part, by a coil.
Dear Torby,
Please kindly provide me information about Hall Effect,
its biasing and which model is appropriate for analog sensing.
Torby How 'bout a link: http://bit.ly/LtnD15
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If high precision is not needed, Hall Effect devices, such as ones from Allegro, can be very useful. There are devices optimized for DC and for AC (50/60Hz line). They also provide a modest degree of isolation and are not outrageously expensive (though, not "cheap"). The AC ones give an offset AC output (typically centered around Vcc/2); for these, you need to read multiple times in one cycle with ADC and numerically process to get peak or RMS.
Jim
theusch 
They also provide a modest degree of isolation...
Then again, while we've designed them in, never on a UL application so I don't know what the rules might be. It has been on modest low-voltage DC and AC draws.
Oops--never mind; CRS. A revisit to the site indicates 2000 to 5000 V.
Well, if you have a copper trace on a board with the Hall device over the trace, then you have exposed high voltage wiring. That makes me uneasy. Solder mask is not a high quality insulator.
Jim
theusch
Well, if you have a copper trace on a board with the Hall device over the trace, then you have exposed high voltage wiring. That makes me uneasy.
All the Allegro devices I've used run the field current through leads on the Allegro chip. There is no "put the device next to the trace".
BTW, for modest mains-type current measuring a PCB-mounted CT (current transformer) works well, provides high isolation, and a proper setup is ULable. IME CTs are also the most common for higher currents, but would not be PCB-mounted. There are other methods. I cannot remember ever having used voltage drop to measure current in mains/AC apps. Perhaps it could be done, if the AVR was totally floating or isolated. Given the nature of OP's inquiry, I would strongly recommend proper isolation.
I agree. Mains isolation is really important.
Jim
