Current Sensing Resistor

[manumenzella]

Hi. I want to make a current sensing device, and I read somewhere (cant remember now) that you could you a small value resistor (about 0.1R) a measure the current drop across it. But how do I calculate the current from that value??? Should I use a simple op amp to amplify the voltage drop (like the 741)???
Thanks!!! Manuel

[ronsimpson]

Manuel,

1 ohm=1 volt for 1 amp. 1 ohm=2 volts for 2 amps.
0.1 ohm=0.1 volt for 1 amp.
I think you can do the math.
Question AC or DC. This may make a difference. How much current? Power Line? (110/220)?
What resolution? If you want to know; is load on (about 0.1 volts) or is load off (about 0 volts) then the PIC can measure directly with no amplifier. If you want to know 1.05 amps then you will need a amplifier! (0 to 0.1 volts on resistor---50x amplifier---0 to 5 volts to PIC.

rons

[sougata]

Hi,

It is important whether you are doing a high side or low side sensing. Low side is the easiest and most frequently used. LM358 is a dual supply amp and works with a single 5 volt supply. Please note that it is not rail-to-rail. That means your op-amp cannot output the entire supply voltage range. If you a using a dual supply then you can use OP07 and pin to pin replacement for LM741 but with better offset voltage characteristic.

Please describe your exact requirements so that some possible schematics can be posted.

[manumenzella]

Hey. Thanks for your answers. I would like to make a battery dircharger / analizer, so the resolution will have to be quite good. The discharge rate will be about 200 - 400 mah. Can you send me any example of using an op-amp to amplify the voltage. Thanks agaaaaaaaaaaaain!!!!!
Manuel