i need simple circuit to measure current using pic
i have a circuit but the resistance is so small like (0.0025Ω) and there are no resistance have this value

 

Hello,

Here is a resistor of 0.002 Ohms:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=TGHGCR0020FE-ND

Bertus

 

It's easy to measure current if you put the resistor on the low side, that is, connected to the ground. Then, you can measure the voltage at the resistor using the ADC of the PIC.

 

You did not state whether you are interested in measuring AC or DC current but I believe from your questions it is DC current measurement that you are after.

hgmjr

 

You don't state what current levels you want to measure, so this won't work for high currents. However, for currents with an order of magnitude in the 1 to 10 A range, you can make a shunt resistor from stainless steel lock wire (used e.g. in aviation work). Harbor Freight sells a spool of safety wire of about 1 mm in diameter; a 10 cm chunk has a resistance of about 9 mΩ. It's also useful as a general purpose wire around the shop. Nichrome and Chromel are also useful wires to make resistors with.

 

If you use a low value of resistance or a low current the output voltage will be low. A op amp can boost this voltage to a more useful value. There are also IC for current measurement ( often with an external resistor ). Hall sensors are another way to convert current to voltage.

 

You can make a low range resistor just by scribbling on a smooth surface with a pencil.

Graphite is a decent conductor.

 

or buy a typical current shunt.. or pencil lead... or this if you are working in a foundry http://www.abb.com/product/seitp322/87658a38b941842dc1256f480034c11c.aspx

 

If the current is going to ground you could use this simple op amp application. Since the - input is a virtual ground it will have no resistance, and have a voltage out for very low current applications. The better the op amp the better it works.

 

Back in the old days, the Simpson 260 series VOM simply used a short length of 10 ga wire between the - 10A and +10A jacks. A pair of tap wires were soldered to this wire at locations so that 50 mv was seen when 10A was flowing throught the large wire.