Shunt resistors and measuring current drawn from a device

[Sceadwian]

Can someone supply me with more information/thoughts on shunt resistors and measuring current draw from a device? I'd like to be able to read the voltage created from the shunt resistors on an industrial rectifier at work (I don't have the stats on it yet though) I'd also like to measure milli/micro amps from some general I/O pins. I know so little about shunts (including creating them) I'd like to use CAT5 cable to create some test shunts, but I lost the link to the page that had a calculator to figure out how long the cable needed to be to get a certin ohmic value. I only want to test basically from 0 to 1 amp. Using my micros (AVR) that'll give me .003ma resolution which sounds like a good test to me. I worked out for my first test I need a .128 ohm resistor. I don't have anything floating around I can really use for that accept for CAT5 cable. For refrence the ADC I'm using is differential 0-2.56 with a 20X gain stage so really 0-.128 volts 8bits so it can measure(reliably) about 1mv differences.

 

[Kral]

Re: Shunt's revisited

Sceadwian,
See the wire tables in the CRC Handbook of Chemistry and Physics (Available at any decent library) for resistance per unit length for various wire sizes. One of the problems with copper wire is that its resistivity changes considerably with temperature. Since you are only using an 8 bit A/D converter, I'm assuming that you're accuracy requirements are not stringent, and therefore this is not a prbolem for you. Another option is to use one of the commercially available shunts (available from Vishay and others) and change the gain of your amplifier to get the propler scaling for maximum resolution from your A/D converter.
Regards,
Kral

 

[mayu_wakui]

Re: Shunt's revisited

here's a link for developing a single phase energy meter. For current sensing the meter uses a shunt, instead of a CT. Maybe the info might get u a kickstart.

https://www.analog.com/en/prod/0,2877,ADE7757,00.html

click on the 'application note' link.