Electronic – What’s the easiest way to implement portable current measurement for a ~200W dc motor

Zetex (now Diodes, stupid name) has a nice little IC which has been mentioned on this site a few times. The ZXCT1009 is a high side current sensor which works as a current divider, though to some extent (low currents) you'll also be able to use it as a multiplier.

The transfer function is

\$ I_{OUT} = \dfrac{R_{SENSE}}{100 \Omega} \cdot I_{SENSE} \$

so with a 1\$\Omega\$ sense resistor you get a \$\div\$100 divider; a 1k\$\Omega\$ resistor will multiply by 10. The output current is limited, but you can use the schematic to roll your own multiplier for higher currents:

Don't make the sense resistor too small; below 10mV voltage drop accuracy decreases fast.

edit (re your comment)
For high side current measurement I think this is an excellent IC. Choose a 100m\$\Omega\$ resistor for \$R_{SENSE}\$ and a 1k\$\Omega\$ for \$R_{OUT}\$ and you'll have a nice 1V/A output.

One option is Vishay's bulk metal foil resistors.

They have many package configurations available, but the VCS series looks good for current sensing applications.

• 2 ppm / C TCR typical
• 0.5% initial tolerance
• 4-terminal configuration
• Much more stable in value after ESD/EOS events, compared to other types.

Pricing and availability will probably drive you crazy. I found a 1 Ohm VCS1625 part at Digikey for $16 each. For 3 A you probably need to go to 0.1 or 0.05 Ohm due to the resistor power limit.