Measuring currents without loosing power?

[Sandman]

Hello, I´m working on a project where I will need to measure currents in a system that can consume no more then 300mW/h. In this power budget the power consumption of all circuitry must be included. The voltages that the measurements will be done at are between 8.5 and 20V and the currents measured will be from 400mA down to as low as one can measure them without to expensive equipment. The problem is this: Using the standard method by measuring the voltage drop over a known resistor the situation is this. If the chosen resistor is close to 1 ohm the power dissipation will be too large since several measurements like this is to be made. And if the resistor is to small the accuracy will decrease too much. And in our case any circuit used to make any measurement will only be supplied by +3.3V so the measurement input must probably be divided down to fit in this range. This will of course decrease the accuracy as well.
Is there anyone that has been into this kind of problem and can suggest a solution? Is there some 'magic' circuit out there for making measurements like this?

//Sandman

 

[Klaus]

Sandman, is this some kind of assignment that has you baffled??
Usually one inserts an ammeter (or milliammeter) into the supply line and measures the current flowing into the connected circuit, whatever circuit that might be. I see no point messing about with 1 ohm resistors and voltage measurements :wink:
Hope tis helps.

 

[Exo]

Why would the accuracy suffer from a smaller resistance value ?
In my digital MM the shunt used is only 0.1 ohm if not less.

Just amplify the voltage drop across it (with opamp for example) to 'stretch' the reading open

 

[ljcox]

Another way to measure current which consumes virtually no energy, is to use a current probe. This is a device that clips over a wire and determines the current by the strength of the magnetic field around the wire.

Len

 

[Russlk]

If you pass the current thru a solonoid coil with a hall-effect device to measure the magnetic field, the power loss will be very small. The dynamic range should be enough to get from microamps to amps. Allegro makes a sensitive, linear hall effect, look it up.