Current to Voltage Converting Using Omp Amps?

Discussion in 'General Electronics Chat' started by crazyengineer, Jul 7, 2011.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    Hello, me again.

    I want to build a circuit that's able to measure the current at a given point of a circuit. I want to do it using omp amps and I thought I could do it by using a difference omp amp and connect a resistor between v2 and v1, measure the potential across the resistor, and use ohm's law to find the voltage, but I realize the resistor will change the current flowing through the circuit.

    Can anyone pitch some ideas please?
  2. someonesdad

    Senior Member

    Jul 7, 2009
    The two usual ways of measuring current are to a) measure the voltage drop across a component and infer the current and b) measure the magnetic field associated with the current. Such methods are used for both DC and AC measurements. For AC measurements of type b), tools such as Hall effect sensors and current transformers are used. For DC measurements, Hall effect sensors are typically used (you could also use a SQUID based on a Josephson junction, but my intuition tells me that's probably an inappropriate tool here :p). There are also more exotic methods like using the magneto-optical Kerr effect, but we're probably getting into the I'm-smoking-too-much-dope region...

    Method a) is known as using a shunt resistor and is equivalent to the 4 wire method of measuring resistance (Kelvin measurement). The trick is to use as small a shunt as possible to avoid interfering with the circuit. You might be able to use an existing circuit component (always a good idea to try this one first).

    If you can measure the resistance of a wire or conductor that's carrying the current, you can use that as a shunt resistor. The usual problem is that typical conductors have nonzero temperature coefficients, so the measured current needs to be corrected for both ambient temperature and temperature rise due to Joule heating. Welcome to engineering. :p If you're lucky, you can find alloys like Manganin or EvanOhm that have low tempcos. Constantan wire from a thermocouple also might be appropriate. Alas, conductors have low resistivities, so the voltage drops will be small, leading to noisy measurements. More engineering...

    For DC currents, method b) requires a Hall effect sensor and perhaps a low MMF magnetic circuit to conduct the flux. Again, more engineering.
    crazyengineer likes this.
  3. iONic

    AAC Fanatic!

    Nov 16, 2007