Measuring Current with an op-amp

Discussion in 'General Electronics Chat' started by crobertsbmw, Nov 23, 2011.

  1. crobertsbmw

    Thread Starter New Member

    Sep 7, 2011
    I want to be able to monitor when the load turns on. It's going to be a low impedance load (oven), and will pull a lot of current. I was thinking that I could just throw in a really small resistor in series and measure the voltage across the resistor. The only problem is that the oven will be ran off of A.C. but the op amp or comparitor will be powered with a 3.3 vdc voltage.

    1) can I tie the DC ground to the ac neutral line or is that a really bad idea?

    2) the voltage across the resistor will only end up being about 2 volts when the oven is on, but with respect to ground it will get up to 110v is that going to be a problem when I am powering this op amp with 3.3 volts? Or does it really only look at the difference on the positive and negative in.

  2. SgtWookie


    Jul 17, 2007
    No, you can't safely do that.

    You either need a current transformer or some type of Hall-effect sensor.
    crobertsbmw likes this.
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    There is no way that you can do this safely in the way that you are imagining. Monitoring circuits like this need to be isolated from the mains, for instance using a transformer, or optically.

    You should also avoid dropping anything like as much as 2V in a resistance for monitoring a high power 110V load. If your oven load were 2kW, the monitoring resistor would burn about 36W, which is hardly convenient.

    My gut reaction is that you would probably be better referring this to a qualified engineer or electrician.
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    My suggestion of an optical device is not much use for current monitoring, as you would need to start with a current transformer in any case to get the volt burden down to something acceptable.

    SgtWookie's Hall-effect idea is simpler and more up-to-date, and avoids risking the noob's trap of the current transformer with open primary.
  5. crobertsbmw

    Thread Starter New Member

    Sep 7, 2011
    Allright, that sounds good. I will have to look into some hall sensors and see what I can come up with.
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    Have you thought about the timescale for this, whatever your sensor?

    Have you access to a standard clampmeter?

    It is also possible to fashion a heathrobinson version as an attachment for an ordinary multimeter.
  7. Wendy


    Mar 24, 2008
    I like the clamp meter idea myself. They are simple, easy to use, and reasonably accurate.
  8. T.Jackson

    New Member

    Nov 22, 2011
    I agree: transformer or some type of Hall-effect sensor.

    May I ask why you want to do this?
  9. Lundwall_Paul


    Oct 18, 2011
    If you realy want to measure this series (Between the coil and source) Where your resistor is. You need to replace that resistor with a current shunt. See A-A-55524SS1 This can be downloaded from the DSCC web site it gives the following part numbers for 10, 20, 30 and 50 amps.

  10. Adjuster

    Well-Known Member

    Dec 26, 2010
    A shunt resistor might be used to make accurate current measurements together with suitable equipment, for instance a low-range AC voltmeter, provided everything is rated for safe AC mains operation. It does not however provide any isolation, and requires the OP to connect things directly to the mains.

    This may be avoided if a magnetic pick-up device is used. If a clamp-type device is available it may not even be necessary to disconnect any wires, provided that the neutral or live lines are accessible separated at some point, rather than together in a cable.

    This is important, for in his original post the OP was talking in terms of tying an op-amp DC line to the mains neutral. It should be made clear that this is not a safe way to do things.

    It is true that for instance a Hall effect device may not have the accuracy available with a precision shunt. Perhaps the OP can comment on what level of accuracy is actually required?
  11. crobertsbmw

    Thread Starter New Member

    Sep 7, 2011
    All I want to know is that the Load is on or off. My load is an oven so it will be pulling a fair amount of current. I just want to be able to detect that the oven is on and then be able to send a signal to my micro-controller. The cheaper and smaller the circuit is the better.

    I was talking to my friend about using a hall effect sensor he was saying that when it is idling it will sit at a voltage in between 0 and Vcc (3.3v), so maybe like 1.5, which is kind of no mans land as far as digital logic goes. Then when the oven is on and pulling current it will oscillate from 0 to Vcc. Is there any other sleek way of doing this that comes to mind?