Current sensing circuit help

Discussion in 'Circuit Help' started by Yuda, Dec 25, 2017.

  1. Yuda

    Yuda

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    I am having a little trouble sensing the current on a high impedance load. I am using Multisim blue to simulate the circuit I built and it works fine on the simulation but it real life it just does not work. the only difference I have in real life is that I am using a transformer less power supply to supply my 14VDC, Yes, I have it in a plastic box and I only have the positive and negative DC leads coming out of it. I am aware of the risks. The problem I am having in real life is when I do not have a load hooked up through my CT transformer I get 1.7VAC on the output of my op amp and connecting my load does not really seem to make much of a difference on the output. In the simulation however when there is no load on my CT transformer I only have 187 mV and then when I connect my high impedance load the voltage shoots up to 5.45VDC which is what I need. By the way, on my simulation I could not find a CT transformer to use so I had to use a voltage transformer. I changed the characteristics to have 35H constant inductance, 1 turn primary, 1500 turns (45.5 ohms) secondary. Also I am using my op amp in a closed loop configuration with 401 gain. Any help on this circuit would be very much appreciated. Also I am new to this forum so if there is anything I left out or if I posted this in the wrong section please let me know. Thank you for your time.

    upload_2017-12-25_17-5-38.png
     

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    Yuda, Dec 25, 2017
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  2. Yuda

    Yuda

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    Yuda, Dec 25, 2017
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  3. Yuda

    Alec_t

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    Does your sim take into account the input bias current of the op-amp and the fact that a LM324 can't pull its output all the way down to ground in that circuit?
     
    Alec_t, Dec 25, 2017
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  4. Yuda

    73's de Edd

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    With R4 at 14.4 MILLION ohms of constriction of your primary input power loop.
    Believing that and not seeing the error, one should also believe in a gnat on a hamster wheel , as being the power source of a Corvette dragster.
     
    73's de Edd, Dec 25, 2017
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  5. Yuda

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    There are a number of problems, but some of them won't affect you until you have an input.

    The problems you'll face without an input include:
    • The gain of 400 is quite high for a single stage. This will amplify problems with offsets and reduce the frequency response.
    • The input offset of 2 to 3 mV will cause an output error of 0.8 to 1.2 V
    • The output claims to go to 0V, but under ANY load it cannot. Even your feedback loop loads the output.
    • Any noise at the inputs will be amplified. Because the op amp's output will respond asymmetrically when near the supply rail, the average output voltage may rise.
    • The input currents don't have an equal impedance to ground. This will cause the input currents to generate an additional offset voltage (that will again be multiplied by 400). This is likely to add a couple of tens of millivolts to the output, unless the transformer's secondary DC resistance is really high.
    • If your power supply voltage has any ripple or noise, variations may cause a small change in the input bias current. Because the input is highly inductive, this will produce an input signal. Combine this with the uneven response to an input signal and you're average output could again rise.
    When you apply an input current, you will face some more problems:he output current will be 1/1500 of the input current of 83uA, this will become a secondary current of 55.6 nA. Given that your input current is typically 45nA, and up to 100nA, your signal is with the range of differences in input bias currents. At the very least you need to null this out.
    • The impedance of different at very low voltages and currents may not be what you expect. Do these diodes have particularly good forward and reverse leakage?
    • Your input signal is AC, and will go below 0V. Whilst this won't exceed the absolute maximum ratings, it will exceed the specified input range.
    What I would do:

    1. Split the rails and bias the inputs from a mid supply point.
    2. Provide an variable offset voltage at one end of the secondary of up to +/- 5mV or so to null out the output.
    3. Reconfigure the circuit as a precision rectifier.
    4. Find a current transformer with a lower ratio so you get a larger signal output
     
    (*steve*), Dec 26, 2017
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  6. Yuda

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Edd, R4 looked like 1.44M to me with my bleary eyes.
     
    (*steve*), Dec 26, 2017
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    73's de Edd likes this.
  7. Yuda

    Yuda

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    I set the resistance that high to simulate a 0.0001 watt load. I wanted to see if my CT transformer would be sensitive enough to measure it, I obviously am not ever going to need that. I was just testing the circuit and messing around.
     
    Yuda, Dec 26, 2017
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  8. Yuda

    Yuda

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    I was messing around with the circuit to see what would happen and I forgot to change that resistor value. I forgot to mention that my load will be dynamic as I am trying to sense devices plugged in to an outlet on my work bench. The devices range from a phone charger to a lamp to a grinder. the reason I started working on this circuit is because I am having difficulty current sensing when I plug in a phone charger. Also thank you for your in depth breakdown of my circuit, I really appreciate it, I just wanted to know if there is any way you could post a circuit that you think I should try? I am still learning the basics of op amps so knowing me I may make a mistake going off the 4 steps you posted.
     
    Yuda, Dec 26, 2017
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  9. Yuda

    Yuda

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    I am not sure if this helps but I copied this from the model of my LM324 from multisim blue. I checked to see if there was anymore information I could send you but this is really all it was showing me.

    .SUBCKT LM224__OPAMP_TEXAS_INSTRUMENTS__1 1 2 3 4 5
    *
    C1 11 12 5.544E-12
    C2 6 7 20.00E-12
    DC 5 53 DX
    DE 54 5 DX
    DLP 90 91 DX
    DLN 92 90 DX
    DP 4 3 DX
    EGND 99 0 POLY(2) (3,0) (4,0) 0 .5 .5
    FB 7 99 POLY(5) VB VC VE VLP VLN 0 15.91E6 -20E6 20E6 20E6 -20E6
    GA 6 0 11 12 125.7E-6
    GCM 0 6 10 99 7.067E-9
    IEE 3 10 DC 10.04E-6
    HLIM 90 0 VLIM 1K
    Q1 11 2 13 QX
    Q2 12 1 14 QX
    R2 6 9 100.0E3
    RC1 4 11 7.957E3
    RC2 4 12 7.957E3
    RE1 13 10 2.773E3
    RE2 14 10 2.773E3
    REE 10 99 19.92E6
    RO1 8 5 50
    RO2 7 99 50
    RP 3 4 30.31E3
    VB 9 0 DC 0
    VC 3 53 DC 2.100
    VE 54 4 DC .6
    VLIM 7 8 DC 0
    VLP 91 0 DC 40
    VLN 0 92 DC 40
    .MODEL DX D(IS=800.0E-18)
     
    Yuda, Dec 26, 2017
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  10. Yuda

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I recommend you read the datasheet.

    The simulation doesn't exactly match the real thing.
     
    (*steve*), Dec 26, 2017
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  11. Yuda

    AnalogKid

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    120 V / 1.44 M = 83 uA

    Another real world thing - power transformers designed for a 6 A primary current do not work down to 0.000083 A. That is a 72,000:1 input current range. It's been a while since I've messed with CT's, but I don't know of any 60 Hz core material that can do that. If there is, then I'll learn something new today. Until then, the transformer probably is just off.

    ak
     
    AnalogKid, Dec 26, 2017
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  12. Yuda

    Yuda

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    Update, I have an iPhone charger as my load and I have tried re wiring my circuit based on what *Steve* suggested however at this point I am positive that I am doing something wrong because my circuit simply can not detect when I plug in my phone charger. If I plug my phone in to my phone charger the circuit works for a few minutes but once the iPhone charger switches to trickle charging the circuit does not work. Is there any chance that someone can post an op amp circuit that is cable of sensing an iPhone charger at idle (just plugged in to mains but not charging a phone)? I have reached the point where my knowledge of op amps is restricting me from going any further because as I said before I am still learning how to work with op amps and I am getting really confused on how to tackle this problem. Also I would like to add that I very much appreciate all of you for taking the time out of your day to help me out, thank you!
     
    Yuda, Dec 28, 2017
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  13. Yuda

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You need to make it more sensitive. A current transformer with a lower turns ratio would be a good start.
     
    (*steve*), Dec 28, 2017
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  14. Yuda

    Yuda

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    How much lower? the ratio is 1:1500 at the moment, should I try to go down to 1:100? Also I am a little confused, I thought the more turns you have on your secondary the more sensitive the transformer is? What am I missing? God I feel like a noob lol.
     
    Yuda, Dec 28, 2017
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  15. Yuda

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    imagine a transformer with n primary turns and m secondary turns.

    If you have an AC voltage v at the primary, the secondary voltage is v x m/n.

    if you have an AC current i at the primary the secondary current is i x n/m.

    So, to achieve more sensitivity in a current transformer the ratio n:m should be lower. You have 1:1500, 1:100 would be 15 times more sensitive.

    The drawback is that the transformer will probably not work up to the high currents your existing transformer works to. This is not a huge problem if you're looking for high sensitivity for low currents.
     
    (*steve*), Dec 28, 2017
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  16. Yuda

    Yuda

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    Ok that clears that up. Also in regards to my current, the maximum amount of current I will need to be able to sense is 20A (20A outlet) which is what my breaker is rated for. I just need an analog signal that is either high when the presence of a load is detected or low when the absence of a load is detected. So given this information, do you think that a CT with 1:100 turns ratio would work? If I am understanding correctly the CT would saturate at higher current but if it would just make my op amp circuit go as far up the positive rail that the op amp I choose is capable of then for my application it would work. So long as I get it sensitive enough to give me a high value (5 to 7VDC) if I were to plug in an led night light or phone charger then it would for sure be able to give me a high value if I were to plug in a heater or fridge and likewise if I unplug the load if my signal drops close to zero then that's all I need. Do you think this is doable? If so what type of circuit should I be looking to build because the circuits I have tried thus far have not been working.
     
    Yuda, Dec 28, 2017
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  17. Yuda

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I would build 2 circuits. One using the 1:100 current transformer, and the other using the 1:1500 current transformer.

    It also might be useful if you can post the circuit you're using now so we can see how you've changed it.

    And... are you hoping to get an accurate measure of the power consumed?
     
    (*steve*), Dec 28, 2017
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  18. Yuda

    Yuda

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    upload_2017-12-28_1-1-16.png
     
    Yuda, Dec 28, 2017
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  19. Yuda

    Yuda

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    I have not tried to build this yet. I have only simulated this circuit. No I do not need an accurate measure of the power consumed, I just need a circuit to detect when a load is connected to the outlet and give me a high value out of an op amp to trigger a transistor to start a relay. The problem im having is just with phone chargers and led night lights. They do not draw much current so the circuits I have tried to build have not been working. The circuit I just posted seems to work on the simulation but I am not sure if it will work practically.
     

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    Yuda, Dec 28, 2017
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  20. Yuda

    Yuda

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    at 240 watts on the mains I am getting 12.6 volts and at 0.1 Watts on the mains I am getting 5.75 with the circuit I just posted.
     
    Yuda, Dec 28, 2017
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