# Current sensing circuit help

#### Yuda

Dec 25, 2017
51
I hooked everything up my breadboard and no matter what I do I can not zero out my op amp. I do not have any load on my CT and adjusting the trimmer I can get the op amp to go as low as 7.41vdc, I attached pictures of my breadboard, power supply, meter and the schematic. Any help would be very much appreciated. Thank you for your time.

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
Are you comparing the output with the potential at the junction of the zener diodes? (Because that's where you should be measuring it).

#### Yuda

Dec 25, 2017
51
I hooked up my meter to the negative rail and the output, I forgot that since I have a split rail I need to measure from the junction of the two zeners. If I remember correctly even when I measured from the junction I was still unable to zero out the op amp but I will correct the issue and post an update a little later.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
If you find that the output is moving closer to zero but you run out of adjustment before it hits zero, you may have to adjust the resistors to get a larger voltage setting from the trimpot (the easiest way is to parallel a high value resistor with the resistor at the end of the trimpot where you run out of adjustment. This will reduce the resistance sightly, giving a slightly extended range)

#### Yuda

Dec 25, 2017
51
I moved the meter from the nagtive rail to the junction of the two zeners and I am now able to zero out my op amp, only problem is I am getting 2.9 volts with a 100watt bulb as my load and I am getting nothing with a phone charger. How can I go about calculating exactly what characteristics I need my CT to have in order to sense low wattage devices such as phone chargers? I have a bag full of CT’s but none of them seem to be doing the trick with low wattage devices. Any suggestions?

#### Yuda

Dec 25, 2017
51
I tried using CT’s with a lower turns ratio with no luck.

#### Yuda

Dec 25, 2017
51
I looked up a breakdown of an iphone charger and it said that it is a switching power supply and it switches the input power some 70,000 a second in order to get the exact output voltage needed (5v). I am starting to think that the CT transformer’s that I have been trying to use do not operate at that frequency, if this is the case would that explain my results? If so what type of CT core should I be using?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507

Now see if you have more sensitivity.

You may have set the zero actually below zero (and with this circuit, the output can no longer go below zero) which will decrease it's sensitivity.

Increase the 100k feedback resistor to increase gain.

And is that trimpot 1MΩ? it should be 1kΩ. With such a high value it will be almost impossible to zero the meter without going too far.

Also put part numbers (R1, R2, R3, ...) on your diagram so we can unambiguously refer to them.

#### Alec_t

Jul 7, 2015
3,287
I looked up a breakdown of an iphone charger and it said that it is a switching power supply and it switches the input power some 70,000 a second
That may be so, but the charger draws the current you are measuring (about 30mA) from a 50/60Hz AC mains supply, so the CT sees only 50/60Hz.

Last edited:

#### Yuda

Dec 25, 2017
51
That may be so, but the charger draws the current you are measuring (about 30mA) from a 50/60Hz AC mains supply, so the CT sees only 50/60Hz.
But how about trying to sense when the iphone charger is on standby (not actively charging a phone but plugged in to mains)? Would I be able to sense the current drawn by the charger itself? So far I have been unable to get my op amps output to move at all when I just plug in a phone charger but if I plug my phone in to the charger I get about 0.5vdc but once I unplug the phone it drops back down to close to zero.

#### Yuda

Dec 25, 2017
51
l think my CT is sensitive enough to detect my phone charger in standby, I just dont have enough voltage being produced on my secondary due to the charger drawing such little current. Therefore my question is can/should I use op amp 1 of my LM324 to amplify the signal from my CT and then feed that in to opamp 2 using the same circuit Steve has been helping me with? How can I go about doing that?

Or, should I leave the circuit the way it is and try to find a different CT transformer that produces a higher voltage on its secondary in the pressence of a small current draw? If this route is a better option, do you have any suggestions for what type of CT transformer could work? Can you possibly help me with the math associated with finding a suitable CT for my application?

#### Yuda

Dec 25, 2017
51

Now see if you have more sensitivity.

You may have set the zero actually below zero (and with this circuit, the output can no longer go below zero) which will decrease it's sensitivity.

Increase the 100k feedback resistor to increase gain.

And is that trimpot 1MΩ? it should be 1kΩ. With such a high value it will be almost impossible to zero the meter without going too far.

Also put part numbers (R1, R2, R3, ...) on your diagram so we can unambiguously refer to them.

I am going to try these changes in about an hour, I will let you know how it goes, thank you for your suggestions!

#### Yuda

Dec 25, 2017
51
I spoke to CR magnetics and I asked them for there input on what CT I should use. They told me that there CT part number CR8410-1000-G with three turns of 18gauge for my primary winding should work. They told me that I should be using a precision rectifier circuit (which I am) but they also gave me a link to a schematic and I wanted to know if the schematic they gave me is all I will need or if I should use the circuit they provided as a buffer for the circuit I currently have? Here is the circuit they sent me.
http://www.crmagnetics.com/Assets/P...tifier Circuit for CT Signal Conditioning.pdf

And here is the data sheet for the CT I purchased from them.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
The precision rectifier you have is better because the op amp is always operating in closed loop conditions.

The protection they use of a pair of back to back zeners is functionally similar to the two diodes you are using (the expected voltages are low enough that it doesn't matter).

The major difference is the 3:1 ratio rather than. 1000:1, or 500:1, or 100:1, or whatever you're using now.

Your power supply will also be a limiting factor.

#### Yuda

Dec 25, 2017
51
Just a little confused, the CT they suggested has a ratio of 1:1000, so they told me to pass my primary 3 times so wouldn't it be 3:1000 ratio? Rite now I am using a CT with 1:1500 ratio, I tried a 1:500 and a 1:100 with no luck. The core's on my CT's do not saturate at low currents so I think that's why it hasn't been working with phone chargers.

#### metalcore

Dec 20, 2017
7
For current sensing, you can use Hall Effect Sensor such as ACS712. In case if the current is AC or switching then you can use a current transformer and if power dissipation is not issued then small value resistor can be used for current sensing.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
I would stick with the 1:100, and place three turns off work through it (or as many as you can).

Make sure you fix the offset circuit.

Also, look for millivolt level output.

And the core doesn't need to saturate for the current transformer to operate. In fact, when it does saturate, you have reached the highest current you can measure.

And the ACS712 is an option.

#### Yuda

Dec 25, 2017
51
I would stick with the 1:100, and place three turns off work through it (or as many as you can).

Make sure you fix the offset circuit.

Also, look for millivolt level output.

And the core doesn't need to saturate for the current transformer to operate. In fact, when it does saturate, you have reached the highest current you can measure.

And the ACS712 is an option.
I tried using the ACS712 but it did not work in the configuration I tried. I purchased the ACS712 on a pre made board that only required me to supply 5VDC, ground, and then it would give me an output. I was unable to get any usable reading due to the fact that the output at idle was 2.5V. If I hook up the output of that circuit (I will post the link to the one I purchased) would I be able to zero out my op amp with the offset circuit you helped me with? If so I can try to use the ACS712 in the circuit you have provided me to see if it will work. The one I would have to use would have to be rated for a minimum of 15A and if I remember correctly the higher the amperage rating the chip is rated for, the less sensitive they become, so there is a bit of a trade off but its worth a shot.

https://www.amazon.com/ACS712-Current-Sensor-Detector-Amperage/dp/B00COD8KJY

#### Yuda

Dec 25, 2017
51
I will try to use the ACS712 again tomorrow, that aside though I am waiting to get the new CT's from CR magnetics. If the ACS712 doesn't work there is still hope for the new CT's since with three turns on my primary they should be able to see about 1mA current draw on my primary. I will keep you all posted, thanks again

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
would I be able to zero out my op amp with the offset circuit you helped me with?

Yes, but you need to change the resistor RS so that the adjustment range is a few millivolts either side of the expected "zero current" output voltage of the ACS712

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