# Need help understanding Current Transformer Sensor

Discussion in 'The Projects Forum' started by shma, Feb 1, 2012.

1. ### shma Thread Starter New Member

Jan 11, 2012
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0
Hello,
I am starting a project to compute the power consumed by load so, I came to know about current transformer sensor but have never used it before. Can I get some idea about how current transformer sensor works? does it only measure current or it can measure power too. I would be using microcontroller to do the calculations and displaying result.
Thanks

Last edited: Feb 1, 2012
2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
A current transformer is typically used to measure AC current. Are you needing to measure AC current or DC current?

hgmjr

3. ### shma Thread Starter New Member

Jan 11, 2012
5
0
I need to measure AC current. I have a confusion, if I need to measure current through certain load lets say bulb, do I need two current sensor or the single one would do it. I just got confused because of two live wires and one neutral wire. Thanks for the reply

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
One current transformer is all you should require to measure the AC current. The signal from the current transformer will be roughly sinusoidal. Either 50 Hertz or 60 Hertz in frequency depending on what country you are in.

Also since you are looking to calculate power you will need to measure the voltage as well.

hgmjr

5. ### CDRIVE Senior Member

Jul 1, 2008
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Keep in mind that a multiplying the AC voltage by the AC current to find power is only valid for non reactive (resistive) loads. Because of this your readings will be 'apparent power' for reactive loads.

6. ### DickCappels Moderator

Aug 21, 2008
2,582
607
If you are going to measure power to resistive loads like a cloths iron or a light blub, then you can just measure current and assume that it is a complete sine wave in phase with a fixed voltage sine wave. In other words: RMS current X K (where K is a constant you come up with!) = Watts.

If you want to measure anything else, especially motors, you will need to take the waveform and the phase of the current waveform with respect to the voltage waveform. One way is to multiply instantaneous voltage times instantaneous current (at the same instant) for small slices of time, with many samples per cycle, then average that over some number of cycles. This process is discussed by Tom Lancaster in the Tech Musings issue at the URL below.
http://www.tinaja.com/glib/muse112.pdf

Last edited: Feb 2, 2012
7. ### BillB3857 Senior Member

Feb 28, 2009
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If using a current transformer, be sure to apply a proper load to the output otherwise voltages can rise to dangerous levels, even to the point of destruction of the current transformer.

8. ### shma Thread Starter New Member

Jan 11, 2012
5
0
I am trying to measure the power of general loads like lamp, and other daily use instrument so, can you give me more knowledge about what to keep on mind for safety while using current transformer sensor. Thanks for all the replies and help..........

Feb 28, 2009
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10. ### shma Thread Starter New Member

Jan 11, 2012
5
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Thanks......Can I get some idea about how can I measure dc current? I am using battery as source to some load. I looked at dc current sensor but they are very expensive. Is there some way to measure dc current. Then, I need to take the measurement to microcontroller and display the current the load is consuming. I have some knowledge about microcontroller.

11. ### shma Thread Starter New Member

Jan 11, 2012
5
0
Thanks......Can I get some idea about how can I measure dc current? I am using battery as source to some load. I looked at dc current sensor but they are very expensive. Is there some way to measure dc current. Then, I need to take the measurement to microcontroller and display the current the load is consuming. I have some knowledge about microcontroller.

12. ### crutschow Expert

Mar 14, 2008
12,537
3,071
If you don't need isolation you can simply place a small resistor (shunt) in series with the load and then amplify the small shunt voltage for the microcontroller input.

Easiest is to place the shunt in the return line since the shunt voltage is referenced to ground, but that does require that the load not have the same common as your microprocessor.

Otherwise you can place the shunt in the plus supply to the load but that requires measuring a small voltage across a resistor sitting at the supply rails. This requires a differential amp with a good common-mode rejection when measuring at the supply voltage (needs rail-rail type amp). There are also dedicated ICs that are designed to measure this shunt voltage at the supply rails for current measurement.

13. ### CDRIVE Senior Member

Jul 1, 2008
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99
Have you considered an Ammeter?

14. ### BillB3857 Senior Member

Feb 28, 2009
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348
Most ammeters won't provide an input to a microprocessor. The low ohm resistor connected to ground on one end is probably the best option. How much current is involved? 1 amp or less or hundreds of amps. Makes a big difference.

15. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Really? When did that happen?

Here, for a \$4.99 it will measure from 100uA (FS) to 10A (FS). This means it's capable of measuring a few microamps.

http://www.harborfreight.com/7-function-digital-multimeter-90899.html

FYI: Prior to the digital age we worked with D'arsonval meters. That's analog to the uninitiated. These meters were usually spec'd at 50uA FS @ 100mV. This movement was used in mutimeters that measured Amps, Volts and Ohms, using internal shunts and multiplier resistors and battery for the Ohms function.

Last edited: Feb 3, 2012
16. ### BillB3857 Senior Member

Feb 28, 2009
2,399
348

I must be showing my age! When I think of Ammeter, I do think of the moving coil/moving vane types. Otherwise, I think DMM. I would say he would be hard pressed to use the Harbor Freight unit as an input to a micorprocessor. If you can think of a unit that would provide proper inputs to a micro within his budget, please enlighten me. Never too old to learn.

17. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Bill, I honestly haven't a clue what you mean by that. What does that mean? Sgt. Wookie uses a Harbor Freight DMM. If it's good enough for him it's good enough for almost anyone.

BTW, that meter is cheap enough to sacrifice for conversion to a permanent panel meter. Also, analog & digital panel meters are readily available.

Some dig & analog panel meters.

http://www.mpja.com/prodinfo.asp?number=12306+ME

http://www.allelectronics.com/index.php?page=seek&id[m]=pattern&id[q]=Dc+Panel+Meter

Chris

Last edited: Feb 4, 2012
18. ### BillB3857 Senior Member

Feb 28, 2009
2,399
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CDrive, enlighten me, please. How do you connect a HF meter to a uProcessor? I do have a Radio Shack meter with serial output, but that's built into the meter.

19. ### crutschow Expert

Mar 14, 2008
12,537
3,071
Originally Posted by BillB3857
If you can think of a unit that would provide proper inputs to a micro within his budget, please enlighten me. Never too old to learn.
It seems clear to me. It's a meter that has a digital output that a microprocessor can read so the data can be processed by the micro.

20. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Bill, tt would appear that the problem here is leakage between my ears. I thought I read the OP's posts thoroughly but evidently not. I totally misinterpreted his requirements. I thought he simply wanted to measure the current draw of a uC, not use a uC to measure current of an another load.... Duh!

My apologies Bill, your brain is working fine. It's mine that's deteriorating. I'm going to go back and read through this thread verrrrrry sloooowly.

Getting old sucks!!

Chris