Working Principle of Current Transducers?

[hkBattousai]

I am working on a project in which I need to sense current through a transmission line. I made a google search on it, and decided to use a current transducer. I searched for current transducers in Farnell.

The current transducers I found have only two pins. They have a a few paged specification sheets, no detailed documentation. Specification sheets mension about supply voltage, etc. I don't understand this only two-pin working principle. I'm totally lost. I search google again for working principle of current transducers, I found physical explanation about halleffect, but nothing about current transducers as a circuit element.

Can anyone help me by explaining how this devices work?

 

[smanches]

Most current transducers are just coils that pick up the magnetic field of the supply wire. You measure this voltage from the two pins.

If you link a more specific transducer, we can give you more specific answers.

 

[hkBattousai]

Most current transducers are just coils that pick up the magnetic field of the supply wire. You measure this voltage from the two pins.

If you link a more specific transducer, we can give you more specific answers.

For example, this transducer:
**broken link removed**

Can you tell me what does these parameters refer to:
Rated voltage range
Operating voltage range
Rated current
Mimimum SPL @ 10cm

What I really need to know is only these two things:
-What range of currents can I sense with this device?
-Is this device linear?
---If not, how can I map measured voltage value to actual amount of flowing current?
---If yes, what is the conversion coefficient between measured voltage and actual current value?

 

[MikeMl]

Current transducers are primarily:

1. Small precision resistor (shunt) inserted into the power line conductors. The line current flows through the shunt creating a (small) voltage drop proportional to the current. The disadvantage is that the small differential voltage drop across the shunt sometimes rides on a large common-mode voltage with respect to measurement ground... This method works for both AC and DC power lines...

2. AC only; a transformer, usually toroidal, which has many turns on the secondary, and only a single turn on the primary. The single turn is created by passing the line carrying the AC current through the hole in the toroid. The AC voltage on the secondary is proportional to the line current. Disadvantage is that the secondary voltage must be converted to a unipolar signal by passing it through a precision rectifier. Advantage is the that the insulation between the line and the secondary subtracts out the common-mode offset mentioned above.

3. Other methods sensing magnetic fields employing the "Hall Effect". Google it.

 

[MikeMl]

That thing is a piezo buzzer. It has nothing to do with current sensing.

 

[crutschow]

2. AC only; a transformer, usually toroidal, which has many turns on the secondary, and only a single turn on the primary. The single turn is created by passing the line carrying the AC current through the hole in the toroid.

It may be obvious but, to clarify, only one wire of the AC pair must be put through the hole. If you put both wires, the magnetic fields cancel and the transformer will have no output.

 

[Reloadron]

I am working on a project in which I need to sense current through a transmission line. I made a google search on it, and decided to use a current transducer. I searched for current transducers in Farnell.

The current transducers I found have only two pins. They have a a few paged specification sheets, no detailed documentation. Specification sheets mension about supply voltage, etc. I don't understand this only two-pin working principle. I'm totally lost. I search google again for working principle of current transducers, I found physical explanation about halleffect, but nothing about current transducers as a circuit element.

Can anyone help me by explaining how this devices work?

I think you really need to explain things like how much current and what is the voltage on this transmission line? Additionally since you use the term "transmission line" the consensus would be AC current so would that be correct? Again, how much current at what voltage and frequency?

Ron

 

[hkBattousai]

I think you really need to explain things like how much current and what is the voltage on this transmission line? Additionally since you use the term "transmission line" the consensus would be AC current so would that be correct? Again, how much current at what voltage and frequency?

Ron

Voltage is 220V 50/60Hz AC, maybe I can adapt it to three-phase supplies later.
I want to measure as much current as I can. If there's a trade off between current and sensitivity, I want to keep sensitivity higher.

 

[crutschow]

Voltage is 220V 50/60Hz AC, maybe I can adapt it to three-phase supplies later.
I want to measure as much current as I can. If there's a trade off between current and sensitivity, I want to keep sensitivity higher.

There is always a trade-off in any measurement between the maximum that can be measured and the sensitivity. Saying you want to measure as much as you can tells me nothing. It depends upon the dynamic range of your measuring system, the noise floor, and the desired measurement resolution of the minimum current. You need to determine these.

So how you going to measure the current value (after it's converted to voltage) and what are you doing with the measurement?

 

[Reloadron]

OK you have a few options. I'll attach a few pictures as examples. The first and likely cheapest option is a current transformer and one of those is the first image. These are sometimes called donuts. They consist of a coil of wire that your conductor passes through (only a single conductor). These devices have numbers on them like in the picture. The picture is a 50/5 meaning when the conductor passing through it is carrying 50 Amps the output wires deliver 5 Amps to a load. Years ago these devices typically drove old analog meters and many still do but they can also drive digital meters with very low input impedance. Their ratios can vary as I use many that are 1,000/5 to measure the current on 3 phase 480 VAC buses. They are simple and rugged devices and their use comes with a cost of how accurate they are and what is called "Burden Voltage". Google is your friend.

The next device pictured is a current transducer. This is a different animal and a more costly animal. Again the current travels along a conductor passing through the device but the current is measured using a hall effect device to measure the magnetic field around the sensor again here Google is your friend. This device outputs a small current of 4 to 20 mA which is proportional to the current being measured. The device is pretty cool in that it has 3 ranges selectable with a top jumper. It can measure 0 to 10, 0 to 20 and 0 to 50 Amps. The device is loop powered meaning a power supply of 24 to maybe 30 volts is in series with it.

Both of these devices are marketed by countless businesses like **broken link removed** I only use CR Magnetics as an example. The Current Transformer pictured is made by Simpson Electric Company and the Current Transducer by Neilsen-Kuljian.

The outputs are scaled for reading. If I have a Current Transformer like pictured that is 50/5 I know that 5 Amps out is actually 50 Amps. Therefore if I take an analog meter that is 5 Amps FS (Full Scale) I can paint or draw a linear scale on the meter face of 0 to 50. The same can be done with a digital indicator device depending on the device. The Current Transducer is about the same. If I set it up to measure 0 to 50 Amps the output will be 4 to 20 mA proportional to the input.

Hope that helps a little as it is brief but an overview.

Note: The order is screwed up for the attached files. The second is the current transformer and the first and third are the current transducer. My Bad.

I also agree with crutschow in that you really need more specifics so what you have here is an overview and that is as good as it gets for an example or two.

Ron

 

[hkBattousai]

Hope that helps a little as it is brief but an overview.

Note: The order is screwed up for the attached files. The second is the current transformer and the first and third are the current transducer. My Bad.

I also agree with crutschow in that you really need more specifics so what you have here is an overview and that is as good as it gets for an example or two.

Ron

Thank you very much for this long explanation and giving your time for me.

There is still one thing that is not clear in my mind.
A "50/5" transformer is supposed to sense current 10 times attenuated. But for what output impedance? The current flowing through the secondary coil of a transformer depends on the load connected, isn't it?

 

[Reloadron]

Thank you very much for this long explanation and giving your time for me.

There is still one thing that is not clear in my mind.
A "50/5" transformer is supposed to sense current 10 times attenuated. But for what output impedance? The current flowing through the secondary coil of a transformer depends on the load connected, isn't it?

Remember I mentioned "burden"? **broken link removed** is a pdf for the CT I pictured. Additionally **broken link removed** contains some very good information. These units are designed to work into a very low impedance. Even the lead length becomes critical as the measuring instrument moves further from the source. So absolutely the accuracy depends on the load.

Current transformers like the one I pictured are very useful but they have their limitations. One sort of trick used in industrial applications is to take the output of a CT like I pictured and run a few turns of the output through a current transducer like the second device I pictured shorting the CT leads after a few turns through the current transducer.

Note in the pdf files where they get not just into lead length but also the AWG of the lead length.

Ron