Getting a signal from a high voltage AC line and compare it using a comparator.

[ssembo]

Guys, i want to get a signal from a high voltage line (240VAC) and compare it such that when a load connected to the 240V line draws more current than the reference, the comparator gives a 5V and when its below, it gives a 0V. its suggested that i use an LM324 op amp but i don't know how. i need a circuit.
Am not any good at electronics, am doing electrical but my project requires this.
thanks.

 

[Reloadron]

You need to do is first determine the level of current you want the circuit to trip on. Then determine a usable sensor to detect the current. like a hall effect sensor for example. Once that is done you can worry about using a comparator like the ones covered here. First things first. Is this for a small personal project or a commercial type design / project?

Ron

 

[MikeMl]

The biggest issue is isolation from the 240V line. I would suggest purchasing an isolated AC current sensor (likely based on Hall Effect or a Current Transformer.

 

[#12]

If Hall effect isn't your cup of tea, you could use a "current transformer". They deliver some amount of voltage per amp, depending on which one you use, and you will rectify that voltage to a DC level and use the LM324 to make decisions about the rectified voltage.

ps, the current transformer provides the isolaton function, too.

 

[4pyros]

How about a Shunt Resistor? What is your current range? Andy

 

[KeepItSimpleStupid]

What about a commercial product? What current range(s)?

 

[ssembo]

Thank you man. This is part of a big project with AC line currents ranging from like 30A - 700A. The reference will depend on where am to connect my circuit. By the way, my issue is mostly about the sensor, can't a shunt help? i don't kno how to use it. Am happy you responding. Thanks

 

[ssembo]

The biggest issue is isolation from the 240V line. I would suggest purchasing an isolated AC current sensor (likely based on Hall Effect or a Current Transformer.

Does a shunt resistor operate on the hall effect principle? am sorry but am not good at electronics at all. Thanks.

 

[ssembo]

If Hall effect isn't your cup of tea, you could use a "current transformer". They deliver some amount of voltage per amp, depending on which one you use, and you will rectify that voltage to a DC level and use the LM324 to make decisions about the rectified voltage.

ps, the current transformer provides the isolaton function, too.

Hello man, good idea but i don't want to hav a bulky circuit. i already have a rectified 5Vdc for the power supply...won't it be too much if i include a rectifying part of a current transformer or is there a way i can use power supply to do the job? thanks

 

[ssembo]

How about a Shunt Resistor? What is your current range? Andy

Hello, line current of 700A - 1.4kA. Please try to explain how this works?

 

[ssembo]

What about a commercial product? What current range(s)?

Hello, 700A - 1.4kA. thanks

 

[KeepItSimpleStupid]

The sensor will basically have to be hall effect or a current transformer. A current transformer doesn't require breaking the circuit. You just pass the conductor through it and it's usually mounted within the electrical enclosure.

Sensors can be purchased that have a voltage output.

In general, the current carrying conductor is placed inside a toroid (a donut shaped transformer) and the secondary is placed across a resistor of a known value. This scales the current to a voltage that's manageable.

Before we get carried away there is the issues of nuscence trips and even whether True RMS values of currents are required. TRMS becomes important if the loads are phase angle fired. Latching the fault, time duration of the fault or even indication of the fault may require pulse stretching if there is an indicator associated with this. Motor start-up current may instantly trip your circuit as well.

I'd like to know the application and/or if your going to be needing 10 or 30,000.

Transducers combined with alarm modules would be the traditional way of doing this UNLESS your incorporating it into a product.

 

[#12]

You have room to apply 700 amps to something and you think a quarter inch rectifier for a current transformer is "bulky"?

 

[Reloadron]

I would suggest you start here and begin to get an understanding on different current measuring devices. It is rather apparent you have not bothered to research any of the initial suggestions. Each device and method has good and bad points, and if you want something to measure the current you plan to measure it should be apparent there will not be a tiny footprint. I suggest you do some homework.

Ron

 

[#12]

That was much more eloquent than the way I said it.

 

[4pyros]

Does a shunt resistor operate on the hall effect principle?

No its a small resistance high power resistor in series with the load. You measure the voltage across the fixed resistor and using ohms law to determine the current. Happy Holidays Andy

 

[Mr RB]

A setup for measuring 240v at 700 to 1400 amps? That's a job for a competent and well paid engineer, using what I would expect to be an expensive professional quality sensor.

 

[Reloadron]

A setup for measuring 240v at 700 to 1400 amps? That's a job for a competent and well paid engineer, using what I would expect to be an expensive professional quality sensor.

Not really as to a sensor. At work I use several 1200:5 CTs on my 480 VAC bus systems. Relatively inexpensive devices. Maybe several months back I posted pictures of a CT I took apart. They are on the home PC and right now I am traveling. However, when going the CT route one needs to get a handle on and understand burden voltage and how it applies to a CT.

I like the newer CT designs that output a 4 - 20 mA current proportional to the actual current. Also something to consider is the waveform a nice sine so RMS current is easy to read?

Next there is uncertainty to consider. How accurate do we need to measure the current? At work +/- 1% is fine on a 1,000 amp bus but that may not always be the case.

Thus I get a little annoyed when someone says I want to measure high voltage high current and fail to spec out what high voltage or current is for their application. One man's high current is 20 amps while another man's may be several thousand amps. I guess it is all relative.

Just My Take
Ron

 

[ssembo]

Guys thanks, am really learning something and am getting to enjoy and appreciate electronics. I am an electrical engineering student doing a project on the issue.

During power system operation the load is continuously varying, and as such distribution transformers undergo cyclic loading and this at times results into overloading and subsequent failure of transformers.

In this project a slave transformer shares the load of master transformer over load and over temperature. A sensor circuit is designed to log the data from the master transformer and if it is found to be in the over load condition immediately the slave transformer will be connected in parallel to the master transformer and the load is shared . i want to use an operational amplifier to compare the load of the transformer with a standard load capacity and an NPN transistor to drive the relay. I want to use a regulated 5V, 750mA power supply. A three terminal voltage regulator is used for voltage regulation .Bridge type full wave rectifier is used to rectify the ac output of secondary 240/12V step down transformer.

The issue is now to pick a signal from the AC line which will hav to be compared with a set reference. When you told me about the hall effect, i did some reading and yah, its the way to go...my problem is am in a small country and we do not get achance of pracctically use those instruments (CTs). isnt there a circuit on board that senses and gives a dc output, i would wish to purchase that. I have to make an order for my brother to bring them for me from that side cause they are not here.

the Tx rating may be 500kVA, 415V line to line. My model will be in single phase.

Thanks.

 

[#12]

The website described as "start here" or CR Magnetics has current transducers that will work. Attached is a catalog page containing one that will work for you.