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

OK, I'll tell you what I would do. This would deviate a little from what #12 suggest.

At work I have several 480 VAC 1,000 amp service lines I like to monitor. Here is what I do. I use a 1200:5 current transformer on each bus. That means when the single line running through the CT (the primary) is at 1,200 amps the CT secondary output is at 5 amps. I take that 5 amp secondary and loop it 4X (four times) through one of these units. So what I now have is 0 to 1,200 amps = either 4 to 20 mA or 0 to 5 volts. Actually in my case I use the 4 to 20 mA and run it through a 500Ω precision resistor so my 0 to 1,200 amps is no0w 2 to 10 volts to use as I please. The reason I loop it 4X is simply to get the second transducer up to full scale of range (5 X 4 = 20).

I also use a data logger to monitor my bus current during certain testing.

Now I have mentioned just as #12 CR Magnetics but a 1200:5 can be had just about anywhere, the same is true of the smaller 20 amp transducer that outputs 0 to 5 volts or 4 to 20 mA. These type units are quite common.

Ron

Reloadron (everyone), hw do u find his idea, i get a 800:5A current transformer for my 500kVA, 415V line to line and put its secondary accros a resister of known value(2ohms), pick a voltage(8.7vac) and rectify it to like 6.6vdc and we set that as the reference. If the line current goes close to or beyond the rated tx current, the comparator gives a 5v otherwise a 0v. Iam not sure of my calculations but how is that idea?

I am using national instrument circuits for the simulations, which one ís the best?

ssembo said:

Reloadron (everyone), hw do u find his idea, i get a 800:5A current transformer for my 500kVA, 415V line to line and put its secondary accros a resister of known value(2ohms), pick a voltage(8.7vac) and rectify it to like 6.6vdc and we set that as the reference. If the line current goes close to or beyond the rated tx current, the comparator gives a 5v otherwise a 0v. Iam not sure of my calculations but how is that idea?

I am using national instrument circuits for the simulations, which one ís the best?

Click to expand...

That method will work. The downside is the inaccuracies. What you have is 0 to 800 amps RMS is equal to 0 to 5 amps RMS. You run the 5 amps through a resistor of 2 ohms so 0 to 5 amps is equal to 0 to 10 volts RMS. You will want about a 75 watt 2 ohm resistor at that point. Max current would be 5 amps * 2 ohms = 50 watts.

The problem is you still at this point have an RMS voltage value proportional to your AC current. So you run it through a full wave bridge. During each alternate half cycle of the AC waveform two of the four diodes in the bridge will conduct and they will have a forward voltage drop. Additionally the bridge output will not be a flat line of nice DC so you add filtering. If you add a cap as a filter the cap will charge to the peak value of the original AC waveform to the bridge rectifier. Without the cap the comparator may not like the waveform.

However, if you just want a trip level for a given current and have a reasonable means to calibrate things then your approach may work for your application. I do it the way I mentioned because I want to reasonably read my currents across a broad range with reasonable accuracy.

Just My Take
Ron

Have loved your take...its good the other way also works, hope my electronics is getting better. Please help give a description of the CT and the other item u use cause the person am sending knows nothing about it.

Now, lets get to the comparator. Am hoping to use lm324 op amp. What can i use to provide a reference voltage to input? I learn't of the NE555N IC. How is it? I am asking for advise on the software to use. Thanks.

OK, we will look at a few pictures of what I am getting at here, therefore three images are attached for your viewing pleasure.

However, first we can take a look at a choice of comparator. You may want to give this link a read. For a simple single comparator I would lean towards the LM311 which is covered in the link. Also here is the data sheet.

The first image is that of an everyday CT (Current Transformer) compliments of my kitchen table. Lets assume (since you can't see otherwise) that it is a 1,000:5 CT. Therefore 0 to 1,000 amps = 0 to 5 amps. I am sure you have a handle on that.

The next device pictured is a nice current transducer very similar to the CT but this device is a 0 to 20 amp device that provides a DC output. There were some such devices linked to earlier in the thread. These current transducers are powered in a 24 VDC current loop where full scale out is 20 mA or in this case 0 to 20 amps = 4 to 20 mA (4-20 being a very popular current loop).

The third picture shows the actual configuration of the two devices. Now something to consider here is that current transformers and current transducers (both our devices) measure current based on amp turns. Now the output from our first CT is 0 to 5 amps, so what I have done on the current transformer, remember the range is 0 to 20 amps is wrap four turns of the primary CT secondary through the second current transducer secondary.

So what we now have3 in our images is 0 to 1000 amps = 0 to 5 amps = 4 to 20 mA in a nice clean DC signal with a good accuracy ratio.

I should point out that the current transducer pictured has a 4 - 20 mA output, however, they can be had with a voltage output. This is simpler and better to drive a comparator circuit. I pictured what I had lying around.

So I take my 4 to 20 mA and run it through a nice precision small 250Ω or 500Ω resistor and get 0 to 1000 amps = either 1 to 5 volts or 2 to 10 volts. Everything is scaled very nice and actually very accurate. I would venture an overall uncertainty of +/- 10 amps at 1000 amps.

Now we get to the comparator. We use a 24 Volt DC small power supply to power our second current transducer for the current loop. We also take that 24 VDC and run it into a simple 12 volt regulator like a LM7812 and use that 12 volts out to power our comparator circuit. The LM7812 will give pretty good regulation so we use maybe a 10 KΩ pot to set our comparator reference.

I should point out that the second device does have terminals on the top above the +/- signs.

Does this help?

Ron

Stepping down 800A to 5a will step up the voltage from 415 to 66KV. Conservation of poower. How is that (rather dangerous) voltage level handled?

@mosaic It's not a power supply transformer. The output has no intention of delivering any serious power.

Mosaic said:

Stepping down 800A to 5a will step up the voltage from 415 to 66KV. Conservation of poower. How is that (rather dangerous) voltage level handled?

Click to expand...

OK, let's look at a few pictures. The first is a CT (Current Transformer) opened up and the second is the CT as normal.

If we look at the CT opened up it is merely a coil wrapped around an iron core. Note the seen wire is the actual secondary of the transformer. The second image shows a cable running through the center of the iron ring (core) and that is the primary. That large current carrying conductor carries the AC current and has a constantly expanding and collapsing magnetic field. The strength of that field is a direct function of the current and is passed on to the secondary turns seen in the first image.

So what we have here is a current (not voltage) ratio in a step down (not up) fashion.

However, there is a caveat to all this. It is very, very unwise to open the secondary of a CT while under a load.

I found a third image in my collection of a mounted working CT. This is a 100:5 CT in a 3 phase 480 VAC circuit I have at work. As you can see there isn't much t it. It simply monitors a phase current sitting there nice and happy.

Hope That Helps
Ron

I have understood the setup but still didnt get the reference thing. Am using my phone, it would be more good if i hard my computer with me. Am enjoying this, let me read the link you gave me. Thanks guys.

still didnt get the reference thing

Click to expand...

Use two resisters across your supply voltage to make your reference voltage.

What can i use to provide a reference voltage to input? I learn't of the NE555N IC.

Click to expand...

The 555 will make pluses its a timer. Andy

The attached is taken from the link. Replace R1 & R2 with a 10 KΩ pot. There isn't much to it.

Ron

Voltage changes

Attached is a circuit so far built. it may not be good cause am already getting problems. When the sensor circuit is connected to the comparator, the voltage across the divider resistors (10kohm) varies with the upper getting to 15V and the lower to -3V which is not the wanted case. i believe it should be in such a way that the voltage across the resistors should be fixed at 6.3v being our reference. please correct me where am wrong. thanks.

I would also like to ask if NI circuits is the best software for this project.

What is XCP1? If that is a current transformer I have no idea how it is configured? I see a single line off whatever it is going to the bridge. I explained in the beginning why you could not just hang a diode bridge out there as you will not get a clean DC level. I suggested and linked to a few devices that would do exactly what you want to do. I see the mains common going to the bridge? Why is that configured that way? I see no filtering at all on the low voltage supply? No caps in there at all? There is no way at all this circuit could work. I am sorry.

What are NI Circuits? National Instruments?

Ron

Reloadron said:

What is XCP1? If that is a current transformer I have no idea how it is configured? I see a single line off whatever it is going to the bridge. I explained in the beginning why you could not just hang a diode bridge out there as you will not get a clean DC level. I suggested and linked to a few devices that would do exactly what you want to do. I see the mains common going to the bridge? Why is that configured that way? I see no filtering at all on the low voltage supply? No caps in there at all? There is no way at all this circuit could work. I am sorry.

What are NI Circuits? National Instruments?

Ron

Click to expand...

XCP1 is a current probe. You just have to set the v/mA and it gives out a voltage output given a current. I found a problem of not geting a transducer (4-20mA) to pick from the CT as have not seen that in this software...National instruments. I used the mains common as a reference since its grounded. The filtering, am going to deal with that.
Otherwise thanks and keep with me.

Attached is a LM7812 regulated power supply and an example of a comparator. The signal in is from any signal that is a representation of your current. When the signal in exceeds whatever you set V ref for the LED will light. The LED should be a standard generic LED. Once you know the V fwd and I fwd of the LED you can select the series resistor for the LED.

You will need to condition or whatever the signal from your current probe. Most I have seen output DC Millivolts as in mV per Amp.

As to the NI software you won't find the unit I mentioned. I don't know of any software that would have units like that in the parts library?

Hope the drawings help
Ron

Ron

Reloadron said:

Attached is a LM7812 regulated power supply and an example of a comparator. The signal in is from any signal that is a representation of your current. When the signal in exceeds whatever you set V ref for the LED will light. The LED should be a standard generic LED. Once you know the V fwd and I fwd of the LED you can select the series resistor for the LED.

You will need to condition or whatever the signal from your current probe. Most I have seen output DC Millivolts as in mV per Amp.

As to the NI software you won't find the unit I mentioned. I don't know of any software that would have units like that in the parts library?

Hope the drawings help
Ron

Ron

Click to expand...

Man, i would like to appreciate every ones contribution to this project. The circuits attached in quote really did a very good work.Have been struggling with them for a long time but finally came up with something good. Thanks very much everyone that helped.

Glad it all worked out for you.

Ron

Thanks.

Have now posted another thread on regulator circuit. You welcome to the discussion. Am enjoying electronics.