HI,
I have got this black PCB mountable CT, the thing is there are no markings, or labels in it. How am I supposed to use it?
Now I am trying the current only sketch, but the output(apparent power and IRMS) I get is very fluctutating.
~San
CT without a name.
Submitted by Guest on Thu, 23/05/2013 - 08:49»
Re: CT without a name.
The only way, without a maker, a type or part number, is to measure it.
If you have a local technical school or university with an electrical laboratory, you could try asking there for help. The test report for the YHDC CT will show you how to do the measurements.
Or you search the Web for one that looks the same, and you hope it is the same.
Re: CT without a name.
The CT I got is from a commercially available wattmeter. At this moment there are no electrical labs at my disposal. So, I guess I will buy a new CT with proper marks. Thank you.
Re: CT without a name.
Found myself another CT, a rather big one with these specifications
Primary : 150 A
Secondary : 5 A
5/3.75 VA
Class : 0.5
50/60Hz 660V
Now, I am confused, what is the VCT of this CT? Is it useful for 220V, 50Hz system and how to calculate burden resistor without knowing the VCT?
P.S I just want to make a wattmeter with some 7-segment displays without radio.
Re: CT without a name.
There's a fundamental misunderstanding there. The clue is in the name - current transformer. It behaves as a current source, not a voltage source (which is what the transformers you're familiar with are). So it produces a current of 5 A per 150 A of primary current, and develops whatever voltage is needed to drive that current into your burden. Like a voltage transformer, you can overload a current transformer, but (guess what) you do so by asking for too much voltage out of it rather than too much current. And while a voltage transformer is quite happy on open circuit, a current transformer is quite happy working into a short circuit.
The rating is 3.75 VA so for 5 A out, your maximum voltage out is 3.75 / 5 = 0.75 V (rms, = 2.12 V p-p). The maximum burden resistance would be 0.15 Ω, 5 W. If you are not using the full 150 A, you could increase the burden resistance somewhat to give you more voltage, otherwise you might need to amplify the output, especially if you are running an Arduino board at 5 V.
WARNING. Do NOT open-circuit any c.t. with primary current flowing. It could develop a quite nasty voltage and flash over, destroying itself in the process. And there is probably enough power from your c.t. to give you a fatal electric shock should you become the burden resistor.
The system voltage is irrelevant (it will be insulated to a much higher voltage than that), it's almost certainly good for 50 Hz or 60 Hz systems.
Re: CT without a name.
Got it. Thanks.
And since 0.75 being a small voltage and 0.15 Ohm value being not available I guess amplifying is the solution?
Re: CT without a name.
It could be. As I said, depending on the maximum primary current that you wish to measure, it might be possible to increase the value of the burden resistor without overloading the c.t. (and thus running into saturation and distortion of the current waveform, which will lead to inaccurate readings). If you've got a basic Arduino / emonTx system working, Calypso-rae has published sketches that will allow you to see a low-resolution picture of the voltage and current waveforms on the serial monitor, or to import the data into a spreadsheet for closer analysis - they are all listed at http://openenergymonitor.org/emon/node/1757
With a bit of advanced programming, you can run your ADC from a 1.1 V internal reference, which might mean you don't need any amplification - that would give you full scale of around 75 A with the 0.15 Ω burden
0.1 Ω high power resistors are fairly readily available, otherwise you might need to make one using resistance wire, or even a thin sheet of brass with a suitable zig-zag path made with saw cuts to trim the value - there are plenty of examples in suppliers' catalogues.
Re: CT without a name.
RW: With a bit of advanced programming, you can run your ADC from a 1.1 V internal reference, which might mean you don't need any amplification - that would give you full scale of around 75 A with the 0.15 Ω burden
I did try this technique, but the results seemed worse than when retaining the standard 0-5V ADC range and increasing the burden resistor to obtain a larger output signal.
I'd be most interested to know whether anyone else has successfully used an Atmel-based measurement system with the 1.1V reference in order to provide a better match for the CT's normal operating zone.
Some constructors of Mk2 Router systems have successfully amplified the output from their CTs to increase the signal size. When I tried this, it seemed to give no improvement. This probably reflects the poor state of my experimental hardware.
Re: CT without a name.
Presumably you also changed the DC offset to 0.55V ?
Re: CT without a name.
Er... The Atmel demonstration circuit in the App.Note runs on the 1.1 V reference with 0.55 V bias, so it must work... Mustn't it?
Re: CT without a name.
Presumably you also changed the DC offset to 0.55V ?
Yes, I got this from the 3.3V rail. The divider needs to be 1:5 and I recall finding two E12 resistor values in this exact ratio. I think they must have been 15K and 75K. I didn't pursue this option because the performance seemed no better than when using the standard 5V arrangement. Maybe I should have persevered for a bit longer ...