adjusting the range of SCT-013-000

Hi!

I am currently using the "SCT-013-000" with the standard 18Ohm resistor and 0-100A range.

This range is actually way to high for my purposes. A range of 0-5A would suit me just fine.

I have seen in some forum posts here that people are achieving a lower range (like 0-50A) by increasing the burden resistor.

On the other hand I read that increasing the burden resistor also increases the voltage on the secondary coil of the CT.

So my question is: Can I somehow convert a "SCT-013-000" into something like a "SCT-013-005" by choosing an adequate burden resistor and get it working with the EmonTX?

Robert Wall's picture

Re: adjusting the range of SCT-013-000

The best advice I can give is read the report on the -000 and then use a test rig as described (or Robin Emley's tools) to check that the voltage waveform you get across your burden resistor is undistorted at your choice of burden resistor value.

But bear in mind that if your c.t. sees a fuse blown or a breaker tripped by a dead short, you could get a damaging voltage spike into the emonTx input.

Riker's picture

Re: adjusting the range of SCT-013-000

Thanks Robert!

So basically if something goes wrong the EmonTX gets fried...

 

I've also seen quite a lot voltage ouput CT's, that deliver 0,33V output at maximum primary current (e.g. the SCT-0400 series). In contrast to the SCT-013 voltage output series, which deliver 1V output at maximum primary current, might this have an dramatic effect on measurement accuracy or would it be negligible?

Robert Wall's picture

Re: adjusting the range of SCT-013-000

A voltage output c.t. is nothing more than a c.t. with the burden resistor inside the casing.

The problem with the current input is it has a very wide dynamic range - from a few tens of milliamps for a phone charger up to 40 A or so for an electric shower (if you have one!). If you use less than the available range of the ADC, which starts out as only 10 bits wide anyway (9 plus the sign if you will), then using half the range drops 1 bit each time, and your ability to measure current at the bottom end of the range disappears.

So if you use a 0.333 V c.t. (1 V p-p approx), the minimum you can measure will be 3.3 times greater than it would be if you were using the full 3.3 V range of the emonTx, for the same accuracy.

The way to decrease the likelihood of damage is to include a series resistor, which won't materially affect the voltage the ADC sees but it will limit the current that the in-built protection diodes have to handle.

Riker's picture

Re: adjusting the range of SCT-013-000

Alright, so 0,33V output type is not the best option to start with in terms of accuracy.

 

If I really got your first answer right, then distortion is probably the bigger issue when increasing the burden resistor value, rather than destroying the EmonTX through overvoltage (unless short-circuits occur)?

Do you have any size recommendations for that series resistor?

 

Robert Wall's picture

Re: adjusting the range of SCT-013-000

Do you have any size recommendations for that series resistor? It has to be large enough to limit the current to that which the protection diodes can handle. The Atmel data sheet tells you that, the c.t saturation curve tells you the maximum current it can deliver, after that it's Ohm's Law. But also note the upper limit for charging the sample & hold capacitor, also on the Atmel data sheet.

I seem to remember doing the sum and a few kOhms was enough.

Lucy's picture

Re: adjusting the range of SCT-013-000

Dear Riker,

You can use SCT-006 for your use. I want to attached the data sheet, but failed,  I can send to you if let me know your Email address.

We are manufacture of SCT-013 split current transformer.

Original company is Beijing Yaohua dechang Electronic Co., Ltd. We have build new factory in Qinhuangdao. Would like to communicate more with so many friends

My Email:  [email protected]

[Edit: The web page for the SCT-006 is http://www.yhdc.com/en/product/348/

For 30 A full scale measurements, a 27 Ω burden should be substituted in the emonTx, with a calibration constant of 29.63, or for a 5 V Arduino a 39 Ω burden is more suitable, with a calibration constant of 20.513. - RW]

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