Split Core CT Primer – ElectroSchematics.com

In terms of affordable split-core current transformers, the YHDC SCT013 series seems to be popular in the world of hobby electronics. I bought some last year. I want to share something useful about those nifty split-core CTs!

❌ Disclaimer: This is not a sponsored write up. I did not intend to promote any brand/product through this post.

⚠ Caution! Risk of electric shock. Be aware of the potential electrical accidents that may occur when working with live wires and hazardous electrical appliances.

⛏ Related Article https://www.electroschemics.com/transformer/

What I picked for this post is the SCT013 – 15A/1V split-core current transformer from the YHDC SCT013 series (no particular reason behind it, though).

See its (SCT013-015) sample datasheet https://www.mcielectronics.cl/website_MCI/static/documents/Datasheet_SCT013.pdf

The SCT013-015 is a 15A (RMS) split-core CT with inbuilt sampling resistor. That means the current is internally passed through a “burden” resistor, so that we can measure the voltage on the end of the wire coming out. So, it is a “voltage output” type device, and the rated output is 1V.

This split-core CT allows you to measure AC current noninvasively by clamping the current sensor around one of the wires where the current flows. Do not clamp both wires as the current flow from one wire will neutralize the current flow from the other wire and the sum will be always zero (you will not measure anything).

On the other end of the CT cable you can see one 3.5 mm TRS audio jack where you can plug in a mating connector to extend the output of the CT. And indeed, with 15A current through the CT, you get about 1V coming out. Note that the CT’s output is available through the Tip (T) and Sleeve (S) of the 3.5mm TRS audio jack. There is no connection to the Ring (R) of the TRS audio jack.

At this point it is worth noting the CT output cable has a pair of wires inside (twin twisted core with an overall braided shield). As observed, the first output (K) of the CT is connected to the Tip and the second output (L) is connected to the sleeve of the TRS audio jack.

Although it is not very relevant to the current discussion, if you have a CT without inbuilt burden resistor (ie, you’ve the current output version) never use it without a proper external burden resistor. This is because, a CT that has no load could develop extreme and damaging voltage within the secondary winding (output) in the presence of primary current. Surprisingly, the SCT013 series CTs without internal burden resistor usually has a built-in transient voltage suppressor (TVS) device to limit the output to a safe value if no burden resistor is wired there across the secondary, whilst the primary (wire carrying the current you want to measure) is energised.

Needless to say, if your CT is a current output type similar to the YHDC SCT013-000 (https://www.elecrow.com/download/SCT013-000_datasheet.pdf), the current signal output needs to be converted to a voltage signal using an external burden resistor.

As you can see in the datasheet snip shown above, the CT has a rated output of 0-50mA for the rated input of 0-100A. The turns ratio is 1:1800, and the recommended maximum burden resistor (sampling resistor) value is 10Ω.

In my napkin back calculation, since this CT is rated for 100A, and the winding ratio is 1:1800, the 100A is converted to 55mA output current, and 55mA through 10Ω leads to 555mV output voltage under full load.

In a different way, if you wanted to add a burden resistor for 1V output at 50mA secondary current, then R=1V/50mA = 20Ω. You could do this with quality resistors in the E12 value series (10Ω x2 will give you the desired resistance).

Returning to the prime thing, the SCT013 series split-core CTs seems to be a budget choice for educational experiments and hobby electronics projects. But it seems like it is clearly made to a price point rather than for quality, thus a very poor pick especially when considering the design of the current clamp mechanism. Be careful, do not hit the CT with anything hard, and/or never drop it on a hard surface – the core is hard enough but very brittle. Moreover, the measurement accuracy is notably piteous when measuring very low currents (it’s hard to make meaningful measurements due to nasty), though it’s usual noise with most low-budget CT based experiments.

This is the inside view of my SCT013-015 split-core CT.

Below is a ‘poor’ microscope capture of the CT PCB. As you can see, the PCB holds one protection diode (WP) – perhaps a TVS diode (https://www.mouser.com/pdfdocs/Semtech_Application_Note_TVS_Diode.pdf), and two resistors wired in parallel (3900 & 1200). The output cable’s red core (sleeve) is soldered to the L output of the CT while the white core (tip) is soldered to the K output. There is no electrical connection on either side of the braided shield underneath the cable’s sheath.

I measured around 54Ω resistance across the tip and sleeve of the TRS plug.

This is the crude schematic drawn by me. It may not look great because of the weird component symbols!

Split-Core CT & Oscilloscope

A quick and dirty idea for current measurement with oscilloscope is to use a split core CT as the “current probe”. The SCT013-015A will sense AC currents up to 15A RMS, enough for me, so I fitted mine with a 3.5mm stereo audio jack and a BNC connector for easy connection to my test equipment(s). This is the crude AC Current Clamp for my oscilloscope (may be modified later). Quite easy right, isn’t it?

I would like to conclude this article now as I’ve a plan to prepare a sequel to this work. In an upcoming article temporarily titled as “AC/DC Current Clamps”, you will find an extension of my CT thoughts. I would also like to update my recently published Sparkplug Wire Sensor project using some funny CT techniques. So, stay tuned!

🤓

Postscript: Optical Current Transformer (OCT)!

Optical current transformers (OCTs) are defined as sensors that directly or indirectly use optical sensing methods to measure electric currents. OCTs are based on Faraday effect, interferometric principle, Bragg Grating and Micromechanical sensors with optical readout. In principle, an OCT uses polarized light which is passed through two separate fiber optic cables one of which is coiled around a current carrying conductor. The magnetic field produced by the current causes a phase shift in the polarized light and the phase angle is thus proportional to the reduction in polarized light intensity at the receiving end.

Further reading:

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