# How much power can be drawn from a CT Clamp?

If I have an AC line powering some device, I can put a CT clamp around either the hot or neutral wire, and measure some low ac voltage and current. I understand this is because the AC line creates a magnetic field due to the right hand rule, and I am essentially harvesting the power with a transformer. I have two questions:

1. Does a CT clamp add more load to the AC line, or is it that the AC line would create that magnetic field no matter what, and I am just harvesting it?

2. How can I get more total power out of a CT clamp? I am not able to loop the input around multiple times, but would like more power. Will more secondary windings do this? widen the ferrite core? I'm not sure.

Any help appreciated.

• CTs convert current to voltage signal for later processing, it's a type of sensor. A sensor must not interfere so much with the quantity needed to be measure. Why do you want more power from them? Sep 8, 2021 at 22:16
• I want to use it as a current transformer to power a small device that will clamp to an ac line Sep 8, 2021 at 22:47
• @LukedukeAnimations Look up the author, Gaikwad. At least two papers are easily found on this topic: "Evaluation of dimensional effect on electromagnetic energy harvesting", which is directly available for free at that link, and also a year later in 2019, "Electromagnetic Energy Harvesting to Power the Micro-power Temperature Sensor", which isn't so freely available on the web.
– jonk
Sep 8, 2021 at 23:55

Question 1 has answers in my question Primary voltage drop in current transformer on this site.

Question 2: CTs have a VA rating just as VTs do. You calculate the maximum burden (resistance) using the VA rating and the rated current. For example, a 5 A, 3.75 VA CT can have a maximum output voltage of 3.75 / 5 = 0.75 V. That means that the maximum output burden (at the rated primary current) will be R = V / I = 0.75 / 5 = 0.15 Ω. Remember that the voltage will fall with falling primary current.

Tip:

• Voltage transformers like open circuits (no load) and don't like short-circuits (overload).
• Current transformers like short circuits (no load) and don't like open circuits.
• So is there any rule to calculate how much load the CT is adding to the line? Sep 9, 2021 at 2:13
• Yes. Power drawn from line = power drawn from CT / efficiency. Efficiency will be close to 1 for a decent CT. I have revised the second paragraph due to late night calculation error. Sep 9, 2021 at 6:53
• I have the simulation here: tinyurl.com/yfstrbw4 . When I increase resistance on the secondary, I can draw more from the primary. Even when I simulate the CT identically, the simulation shows that increasing resistance on secondary would increase voltage and current in the secondary. IRL, when I increase resistance, voltage drop across the resistor increases to a max of what it would be as an open circuit, in my case (5v). As close as I can measure current stays the same. Is this because the CT is weakly coupled bc few primary turns? or what is going on here? Sep 14, 2021 at 0:23

A CT might be able to draw 100 mW of linear power from a few kW power source drawing 10A so it's not efficient nor free energy unless tapped into someone else's line.

A novel method I read about storing extra energy is using a nano crystalline CT with very high mu that would normally saturated 3/4 of the time is counteracted with a control coil to prevent saturation but still stored more energy than put back might boost up to 46%. Their CT with a lot of effort (!) was able to store almost 1/2 W. But the impedance must be matched to get MPT (max power transfer with 50% thru)

The limitation is the thermal resistance of thousands of windings to create 1/2 W with matched impedance from N^2 and the voltage available into a linear load, not LEDs. Plus the ability to added a second winding to counteract the saturation effects that kill the inductance and ability to store energy. But the linear load gets an irregular pulse response and not a sine wave. That's the challenge.

• Any links to this novel method or things I can google? Sep 9, 2021 at 2:12