How a CT works. Like, physically.

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We live in a world so complicated that we don’t know how objects we use every single day of our lives…work. The things every single one of us use constantly: phones, the internet, and current transformers. Okay, maybe that last one is limited to the kinds of people who click this article. But just like a sign that says “you are here, Steve” it only has to be accurate for its audience. This article’s audience expands beyond Steves, just in case all the Stephens out there were worried.

Current Transformers: From the Eyes of an HVAC Technician

Functional Devices offers a variety of CTs, or Current Transformers. We actually tend to use the catch-all term “Current Sensor.” The general purpose of a Current Sensor is to sense current. Engineers have quite the knack for naming things. Did you know an engineer named Functional Devices, Inc? Almost all Current Sensors work by threading a current carrying wire through a relatively large hole, or aperture, in the center of the device. The Current Sensor reads the current then converts the data into a usable form for the device that is receiving the data. Most building automation controllers are simply looking for a go/no-go. For that, a simple “Current Switch” is a good choice. There is an electrical contact closure that opens or closes depending on whether there is current flowing through the aperture. Some models have LEDs that turn on and off to indicate which state the contact is in.

LEDs? Alright, what kind of voltage does it take?

There is no need to plug in any power source to our Current Switches to get the LEDs to work. And no, there are no batteries in these devices. So…how the heck are the LEDs turning on? Don’t tell the building owner but we’re actually stealing power from the load. That’s right, the current carrying wire going through the aperture is what’s powering the Current Sensor. There is a term called “insertion loss” which describes any voltage drops across the current carrying wire due to the Current Sensor stealing power. Let me assure you that our Current Sensors draw so little power that any sort of effect they have on the line is negligible. Now, if you’re putting 10,000 of our Current Sensors on the same wire…give me a call and let me know what happens.


A voltage transformer is the best way humanity has discovered to convert voltages up and down with minimal power loss. The fundamental principle of a transformer is this: current flow in a wire induces current flow in nearby wires. And that’s the principle used for current transformers, or CTs. I’m going to yada yada over material science, distance equations, AC vs DC, and leave that to a real physics website.

Let’s imagine we have two lengths of wire sitting on a table parallel to each other. On the right one, let’s connect a little lightbulb to the wire. It’s not emitting light because…well why would it? There’s no power applied. But now let’s plug in an AC power source to the left wire. There’s absolutely no physical connection between the left wire with the power source and right wire with the lightbulb. Yet, the lightbulb starts to dimly glow. How can this be? Electromagnetism. It’s magic, but it works. What if we wanted the light to glow twice as bright? We could lay down a third bit of wire in parallel with the other two and apply power to it as well. With now two current carrying wires, the lightbulb wire would be getting a twice as strong magnetic field, inducing a current flow twice as strong. Or, critically, we could keep the two-wire setup and wrap the power source wire. You can imagine if you were holding a current carrying wire in your hand, going across your palm would be, say, 1 Amp. But if you then took that current carrying wire and wrapped it around your hand once more, going across your palm would be 1 Amp the first time, then that 1 Amp again in the second wrapping. What you’ve got in your palm is functionally equivalent to having two current carrying wires of 1 Amp each.

Okay, it’s magic. Let’s wrap this up.

Good play on words, title. We know by increasing the wrappings of our little desktop wire setup, we can get a lightbulb to light up brighter. That equates to more current sucked “wirelessly” from the left wire to the right wire. That’s right, your phone’s wireless charger is just hidden wires. Add more and more wraps and you get more and more current. And that’s pretty much the starting point of every Current Sensor. You’ve got your one 120V wire carrying current to your 1/4HP blower motor. That’s equivalent to the single wire laying across the table. Then inside our Current Sensor is a wire wrapped many times to amplify that current into something usable. Nonintrusive, inexpensive, no need to run extra wire to the sensor, and it’s easy to use. The perfect device for any contractor.

Be sure to check out our full line of Current Switches and Current Transducers.

Noah Smith
Noah Smith

Noah Smith, or as his friends call him "Noah", is a design engineer at Functional Devices. He graduated from Indiana University in 2018. Just kidding, he graduated from the superior Purdue University with a Bachelors degree in Electrical Engineering in 2018.

Outside of work, Noah spends his time fantasizing about going back to work. He is a seasoned traveler, having visited almost two states. His weekends are spent with his wife and what has been described as "too many cats."

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