In many building automation and electrical control applications, you’ll encounter loads whose current draw is too small to trigger a standard current switch or sensor. In this post, we’ll show a clever, practical technique to reliably detect these low-current circuits and explain exactly how and why it works.
By looping the conductor through a current sensor multiple times (multiplying the perceived current), you can bring even sub-threshold currents into range without changing the circuitry itself. Along the way, you’ll learn key principles, important limits, and real-world examples using Functional Devices’ sensors.
At Functional Devices, we’ve been designing and manufacturing high-quality relays, current sensors, transducers, power supplies, and lighting controls in the USA since 1969. Our goal is to make installation simple and performance reliable for contractors, engineers, and integrators. If you’re facing challenges with low-current sensing or need help choosing the right device, explore our full lineup of current sensors or reach out to our technical team. We’re here to help.
Why “Below Threshold” Is a Problem
Modern systems often include efficient devices like Electronically Commutated Motors (ECMs) and LED lighting and low-wattage power supplies. These loads may draw only a few tenths of an amp — well under the trip point of many current switches. The result: the sensor never “sees” enough current to actuate, leaving the switch open even though a load is present.
We produce several current-sensing products: current switches, current transducers, and combo relay/sensor devices. Although these cover most standard needs, when the load is too light, you need a workaround.
The Looping Trick: How It Works
The principle is simple: pass the current-carrying conductor through the sensor’s aperture more than once. Each pass adds to the effective current seen by the sensor. If you double-loop, the sensor “sees” twice the current; triple-loop, triple; and so on (so long as you don’t exceed the sensor’s rating).
Real-World Example: RIBXGTA
Using Functional Devices’ RIBXGTA current switch (which has a minimum sensing threshold of 0.75 A), here’s how it plays out:
- With a single pass, the conductor carries 0.20 A. That’s below 0.75 A, so the switch does not trip. The LED remains dim, indicating “below threshold.”
- Now loop the same conductor 4 times through the aperture. The sensor “sees” 0.20 A × 4 = 0.80 A, which is above the 0.75 A threshold. The switch closes, the LED lights solid, and the output contacts show low resistance.
This trick works not just for switches, but also for current transducers, which output an analog voltage proportional to the sensed current. Looping increases the sensed current, and thus increases the analog output proportionally.
Limitations & Best Practices
While the looping trick is effective, it must be used carefully:
- Don’t exceed the sensor’s rated current. The boosting effect works only within safe limits.
- Watch magnetic coupling and noise. Multiple loops may introduce additional inductance or coupling—test with your system.
- Maintain consistent passes. All loops should lie cleanly and evenly through the aperture; avoid twisted or overlapping passes.
- Verify with a multimeter. After looping, confirm the switch closure or transducer output to ensure expected behavior.
- Mind physical constraints. The aperture size and cable routing may limit how many loops you can practically use.
This technique is often the simplest way to bring low-current circuits into the reliable sensing range without needing a more sensitive or custom sensor.
When & Why Use This in Your Designs
Here are some applications where using the looping trick is helpful:
- In retrofits where replacing wiring or sensors is impractical
- For energy-efficient systems where loads are very light
- In cases where you want to avoid more expensive sensors with ultra-low thresholds
- When system reliability matters but you also want to minimize cost & complexity
By leveraging this looping trick, control engineers and integrators can expand the usable range of standard current sensors and switches in many practical applications.
Functional Devices: Helping Customers for Over 50 Years
Detecting currents below a sensor’s threshold doesn’t have to force you into specialty devices. With the looping technique, you can multiply what the sensor “sees” and reliably sense light loads using the same hardware you already have.
At Functional Devices, our products are built to simplify installation and keep your systems running reliably, whether you’re working with relays, current sensors, power supplies, or lighting controls. If you’re monitoring LED lighting circuits or ECMs that draw very little current, our solutions can help you achieve accurate detection without costly redesigns. Explore our line of current sensors or connect with our support team for expert guidance on your next project.
About Functional Devices, Inc.
Functional Devices, Inc., located in the United States of America, has been designing and manufacturing quality electronic devices since 1969. Our mission is to enhance lives in buildings and beyond. We do so by designing and manufacturing reliable, high-quality products for the building automation industry. Our suite of product offerings include RIB relays, current sensors, power controls, power supplies, transformers, lighting controls, and more.
We test 100% of our products, which leads to less than 1 out of every 16,000 products experiencing a failure in the field.
Simply put, we provide users of our various products confidence and peace of mind in every box.