Fuse Guard: A Circuit for Monitoring a Fuse with a Flashing LED

Never miss a blown fuse again with the Fuse Guard! This straightforward four-component circuit ensures you’re instantly alerted when a mains fuse fails, using an active flashing LED — no complex measuring needed, just smart signaling!

The Fuse Guard

The term “circuit” is almost an exaggeration for this four-component solution. The fuse guard is connected in parallel to a mains fuse. If the fuse is OK, no voltage drops across it and everything is fine. However, if the fuse is triggered or “blown,” almost the full mains voltage of 230 V≈ is present at the monitoring electronics via the connected load. All that is then needed to make an ordinary flashing LED light up is a little current or voltage limitation and rectification.

Current Limitation

Figure 1 shows the circuit. When the fuse is triggered, the mains voltage is applied to R1 and the cathode of D3. R1 has a dual function: On the one hand, its resistance simply limits the current flowing. On the other hand, it also indirectly limits the voltage drop across the LED. If the current is small enough, the voltage dropping across D1 also remains small enough (in the 5 V range), which is beneficial for the blinking LED and its integrated electronics. An explicit voltage limiter is therefore not required. Diode D2, which is anti-parallel to D1, reduces the potential reverse voltage to a maximum of -0.7 V, which is not dangerous for D1.

Essentially, the circuit would be complete at this point, and three components would be sufficient for the function as a fuse monitor. However, with D3, another diode ensures that only half-waves generate current at R1, so only half the power loss occurs. At a voltage of 230 V≈, only about 3 mA_eff flows, which results in a dissipation of just under 0.7 W. Therefore, a 1 W version that is suitable for mains voltage is sufficient for R1.

Assembly

The fuse guard can easily be set up on a small experimental strip-board (Figure 2 and Figure 3). According to the IPC2221 standard, the distances between the strips are, in principle, sufficient for mains voltage, at least in dry ambient conditions. Nevertheless, the areas between which mains voltage is present should ideally have larger distances in the range of ≥2 mm. To achieve this, it may be necessary to remove some copper from the conductor tracks.

As already mentioned, the fuse guard is connected in parallel to the mains fuse to be monitored. If a fuse blows, causing a power outage, the circuit becomes active and flashes. Red or yellow are suitable LED colors — a green LED would misleadingly imply that the fuse is “good.” Caution with mains voltage: Since the circuit is not electrically isolated from the mains, it must be installed so that it is safe from touch. Elektor does not like to lose readers to Zeus, who, as you know, is the one responsible for lightning!

Editor's Note: This article (240272-01), translated by Jörg Starkmuth, will appear in the Elektor Circuit Special 2025.

At Elektor, we publish clever and conversation-starting projects from engineers and makers around the world. The Fuse Guard is a great example — simple, effective, and clearly sparking debate! We appreciate that the author includes a clear mains voltage warning, and as always, we recommend working with caution and using components rated for the job. Safety always comes first. When we publish community projects, our goal is to inspire thinking and open discussion. We encourage everyone to engage critically, and to use the Discussion section to (kindly!) share thoughts, improvements, or safety tips. Let’s keep learning from each other and building cool stuff — safely. — The Elektor Editorial Team

Questions About the Fuse Guard?