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Over-Current Protection Simple Electronic Projects with Julian Edgar This very simple project can sound an alarm or disconnect the load when a low-voltage DC current flow exceeds a preset value. T here are many applications where a device needs to be shut off, or a warning given, if a load draws excessive current. This little project can be configured to activate at any current level from about 1A to 20A, costs almost nothing and is suitable for a wide range of low-voltage DC circuits. Example uses include: • an over-current warning or cutout for battery-operated power tools • switching off a motorised door, gate or similar if an obstruction is met while it is moving • protecting simple power supplies • protecting analog model railway controllers if a derailment occurs that short circuits the supply The approach Conventional over-current monitoring is usually done by sensing the voltage drop across a resistor in series with the load. As the current flow increases, so does the voltage across the resistor. However, to minimise the voltage drop (and power dissipation in the resistor), the resistor’s value is usually very low. This small voltage needs to be amplified by additional circuity before being compared to a fixed voltage that corresponds to the maximum allowable current. However, in this project, the current flow is sensed completely differently. Instead of the resistor/amplifier/ comparator approach, a simple reed switch is used. A reed switch closes when subjected to a magnetic field. The magnetic field is normally provided by a magnet being brought close to the switch. Instead, we place a coil of wire around the reed switch. The coil is placed in series with the load, so the full load current passes through this coil. The strength of the magnetic field generated by this coil depends on the The Jaycar SM1002 reed switch closes when a magnetic field is present. This can be provided by either a magnet or coil of wire. siliconchip.com.au current flowing through the winding and its number of turns. When the current reaches a level that develops a sufficiently strong magnetic field, the switch closes. That can sound an alarm, or via a latching relay, disconnect the load. If we want to alter the current at which the reed switch closes, we simply change the number of windings around the switch. Reed switches vary in their specifications, so (say) six turns around one switch may cause the switch to close at 2A, but with another switch, the same six turns may cause the switch to close at 3A. The trick is to test the switch until you get the behaviour you want. We are using the fairly typical Jaycar SM1002 reed switch. It has a glass envelope, is 16mm long and 2mm in diameter, and is rated to handle 0.5A (500mA). If substituting another, make sure it’s a normally-open type. Calibrating the reed switch The lower the number of turns around the reed switch, the higher the current at which the switch trips. The minimum number of turns is one (used in the power tool application covered shortly), and the maximum is mostly dictated by how many you can fit around the reed switch. Using 0.5mm diameter enamelled wire, it’s fairly easy to fit 16 turns on the switch. This gives a trigger point of about 1A. Using a single turn results in a trigger point of about 20A. Because the load current all passes through the coil, using overly thin wire will increase the voltage drop and power dissipation in the coil. However, this works out well because higher current values require fewer turns, allowing the use of thicker wire. For example, 16 turns of 0.5mm wire gives a measured voltage drop of only 1.4mV at 1A. Use the thickest wire that still allows a sufficient number of turns to be wound around the reed switch. It is very important to note that reed switches are fragile – the glass envelope breaks easily. Do not wind the coil directly on the reed switch! Instead, wind it around a former like the shaft of a small screwdriver or a drill bit. If you are using the Jaycar reed switch, a former diameter of just under 2mm works well, and the resulting coil will be a friction fit over the reed switch. Calibrating the device So, how do we calibrate the reed switch to trigger at our desired current? The easiest approach is to use a variable bench power supply with a current readout. Connect your multimeter across the reed switch with the multimeter in continuity mode (ie, it sounds a buzzer when the reed switch closes), which is best done using alligator clip leads. Place the wound coil around the reed switch. Connect the coil switch in series with the power supply & load; the load can be one or more wire-wound resistors (you may not need a load if your bench supply has current limiting). Starting at zero current and voltage, increase the voltage while watching the current display. When the multimeter sounds its buzzer, Rugged reed switches Reed switches are also available in fully encapsulated plastic packages, with the glass reed switch concealed inside. Usually, such switches are sold with a matching magnet for security system applications. We tested some reed switches like these, and got good results, so if you’re concerned about the fragility of the glass switches, you could try one of these. But they’re more expensive. 16 turns of 0.5mm diameter enamelled copper wire on the reed switch. This gives a switching current of about 1A with the Jaycar SM1002 reed switch. Australia's electronics magazine November 2025  79 indicating that the reed switch has closed, take note of the current reading. If you need a higher current trip point, reduce the number of turns on the coil. If you would like a lower current trip point, add more turns. If you don’t have a variable power supply, you could use a resistor bank that gives the calculated correct current flow and then alter the number of turns until the reed switch closes. For example, if you want the switch to close at 2A and you are using a 12V supply, use wire-wound resistors that provide a 6W load (12V ÷ 2A). In this case, the resistors will need to dissipate 24W, so you could use six 1W 5W resistors in series. They’ll still get hot, though, so only keep the circuit powered briefly on each test. Technically, the relationship between the trip point and number of turns around the reed switch should be linear, but I did find some variation during my testing. Perhaps this was because the coils were not always identical except in the number of turns. The reed switch ‘naturally’ has hysteresis – the switch-off current is considerably lower than the switch-on current. For example, the switch may close at 1A and open at 600mA. Alarms and disconnects Sounding an alarm when current flow exceeds the set point is very easy – as shown in Fig.1, you just need to wire a buzzer in series with the reed switch and connect both across a voltage source. Choose the buzzer voltage to match the supply or, if using a buzzer with a lower operating voltage, use a series resistor to drop the voltage to suit. Disconnecting the load when the setpoint is reached is a little more complex. Fig.2 shows my approach. A relay disconnects the load if the setpoint is exceeded. However, if that were the entire circuit, the relay would operate, the load would disconnect, the current would drop to zero, the reed switch would open, and then the process would repeat! To avoid this, we use a relay with an additional set of contacts that causes the relay to latch (ie, to stay engaged) once it has been pulled in. This is achieved by wiring the relay’s second common (C) and normally open (NO) set of contacts in parallel with the reed switch. A momentary reset button opens this circuit, causing the relay to drop out, or you could power-­cycle the device to reset it. Note that a diode is placed across the relay coil, protecting the reed switch’s contacts against the inductive spike from the relay’s coil. 80 Silicon Chip This Makita 18V battery-operated drill comes apart easily, with normal Philips head screws holding the two halves of the body together. Fig.1: the over-current buzzer circuit. When sufficient current flow occurs through the coil, the reed switch closes, activating the warning buzzer. This will work with a device powered by a DC mains supply in place of the battery. Fig.2: the over-current disconnect circuit. When sufficient current flows through the reed switch, it closes, pulling in the relay and disconnecting the load. The relay latches in that state, with the system able to be reset by pressing the pushbutton or by cycling power (eg, removing the battery). Australia's electronics magazine siliconchip.com.au An optional buzzer can be wired in parallel with the relay’s coil so the user knows why the power was cut. The red and black wires going to the motor are the only ones we need to access. Either the negative or positive connection to the motor can be cut (I cut the negative as it was easier). I used normal multi-stranded cable tinned... Adding an over-current alarm to a battery drill ... with solder to rejoin the wires and ensured the coil turns could not short together. The reed switch has been slipped into place to show how it will fit. The motor and buzzer power/ground connections, insulated with tape. The tape will later be wrapped around the reed switch as well, leaving only its connections exposed. I used a 12V buzzer with a 100W dropping resistor to suit the measured 19.5V supply. It was easily loud enough to be heard through the case with the drill running (otherwise, make a small hole in the case). After being tested, the bare connections can be covered with silicone sealant. siliconchip.com.au Australia's electronics magazine If you have been using power tools for a long time, it’s likely you’ve developed a good feel for their use. For example, when you are drilling a large hole, you start with a smaller drill bit and you’ll also know to go gently when you move onto the big drill bit. However, people who are new to power tools literally have no idea about these things! Instead, they’ll work a power tool until it goes up in a puff of smoke. I’ve seen it happen... To prevent that, you can add this over-current alarm to a battery-­operated drill, which causes a piezo buzzer to sound long before the drill stalls. It can even sound a more subdued, pulsating warning as the drill load gets close to being excessive, with brief current spikes being just enough to momentarily close the reed switch. Simply use the circuit shown in Fig.1, with the buzzer powered by the drill’s motor power feed. Here, a single turn of wire around the reed switch worked well in giving an alarm prior to the drill stalling, and the alarm does not sound under normal loads. However, that was pure luck; in some cases, adjustments may be required to get a suitable result. One approach is to open up the drill, cut a wire to the motor and extend the cut ends outside the case. Close the drill up again, and you have an easy way of trialling different numbers of turns around the reed switch. To load the drill, lock a straight shaft in the drill chuck and then clamp this shaft between two pieces of wood in a bench vice. By tightening the vice, you can vary the load. If the drill has a two-position gearbox, always test on the faster speed (lower torque). Never try to load the drill when it is disassembled – the motor could leap from the casing and cause injury. Many battery drills develop a lot of torque, so you will need a firm hand as you increase the load on the drill by clamping the blocks more tightly around the spinning shaft. Test in short bursts, for your sake, as well as the motor’s. Conclusion This is a simple and inexpensive modification that can protect tools or other devices from being overloaded and damaged. There also isn’t a lot to go wrong – the parts should last essentially forever, SC especially if rarely triggered! November 2025  81