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Constructional Project Project by John Clarke DC Supply Protectors Any one of these three simple, inexpensive circuits will protect your equipment from damage due to an incorrectly connected or malfunctioning power supply. They protect against a higher than expected voltage or a reverse polarity supply and have very little effect on the voltage applied to the device. M any devices are powered using a mains plugpack or power ‘brick’. All is well if you use the proper supply and it is wired correctly. However, damage can occur if the wrong supply is used or it is miswired, applying either the wrong polarity voltage or an excessively high voltage to the item to be powered. That is an especially big problem if you haven’t used the device for many years, have moved, if you’ve had to buy a replacement power supply (due to failure or loss), or someone else is using it who is not familiar with the correct supply. Our Supply Voltage Protectors prevent damage to equipment in the case of an incorrect input voltage. They switch off power to the equipment if the input voltage is too high and prevent current flow if the polarity is incorrect. A supply that produces more voltage than a piece of equipment expects can damage its internal components. Applying reverse polarity to a circuit can also irreparably damage parts, such as ICs and electrolytic capacitors unless the circuitry already includes reverse polarity protection. Such protection (eg, a diode) often reduces the voltage available to the device. However, the designs presented here are different, as they use a Mosfet instead that loses very little (basically no) voltage. Q1 IPP80P03P4L-07 or SUP60061EL-GE3 D S K ZD1 G 15V A OVERVOLTAGE ADJUST VR1 50kW 3.9kW REF1 TL431 + 10kW 3.9kW A SC Ó2025 TL431 1N4148 K A REF A E l K K + POWER LED1 – 2.2kW G K S ZD 2 CON2 Q2 IPP80N06S4L OR FDP027N08B-F102 13V A K 1N4148 K Q4 BC546 10kW A K B K A Q1 , Q2 Q3 , Q4 LEDS ZD1,ZD2 A K C 1 0 kW B D1 A REF l D A Mirror Version of TL431 A 2.2kW C K 2.5V TL431 10kW Q3 BC556 10kW CON1 REF E B OUTPUT A REF – K LED2 1 0 kW INPUT 10kW OVERVOLTAGE 100nF E C G D D S SUPPLY PROTECTOR (ADJUSTABLE THROUGH-HOLE VERSION) Fig.1: the adjustable TH version uses Mosfet Q1 for reverse-polarity protection and Q2/REF1 for over-voltage protection. 46 Practical Electronics | May | 2025 DC Supply Protectors These Protectors can be built as standalone devices. Still, as they are relatively compact and inexpensive, they should ideally be installed within existing equipment. That way, nothing bad should happen unless you connect a supply that’s outside the usual ratings. We are presenting three versions of the circuit. Two have a trimpot adjustment to set the overvoltage protection threshold; both can be used with plugpacks that produce up to 27V DC. One of those two versions uses surface-­mounting parts, so it is smaller than the other versions and can be squeezed into tighter spaces. It is rated to handle up to 3A. This version can have the overvoltage set as low as 3V. The through-hole equivalent can handle more current, up to 7A. However, it needs at least 5V to operate. The third version is slightly cheaper to build but can only be adjusted in voltage steps determined by zener diode reverse breakdown or avalanche values. It can handle up to 50V. One advantage of this version is that it doesn’t require a setup procedure; you simply build it, install it and away you go. Its minimum overvoltage protection setting is 7.5V. Circuit details Figs.1-3 show the circuits of all three versions. They all provide reverse-­ polarity protection using P-channel Mosfet Q1. With a correct polarity connection, Q1 conducts initially via the intrinsic diode within the Mosfet, allowing current to flow and voltage to appear at the source. The 10kW resistor then pulls the gate to ground, so Q1’s channel is switched on. This allows current to bypass the intrinsic diode, greatly reducing the voltage across it. Zener diode ZD1 prevents the gate-to-source voltage from exceeding the maximum specification of 10V for the surface-mount device (SMD) version and 15V for the through-hole (TH) version. On the other hand, if the polarity of the input supply is reversed, Q1’s intrinsic diode is reverse-biased and the gate voltage is the same as the source. So the Mosfet remains off, and no current flows to the load. Adjustable over-voltage protector circuit Figs.1 & 2 are the adjustable overvoltage protector circuits. The circuit Practical Electronics | May | 2025 is the same for the TH (Fig.1) and SMD (Fig.2) versions of the board, except that some parts have different codes/ packages and some ½W TH resistors are replaced with two parallel ¼W SMD resistors (where a higher power rating is required). This overvoltage protection circuitry comprises N-channel Mosfet Q2, shunt regulator IC REF1 and bipolar transistors Q3 & Q4. Q2 is usually held on via gate voltage applied through the 10kW resistor from the positive supply. In this case, it has a low resistance between its drain and source, connecting the ground terminal of CON1 to the negative side of CON2, so current can flow between the load and supply. Zener diode ZD2 protects the gate of Q2 from excessive voltage. A 10V zener is used for the SMD version, while a 13V zener diode is used for the TH version, reflecting the ratings of the selected Mosfet types. Power indicator LED1 is lit by current flow through the 2.2kW resistor to the negative side of the supply. For the SMD version, the 2.2kW resistor is instead two 4.7kW resistors in parallel, in case the supply voltage is at the higher end of the allowable range. The TL431 adjustable shunt voltage reference IC, REF1, detects an overvoltage condition from the input supply. Fig.4 shows the circuitry within the TL431, which includes a 2.5V reference, an op amp, an output transistor and a protection diode. When used as a voltage reference, the REF input connects to the cathode, placing the TL431 in a negative feedback configuration, where it regulates its ‘cathode’ voltage to match its internal reference voltage. If you want a higher cathode voltage, a voltage divider is included between the cathode and anode, with the divided voltage applied to the REF input. For our circuit, we instead apply a voltage to the REF terminal via a resistance. In this case, the TL431 operates in open-loop mode without any feedback to maintain the reference voltage. This arrangement uses the internal op amp as a comparator, switching the output transistor off if the input voltage (Vin) is lower than the reference voltage, or on otherwise. When the transistor is off, the cathode connection is pulled to the supply voltage via Rsupply. In contrast, when the transistor is switched on, the cathode Features & Specifications Adjustable through-hole version ● Overvoltage protection threshold: 5-27V (3-27V if SMD TL431 is used) ● Input voltage range: 5-27V ● Maximum current: 7A Adjustable SMD version ● Overvoltage protection threshold: 3-27V ● Input voltage range: 3-27V ● Maximum current: 3A Fixed through-hole version ● Overvoltage protection threshold: 7.5-47.7V ● Input voltage range: 5-50V ● Maximum current: 7A Kits from Silicon Chip Adjustable SMD Version (SC6948, £10.50 + P&P): includes the PCB and all onboard parts. Adjustable TH Version (SC6949, £13.50 + P&P): includes the PCB and all onboard parts with both SOT-23 & TO-92 TL431 ICs. Fixed TH Version (SC6950, £12 + P&P): comes with the PCB and all onboard parts except ZD3 and R1-R7. 47 Constructional Project Fig.2: the SMD adjustable version of the circuit is very similar to the one shown in Fig.1. The main differences are the use of alternative devices and the doubling of some resistors for increased power handling. connection is held close to the anode voltage of 0V. The state of the output, whether high or low, depends on the voltage applied to the REF input. In our circuit (Fig.1 or Fig.2), this voltage is from the divider connected across the input supply formed by trimpot VR1 and a 3.9kW fixed resistor (or two 7.5kW resistors in parallel, giving 3.75kW). When this divided (reduced) voltage is below the 2.5V reference, the cathode of REF1 is pulled high. When the divided voltage is above 2.5V, the cathode is pulled low, near to ground potential. Trimpot VR1, in conjunction with the 3.9kW resistor, sets the overvoltage threshold. When the threshold is reached, the cathode of REF1 goes low, so transistor Q3 switches on. It in turn switches on transistor Q4, which pulls the gate of Mosfet Q2 low to switch it off. In this condition, the high collector voltage of Q3 pulls the adjust terminal of REF1 higher again via diode D1 and the 10kW resistor. This provides voltage hysteresis, ensuring that REF1’s cathode remains low until the supply voltage drops significantly below the overvoltage setting. The 100nF capacitor between the base and emitter of Q3 is included to prevent the circuit from initially latching into a voltage overload state at power-up. Immediately after power is applied to CON1, REF1 would momentarily conduct current that would otherwise switch on Q3 and latch REF1 on if it were not for the capacitor momentarily holding Q3 off. LED2 is the overvoltage indicator and it lights under two conditions. If the input supply exceeds ZD2’s breakdown voltage, current will flow through LED2, its 2.2kW series resistor and ZD2. However, if the overvoltage threshold is set below ZD2’s breakdown voltAn enlarged photo of the underside of the Adjustable SMD version of the DC Supply Protector. Compared to the other versions, this one has components mounted on both sides of the PCB. 48 age, LED2 will only light when there is an overvoltage shutdown, via NPN transistor Q4. Overvoltage shutdown is indicated when LED2 is on and LED1 is off. When reverse polarity protection is active, both LEDs will be off despite the input power supply being switched on. TL431 limitations One thing to note when using the TL431 in the TO-92 through-hole package is that the threshold between switching high or low is closer to 2V than 2.5V. The likely reason is that the reference requires a minimum current to produce the 2.5V reference, which is only available in closed-loop mode. In open-loop mode, the reference is operating further down the threshold knee of the voltage versus current curve. This threshold also varies with temperature, although provided the temperature does not vary over a wide range, the resulting accuracy will be satisfactory. For more information on using the TL431 in open-loop mode for undervoltage and overvoltage detection, see the Texas Instruments Application Report SLVA987A PDF at www.ti.com/lit/pdf/slva987 The SMD version of the TL431 does not appear to suffer the same problem, as it shows a very sharp voltage-­versuscurrent threshold voltage curve even at very low currents. For this reason, the TH PCB has provision for using Practical Electronics | May | 2025 DC Supply Protectors Q1 IPP80P03P4L-07 or SUP60061EL-GE3 D S K K ZD1 15 V G A ZD3 A R3 R4 R5 R6 A 10kW – Q3 BC546 K CON1 D B A SCR1 C106D R2 A K G LEDS K CON2 10k W E 150 W ZD1,ZD2, ZD3 – R7 R1 C + l LED1 9. 1k W + SC LED2 OUTPUT A POWER K INPUT Ó2024 l OVERVOLTAGE K 10nF G S ZD2 13V K Q2 IPP80N06S4L OR FDP027N08B-F102 A 470W SUPPLY PROTECTOR (FIXED OVERVOLTAGE) B E G C Q 1, Q 2 C106D BC546 A A G K D D S Fig.3: the fixed overvoltage version requires you to select values for resistors R1-R7 depending on the threshold voltage you want; see Table 1 overleaf. It uses an SCR and zener diode for the over-voltage function rather than a TL431. the surface-mount version instead of the TO-92 package version. Fixed overvoltage protector circuit Fig.3 is the fixed overvoltage protector circuit. This circuit includes reverse polarity protection using Pchannel Mosfet Q1 in the same way as Figs.1 & 2. The overvoltage protection also uses N-channel Mosfet Q2, although Q2 is controlled differently in this circuit. Instead of an adjustable overvoltage threshold controlled by a TL431 shunt reference IC and trimpot, the threshold is set and detected by zener diode ZD3. If the voltage applied to the zener is above the overvoltage threshold and it conducts, silicon-controlled rectifier SCR1 is triggered to switch off Mosfet Q2. A 10nF capacitor is included across the SCR to prevent it from latching on due to a rapid rise in voltage (dV/dt) as power is initially applied to CON1. Any voltage rise faster than 8V/μs will likely switch the SCR on. The 10nF capacitor slows down the voltage rise. Mosfet Q2 is normally held on via the gate voltage applied by the 10kW gate resistor and paralleled resistors R3-R6. Zener diode ZD2 protects the gate from excessive voltage. With Q2 on, a low-resistance connection exists between the drain and source, connecting the ground of CON1 to the negative side of CON2. Practical Electronics | May | 2025 In that case, the power LED (LED1) lights due to the current flow through R7 to the negative side of the supply. Transistor Q3 is typically switched on by the bias current from the positive supply via resistors R3-R6 and R1. With Q3 on, current can flow through ZD3 at its collector and the 150W resistor at its emitter, but only if the supply voltage exceeds ZD3’s breakdown voltage. 4mA needs to flow through ZD3 before the voltage across the 150W resistor reaches 0.6V, which is just sufficient to trigger SCR1 via its 470W gate resistance. Thus, when the SCR switches on and disconnects the load, the supply voltage is ZD3’s rated breakdown voltage plus the 0.6V required across the 150W resistor. When SCR1 latches on, there is about 1V between its anode (A) and cathode (k), so Q3 switches off. With SCR1 on, the low voltage at Q2’s gate switches it off, disconnecting the ground supply at CON2. LED1 is now off, while the low voltage across SCR1 causes LED2 to light, with current flowing through the 9.1kW resistor to the switched-on SCR. Note that LED2 will also light when the voltage across ZD2 reaches its breakdown of 13V. As the supply voltage rises, LED2 brightens as more current flows through the LED via the 9.1kW resistor and ZD2. Overvoltage shutdown is indicated when LED2 is lit but LED1 is off. The voltage divider formed with R1 and R2 ensures that Q3’s base is well below 0.6V, keeping Q3 off when SCR1 is on. With Q3 off, the gate drive to SCR1 is off, but the SCR remains latched on due to the current flowing through it. Resistors R3 to R6 provide the required 5mA latching and holding current to ensure it stays on in this condition. Fig.4: the basic circuitry within a TL431 voltage reference. Usually, the REF terminal is connected to a divider between the anode and cathode (closed-loop mode). Here, we are using it in open-loop mode, as a voltage detector. 49 Constructional Project Table 1 – resistance values for fixed TH version 13V 13.7V 30kΩ 10kΩ 8.2kΩ 8.2kΩ 6.8kΩ × 1.2kΩ If you are wondering why we need Q3 instead of ZD3 connecting directly in series with the 150W resistor, it is because ZD3 could be damaged by excessive current as the supply voltage rises well above its breakdown voltage. For example, if ZD3 is a 12V zener diode, it will conduct 4mA when the supply is at 12.6V but 186mA at 40V. In that case, it would be running well above its power rating. Additionally, the 150W resistor would be dissipating just over 5W. Having transistor Q3 means that all this current stops once the overvoltage threshold is reached, preventing high dissipation in ZD3 and the 150W resistor. 12V 12.7V 27kΩ 8.2kΩ 6.8kΩ 6.8kΩ 6.8kΩ × 1kΩ 11V 11.7V 24kΩ 8.2kΩ 5.6kΩ 6.8kΩ 6.8kΩ × 1kΩ Zener diode biasing 10V 10.7V 18kΩ 6.2kΩ 6.8kΩ 5.6kΩ 5.6kΩ × 910Ω ZD3 Vovl R1 R2 R3 R4 R5 R6 R7 47V 47.7V 130kΩ 13kΩ 18kΩ 18kΩ × × 8.2kΩ 43V 43.7V 110kΩ 13kΩ 16kΩ 16kΩ × × 6.8kΩ 39V 39.7V 100kΩ 13kΩ 15kΩ 15kΩ × × 5.6kΩ 36V 36.7V 91kΩ 13kΩ 16kΩ 13kΩ × × 4.3kΩ 30V 30.7V 75kΩ 13kΩ 12kΩ 12kΩ × × 3.0kΩ 27V 27.7V 68kΩ 13kΩ 5.6kΩ × × × 2.4kΩ 24V 24.7V 62kΩ 13kΩ 4.7kΩ × × × 2.2kΩ 22V 22.7V 56kΩ 13kΩ 8.2kΩ 10kΩ × × 2.0kΩ 20V 20.7V 51kΩ 13kΩ 8.2kΩ 8.2kΩ × × 1.8kΩ 16V 16.7V 36kΩ 10kΩ 10kΩ 10kΩ 8.2kΩ × 1.3kΩ 15V 15.7V 33kΩ 10kΩ 10kΩ 8.2kΩ 8.2kΩ × 1.3kΩ 9.1V 9.8V 15kΩ 4.3kΩ 5.6kΩ 5.6kΩ 5.6kΩ × 820Ω 8.2V 8.9V 12kΩ 4.3kΩ 4.7kΩ 4.7kΩ 4.7kΩ × 750Ω 7.5V 8.2V 7.5kΩ 2.4kΩ 5.6kΩ 5.6kΩ 5.6kΩ 5.6kΩ 620Ω 6.8V 7.5V 3.6kΩ 1.2kΩ 4.7kΩ 4.7kΩ 5.6kΩ 5.6kΩ 560Ω White = ½W, yellow = 1W, × = not fitted Fig.5: a typical V/I curve for a zener diode. 50 The 150W resistor could be increased in value, but that would mean that the overvoltage threshold would occur at a much lower voltage than the zener diode breakdown voltage. This would be less consistent than using the zener at the steeper region of its conduction curve. Fig.5 shows a typical zener diode V/I curve. In the forward direction (current flowing from anode to cathode), it acts like a regular diode, conducting current with 0.6-0.7V voltage across it. In the reverse direction, the zener initially acts like a diode, blocking current with minimal leakage current. However, beyond a certain voltage, the ‘leakage’ current increases and then it begins conducting significant reverse current. This is the reverse breakdown mode, which provides a relatively steep VI curve beyond the knee region. Each zener diode is characterised at a particular current for its zener voltage. If the zener diode is operated at a current much less than that, the voltage across it will also be lower. For our circuit, we want the zener diode operating more in the linear region, where the V/I curve is steep, rather than in the knee region and preferably near to the reference current for the zener. The recommended BZX79Cxx series of zener diodes for our circuit are characterised for a 5mA reference current between 2.4V to 24V, or 2mA above that. The 4mA current for the zener diode in our circuit is a reasonable compromise between those. Practical Electronics | May | 2025 DC Supply Protectors Resistance value calculations The resistance values required for resistors R1 to R7 depend on the overload voltage (Vovl), the maximum input voltage (Vmax) and the latching and holding current for SCR1. Resistor power ratings, LED currents, transistor Q3’s base current and ZD3’s current need to be considered. Table 1 shows the resistor values and wattage ratings for various overvoltage thresholds and a 50V maximum applied input voltage. A panel describes the calculations used to formulate that table in more detail. SMD adjustable version The SMD adjustable version is built using a double-sided plated-through PCB coded 08106241 that measures 51 × 23mm. As shown in the overlay diagrams (Fig.6), all the SMD parts except the two LEDs mount on one side of the PCB, with the through-hole parts such as the two screw terminals and trimpot on the other side. Begin by soldering the SMDs. That can be done by soldering one lead of the component first, holding it in place with tweezers. Once it is aligned and positioned correctly (by remelting the solder if necessary), the remaining lead(s) are then soldered. A good light and a magnifying glass are very useful for this task. You will need to identify the parts first. The resistors are marked with three or four digit codes as shown in the parts list. The 100nF capacitor will not be marked. The smaller semiconductors in SOT-23 packages also have component markings, as per the parts list (although they can vary). The 10V zener diodes are cylindrical with blue markings at the cathode (k) end. Diode D1 also has a polarity stripe at the cathode end. Note that the TL431 can have alternative pinouts, with the standard pinout having the cathode at left and reference at right when the anode pin is at the top. The mirrored pinout has the cathode and reference pins transposed. We have provided for both orientations on the PCB by having a 6-pad footprint instead of just the three required for one pinout of the device. The TL431 must be orientated according to the pinout of the device used. We have marked the pins on the PCB overlay showing the anode, cathode and reference pads. The parts supplied in our kit should be the mirror pin version. The way to check this is to use a multimeter on its diode test across the cathode and reference pins. You should get a reading of one diode drop (around 0.7V) when the red probe is on the REF pin and the black probe on the cathode pin. You can then orientate it correctly on the PCB and solder it in place. While doing that, be careful not to let solder bridge the used and unused pads. If that happens, use a bit of solder wicking braid can be used to remove the excess solder (adding flux paste will make it easier). When installing the diodes, make sure these are orientated correctly. The anode (A) and cathode (k) orientations are marked on the PCB overlay. Once all the surface mount parts have been soldered in place on the one side, flip it over and fit the LEDs, taking care to place each with its cor- rect orientation (checked as mentioned earlier) and in the correct position with regard to colour. These are green for power and red for overvoltage, although you are free to customise the colours if desired. Ideally, the surface mount LEDs should be tested using the diode test mode of a multimeter. With the red probe on the anode and black lead on the cathode, the LED should light and show its colour. We used green for power and red for overload. There is often a stripe or dot on the cathode but we have seen LEDs with a marking on the anode, so it’s better to test them. The trimpot is installed with the top screw adjustment orientated as shown. This provides an increasing overvoltage threshold with clockwise rotation. The two screw terminals should be mounted with the wire entry toward the outside of the PCB at each end. TH adjustable version The through-hole adjustable version is built on a double-sided plated-­ through PCB that’s 08106242 and measures 70.5 × 35.5mm. Refer to Fig.7, the PCB overlay diagram, during the assembly process. If you are using the SMD TL431 version, install it first, but be careful as they can have alternative pinouts with the reference and cathode transposed. See the instructions a few paragraphs above on determining which pinout you have, aligning it with the PCB markings and soldering it. The zener diodes and diode D1 can be fitted next. ZD1 is a 15V type, while ZD2 is rated at 13V. These each need to be orientated as shown in Fig.7, Fig.6 (left): the overlay diagrams for the SMD adjustable version of the Supply Protector (shown at 150% actual size). Fig.7 (upper right): the PCB overlay diagram for the through-hole adjustable version. Fig.8 (lower right): the PCB overlay diagram for the through-hole fixed overvoltage version. Practical Electronics | May | 2025 51 Constructional Project Resistance value calculations Table 1 shows the required resistance values and power ratings for the Fixed Protector for overvoltage thresholds from 7.5V to 47.7V with a maximum input voltage of 50V. There are no satisfactory resistance values to meet all requirements for overvoltage thresholds below 7.5V, so if you require a threshold that low, build one of the other versions. R3 to R6 calculations The total resistance for R3 to R6 is calculated first. This resistance provides current for SCR1 and the base of transistor Q3 via R1. Up to four resistors can be paralleled for a sufficient power rating and to achieve the required resistance. The latching and holding current required for SCR1 to remain in conduction is 5mA. This satisfies the worst-case latching current and the worst-case holding current at 25°C. The total resistance, R, required is the overload voltage threshold (Vovl) minus one volt (the SCR anode-to-cathode on-voltage), divided by 5mA, ie, R = (Vovl − 1V) ÷ 5mA. The total power rating required is the maximum operating voltage for the circuit (eg, 50V) minus 1V squared and then divided by the resistance, ie, (Vmax − 1V)2 ÷ R. The required power rating can be reduced by spreading it between two to four resistors in parallel. If all those resistors have the same value, they share the dissipation equally. If different, each resistor will need to be assessed for its share of the dissipation. R1 & R2 value calculations Resistor R1 drives the base of Q3, which must saturate when conducting 4mA. This 4mA is the current that flows through ZD3, Q3 and the 150Ω resistor at the overvoltage threshold. We drive Q3’s base with 250μA (1/16th the collector current) or more to ensure Q3 goes into saturation. Resistor R2, between the base of Q3 and ground, is necessary since it reduces the base voltage to less than 0.3V due to divider action with R1 once SCR1 is latched. Typically, SCR1 will have about 1V across, so provided that R1 is at least triple R2’s value, that will be reduced to 250mV or less. That prevents Q3 from conducting through ZD3 once overvoltage has been detected and SCR1 latches on. For overvoltage settings of 20.7V and above, we set R2 so 100μA flows through it at the overload voltage threshold. At this threshold, there will be 0.66V between the base and emitter of Q3 and 0.6V at the emitter of Q3, giving a total of 1.26V across R2. For an approximate 100μA current, R2 needs to be 12.6kΩ (13kΩ is the closest E24 value). 13kΩ gives 96.9μA, close enough to 100μA. When calculating the value for R1, this 100μA needs to be included since this bypasses the current from Q3’s base. So, instead of R1 supplying 250μA to Q3’s base, it needs to supply 350μA in total. R1 is calculated as the overload voltage threshold (Vovl) minus the 1.26V at Q3’s base, divided by 350μA. Since R1 is in series with the R3-R6, the parallel value of R3-R6 is then subtracted from this to get R1’s value, ie, R1 = (Vovl − 1.26V) ÷ 350μA − (R3 || R4 || R5 || R6). If the calculated value for R1 is less than three times the value of R2, the current through R2 needs to be increased and the equations reworked. For example, to get 200μA through R2, R2 = 1.26V ÷ 200μA = 6.3kΩ (use 6.2kΩ). Then R1 = (Vovl − 1.26V) ÷ 450μA, where 450μA is the 200μA R2 current plus the 250μA required for Q3’s base. LED current LED1 switches off above the overvoltage threshold, so the maximum LED current will occur with the supply at the overvoltage setting. Assuming 10mA is a suitable maximum current, the value for R7 is the overload voltage minus the 2V across the LED, divided by 10mA, ie, R7 = (Vovl − 2V) ÷ 10mA. The power rating for R7 also needs to be considered, so its value needs to be greater than (Vovl − 2V)2 ÷ 250mW, where 250mW is a conservative derating for a 500mW resistor. If this calculation gives a higher value than the above, the maximum LED current will be below 10mA to avoid overheating the current-limiting resistor. The overvoltage LED (LED2) series resistor value is calculated similarly; only this time, the maximum input supply voltage is used in the calculation. That’s because LED2 will light from the overvoltage threshold to the maximum input supply voltage, Vmax. So the calculation is Vmax minus the voltage across LED2 and SCR1, divided by 10mA, ie, R = (Vmax − 3V) ÷ 10mA. Similarly, the minimum value, considering the resistor power rating, is (Vmax − 3V)2 ÷ 250mW. We selected a 9.1kΩ 1/2W resistor for a Vmax of 50V. 52 with the cathode band toward the top. The resistors can be mounted next; check each value with a multimeter to be sure the correct value is installed in each place. The two LEDs are installed with the tops of their domes about 12mm above the top of the PCB. Check which colour the diode is before installing it, using the diode test mode on a multimeter if the lenses aren’t tinted. We used a green LED for power (LED1) and red for overvolage (LED2). In each case, the longer lead is the anode. Next, fit transistors Q1-Q4, being careful that each is placed in the correct position (check their part codes against Fig.7 and the PCB overlay). If using the TO-92 package version of the TL431 (REF1), you can also fit it now. Follow by mounting the 100nF capacitor. The trimpot is installed with the screw adjustment orientated as shown, providing an increasing overvoltage threshold with clockwise rotation. The two screw terminals are mounted with the wire entry toward the outside of the PCB at each end. TH fixed overvoltage version The through-hole fixed overvoltage version is built on a double-sided, plated-through PCB coded 08106243 that measures 70.5 × 35.5mm. The PCB overlay diagram for this version is Fig.8. First, the values for resistors R1-R7 need to be selected using Table 1, based on the required overvoltage threshold. The required voltage rating for ZD3 is also listed in that table. Note that resistors R3-R6 may need to be 1W types (if shown in yellow in Table 1), and not all four of these resistors are necessarily used for all possible threshold voltages. The zener diodes and diode D1 can be fitted now. ZD1 is rated at 15V, ZD2 is a 13V type, while ZD3 is as per Table 1. These each need to be orientated as shown in Fig.8, with the cathode band toward the top. The resistors can be mounted next; check each value with a multimeter to be sure the correct value is used in each location. The two LEDs are installed with the tops of their domes about 12mm above the top of the PCB. Check which colour the diode is before installing it, using the diode test on a multimeter if the lenses aren’t tinted. We used green Practical Electronics | May | 2025 DC Supply Protectors for the power LED (LED1) and red for overvoltage (LED2). In each case, the longer lead is the anode. Be sure when mounting Q1 to Q3 that each is placed in the correct position and orientation. The SCR goes in with the metal tab side towards R3-R6. The trimpot should be installed with the screw adjustment orientated as shown, providing an increasing overvoltage threshold with clockwise rotation. The two screw terminals are mounted with the wire entry toward the outside of the PCB at each end. Testing If you have an adjustable power supply, you can apply power to the input and check that the power LED lights and the overvoltage switch-off function operates at the desired voltage. This is preset with the fixed version or can be changed using VR1 for the adjustable versions. Once the overvoltage threshold has been reached, the power LED goes off and the overvoltage LED lights up.The supply will need to be switched off or significantly reduced before power is restored to the output. Also remember that the overvoltage LED may light once the supply voltage exceeds ZD2’s breakdown voltage. Overvoltage shutdown is indicated when the power LED (LED1) is off and the overvoltage LED (LED2) is lit, but not when both LEDs are alight. For the adjustable versions, you can set the overvoltage threshold approximately by measuring the resistance across VR1 when the power is off. Divide the VR1 resistance by 3.9kW, add one, then multiply by 2V if you used a TO-92 TL431 or 2.5V if you used the SMD version. The formula is Vovl = (R(VR1) ÷ 3.75kW + 1) × Vref. That will tell you roughly what voltage it will cut out at, within about 1V. For the reverse calculation, to determine what resistance you need across VR1 for an approximate voltage threshold, divide the desired threshold by 2V (TO-92 TL431) or 2.5V (SMD TL431), then subtract one and multiply by 3.9kW (3.75kW for the SMD version) The formula is R(VR1) = (Vovl ÷ Vref − 1) × 3.9kW. To set it more accurately, you will need an adjustable power supply or make a basic one using a wirewound 1kW potentiometer connected across a fixed supply (but be careful not to exceed its power rating). PE Practical Electronics | May | 2025 Parts List – DC Supply Protectors Common between all versions 2 2-way PCB mount screw terminals with 5mm or 5.08mm spacing (CON1, CON2) SMD Adjustable Version 1 double-sided, plated-through PCB coded 08106241, 51 × 23mm 1 100nF 50V X7R ceramic capacitor, SMD 3216/1206 size 1 50kΩ multiturn top-adjust trimpot, Bourns 3296W style (VR1) Semiconductors 1 AO3401(A) 30V 4A P-channel logic-level Mosfet, SOT-23 (Q1; marking: X15V) 1 AO3400 30V 5.8A N-channel logic-level Mosfet, SOT-23 (Q2; marking: XORB) 1 BC856C 65V 100mA PNP transistor, SOT-23 (Q3; marking: 9AC) 1 BC846C 65V 100mA NPN transistor, SOT-23 (Q4; marking: 1C) 1 TL431 adjustable shunt voltage reference, SOT-23 (REF1; marking: 431) 🔴 1 1N4148WS 75V 150mA switching diode, SOD-323 (D1) 2 BZV55-C10 10V 500mW zener diodes, SOD-80C (ZD1, ZD2) 1 green SMD LED, M3216/1206 size (LED1) 1 red SMD LED, M3216/1206 (LED2) Resistors (all M3216/1206 size 1/4W 1% SMD) 4 7.5kΩ (code 7501 or 752) 4 4.7kΩ (code 4701 or 472) 7 10kΩ (code 1002 or 103) Through-Hole Adjustable Version 1 double-sided, plated-through PCB coded 08106242, 70.5 × 35.5mm 1 100nF 63V/100V MKT polyester capacitor 1 50kΩ multiturn top-adjust trimpot, Bourns 3296W style (VR1) Semiconductors 1 IPP80P03P4L-07 or SUP60061EL-GE3 P-channel logic-level Mosfet, TO-220 (Q1) 1 IPP80N06S4L or FDP027N08B-F102 N-channel logic level Mosfet, TO-220 (Q2) 1 BC556 65V 100mA PNP transistor, TO-92 (Q3) 1 BC546 65V 100mA NPN transistor, TO-92 (Q4) 1 TL431 adjustable shunt voltage reference, TO-92 (REF1) OR 1 TL431 adjustable shunt voltage reference, SOT-23 (REF1; marking: 431) 🔴 1 1N4148 75V 200mA signal diode (D1) 1 15V 500mW or 1W zener diode (ZD1) 1 13V 500mW or 1W zener diode (ZD2) 1 3mm green LED (LED1) 1 3mm red LED (LED2) Resistors (all ½W metal film, 1%) 2 3.9kΩ 2 2.2kΩ 7 10kΩ Through-Hole Fixed Overvoltage Version 1 double-sided, plated-through PCB coded 08106243, 70.5 × 35.5mm 1 10nF 63V/100V MKT polyester capacitor Semiconductors 1 IPP80P03P4L-07 or SUP60061EL-GE3 P-channel logic-level Mosfet, TO-220 (Q1) 1 IPP80N06S4L or FDP027N08B-F102 N-channel logic level Mosfet, TO-220 (Q2) 1 BC546 65V 100mA NPN transistor, TO-92 (Q3) 1 C106B 200V or C106D 400V 4A SCR, TO-126/TO-225AA (SCR1) 1 15V 500mW or 1W zener diode (ZD1) 1 13V 500mW or 1W zener diode (ZD2) 1 BZX79Cxx 500mW (2mA or 5mA reference current) zener diode (ZD3) [See Table 1 for voltage rating] 1 3mm green LED (LED1) 1 3mm red LED (LED2) Resistors (all ½W metal film, 1%) 1 9.1kΩ 1 470Ω 1 150Ω 2 10kΩ R1-R7: see Table 1 🔴 TL431QDBZR, TL431FDT or TL431SDT have the standard pinout; TL431MFDT or TL431MSDT have the mirrored pinout 53