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Constructional Project Project by Stefan Keller -Tuberg This device will disconnect a load from its power supply if the voltage is reversed or too high. It also disconnects the load if it draws current above the adjustable trip level. Its dual-rail support means it can work with devices like audio amplifiers with a split (positive and negative) supply. I Dual-Rail Load Protector n May 2025, we published three DC Supply Protectors that guard against reversed or excessive supply voltages, but they could only handle a singlerail supply. This design provides even more functions, extends the reverse/overvoltage protection to split rails and adds adjustable current limits with automatic or manual resetting. If you’ve built something that uses flying power leads, you may already have had a close call mixing up polarities. Or have you ever forgotten to check that you’re using the right supply to power a device? If any of these ring true, this design might help avoid a catastrophe by introducing power supply protection. It’s so versatile that you’ll think of many applications for it. The overvoltage cut-out levels can be set between ±5V and ±19V (or 5-38V for the single-rail version). If the supply overshoots the protection level, this device will rapidly interrupt it. The overcurrent thresholds are set by a current sense resistor and trimpot. The sense resistance is chosen so the voltage drop is approximately 50mV at the nominal protection level. The trimpot range permits adjustment from zero up to twice the nominal current level. When the current limit is reached, it interrupts the offending power rail 60 by turning it off completely, similar to a fuse blowing. This minimises the chance of damage due to a fault compared to simply holding the current at the threshold by reducing the voltage, as a current-limited bench supply would do. Also, if the device is unattended when it fails, interrupting rather than limiting the power delivered could help avoid an even larger disaster. It can be set so that when the overcurrent circuit trips, it will automatically reconnect after a two-second delay or require manual intervention (a button press). If your dual-rail application is asymmetric, you can set different overvoltage and overcurrent thresholds for each rail. Depending on the Mosfets used, it can handle up to 4-7A per rail without heatsinking. Adding heatsinks to the Mosfets will allow them to handle more, up to 10A for the higher-current Mosfets specified. Due to the design’s modularity, you only need to populate the required features. To start with, you can equip it to suit single or dual supply rails. Configuring it for a single rail saves a few components and doubles the single supply maximum voltage to 36V. If your device to be protected has more than one positive or more than Fig.1: some of the different ways the Supply Protector can be used. If the maximum voltage of ±18V for the dualrail version is not enough for your application, you can stack two boards to double that, as shown on the far right. Practical Electronics | September | 2025 Dual-Rail Load Protector Features & Specifications ● Voltage range: 4-36V DC or ±4-18V DC (±4-36V DC with two boards) ● Over-voltage cut-out: 5-38V DC or ±5-19V DC (±5-38V DC with two boards) ● Voltage withstand: up to ±60V at either input or across both inputs ● Current capability: 7A+ without heatsinking (more with heatsinks) ● Voltage insertion loss: typically <300mV <at> 10A ● Over-current protection: disconnects rails independently if current draw exceeds a set threshold ● Over-current reset: automatically after two seconds or manually via pushbutton 🔹 🔹 🔹 🔹 exact values depend on parts used one negative rail, you can common the grounds and use two or more of these boards to protect them all, including dual-rail applications operating up to ±36V, as shown in Fig.1. You can leave some components off if you don’t need overcurrent protection. You can also leave off the overvoltage sections if you don’t want that feature. The circuit is arranged in three sections, each supporting one or two power rails. Reverse polarity protection The first section of the circuit, shown in Fig.2, uses Mosfets Q1 & Q2 like ‘ideal diodes’. They have a very low voltage Practical Electronics | September | 2025 drop when forward-biased but a high impedance when reverse-­biased. If you accidentally connect the input voltages with the wrong polarity, the internal body diodes of Mosfets Q1 and Q2 will be reverse-biased, and no current can flow. Q1 and Q2 remain off, protecting all the downstream components from the abnormal condition. The specified Mosfets have reverse voltage ratings up to 60V, offering plenty of protection against accidental power supply reversal. However, without protection, the Mosfets could be damaged by gate-tosource voltages exceeding 20V. Zener diodes ZD1 and ZD2 ensure that the voltage between the gate and source of each Mosfet cannot exceed 15V. Other Mosfets in the design have similar protections. When the input voltage polarities are correct, the internal diodes of Q1 & Q2 are forward-biased. As current starts to flow, 47kW resistors pull the Mosfet gates to ground, so they switch on. As the gate bias exceeds 2-4V and the Mosfet channel resistance drops, the internal protection diodes will be shunted, so very little voltage will be lost across the Mosfets. The Supply Protector’s minimum voltage rating of 4V is because that is the minimum voltage at which the Mosfets used are guaranteed to switch on and conduct sufficient current. Over-voltage protection The following section deals with over-voltages. Zener diodes ZD3 and ZD6 set a fixed value for each rail’s protection threshold. The knee voltage for 1W zener diodes rated above 5.6V occurs around 3.5-5% below the nominal zener voltage. As the supply voltage reaches this level, they will start to break down. Lower voltage zener diodes have a more rounded knee, so the difference from nominal can be larger. When enough current flows to develop 0.6V at the gate of the associated SCR, it will trigger and switch off either Mosfet Q4, in series with the positive rail, or Mosfet Q5 in the negative rail, disconnecting and protecting the downstream circuitry and the load. The SCR will remain latched until the supply voltage is removed. Providing the applied voltage remains below the Mosfet specification (55V or 60V), the unit will tolerate the condition indefinitely, and the device you’re protecting will stay safe. Most applications won’t require fine overvoltage threshold adjustment, so you can simply set it by selecting the nearest zener. Two extra diodes labelled D4 and D5, in series with the zeners, allow the threshold to be tweaked. Usually, they are replaced with wire links, but if required, regular or schottky diodes can be fitted to increase the overvoltage trip thresholds by 0.3V (SB140/1N5819) or 0.6V (1N4004). Op amp IC1 has an absolute maximum limit of 40V, the highest overvoltage threshold supported. In practice, the trip points should be no more than ±19V for one dual-rail device or 38V for a single-rail version, giving a small safety margin. The 220μF and 3300μF electrolytic capacitors are to counteract the effects of power source inductance. At switchon, many devices cause a momentary current surge as the supply bypass 61 Constructional Project Fig.2: the Supply Protector circuit has mostly independent positive and negative sections with three stages each. The first is reverse polarity protection (using Mosfets Q1 & Q2), followed by overvoltage protection (Mosfets Q4 & Q5), then overcurrent protection (Mosfets Q10 & Q11). The only sections shared between the positive and negative rails are the half-supply generator (IC1d), reset oscillator (IC1c) and reset switch. capacitors charge. This high current pulse can interact with inductances in the wiring etc, causing ‘ringing’ (oscillation), which causes an overshoot voltage to appear on the affected power rail, sometimes a significant one. One of my test supplies caused damped oscillations with a frequency of around 2MHz, resulting in a peak overshoot voltage of around 50% above the nominal supply. This persistently tripped the overvoltage protection at power-on. The electrolytic capacitors dampen power-on overshoot to avoid false overvoltage trips. In severe cases, you may need to increase the value of the 220μF parts, although they should be sufficient for most cases. It is usually more severe with a longer input power cable. Overcurrent protection The third section of the circuit pro62 vides overcurrent protection. The load current is monitored by the voltage drop across the ‘+sense’ and ‘-sense’ resistors. For the positive rail, LED15, VR1 and one ‘+bias’ resistor set an adjustable reference voltage at pin 3 of IC1a that is a couple of volts below the +ve rail voltage. LED17 and the other ‘+bias’ resistor create another voltage at IC1’s pin 2 that varies with the ‘+sense’ voltage drop. IC1a compares these voltages; its output is low when the sensed current is below the setpoint, so Mosfet Q10 is usually on. If the current setpoint is exceeded, IC1a’s output goes high, switching off Q10 and disconnecting the load, while also lighting overcurrent indicator LED21. Op amp IC1b, Mosfet Q11 and LED22 function similarly for the negative rail. The purposes of LED15 and LED17 aren’t to emit light; they provide consistent voltage drops so the op amp inputs remain within the chip’s common-­mode range, which does not go up to the positive rail. The fact that LEDs have a higher voltage drop than a regular silicon diode (around 1.8V rather than 0.7V) is useful in this application. The voltage across the ‘+sense’ resistors is approximately 50mV at the nominal overcurrent trip point. VR1 is for fine-tuning; its 100W value means that with 1mA flowing through it, a full trimpot rotation will cover twice the nominal voltage range expected across ‘+sense’. Note that the ‘+set’ LED (LED15) usually goes out when the overcurrent LED (LED21) lights. However, there are cases where the current is near the overcurrent set point where both could light. So if you notice that, it’s normal. The overcurrent protection only interrupts the rail experiencing the overload. When that happens for the Practical Electronics | September | 2025 Dual-Rail Load Protector positive rail, D19 provides a feedback path to latch the state even after Q10 interrupts the current and the ‘+sense’ voltage drop falls back to zero. It will remain off until the condition is reset by NPN transistor Q8 switching on and pulling pin 3 of IC1a below the pin 2 level. When the output of IC1a goes high, another NPN transistor (Q9) inverts the transition to create a falling edge. This is combined with any falling edge from IC1b by diode D24. These are AC-coupled to IC1c by a 1nF capacitor and diode D29, which works as an overcurrent reset monostable. A two-second delay is provided by the 1μF capacitor and 2.2MW resistor. The 100kΩ resistor at pin 9 of IC1c prevents damaging input currents when pins 9 and 10 differ by more than 5.5V. When the monostable times out (if enabled) or the reset pushbutton is pressed, Q6, Q7 and Q8 temporarily Practical Electronics | September | 2025 shift the voltage levels at the inputs of IC1a and IC1b. This forces them out of their latched states, re-enabling Mosfets Q10 and Q11. If the auto-reset feature is enabled, voltage to the load will be restored two seconds after it trips. If the overcurrent condition persists, the trigger-­ delay-restore process will repeat indefinitely every two seconds (or until the fault clears). This monostable arrangement requires a reference voltage at the midpoint of the IC’s power supply. We could have used the GND line for this reference, but that would mean the circuit would only work with symmetrical dual rails, reducing its flexibility. So IC1d synthesises a mid-rail voltage (halfway between +ve and -ve) that self-adjusts without needing different configurations. The 100nF capacitor and 1MW resistor connected to header CON3 ensure that resets are only momentary, even if the pushbutton is held down. This quickly rearms the overcurrent protection while preventing the output from being held on continuously if excessive current continues to flow. The 100nF capacitor at IC1c’s output works similarly for monostable-­initiated resets. We don’t care about the brightness of LED15-LED18 since, as mentioned, they are not indicators. However, it does matter for LED7, LED8, LED21 and LED22. The circuit shows 22kW current limiting resistors for them, suiting high-brightness LEDs. If using regular LEDs, reduce the values to around 5.6kW for more current. Alternatively, if they’re too bright (which may happen with higher-­voltage single-rail applications), increase the series resistances. Diodes D26 and D27 across the output terminals are normally reverse-­ 63 Constructional Project biased. These protect the circuit from inductive loads or long output power leads. Any inductance in these can cause a reverse voltage spike when the load current is interrupted (as can certain capacitor configurations in the load). These diodes will safely dissipate that energy. Component selection The PCB is designed for miniature 1/8W resistors, 3.5mm long. You can get them from element14, DigiKey or Mouser. You can use more common 1/4W resistors, but you will need to stand them up (at least partially). The current sense resistors will typically be below 0.1W. element14, DigiKey and Mouser have wide ranges of low-value ‘current sense’ resistors, many of which will be suitable, even if their power ratings are higher than necessary. Alternatively, parallel two or more resistors to create a lower resistance. The PCB holes are large enough for the leads of current-sense resistors or multiple regular resistors. The parts list gives information on supported capacitor lead pitches (although you can bend them if you have to) and suggested Mosfet types. However, many more suitable ones will be available. If substituting other Mosfets, pay particular attention to their maximum on-resistance, Vgs threshold voltage and reverse breakdown voltage. Particularly for P-channel Mosfets, cost-effective options with a low on-­resistance aren’t common. The higher the maximum on-resistance, the hotter the Mosfets will run. Heatsinking Using, say, IRF1018E and IRF4905 Mosfets, at 4A current draw and 10V or higher, they will dissipate 135mW and 320mW each, respectively. The temperature of a TO-220 package in free air rises by around 70°C/W, so without heatsinking, they will rise to 9°C and 22°C above ambient. Note that the ambient temperature is the air temperature within the enclosure, which could be significantly higher than room temperature. The Mosfet on-resistances could be a little higher when using rail voltages significantly below 10V, so for lower operating voltages, pay close attention to the Mosfet temperatures during testing. If they become too warm to touch comfortably, they require heatsinks. If you know or suspect you’ll need heatsinks in advance, it will be easiest to fabricate and mount the transistors onto them before soldering the transistors to the PCB. You can fashion heatsinks from 3mm aluminium using three separate bars or angles for the three rows of Mosfets. Cut the material to fit comfortably within the component footprint. For a dual rail application, mount two Mosfets per angle with their centre holes spaced 18mm apart. Use insulators and Nylon bushes and/or screws; insulating each Mosfet from its heatsink is the best practice. You won’t necessarily require large heatsinks; it depends on how much power needs to be dissipated. For maximum heat dissipation, bridge the tops of the three aluminium angles with a commercial heatsink. Construction Figs.3 & 4: the dual-rail version of the Supply Protector uses all the parts on the PCB, although some sections can be omitted. Parts that can be left off if you don’t need over-current protection are shown, in Figs.7 & 8. Soldering heavyduty 1mm2 wires to the underside of the board, as shown here, will reduce the resistance of the current-carrying tracks. That will lower the voltage drop between the input and output and allow the PCB to handle more than 5A. 64 Start by selecting the values of the current sense and bias resistors, and over-voltage threshold zeners, using either the panel or Tables 1 & 2 overleaf. The Supply Protector is built on a double-sided 96 × 69mm PCB coded 18109241. If you’re building the full dual-rail version, fit all the components shown in Fig.3, while if you want to make the single-rail version, fit just the components shown in Fig.5 or Fig.6. Figs.7 & 8 show further variations, which experienced constructors Practical Electronics | September | 2025 Dual-Rail Load Protector could combine with one of the single-­ rail variants if desired. The two adjustment diodes, D4 and D5, are generally not required and can be replaced with wire links. If you need to fine-tune the trip voltage, you can fit diodes instead, as explained earlier. We suggest constructing in two stages: build and test the reverse-­ polarity and overvoltage protection sections before adding the remaining components. Roughly speaking, the reverse and overvoltage protection components are below and to the left of the 3300μF electrolytic capacitor in the middle of the board, not including the two 3300μF capacitors (use Fig.7 as a guide). Pay close attention to the orientation of the transistors, diodes, LEDs and electrolytic capacitors. The two SCRs should face in opposite directions. Start by fitting the lowerprofile components like the diodes and resistors that will lie flat on the board (see Table 3), then the capacitors, then the taller components like the Mosfets. If you want to minimise the voltage drop across the device, or will be using it at high currents, you can solder extra wires to the underside as shown in Fig.4. That should not be necessary for applications up to around 5A per rail, though. Initial testing The easiest way to verify the correct operation of the reverse polarity protection is with a variable power supply. Apply power to the input connector in reverse but starting at 0V. Ramp the voltage slowly up to -1V and monitor the “+rail” and “–rail” test points with a multimeter to verify the absence of any voltage. It is working if the supply reaches -1V and there is no voltage on those test points. You could use one AA or AAA cell if you don’t have a variable power supply. Note that if you previously applied power in the correct direction, your multimeter may read the residual charge on the 220μF capacitors. Next, verify the overvoltage protection threshold by switching off the variable power supply and reconnecting the supply with the correct polarity. As you ramp the variable power supply up, by the time it reaches 1V, a voltage will be detectable on both test points. Practical Electronics | September | 2025 The fully assembled Dual Rail Supply Protector PCB with all features available. Note the use of smaller-than-usual resistors to keep it compact. Figs.5 & 6: these overlay diagrams shows the board fitted with just the components needed for a single rail Supply Protector. You need to add wire links where shown in red. When building any of these versions, watch the orientation of the IC, diodes and Mosfets, as they must all be correct. 65 Constructional Project Pluggable terminal blocks for the inputs and outputs make connecting the wires easy. The board can be mounted using a tapped spacer in each corner. Figs.7 & 8: these overlays shows which components you can leave off (or link out) if you don’t want either the overcurrent or over-voltage protection feature. If building a single-rail version, you will need to refer to Figs.5 & 6 as well, and figure out which components to leave off or link out. 66 The readings will initially be around 0.7V below the variable power supply level. Once you reach 3-4V, the test point voltages should rise to the input voltage. There are two additional test points labelled “+rail prot” and “–rail prot”, which you can now monitor. Continue increasing the variable supply towards the protection threshold. As you pass the threshold, each overvoltage protection LED should illuminate and then, at a fractionally higher input voltage, the “+rail prot” and “-rail prot” test points should start falling back to zero. The actual tripping thresholds may differ from the calculated value due to component tolerances and the zener knees. Remember not to ramp the input voltage past the ratings of the 220μF capacitors, and be very careful not to ramp your variable supply past 40V (or ±20V) until you are sure the overvoltage protection in both rails is working correctly, or you risk damaging IC1 (if fitted). If either rail’s protection hasn’t kicked in by 1V beyond the calculated trip point, it’s either not working, or the zeners are wrong. To reset after the overvoltage protection has tripped, return the variable supply to 0V or temporarily disconnect it. With the overvoltage protection section working, you can finish fitting components to the PCB, starting with IC1; ensure its pin 1 indicator goes at upper right as shown in the overlay diagram. Don’t mount the two trimpots yet. If your board needs cleaning because it’s covered in flux residue, submerge it under isopropyl alcohol or methylated spirits and gently rub it with an old toothbrush. Wait for it to dry, then mount and solder the trimpots. Check that Mosfets are electrically isolated from any heatsink metal. To verify the overcurrent trip circuit, connect the power rail (or rails) to a variable supply but don’t yet connect a load. Adjust the power supply to the same voltage you used to calculate the bias resistances. If either overcurrent trip LED is already illuminated, slowly wind the trimpots clockwise until they extinguish automatically, use the pushbutton reset or short the upper two pins of CON3. If either of the overcurrent trip LEDs fails to extinguish, there is a Practical Electronics | September | 2025 Dual-Rail Load Protector problem with the reset or overcurrent circuit. If you’re using auto-reset, check the output of IC1d at the “VG” (virtual ground) test point is near ground level for a symmetrical dual-rail version, or otherwise approximately midway between the power rails. Assuming both overcurrent LEDs are off and the set LEDs are on, wind each trimpot anti-clockwise until either the trimpot is fully anti-­clockwise or the corresponding set LED switches off and the overcurrent LED comes on. When you’ve reached this point, nudge the trimpot clockwise (and press the reset button if equipped) so both the overcurrent LEDs are again off. The overcurrent protection circuits will now be armed at a very small current threshold. Use a low-value resistor (say 100W or less) between ground and each output rail to verify that the overcurrent protection triggers. The corresponding overcurrent LED should illuminate instantaneously, and the associated set LED should extinguish. If your overcurrent trip point exceeds the normal operating current by an amp or more, connect a dummy load that will draw current just below the overcurrent tripping point and adjust VR1 and VR2 so the load remains turned on. Otherwise, use your intended load to set the overcurrent trip point. Finally, run the device for ten minutes while monitoring the temperature of the Mosfets. If the Mosfets become too warm to touch comfortably, turn off the power and fit heatsinks before using it. Wire it up, and you can sit back and relax, knowing your load device is protected! Parts List – DC Supply Protectors (all features) 1 double-sided PCB coded 18109241, 96 × 69mm 2 100W miniature top-adjust trimpots (VR1, VR2) 2 3-way 5.08mm pitch pluggable terminal blocks (CON1, CON2) [Farnell 3797901 + 2452499 or DigiKey 5EHDVC-03P + 5ESDV-03P] 1 3-way pin header and jumper shunt (CON3) 1 NO momentary pushbutton (optional) 4 M3 × 6mm panhead machine screws and matching spacers Semiconductors 1 NCS20074 quad rail-to-rail output op amp, SOIC-14 (IC1) 3 high-current P-channel Mosfets, TO-220 (Q1, Q4, Q10) ★ 3 high-current N-channel Mosfets, TO-220 (Q2, Q5, Q11) ★ 2 BC556 45V 100mA PNP transistors, TO-92 (Q3, Q7) 3 BC546 45V 100mA NPN transistors, TO-92 (Q6, Q8, Q9) 2 C106 SCRs, TO-126 (Q12, Q13) 8 high-brightness 3mm LEDs (LED7-8, LED15-18, LED21-22) [Vishay TLLK4401] 6 15V 1A zener diodes, DO-41 (ZD1, ZD2, ZD11, ZD12, ZD23, ZD25) 2 1A zener diodes, DO-41, values to suit application – see Table 1 (ZD3, ZD6) 2 1N5819 or SB140 40V 1A schottky diodes (D13 & D14) 6 1N4148 75V 200mA diodes, DO-35 (D19-D20, D24, D28-D30) 2 1N4004 400V 1A diodes, DO-41 (D26, D27) 2 extra diodes to fine-tune over-voltage thresholds (D4, D5; optional, see text) ★ suitable types include IRF4905 (up to ±55V & 4A), IPP80P03P4L-07 (±30V & 7A) and SUP90P06-09L-E3 (±60V & 7A) ★ suitable types include IRF1018E (7A), CSD18534KCS (7A), DIT050N06 (4A), STP60NF06 (5A) & IPP80N06S4L (8A; all can handle up to ±60V, ratings are without heatsinks and are only a guide) Capacitors 2 3300μF 50V electrolytic (5mm or 7.5mm lead pitch, diameter ≤ 18mm) 2 220μF 63V low-ESR electrolytic (3.5mm or 5mm lead pitch) 1 1μF 50V ceramic or MKT 9 100nF 50V ceramic or MKT 2 10nF 50V ceramic or MKT 1 1nF 50V ceramic or MKT Resistors (all ⅛W 5% miniature axial unless noted) ♦ 1 2.2MW 9 47kW 4 3.3kW 1W 5% 3 1MW 9 22kW 2 910W 1 330kW 1 15kW 4 Rbias resistors – see Table 1 2 150kW 6 10kW 2 Rsense resistors – see Table 2 3 100kW ♦ regular ¼W resistors can be used but they won’t sit flat on the PCB Table 1 – zener diode values Table 2 – current sense resistors Trip ZD3/ZD6 Bias resistors Adjustment range Sense resistor ~5V 5.1V 3.0kW 0-1A 100mW 1/8W ~5.5V 5.6V 3.3kW 0-2A 50mW 1/4W ~7.25V 6.8V 5.1kW 0-3A 33mW 1/4W ~10.3V 10V 8.2kW 0-4A 25mW 1/2W ~13V 13V 11kW 0-6A 16mW 1/2W ~15.1V 15V 13kW 0-8A 12mW 2/3W ~18V 18V 16kW 0-10A 10mW 1W ~20V 20V 18kW ~23.8V 24V 22kW ~29.5V 30V 27kW Tables 1, 2 and the panel on the next page are used to determine the best values of various components to suit your needs. ~38V 39V 36kW Practical Electronics | September | 2025 Table 3 – resistor colour codes 67 NEW! Constructional Project Calculating component values Several component values should be selected to suit your application as follows. Overvoltage trip point 5-year collections 2019-2023 All 60 issues from Jan 2019 to Dec 2023 for just £49.95 PDF files ready for immediate download 2018-2022 All 60 issues from Jan 2018 to Dec 2022 for just £49.95 PDF files ready for immediate download 2017-2021 All 60 issues from Jan 2017 to Dec 2021 for just £49.95 PDF files ready for immediate download 2016-2020 All 60 issues from Jan 2016 to Dec 2020 for just £44.95 PDF files ready for immediate download See page 69 for further details and other great back-issue offers. Purchase and download at: www.electronpublishing.com 68 First, you must determine the highest voltage that’s safe to apply to the load. If unsure, measure the output of the existing power supply and add a safety margin. Zener diodes ZD3 (‘+OVtrip’) and ZD6 (-OVtrip’) set the overvoltage trip point for each rail in combination with the 3.3kW resistors. The SCRs will trip when their trigger input reaches approximately 0.6V. Allowing for a voltage drop of about 100mV across the resistors, the required zener voltage is (Trip – 0.7V) × 1.05. As mentioned earlier, low-voltage zeners may trigger at lower voltages than expected. Also, typical zeners diodes have 5% tolerances. In the middle of the voltage range (eg, around ±15V), you can generally get away with a zener diode that has a voltage rating close to the desired trip point, as the 0.7V and 5% factors cancel out. Because the expected overvoltage trip point lies within a range, and zeners are only available in certain preferred values, you may need to use adjustment diodes if you require high precision. Adding a schottky diode for D4 or D5 (like a BAT85, SB140 or 1N5819) will increase that rail’s trip point by around 0.3V, while adding a silicon diode (like a 1N4148 or 1N4004) will increase it by around 0.6V. Don’t use zeners below 4.3V or above 19V (for a dual-rail configuration) or 39V (for single-rail operation). You can use different values for the two zeners for asymmetric applications. Ensure that the 3300μF output capacitors have voltage ratings above the trip points. For example, if you have ±18.1V overvoltage protection thresholds, select 25V capacitors. Because the 220μF capacitors after Mosfets Q1 & Q2 are on the unprotected side of the overvoltage protection circuit, they will experience any overvoltage, so their voltage ratings should exceed the highest expected input voltage. We recommend using 50V or 63V rated capacitors there, although you might get away with 35V caps in some cases. Overcurrent trip point The ‘+sense’ and ‘-sense’ resistors are used to monitor the current in each rail. The overcurrent trip is calculated for a sense resistor voltage drop of about 50mV, although the trimpots let you set it up to 100mV. Use Ohm’s law, R = V/I, and the power formula, P = VI, to calculate the required resistances and power ratings. Let’s use 2A as an example. For a 50mV drop, the formulas give R = 25mW (0.05V ÷ 2A). If you can’t find a resistor with the calculated value, round the resistance to the closest available value. A 0.022W, 0.025W or 0.033W resistor would be suitable in this case. We calculate the power at 100mV as we don’t want the resistor to overheat if the trimpot is set to maximum, so P = 200mW (0.1V × 2A). Ideally, the resistor should have close to twice the power rating (to account for elevated ambient temperatures etc), so in this case, use a ½W or 0.6W resistor. If your application has asymmetric current requirements, you can choose different values for the two resistors. Bias resistances Four resistors are labelled ‘+bias’ or ‘-bias’. The bias resistors are selected so that about 1mA flows through them when the supply is at its nominal (not overvoltage trip) level. The series LEDs have a forward voltage drop of around 1.8-2V, so consider that when calculating the resistor values. The exact drop doesn’t matter as long as the four LEDs (LED15-LED18) are the same type, so the voltage drops are similar. Red, orange or yellow LEDs with a forward voltage drop below 2.3V will work. You can measure the LED’s forward-biased drop using a digital multimeter’s diode testing function. Say the nominal power supply is ±12V and you have red LEDs with a 1.6V forward voltage. The required resistance will be R = (12V – 1.6V) ÷ 0.001A = 10.4kW. Choose the nearest available resistance, 10kW in this case. If you have an application with asymmetric voltage rails, the ‘+bias’ and PE ‘-bias’ resistances may differ. Practical Electronics | September | 2025