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Discrete Ideal This deceptively simple circuit uses just a handful of transistors, diodes and resistors. But it still provides a very useful function: active rectification of the output of a centre-tapped transformer or combining two DC supplies with low losses. It is much more efficient than a bridge rectifier or diodes at higher currents, producing less heat without costing much more. Project by Phil Prosser & Ian Ashford T he Ideal Diode Bridge Rectifiers project, published in December 2023 (siliconchip.au/Article/16043), included six different PCB designs to suit different situations. It was popular, with many built, but two aspects of that design bothered me (and others). Firstly, it used a pretty expensive custom IC, with the SMD version being a bit tricky to solder. Secondly, despite that expense, it could only handle rectifying the output of a single transformer secondary. So you couldn’t use it at all with a centre-tapped secondary, and two complete boards were required to derive split rails from a transformer with separate secondaries, doubling the cost. Wouldn’t it be nice to have a direct drop-in replacement for a bridge rectifier that could handle single, dual or tapped secondaries? And it’d be great to use standard parts, so we don’t need to source that expensive IC. Reader Ian Ashford sent us a circuit design he uses for dual-rail rectification but didn’t have a PCB design. When the Editor asked me if I wanted to turn it into a full-on project, there was only one possible answer to that! Ian and I performed further testing, development and tweaking, finally arriving at this very flexible, robust and useful circuit. So, this project is a collaboration that follows the ideal rectifier theme but with a different focus from the previous design. When to use this design As well as rectifying a transformer’s output(s), this design is also suitable for combining DC supplies with low losses, eg, combining the output of a solar panel and a battery, or a solar panel and wind generator. While it costs a little more than a bridge rectifier to build, it is significantly more efficient at higher currents and has a much lower voltage loss. So it’s ideal for high-power devices like power supplies and audio amplifiers. Its only real drawbacks are a limited voltage handling capability (up to ±40V or +80V) and the fact that it’s larger than a 35A bridge rectifier, so you’ll need room to fit the PCB. This project uses high-current, low RDS(on) Mosfets. To keep the circuit simple, we have used P-channel Mosfets on the positive rail and N-channel Mosfets on the negative rail. If your current demands are only modest, you could use the ubiquitous IRF9540 (P-channel) and IRF540 (N-channel) power Mosfets, which are available from Altronics and Jaycar. They can handle up to about 5A. Much more significant currents can be handled using the devices in the parts list, which are not all that expensive but are unlikely to be available from your local shop (but kits are available). All the other parts in the design are bog-standard, and you will surely have them in your parts drawer or at your local shop. Design process Between Ian’s initial email with the circuit he uses in DC and low-power Figs.1(a) & (b): the two main ways to use the Discrete Ideal Bridge Rectifier. At the top, a centre-tapped transformer secondary winding is used to generate split (positive and negative) rails. Two separate secondaries can also be used if they are connected in series. The connections at right show how to use the same board to combine the outputs of two DC supplies (the solar panel and battery are just examples). OUT+ will be fed by whichever has a higher voltage. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au Bridge Rectifier » Generates split rails (positive and negative DC supplies) from a single centre-tapped transformer secondary (or two secondaries wired in series) » It can also be used to combine two DC supplies (whichever has a higher voltage feeds the load) » Maximum output voltage: ±40V or +80V (transformer applications), +40V (combining DC supplies) » Maximum current: 10A RMS without heatsinking, more with heatsinking » Typical voltage drop: <100mV input-to-output » Typical dissipation: 1.7W <at> 5A RMS, 6.8W <at> 10A RMS AC applications and the final design presented here, we exchanged many ideas, questions and refinements. Some requirements we decided on are: ; A low part count was important. ; The design had to ‘just work’ without tweaks. ; Reverse current when Mosfets switch on and off had to be minimal in all applications. Many ideas were shared, and challenges were presented in every direction. In the process, the conceptual circuit grew to something larger and more complicated than was strictly necessary. It was at this stage that we tabled those design goals. Ian was keen to keep the size of the board down, so we designed a throughhole version and an alternative that uses some SMDs to fit in tighter spaces. We realised that this would never be the size of a conventional bridge rectifier, so we just aimed to produce reasonably-sized boards that would likely fit into an existing chassis but that aren’t too fiddly to build either. The final design is vastly ‘tighter’ than the test board. I often build a prototype board that is purely functional and worry about improving the layout later, once I’ve proven it works. In discussing what changes were warranted to Ian’s concept, achieving a design that ‘just worked’ became important. That led to the introduction of constant current sources as loads in the design. It makes the operation largely independent of supply rail voltage and allows constructors to use the Ideal Bridge with 9-25V AC transformers without any changes. We also changed the sense circuit siliconchip.com.au to only switch on the Mosfet when the input is at a programmed voltage above the rectified output. This blocks reverse currents and allows it to be safely used for combining DC supplies, which might be very close in voltage at times. The resulting circuit is simple and works well. We’ll get to a couple of subtleties later in the description, once we’ve gone over its operating principle. Two versions There are two PCBs for this project: an SMD version and a throughhole version. They use the same circuit. The SMD version is smaller than the through-hole version, which may be helpful in some circumstances. It doesn’t use any tiny parts (the resistors are M3216/1206 and there are SOT23 transistors), so it isn’t hard to assemble. Both versions use the same TO-220 (through-hole) Mosfets. That is because it makes it easy to add flag heatsinks if necessary for your application. High-current SMD Mosfets are available, but they are trickier to heatsink if necessary and will take up more room than a TO-220 in this application. Design limitations This circuit is suitable for rectifying the output of dual or split secondary transformers where the junction of the windings from the ground point for output capacitors, as shown in Fig.1(a). This design will work if you have a transformer with a single secondary Australia's electronics magazine winding, but the switching could be noisy. ICs like the LT4320 used in the December 2023 designs switch the bottom Mosfets on for a full half-cycle to ensure clean switching. So, for that sort of application, we recommend you build one of the designs we published then (kits are available at siliconchip. au/Shop/?article=16043). Regarding how much current the board can handle, P-channel Mosfets typically have a higher RDS(on) figure than N-channel Mosfets. This means that the positive-rail Mosfets will be the limiting factor in how much current can be drawn due to their voltage drop and consequent power dissipation. We have avoided the complexity of a gate drive boost circuit there. Using one would have allowed us to use four identical N-channel Mosfets, but we didn’t think that was worth the extra parts and possibly new failure modes. Up to about 10A, the Mosfets will not require heatsinking, although it wouldn’t hurt to add small flag heatsinks above 5A. Above 10A, you must add a substantial flag heatsink on each Mosfet. Decent flag heatsinks should let it handle at least 15A. Beyond that, you might need a more serious cooling solution, like forced airflow over heatsinks. Circuit details The circuit is shown in Fig.2. Unlike the previous Ideal Bridge Rectifier, this circuit can have its inputs connected across a single secondary or a pair of series-connected secondaries to generate split supply rails. In those cases, the secondary winding’s September 2024  79 centre tap does not connect to this circuit. Instead, it connects to the output capacitor bank ground and the load’s ground, as shown in Fig.1(b). So that it can produce split rails, it contains two similar sections stacked on top of each other. They would be identical except that they have opposite polarities to handle current flowing in opposite directions. The upper section uses two P-channel Mosfets, four PNP bipolar junction transistors (BJTs) and two NPN BJTs. The lower section has two N-channel Mosfets, four NPN BJTs and two PNP BJTs. Each of the four sections senses the input AC voltage at one terminal. When it is about 34mV greater in magnitude than the output voltage (higher than the positive rail or lower than the negative rail), the corresponding Mosfet is switched on by driving its gate with an appropriate voltage. We only want the Mosfet on when the input exceeds the output by a small margin to ensure that the Mosfet is off when these voltages are equal and that there is no chance the Mosfet is on as the input voltage magnitude drops below the output. If that were to occur, current would reverse and flow from the capacitor bank through the transformer, creating current spikes and a great deal of electrical noise, plus possibly overheating the Mosfets. Fig.2: the Ideal Bridge Rectifier circuit comprises two identical sections at the top to deliver current to the DC OUT+ terminal, with two more sections below to handle current flow through the DC OUT− terminal. The lower sections are ‘mirror images’ of the upper sections, with components of opposite polarity (NPN transistors instead of PNP etc). The circuit is the same for the TH and SMD versions; the alternative devices are direct equivalents except for their packages. 80 Silicon Chip Australia's electronics magazine siliconchip.com.au As the four separate sections all work the same way, let’s concentrate on the one shown in the upper-left corner of Fig.2. The voltage sensing circuit comprises diodes D1 and D2 plus PNP transistors Q5 and Q6. Q5 acts as a diode, since its base and collector are joined. Ignoring the 68W resistor for now, with a constant current flowing through these transistors, both will conduct if the AC input voltage at CON3 is the same as the DC output voltage at CON1. If Q6 is on, Mosfet Q1’s gate voltage is high, and it is off. As the input voltage increases, Q6 switches off, so the gate of Mosfet Q1 is pulled low by its 22kW collector resistor – see Scope 1. The 68W resistor is important as it alters how the comparator works. The total current through the two 22kW collector resistors is determined by a constant current sink comprising NPN transistors Q7 and Q8. On the positive cycle for the AC1 input, about 0.5mA is drawn through each of these resistors (as well as the matching pair for Q9 & Q10). This 0.5mA flows through the transistor and diode pairs Q5/D1 and Q6/ D2, which drop the voltage by about 1.2V, but on the AC input path, it also flows through the 68W resistor, dropping 34mV or so in the process. This extra voltage drop means we draw more current from the base of Q6 than Q5 until the AC input is 34mV above the output voltage. Mosfet Q1 remains switched off until that condition is met. Once the input exceeds the output by 34mV, Q6 starts switching off and the Mosfet switches on. This charges the output capacitors until they get to 34mV below the input. Essentially, the circuit contains a negative feedback loop, where Q5 and Q6 try to maintain a 34mV difference across the Mosfet by controlling its gate voltage. Without the 68W resistor, they would try to maintain 0V across the Mosfet, and due to various tolerances in the circuit, the Mosfet might be held on all the time, which is not what we want! As a result, at lower load currents, we are not simply switching the Mosfet hard on and off; instead, it is operating in linear mode with a low voltage drop across it due to the negative feedback. Part of that voltage drop is a result of the RDS(on) of the Mosfet, while part is from the gate voltage being moderated, siliconchip.com.au Scope 1: an oscilloscope grab of the Ideal Bridge in operation, showing rectification of the voltage at the AC1 terminal. The pink trace is the output voltage at 5A, cyan is the AC input voltage, and yellow is the gate drive for Mosfet Q1, which peaks at about -8V. Scope 2: the Discrete Ideal Bridge starting into two 35,000μF capacitor banks. This is a pretty brutal thing to do to any bridge. Usually, you would use a soft-start circuit to keep the initial current surge under control. Still, the Bridge survived it! which we can see in the oscilloscope screen grabs. As the load current increases, we see the sense circuit driving the Mosfet harder, ie, its Vgs increasing until it is 12V, at which point the gate protection zener diode (ZD1 in this case) conducts to prevent the Mosfet gate from being driven beyond its ratings. If you look at the scope images (especially Scope 4), you will see that when drawing high currents, the circuit transitions from the linear feedback operation to driving the Mosfet fully on with 12V. This occurs because the voltage drop across the Mosfet exceeds 34mV due to its minimum RDS(on). As a result of the way we are driving the Mosfet, there is little value in utilising ultra-low RDS(on) Mosfets in a dual-rail bridge. 10mW or so is fine. We felt this was the sweet spot at which the voltage drop across the Mosfets is defined by the feedback loop up to about 5-6A. Because of how Mosfets are made, P-channel Mosfets tend to have a higher RDS(on). The constant current sink based around Q7 & Q8 is a standard two-­ transistor current source/sink configuration. We could have tied this to the output ground and reduced the dissipation in transistor Q7, but we chose to tie it to the negative output rail for the positive rail comparators and positive rail for the negative comparators. This is because it gives maximum gate drive to the Mosfets for low-­ voltage operation, especially during startup when massive currents are often drawn for charging capacitor Silicon Chip kcaBBack Issues $10.00 + post January 1997 to October 2021 $11.50 + post November 2021 to September 2023 $12.50 + post October 2023 onwards All back issues after February 2015 are in stock, while most from January 1997 to December 2014 are available. For a full list of all available issues, visit: siliconchip.com. au/Shop/2 PDF versions are available for all issues at siliconchip.com.au/Shop/12 We also sell photocopies of individual articles for those who don’t have a computer Australia's electronics magazine September 2024  81 Scope 3: a close-up of the rectified output. Again, pink is the output, cyan is the AC input, and yellow is the gate drive. This neatly shows the Mosfet switching as the AC input voltage slightly exceeds the DC output voltage. Scope 4: the negative rail behaviour, which is a ‘mirror image’ of the positive rail. Driving the rectified 12V AC into a 1W load is clearly giving the transformer a workout, as seen by the flattened top and bottom of the cyan waveform. Under these conditions, it would be advisable to mount a flag heatsink to each Mosfet as they individually dissipate about 1.5W. banks. This reduces dissipation in the Mosfets during these high-stress phases of operation. It also has the benefit of the PCB not needing a GND connection. If you look at Scope 2, which shows the startup behaviour, the Mosfet has over 10V of gate drive in the first cycle of operation. One benefit of using a constant current source/sink is that the circuit’s behaviour is mostly independent of the operating voltage, as long as it’s above the minimum threshold required to bias on the Mosfets. The 22kW resistors in the circuit allow one current source/sink to drive the sense amplifiers for both input rails. The actual value of these resistors is not that important, although we don’t want a large voltage drop across them so that we can use the Ideal Bridge at modest AC input voltages. For the PCB layout we need to consider the thermal characteristics of D1, D2, Q5 and Q6 (the sense amplifier). Silicon diodes have a -2.1mV/°C thermal coefficient for their forward voltage drop, so for every 1°C increase in temperature of a diode junction, its forward voltage falls by 2.1mV. This means that if one diode is hotter than the other, we will get an error in the switching voltage. A similar effect is seen with the base-emitter voltages of Q5 and Q6. For this reason, we have placed the diodes right next Application Max current Low-Current Full Bridge 2-3A no heatsink Max voltage N-channel P-channel Source/comments 40V IRF540 IRF9540 Altronics & Jaycar IRFB4410ZPBF SUP70101EL-GE3 IRF135B203 IXTP76P10T ±40V High-Current Full Bridge 10A no heatsink DC Combining 5A no heatsink DC Combining 10A no heatsink to one another, and placed the transistors so they can be glued together. This will ensure our switching margins are stable even as the board heats and cools during use. The ‘sense’ transistors (Q5 & Q6, Q9 & Q10 etc) only ever have 12V across their collector-emitter junctions, so we have specified standard BC546-9 or 556-9 devices (or their SMD equivalents, BC8xx). However, the current source/sink transistors will have the full dual rail voltage across them, which could be up to ±40V or 80V total. Therefore, we have specified MPSA42/92 transistors for these (or the SMD equivalents, MMBTAx2). These standard high-voltage, lowpower devices are available from all the larger online suppliers. If you have ±25V or lower voltage rails, you could use BC546/556/846/856 transistors there instead. It is important to consider that the BC546/BC556 have the opposite pinout to the MPSA42/92 transitors, so you would need to install them backwards if you do this. Luckily, for the SMD transistors, the BC846/856 series SMD pinouts are the same as the MMBTA42/92 pinouts, so they are a direct swap for applications below ±25V. Note that the 47kW resistor values were chosen to allow operation from low voltages to about ±40V at the output. At the upper limit, the 47kW resistors will dissipate 130mW each. While that is well within the ratings of a 1/4W resistor, we have specified 1/2W resistors just to be safe. If you will only use this bridge at the higher end of its voltage range, you could increase those resistor values slightly to, say, 68kW. That will reduce their dissipation to a maximum ±30V As above 12-24V Not required 12-24V Not required Table 1 – examples of suitable MosfetsAustralia's electronics magazine SUP90P06 Mouser, DigiKey & Silicon Chip kit IXTP96P085T IRF9540 Altronics & Jaycar 100mV/A drop SUP90P06-09L-E3 Mouser & DigiKey 7.4mV/A drop SUP70101EL-GE3 Mouser & DigiKey 11.4mV/A drop IRF4905 Mouser & DigiKey siliconchip.com.au of 94mW, so 1/4W resistors should be fine. You could also lower their values for low-voltage applications, although that shouldn’t be necessary. Startup behaviour Scope 2 shows the circuit starting up when AC power is first applied. On that first cycle, the AC input blue trace goes negative. This charges the negative capacitor to about 5V, although we don’t have a plot of the negative rail here – we know that the negative and positive rails will be about the same. The Mosfet body diode conducts on this cycle in the absence of voltage at the Mosfet gate (due to the low initial voltage). Once there are a few volts on the output rails, the constant current source/sink and BJT-based voltage sense circuits kick in. By the time we are into the first positive excursion of the AC1 input in cyan, we can see the gate drive pulling the gate low (in yellow), having already charged the large capacitor bank enough in the first cycle. Indeed, the gate voltage on that P-channel Mosfet goes below 0V, being pulled toward the negative rail, and we see a full 12V on that P-channel Mosfet gate in the first real cycle of operation. This shows the benefit of connecting the current source/sink to the opposite rail rather than ground. I love the simplicity of circuits like this, which squeeze more out of a handful of components than seems reasonable. I also like going back to basics and using BJTs in the current sink and sense amplifier. PCB layout We touched on some PCB layout considerations earlier. There are a few aspects of the PCB design that are very important: Parts List – Discrete Ideal Bridge Rectifier 4 6.3mm pitch PCB-mount vertical spade connectors (CON1-CON4) [Altronics H2094, Jaycar PT4914] 2 SUP70101EL 100V 120A P-channel Mosfets, TO-220 (Q1, Q2) 2 IRFB4410ZPBF 100V 97A N-channel Mosfets, TO-220 (Q3, Q4) Resistors (1% ¼W axial – TH version | 1% ¼W M3216/1206 – SMD version) 4 100kW 2 47kW 0.5/0.6W (5% OK) 8 22kW 2 330W 4 68W Through-hole version 1 double-sided PCB coded 18108241, 87.5 × 45.5mm 4 BC556/7/8/9 100mA PNP transistors, TO-92 (Q5-Q6, Q9-Q10) 2 MPSA42 300V 500mA NPN transistors, TO-92 (Q7, Q8) 2 MPSA92 300V 500mA PNP transistors, TO-92 (Q15, Q16) 4 BC546/7/8/9 100mA NPN transistors, TO-92 (Q17-Q20) 4 12V 0.4W zener diodes, DO-35 (ZD1-ZD4) [Altronics Z0332] 12 1N4148 75V 200mA diodes, DO-35 (D1-D12) SMD version 1 double-sided PCB coded 18108242, 54.5 × 54.5mm 4 BC856/7/8/9 100mA PNP transistors, SOT-23 (Q5-Q6, Q9-Q10) 2 MMBTA42 300V 500mA NPN transistors, SOT-23 (Q7, Q8) 2 MMBTA92 300V 500mA PNP transistors, SOT-23 (Q15, Q16) 4 BC846/7/8/9 100mA NPN transistors, SOT-23 (Q17-Q20) 4 12V ¼W zener diodes, SOT-23 (ZD1-ZD4) [BZX84C12] 12 1N4148WS 75V 150mA diodes, SOD-323 (D1-D12) [Altronics Y0162] For combining DC supplies, halve the numbers of all components except the PCB and spade connectors. – TH version kit (SC6987, $30) – SMD version kit (SC6988, $27.50) ● The layout of the current sense amplifier with its two transistors, two 1N4148 diodes and 68W resistor is kept very tight as it must accurately sense small voltages with relatively low bias currents. ● The sense transistor pairs, like Q5 and Q6, are face-to-face, so you can super glue these together to keep them as tightly thermally coupled as possible (or add a smear of thermal paste between them). On the SMD version, these parts are tight against one another. The SMD version of the Discrete Ideal Bridge Rectifier is 54.5 × 54.5mm, while the through-hole only is a bit larger at 45.5 × 87.5mm (not shown to scale). siliconchip.com.au Australia's electronics magazine Both kits include the PCB and everything that mounts on it ● The pairs of 1N4148 diodes (D1 & D2) are right next to one another, so they stay at similar temperatures. ● The path from the AC inputs through the Mosfets and to the DC outputs is kept as short as possible and uses large copper fills to maximise the current carrying capacity of the PCB. PCBs do not have a fixed ‘current rating’, but we must ensure that the voltage drop and heating in the tracks is reasonable at any current likely to be drawn. At the AC1 input, which has the thinnest connection to the Mosfet, we have parallel copper on the top and bottom layers of the PCB. September 2024  83 83 Fig.3(a) & (b): the full-populated through-hole version of the PCB (left) and the reduced version for combining DC supplies only (right). The full version can also be used to combine DC supplies. Watch the diode and Mosfet orientations, and remember that Q7/Q8 and Q15/Q16 need to be reversed if you are using BC546/BC556 transistors instead for lower voltage applications, compared to what’s shown here. Mosfet selection We have included 100V low-RDS(on) Mosfets in the parts list. They only cost a few dollars each and work well. If selecting alternative Mosfets, look for a voltage rating well above the rail voltage you want; we feel that 80-100V is about right. Select an RDS(on) of 10mW or less. The P-channel Mosfet will usually have a higher RDS(on); there is little point in selecting N-channel Mosfets with a significantly lower on-­ resistance than the P-channel devices you will be using. For lower currents, you can get away with less expensive Mosfets. Even though the savings in dissipation won’t be as great, the reduction in voltage loss can still make this design very beneficial in lower-current designs. For example, we used IRF540/ IRF9540 Mosfets from Altronics in some tests, and it was fine up to about 3A, still giving a much lower voltage drop than a conventional bridge. Table 1 includes some advice on Mosfet selection. Construction The through-hole version is built on a double-sided PCB coded 18108241 that measures 54.5 × 87.5mm, while the SMD version is coded 18108242 and is a bit smaller at 54.5 × 54.5mm. For the former, refer to the Fig.3(a) PCB overlay diagram, while Fig.4(a) is the overlay for the SMD version. The smallest SMD parts are the SOT23 transistors and SOD-323 diodes. These are large enough that they are not too challenging if you have a desk magnifier and a reasonably good soldering iron. If you are using it to combine solar panels or DC power sources, you can leave off all the negative rail parts, shown in a dashed box in Fig.1. These Figs.4(a) & (b): the SMD versions of the PCB, with the full version on the left and the DC combining version only on the right. If substituting BC846/BC856 transistors for the MMBTA types, you don’t need to change how they are fitted to the board. Only diodes D1-D12 and the Mosfets could be easily installed backwards, so ensure they aren’t. 84 Silicon Chip Australia's electronics magazine versions are shown in the alternative overlay diagrams, Figs.3(b) & 4(b). Start by fitting all the resistors. Follow with the diodes, making sure you orientate them correctly, with the cathode stripes facing as in the relevant PCB overlay diagram. We found that for the SOD-323 SMD diodes we got, it was tough to tell which end was the cathode. If unsure, use a magnifier or a DMM set on diode test mode. Next, solder the signal transistors in place. As mentioned earlier, if you are using this at low voltages only, you can use all BC546/556/846/856 transistors throughout. If you do this, remember that the through-hole devices for Q7, Q8, Q15 & Q16 must be rotated by 180°, as the MPSA42/92 types have a different pinout. Mount the 12V zener diodes next. The SMD SOT-23 parts are small and in the same packages as the bipolar transistors, so make sure you don’t mix them up. Place them with tweezers and tack one leg, allowing you to adjust it (if necessary) by reheating the initial joint before soldering the remaining leads. Fit the power Mosfets next. Watch the layout here, as they face in alternate directions on the board to optimise the track layout. Also, don’t get the two different types mixed up. Tack one leg of each and fiddle them so they are neatly aligned and the same height, then solder the remaining leads. Finally, mount the 6mm connectors. You could solder wires directly to the board, but we reckon using crimp spade lugs is much neater. Testing We suggest testing the board in two siliconchip.com.au PIC Programming Adaptor Our kit includes everything required to build the Programming Adaptor, including the Raspberry Pi Pico. The parts for the optional USB power supply are not included. Use the Adaptor with an in-circuit programmer such as the Microchip PICkit or Snap to directly program DIP microcontrollers. Supports most newer 8-bit PICs and most 16-bit & 32-bit PICs with 8-40 pins. Tested PICs include: 16F15213/4, 16F15323, 16F18146, 16F18857, 16F18877, 16(L)F1455, 16F1459, 16F1709, dsPIC33FJ256GP802, PIC24FJ256GA702, PIC32MX170F256B and PIC32MX270F256B Learn how to build it from the article in the September 2023 issue of Silicon Chip (siliconchip.au/Article/15943). And see our article in the October 2023 issue about different TFQP adaptors that can be used with the Programmer (siliconchip.au/Article/15977). Complete kit available from $55 + postage siliconchip.com.au/Shop/20/6774 – Catalog SC6774 halves. The following steps test the two positive sections. 1. Connect the Bridge outputs to an electrolytic capacitor of at least 470μF. Make sure you get the polarity correct. 2. Connect the negative of a 12-24V power supply to the negative of your capacitor and the positive to either of the AC inputs. If you can set a current limit, set it to a few hundred milliamps. 3. Switch on the supply and check that the capacitor charges up to the input voltage. 4. Put a 100W 1W resistor (or similar) across the capacitor and check that the voltage across it does not droop significantly (no more than 100mV). This verifies that the appropriate Mosfets are on; otherwise, the voltage would drop by 600mV or more. It also confirms there are no catastrophic shorts, or you would get smoke. Now test the other AC input using the same method. If you run into trouble in either case, go through the following checklist below: 1. Is your power supply going into siliconchip.com.au current limiting? Use a multimeter to check for the expected voltage at the AC input. 2. Are your Mosfets the right way around? 3. Check that the diodes are all orientated correctly. If any are wrong, the Rectifier will not work. 4. Check your soldering and look for solder bridges. 5. Check that the current sink and source work by measuring the voltage between the base and emitter pins of Q8 and Q16. The reading should be close to 0.6V in both cases. Also check for a ~600mV Vbe on Q7 and Q15. If the readings are low, check that the associated 47kW resistors are OK. 6. Check the voltage across the zener diodes. Are they the right way around? If the capacitor bank is charged up and there is no load resistor, the voltage across them should be low, while you should get a reading of several volts with the 100W resistor across the capacitor. 7. If the behaviour is correct for one AC input of the Bridge but not the other, check the circuitry around the misbehaving input and compare voltages to the other half. Australia's electronics magazine 8. If both inputs don’t work, you have a systematic problem since they are essentially independent. Having tested it with one polarity, switch off the supply and connect its positive output to DC OUT+ on the Bridge and the negative of your power supply to one of the input terminals. You should see the capacitor charge up to the input voltage again. Proceed with testing in this configuration as above. Using it Once installed, it will pretty well look after itself. Refer to Figs.1(a) & (b) to see how the connections should be made. If you expect to draw continuous high currents from the power supply, you will probably want to put some flag heatsinks on the Mosfets. Aside from that, you should find that it just works. Remember that you may need a mains soft-starting system if you have a really substantial capacitor bank and low-impedance transformer like in a big audio amplifier. We published such a design in April 2012 (“SoftStarter”; siliconchip.au/ Article/705). SC September 2024  85