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Active Mains Soft Starter Part T wo by John Clarke Our Active Mains Soft Starter, introduced last month, is ideal for eliminating the switch-on kick from power tools rated up to 750W. You can also use it to avoid high inrush currents for stationary equipment that can trip circuit breakers or wear out switches. This article covers the assembly, testing, adjustment and calibration of this new Soft Starter. T he Active Mains Soft Starter uses a combination of an NTC thermistor and a MOSFET to provide an adjustable soft-starting period. Using a MOSFET means the thermistor experiences little heating, so repeated starts (within reason) do not degrade the effectiveness of the Soft Starter. Both the MOSFET and the thermistor are bypassed by a relay after soft starting so that there is very little power loss or heating within the Soft Starter, even with a high load current draw. It is housed in a conveniently compact 17.1 × 12.1 × 5.5cm plastic enclosure with an IEC mains input socket, GPO/mains output and three (optional) neon indicators to show what it is doing. Because it monitors the load current, it is automatically activated whenever the load appliance is switched on, even if the Soft Starter is already powered. That means you can use the trigger or switch on power tools to activate them. Or, you can simply switch it on at the wall, which is handy if you have multiple devices connected to the output (eg, via a power board). Having described what it does and how it works, let’s move on to building it. Construction Most of the parts mount on a double-­ sided, plated-through PCB coded 10110221 that measures 159 × 109mm and is avaiable form thr PE PCB Warning: Mains Voltage The entire circuit of the Active Soft Starter floats at mains potential and could be lethal should you make contact with it. Don’t assume that because we use isolation between different parts of the circuit that some parts are safe to touch – they are not! The isolation between parts of the circuit is to allow for the differing voltage potentials in parts of the circuit rather than for safety. 36 Service. Once assembled, it is housed in a polycarbonate or ABS enclosure measuring 171 × 121 × 55mm. The only off-board parts are the IEC mains input socket, mains output socket and three neon indicators. Fig.7 shows the parts layout on the PCB. Begin by installing the surface-­ mounting dual op amp (IC2). You will need a soldering iron with a fine tip (or a regular tip and some flux paste), a magnifier (if you do not have excellent vision) and good lighting. Solder the IC to its PCB pads by first placing it with the pin 1 locating dot to the top left and aligning the IC leads to the corresponding pads. Then solder a corner pin and check that it is still aligned correctly. If it needs to be realigned, re-melt the soldered connection and gently nudge the IC into alignment. When you’re sure it’s placed correctly, solder all the IC pins. Any solder that runs between and bridges two pins can be removed with solder wicking braid (adding extra flux paste is recommended). Note that pins 6 and Practical Electronics | March | 2024 Fig.7: assembly is straightforward, with most parts LIVE mounting on the PCB, as shown here. Q1 has no mounting hole and is adhered to the PCB using double-sided adhesive thermal tape. Because of supply constraints, we have designed the board to accept two different types of current transformer, with either two or three pins. Note that the three TVSs are bidirectional, so their orientations are not critical. 7 are joined on the PCB, so a bridge between them won’t matter. Fit the resistors next. Table 1 shows their colour-code values, but we recommend using a digital multimeter (DMM) to check each resistor before soldering it in place. Three resistor types are used; one is a 1kW 5W wirewound, six are 1W types, and the remainder are smaller 1/2W resistors. Mount the 5W resistor with a gap of about 1mm from the PCB to allow air to circulate. Diodes D1-D3 and zener diodes ZD1-ZD3 are next on the list. Ensure they are oriented correctly and the types are not mixed up before soldering their leads. TVS1-TVS3 can also be installed now. These are bi-directional (AC) devices, so they can be installed either way around on the PCB. Make sure the correct type number for each TVS is inserted in the specified location. Mount the remaining ICs, taking care to get the correct IC in each place and with the proper orientation. We used a socket for IC1, although you could solder it directly to the PCB, assuming it has already been programmed. IC3 and IC4 both have six pins, so don’t get them mixed up. On the PCB, pin 5 of IC4 has only a tiny pad to provide an increased creepage distance between pins 4 and 6. You can fit the capacitors next, of which there are four types: the mains X2-rated capacitors, electrolytic capacitors, MKT polyester and Practical Electronics | March | 2024 a multi-layer ceramic. Note that the electrolytic capacitors need to be oriented correctly since they are polarised, while the others can be installed either way around. Remember that the 100nF capacitors could be labelled as 104 (10pF × 104), while the 4.7nF capacitor could be labelled 472 (47pF × 102) and the 1μF ceramic capacitor could be labelled as 105 (10pF × 105). Next, install potentiometer VR1 and thermistor NTC1. Bridge rectifier BR1 is next; its positive lead is spaced wider than the remaining leads, so it will only fit in one way. MOSFET Q1 can also be fitted now. Bend its leads by 90° about 5mm from the package and secure the metal tab to the PCB using some double-sided thermal transfer tape before soldering the leads. Because the tracks are thin near the pads for the MOSFET leads, build up their exposed copper tracks on the underside of the PCB with solder. Install CON1 to CON4 next, as well as the current transformer (T1). Depending on which type of transformer you have, it might have two or three leads. The PCB will accommodate either type. The next step is to install relay RLY1 with its coil terminals toward CON4. The relay is secured using 15mm-long M3 screws and nuts, with each screw inserted from the underside of the PCB. Winding transformer T2 The windings on the toroidal ferrite core for T2 are made with 0.25mmdiameter enamelled copper wire, as shown in Fig.8. The primary has 10 turns, while the secondary has 48 turns. Cut a 125mm length for the primary and 1m for the secondary, and wind on each side-by-side; the winding directions are unimportant. The windings must be separated by at least 3mm at each end. Mount the finished transformer on the PCB with two cable ties that both secure the toroid and keep the primary and secondary windings separated, so make sure they go between the windings. The third cable tie holds down the toroid in the middle of the secondary winding – see Fig.7 above. Pass the primary and secondary wires through the PCB pads and strip off the Fig.8: wind T2 as shown here, keeping the windings neat and close together and ensuring at least 3mm of separation between the primary and secondary at either end. 37 Table 1: Resistor Colour Codes Fig.9: these are the required cut-outs in the side of the case and the lid. You can download this diagram as a PDF from the March 2024 page of the PE website and print it to use as a template. Be careful making the IEC cut-out and neon holes brcause if they are too large, the parts will fall out. Try to avoid the GPO* cut-out coming too close to the separate hole as, if the plastic in between is thin, it could break. *UK users will need a UK socket cut-out. insulation at all four ends to allow the wires to be soldered. The insulation can be burnt off with a hot soldering iron, by holding a blob of hot solder over the wire ends for a few seconds. Otherwise, you can scrape the insulation off with a sharp hobby or craft knife. Final assembly The Soft Starter PCB is secured to the base of the enclosure using 6mm-long M3 machine screws that screw into the integral brass inserts. But before attaching the PCB, the IEC connector cut-out will need to be made in the side of the enclosure. You will also need to drill holes in the lid for the GPO/mains socket and neon indicators. Fig.9 is a template for the required cut-outs. You can photocopy it from the magazine at 1:1 scale or download a PDF from the March 2024 page of 38 the PE website and print it out (make sure to print it at ‘actual size’) – see: https://bit.ly/pe-downloads The large cut-outs for the mains GPO/mains socket and IEC connector can be made by drilling a series of small holes around the inside perimeter, knocking out the centre piece and filing the outline to a smooth finish. If you use Jaycar neon indicators, the holes must be sized so that they stay clipped in place when inserted into the cut-out. So take care with the hole size; the inside of the hole will need a slight chamfer to reduce the panel thickness so that the clips can spring outward to secure each neon. The Altronics neon indicators are secured with a nut threaded onto the plastic housing instead of clips. Once the drilling and filing are complete, install the IEC connector. The PCB can then be placed inside the case, but don’t secure it just yet. First, the IEC connector must be secured using countersunk nylon M3 × 10mm screws, although you can use metal nuts. You may need to cut away some of the internal ribs in the case to allow the nuts to fit as we had to for the prototype (you can just see this in the photo overleaf). The nylon screws are essential as they avoid the possibility of the screws becoming live (at mains voltage) should a mains wire inside the enclosure come adrift and contact a screw holding the IEC connector. Before attaching the mains/GPO socket and neon indicators, you can print out the front panel label shown in Fig.10. You can also download it as a PDF from from the March 2024 page of the PE website. Practical Electronics | March | 2024 Fig.10: there are two different versions of the front panel artwork that you can download, either with the neon holes marked (as shown here) or without, if you’d prefer not to fit them. The download includes two versions of the front panel. One does not have the three neon indicator holes, and is included if you prefer not to use them. The wiring is also simplified when not using neon indicators. All wiring must be run as shown in Fig.11, using mains-rated cable. Be sure to use 10A cable for the thicker wires shown in Fig.11; brown wire must be used for the live wiring while the blue wire is used for neutral. Green/yellow-striped wire is used for the earth wiring only, and the earth lead from the IEC socket must go straight to the GPO/mains outlet. The thinner wires shown (without a red asterisk) can use lighter-duty 7.5A mains wire, or simply 10A wiring throughout if you prefer. Be sure to insulate all the connections with heatshrink tubing for safety and cable tie the wires as shown, to prevent any wire breakages coming adrift. Use 10mm-diameter heatshrink around the bodies of the neon indicators, 5mm for the wires to the IEC connector (red or brown for live, blue or black for neutral, and green for Earth) and 3mm for the wires to the relay (similar colour coding). Note how the relay contact connections are made using 4.8mm spade crimp lugs while the relay coil wires are soldered. Try to avoid melting the surrounding relay plastic housing while doing that, and be sure to insulate the joints afterwards with heatshrink tubing. The wires to the IEC socket are also soldered and then insulated. Practical Electronics | March | 2024 Secure the live and neutral leads to the GPO/mains outlet using cable ties that pass through the holes in its moulding. Also, use neutral-cure silicone (eg, roof and gutter silicone) to cover the live bus piece that connects the live pin to the fuse at the rear of the IEC connector as it is live, and there is no good reason for it to be exposed. Take great care when making the connections to the mains socket (GPO). In particular, be sure to run the leads to their correct terminals (the GPO/ mains socket has the L, N and E clearly labelled) and do the screws up tightly so that the leads are held securely. Similarly, ensure that the wires to the screw terminals are firmly secured. Testing Always attach the lid using at least two screws at diagonal locations before switching on the power. Before applying power, check your wiring carefully and ensure all mains connections are covered in heatshrink tubing, and the wiring is cable tied. Then install the 10A fuse inside the fuse holder and verify that IC1 is plugged into its socket and correctly oriented. The relay wires are cabletied to other mains wires after installation in the case. 39 Fig.11: be very careful to run all the wiring as shown here, including using the colours shown, adding all the required insulation and the cable ties as indicated. All wires can be run using 10A mains-rated cable, or you can use 7.5A-rated cable for the thinner wires shown (without the red asterisks) if desired. LIVE LIVE Should you forget to install IC1 before powering up, the 4.7nF capacitor at the pin 4 connection could be left with a remnant voltage when you switch off the power. This can destroy IC1 when it is plugged in. So if you power it up without IC1 plugged in, wait for a few minutes with power off and check that the voltage between pins 4 and 8 is less than 1V before plugging in the IC. Typically, VR1 would be set to midtravel for a nominal one-second softstart period. If set full anti-clockwise, VR1 gives a 9.5s soft-start period while near full-clockwise gives a half-second soft start period. 40 Calibration Rotating VR1 fully clockwise has the Soft Starter enter another mode. This is used to measure the voltage from the precision rectifier when no appliance is connected. This is the offset voltage that needs to be taken into consideration when detecting whether there is current flow or not when an appliance is detected. Typically, the output of IC2a (the full-wave rectified current waveform) will not sit at the negative supply at pin 4 with no load, but will be slightly positive. This offset can be measured and taken into account by IC1. This setting only needs to be done if the soft start circuit does not correctly detect when the appliance is off. To set this offset, with the power off and unplugged from the wall, rotate VR1 fully clockwise. No appliance should be plugged into the Soft Starter’s mains/GPO outlet. Attach the lid, power it up and wait a few seconds before switching it off. This will let it store the DC voltage produced by IC2 when no current is measured. Unplug it, remove the lid and rotate VR1 back from fully clockwise to the desired soft-start period. As mentioned earlier, somewhere mid-way will give a suitable soft-start duration of one or two seconds for most Practical Electronics | March | 2024 www.poscope.com/epe The completed unit just before the lid is attached. The numerous cable ties mean that even if a wire breaks off, it can’t make contact and damage other parts of the circuit or create a shock hazard. situations. However, other periods are available depending on your appliance’s requirements. Choosing the soft-start period The available periods are 9.5, 5.5, 2.0, 1.0, 0.625 or 0.5 seconds, adjusted using VR1. You can use the slower rates for soft-starting capacitive loads if you are not concerned about how long it will take to power up the load. The 9.5s startup period is probably too long for most cases, but a 5.5s period is a good option. The finished Active Mains Soft Starter is easy to use, just plug your desired appliance into the GPO on the front panel and then connect the Soft Starter to mains power via the IEC plug on its side. (UK constructors will use a UK mains outlet.) Practical Electronics | March | 2024 For power tools, the best period depends on the time the tool takes to get up to full speed and the acceptable amount of movement the tool makes during starting. A shorter duration will produce more tool movement than a longer duration, but will let you get to work faster. If the period is longer than necessary, you will need to wait longer for the tool to be ready to use. 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