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Brownout Protector By JOHN CLARKE Protects AC motors against low AC mains voltage What is a “brownout”? This rather graphically describes what happens to your lights when the AC mains voltage drops dramatically – they get very dim. But apart from dim lights, brownouts are a fatal hazard to induction motors, as used in air conditioners, pumps, dishwashers and a lot of other appliances. Y EARS AGO, BROWNOUTS were quite rare and generally confined to rural districts where the power lines had very long runs. A falling tree or an electrocuted possum might cause the mains voltage to drop to a low level and lights would go dim. This has always been a hazard for the induction motors used in pumps and refrigerators. Nowadays though, because the electricity grid is running much closer to total capacity, brownouts can be experienced much more commonly in the cities and suburbs. Our own offices in the Sydney suburb of Brookvale have had brownouts on a number of occasions in the last year or so. On each Main Features • Adjustable threshold voltage • Switches up to 2300W • Power indication • Brownout indication • Rugged sealed enclosure 60  Silicon Chip occasion, we have made sure that the air conditioner, fridges, compressors and other machinery in the building were turned off until full AC mains supply was restored. Had we not done so, all the motors in that equipment were liable to burnout. So how many motors in your home are at risk right now if a brownout was to occur? The list can be quite long: fridge, freezer, washing machine, dishwasher, air conditioner, pool pump, spa pump and perhaps one or two garage door openers; typical of many homes. All this equipment could attempt to turn on during a brownout and the motor(s) would probably burn out. Specifications Standby power consumption: <5W with relay on Maximum Control Power: 2300W Brownout threshold voltage: typically set to 200V Switch on delay: 5 seconds Maybe your insurance policy covers motor burnouts but you would need to read the fine print. The insurance company might also look askance at your claim if there was more than one motor burnout or if the appliances were more than a few years old. Why do motors burn out? When induction motors are starting up they draw very heavy current for a second or two and when they are up to speed, the current drops back to reasonable levels. However, if the AC mains voltage is low, the induction motor may not develop enough torque to come up to full speed. In all of the appliances listed above, the motor starts with a heavy load so it is at particular risk if those starting currents do not reduce quickly. Those motors with a starting winding (switched out by a centrifugal switch) are at particular risk because those windings are only intended for very intermittent use. By the way, some motors do have thermal cut-outs but these cannot be regarded as a panacea – they are more correctly regarded as fire preventive siliconchip.com.au rather than protecting the motor from any damage. So there you have the reasoning behind our Brownout Protector. If you have a couple brownouts every year, you need protection for your appliances. You cannot rely on the possibility that you will be at home or awake when a brownout occurs and that you will be able to turn off all of the at-risk appliances before they are damaged. And unless the appliances are all in a single location (unlikely!), you need one Brownout Protector for each appliance you wish to protect. Above: the Brownout Protector is housed in a rugged ABS plastic case with a clear lid. It can be used with induction motors rated up to 2300W and you will probably need one for each appliance you wish to protect. Features The SILICON CHIP Brownout Protector provides constant protection for any single-phase induction motor, disconnecting power when the AC mains voltage drops below a preset level and then reconnecting it when the voltage returns to normal. The cost of this protection is far less than the likely cost of repair and replacement of a typical small induction motor. It may be used with induction motors rated up to 2.3kW (10A). siliconchip.com.au Power is applied to the unit via a switched IEC connector attached to one end of the case. Note that this connector and its internal mounting plate must be secured using Nylon screws to ensure safety. December 2008  61 GPO F1 10A A SLOW BLOW S1 12.6V K 0V T1 12.6V/7VA E K D5 A K A K K A A 10 F 16V A A  LED1  LED2 K K 100k 10 F 16V 560 VR1 50k 2 TP1 8 3 1 IC1a IC1: LM358 10k ZD1 3.9V 100k A IC1b 6 E 100nF 7 2.2k C B E Q1 BC337 470 SC  2008 B K A E D1–D6: 1N4004 A BROWNOUT PROTECTOR A K A 7812 C GND K D7: 1N4148 ZD1 BC337 LEDS A SET VR1 SO DC VOLTS AT TP1 = (Vmains/100) E.G., 230V/100 = 2.3V SET VR2 SO DC VOLTS AT TP2 = (Brownout Volts/100) E.G., 200V/100 = 2.0V Q2 BC337 K 4 VR2 50k B D7 5 TP2 K C 10k +12V C2 100 F 16V 2.2k POWER TP GND A 100 F 16V D6 2.2k C1 470 F 25V E K GND A N A 30A AC CONTACTS +12V OUT IN D1–D4 6.3V 240V N RLY1 REG1 7812 K IN GND OUT Fig.1: the circuit monitors the rectified DC voltage at the output of bridge rectifier D1-D4. This voltage is fed via VR1 to voltage follower IC1a which in turn drives comparator stage IC1b. IC1b then drives transistor Q1 to control RLY1. Note that since the year 2000, the electricity suppliers are obliged to follow Australian Standard AS60038 where mains voltage should be 230VAC with a tolerance of +10% and -6%. That means that the voltage could drop to 216V at the lower tolerance limit. Our circuit sets the switching threshold to 200VAC to avoid nuisance tripping during normal supply conditions. A heavy-duty relay does the switching. While ever the mains voltage is normal, the relay contacts are closed and power is available to the load (motor). If the mains voltage drops below 200VAC for more than five seconds, the relay contacts open to protect the motor. The relay contacts are rated for inrush currents of up to 65A – ideal for switching power to a motor which is pulling heavy starting currents. Circuit details The full circuit is shown in Fig.1. It comprises just a few low-cost components. These include a dual op amp (IC1), a couple of transistors, a 12V 62  Silicon Chip regulator and the heavy-duty relay. Power for the circuit is derived from the mains via a 12.6VAC stepdown transformer, T1. This drives a full-wave rectifier using diodes D1-D4 and a further diode, D5, before filtering with a 470μF capacitor (C1). The resultant nominal 17V DC is applied to the 12V 7812 3-terminal regulator (REG1). REG1 provides the 12V supply for IC1 and the 12V relay. Brownout detection To detect a brownout condition, the circuit needs to monitor the AC voltage from the transformer secondary winding. In practice, we don’t do this directly but instead monitor the rectified DC waveform at the anode of diode D5. This is filtered using a 100kΩ resistor and by a 100μF capacitor (C2) which is shunted by 50kΩ trimpot VR1. The resulting DC voltage across C2 is about 3.6V. Note that this voltage does not necessarily track the 16V or so that appears across capacitor C1. This is because C1 charges to the peak of the rectified 12.6V waveform whereas the 100kΩ resistor, trimpot VR1 and 100μF capacitor (C2) form an averaging filter to give a lower voltage (Vp x 0.636 x 150kΩ/50kΩ ~3.6V). OK, so why go to all this trouble rather than just monitoring the DC voltage across capacitor C1? After all, if the mains voltage varies, the voltage across C1 will vary in proportion, will it not? The reason for using the averaging filter method is twofold. First, the actual AC waveform of the mains supply is usually “flat-topped” due to the loading effects of gas discharge lighting (eg, fluorescents) and the capacitor-input power supplies used in all computers and most electronic equipment. Using the peak of the waveform to represent the actual mains voltage is not sufficiently accurate because the degree of “flat-topping” varies during the day, depending on whether it is peak or off-peak period. Second, when the relay switches on and off, it causes a considerable variation in the voltage across C1. For example, across C1 we measured siliconchip.com.au 15.8V with the relay energised (on) and 17.45VDC with the relay off, a variation of more than +10%. By contrast, the variation in the “averaged” voltage across C2 was 3.6V with the relay on and 3.75V with the relay off, a variation of just over 4%. This is important because in the worst case, the brownout detector needs to respond to an actual variation in mains voltage from 216V (the normal minimum mains voltage) to 200V (the switching threshold). This is a variation of only 7.5% and we don’t want the circuit being confused by variations in the supply waveform. Trimpot VR1 is included so that the sample voltage fed to op amp IC1a is exactly 1/100th of the mains AC voltage value. To give an example, if the mains voltage is 230VAC, trimpot VR1 is adjusted so the DC voltage at the output of IC1a, at TP1, is exactly 2.3V. This is part of the calibration procedure and just why we do this will become clear in a little while. The voltage at TP1 is fed to the noninverting input (pin 5) of op amp IC1b which is connected as a comparator. A nominal 3.9V reference is provided by zener diode (ZD1) which is fed via a 560Ω resistor from the +12V supply. Trimpot VR2 sets the switching threshold for IC1b and its wiper is connected to IC1b’s pin 6 inverting input. Pin 6 is set to about 2.00V (representing a brownout threshold detection point of 200VAC). So with a normal mains voltage, pin 5 will be at 2.3V (representing a 230VAC mains voltage). This voltage is higher than the 2V at pin 6 and so the output of IC1b will be high (close to 12V). This switches on transistor Q1 which powers the relay (RLY1). The relay’s contacts supply power to the appliance connected to the GPO. When IC1b’s output is high, diode D7 will be reverse biased and so the 100kΩ resistor at pin 5 does not affect circuit operation. However, should the mains voltage drop to just below 200VAC, the voltage at pin 5 will go below the 2V threshold set at pin 6 and so pin 7 of IC1b will go low. This will switch off transistor Q1 and the relay, to disconnect power from the load. Diode D6 quenches the back-EMF from the relay when its magnetic field collapses, protecting Q1 from damage. Simultaneously, transistor Q2 switches on to light the brownout siliconchip.com.au Parts List 1 PC board, code 10112081, 152 x 108mm 1 IP65 ABS enclosure with clear lid, 171 x 121 x 55mm (Jaycar HB-6248 or equivalent) 1 2853 12.6V 7VA mains transformer (T1) 1 12V coil relay with 30A 220VAC contacts (Jaycar SY-4040 or equivalent) 1 IEC snap-fit chassis fused male connector with switch 1 10A M205 slow-blow fuse (F1) 1 10A IEC mains cord 1 10A mains panel socket with side wire entry 1 20°C per watt heatsink (19 x 19 x 10mm) 2 2-way PC-mount screw terminal blocks, 5.08mm spacing 1 72 x 27mm sheet of 1mm aluminium or steel 5 6.4mm insulated spade connectors for 1mm2 wire 1 6.4mm piggyback spade connector for 1mm2 wire 4 4.8mm spade connectors for 1mm2 wire 1 5.3mm ID eyelet terminal for 1mm2 wire 2 M4 x 10mm screws 4 M3 x 6mm screws 3 M3 x 10mm screws 4 M3 x 15mm Nylon countersunk screws 2 M4 nuts 7 M3 nuts 3 3mm ID star washers indicator, LED2, via a series 2.2kΩ current-limiting resistor. Hysteresis When IC1b’s output is low, D7 conducts and pulls pin 5 even lower than 2V due to the voltage divider action of the 100kΩ and 10kΩ resistors. For example, if the voltage at TP1 is at slightly less than 2V, the output of IC1b will very close to 0V. The anode of D1 will be about 0.5V and so the divider action caused by the 10kΩ resistor connecting to 2V and the 100kΩ resistor connecting to 0.5V will give a voltage at pin 5 of (2.00 - 0.5V) x 100/110 + 0.5V, or 1.86V. This is a drop in voltage of 140mV. So instead of pin 5 now being at 2V, 2 4mm ID star washers 2 4mm ID flat washers 9 100mm cable ties 1 100mm length of 10A blue mains wire 1 150mm length of 10A brown mains wire 1 150mm length of 10A green/ yellow mains wire 1 100mm length of medium duty hookup wire 1 100mm length of 10mm heatshrink tubing 3 PC stakes 2 50kΩ horizontal trimpots (code 503) (VR1,VR2) Semiconductors 1 LM358 dual op amp (IC1) 1 7812 12V regulator (REG1) 2 BC337 NPN transistors (Q1,Q2) 1 3mm green LED (LED1) 1 3mm RED LED (LED2) 1 3.9V 1W zener diode (ZD1) 6 1N4004 1A diodes (D1-D6) 1 1N4148 switching diode (D7) Capacitors 1 470μF 25V PC electrolytic 2 100μF 16V PC electrolytic 2 10μF 16V PC electrolytic 1 100nF MKT polyester (code 104 or 100n) Resistors (1/4W, 1%) 2 100kΩ 1 560Ω 1 10kΩ 1 470Ω 3 2.2kΩ the action of the 100kΩ resistor, diode D7 and the 10kΩ resistor reduces the voltage by about 140mV, ie, to 1.86V. Before IC1b’s output can go high again, the mains voltage would have to rise by the extra amount to make up this 140mV difference. This requires an increase in mains voltage of 14VAC. In practice though, because the average voltage at TP1 is higher when the relay is off compared to when it is on, the extra voltage required from the mains for the relay to switch back on again is about 10V. This voltage difference effect is called “hysteresis” and is included to prevent the relay from rapidly switching on and off at the brownout threshold. December 2008  63 4004 4004 2.2k 100k 470 F H CTI WS TU O N W OR B TP GND TP1 100 F 100nF 1 560 ZD1 TP2 2.2k 1 2 2 RLY1 2 IEC MAINS CONNECTOR WITH SWITCH AND FUSE (REAR VIEW) MAINS WIRING CONNECTORS: 1: 6.4mm INSULATED SPADE CONNECTORS 2: 4.8mm INSULATED SPADE CONNECTORS 3: 6.4mm PIGGYBACK SPADE CONNECTOR 100 F 10k LED2 BROWN OUT CON2 4004 2 D7 470 VR2 50k 3V9 3 4148 IC1 LM358 M3 x 10mm SCREW WITH LOCK WASHER & NUT 1 10 F 10k VR1 50k 1 K LED1 POWER 10 F 2.2k 2853 N A CON1 T1 100k 4004 4004 REG1 7812 PRIMARY A SEE DETAIL DIAGRAM SECONDARY GPO (REAR VIEW) E D1 D2 D3 D4 D5 4004 18021101 A K D6 1 Q1 Q2 M4 x 10mm SCREWS WITH FLAT & LOCK WASHERS, NUTS NOTE: ALL WIRING TO THE IEC CONNECTOR, THE GPO, AND THE OUTPUT CONTACTS ON THE RELAY (1) MUST BE RUN USING 240VAC CABLE Fig.2: follow this diagram to assemble the PC board and complete the wiring. Mains-rated cable is used for all wiring to the GPO, IEC connector and relay output contacts and this wiring must be secured using cable ties. Provided that the mains voltage remains below the brownout threshold, the relay will remain off. In fact, the relay remains off at any voltage below the threshold, including voltages down to 0VAC. A power-on delay is included so that the relay only switches on about five seconds after power is applied. This delay is due to the values of the 100kΩ and 100μF filter components that monitor the average voltage from the rectifier. These are sufficiently large so that it takes time for the 100μF capacitor to charge up to above the voltage provided at TP2. This delay is also important to allow for the inevitable momentary drop in mains voltage which is caused by the high surge currents every time an induction motor starts up. Normally, these high currents only last a second or two, depending on the appliance, and we want to be sure that they do 64  Silicon Chip not cause the Brownout Protector to erroneously switch off the power. CRIMP EYELET STAR WASHERS TRANSFORMER MOUNTING FOOT Construction The Brownout Protector is housed in a weatherproof ABS enclosure (171 x 121 x 55mm) with a transparent lid and neoprene lid-sealing gasket. The box is designed to meet the IP65 dust and moisture ingress standard, although this standard is compromised somewhat by the addition of the GPO and IEC socket. All of the parts, except the GPO and IEC connector, are assembled onto a PC board coded 10112081 and measuring 152 x 108mm. This board has corner cut-outs at one end to allow it to sit on the base of the box. The IEC mains input socket with on/ off switch and integral fuse is mounted in one end of the case and a 3-pin AC socket is mounted on the transparent lid. The two LEDs on the PC board can M3 NUT PC BOARD M3 x 10mm SCREW Fig.3: an M3 x 10mm screw & nut, two M3 star washers and a crimp eyelet are used to secure the earth wire to the transfomer frame. be clearly seen through the transparent lid so the overall assembly is very straightforward. The complete wiring diagram is shown in Fig.2. Begin construction by checking the PC board for any defects such as shorted or broken tracks. That done, check that the hole sizes are correct. The holes for the four corner mounting screws, for REG1 and for the transformer mounting points need to be 3mm in diameter, while the holes for siliconchip.com.au the relay mounting screws should be 4mm in diameter. Check also that the main PC board is cut and shaped to size so that it fits into the box. Insert the resistors first, taking care to place each in its correct position. Use the resistor colour code table when selecting each value. You can also use your digital multimeter to check each resistor before installing it. Next, install PC stakes for test points TP1, TP2 & TP GND. That done, install the 1N4004 diodes (D1-D6), the 1N4148 diode (D7) and zener diode ZD1, taking care with their orientation. IC1 can be mounted next (watch its orientation), followed by the capacitors. Note that the electrolytic types must be oriented as shown. The 3-terminal regulator (REG1) is mounted on the PC board with a small finned heatsink. It leads need to be bent to fit into the holes provided and then it is secured on the heatsink with an M3 x 10mm screw and nut and its leads soldered. Next, install trimpots VR1 & VR2, transistors Q1 & Q2, LEDs 1 & 2 and the two 2-way screw terminals CON1 & CON2. The transistors and LEDs sit a few millimetres above the PC board. The relay is secured using M4 screws and nuts while the transformer is attached using M3 screws and nuts. The transformer must be earthed and this is achieved using a short green/ yellow earth wire with crimped eyelet. This is attached to one of the transformer mounting feet with two star washers, above and below the eyelet – see Fig.3. Note that the enamel must be scraped from the transformer foot to ensure good contact. The IEC fused male socket and switch is a snap-in type intended for use with a mounting plate thickness of about 1mm. Unfortunately, the specified IP65 box has a wall thickness of 3mm so the socket cannot be mounted directly to it. Instead, the IEC socket is first mounted on a 1mm thick metal plate and this plate is then secured to This is the view inside the completed unit. Take care to ensure that the GPO is wired correctly and that the mains earth leads are properly terminated. the inside of the box using four Nylon screws and metal nuts. As a result, the flange of the IEC socket is mounted flush with the surface of the box, giving a neat finish. Diagrams for the metal plate, the box cut-out and the socket cutout in the box lid are shown in Figs.4-6. Note that the end of the box for the IEC cut-out is best located at the same end as the Table 1: Resistor Colour Codes o o o o o o siliconchip.com.au No.   2   1   3   1   1 Value 100kΩ 10kΩ 2.2kΩ 560Ω 470Ω 4-Band Code (1%) brown black yellow brown brown black orange brown red red red brown green blue brown brown yellow violet brown brown 5-Band Code (1%) brown black black orange brown brown black black red brown red red black brown brown green blue black black brown yellow violet black black brown December 2008  65 The PC board is secured to the bottom of the case using self-tapping screws that go into integral standoffs. The IEC socket is attached by first clipping it to an aluminium mounting plate (see Fig.5), then fitting it inside the case and securing the plate using four Nylon screws and metal nuts (see photos). two sets of mounting bushes located on the base of the box (see photo). Note also that the cutout in the box should be just enough to provide clearance for the flange of the IEC socket. Once the IEC connector has been secured in place, you can install the PC board. To do this, you will need to first slide the edge of the PC board under the IEC connector. The PC board is secured using for M3 x 6mm screws into the integral threaded mounting bushes on the base of the box. Wiring All wiring must use 250VAC 10A rated wire except for the relay coil wires to CON2. Brown wires are used for the Active and the blue for the Neutral. The green/yellow-striped wire is for the Earth wiring and must not be used for any other wiring. Note that the mains wires termi66  Silicon Chip nated at the IEC socket and on the relay will need to use insulated crimp connectors. You must use a ratchetdriven crimp connector to fit these. Do not use a cheap automotive-style crimp tool, as this will not give reliable connections. Note that the crimp connections to the relay will need to be bent over slightly so that the lid can fit without fouling. All wiring must also be secured with cable ties to prevent a loose wire moving and making contact with the low-voltage components on the PC board. We did this with nine cable ties, as can be seen in the photographs. The two Neutral wires are also tied to the 3-pin socket using the holes on its moulding. Initial checks Before doing anything else, use your multimeter (set to a low ohms range) to check between the earth pin of the IEC connector and the earth outlet of the GPO. You should get a reading of zero ohms here (this checks the integrity of the earth connection). Similarly, you should get a reading of zero ohms between the earth pin of the IEC connector and the transformer frame. Having verified the earth connection, fit the 10A fuse to the fuseholder in the IEC socket. Note that this fuse should be a slow-blow type. Testing When you are testing and making adjustments, the Brownout Protector will need to be operated with the lid open. You must take care not to touch any of the connections in the 250VAC section when it is plugged into a wall socket, even though they are insulated by the crimp connectors (it is wise to be careful). This includes the wiring to the GPO, IEC connector, transformer primary and relay contacts. First, set your multimeter to read up to 250VAC and insert its insulated siliconchip.com.au Points To Check 4.5mm DIAM. 26 33.5 14 10.9 16.75 40 (BOX LID) (1) Be sure to use the specified ABS plastic case & note that Nylon screws must be used to secure the IEC connector plate. (2) Use mains-rated cable for all connections to the IEC socket, the GPO and the relay contacts. Secure this wiring using cable ties – see photos. (3) Use insulated spade connectors to terminate the leads to the IEC connector and to the relay contacts. A ratchetdriven crimping tool is necessary to fit the spade connectors. (4) Do not touch any part of the 250VAC wiring while this device is plugged into the mains. Also, DO NOT attempt to build this device unless you know what you are doing and are familiar with high-voltage wiring. 5 10 A A (END OF BOX) A 72 47 13.5 siliconchip.com.au 10 A A Further testing can be done if you have access to a Variac. This can be used to reduce the mains voltage to check that the brownout detection operates at the required voltage. If you do not have access to a Variac, then you can adjust VR1 so that the TP1 voltage drops just below the TP2 voltage. When it does, check that the relay switches off and that the brownout LED lights. Return VR1 to its correct position after this test and secure the lid with the four screws. That completes the setting up. The Brownout Protector can now be used 6 probes into the Active and Neutral terminals of a mains outlet. Measure the mains voltage, then remove the probes from the mains outlet and switch on the Brownout Protector. Wait for the relay to switch on, then measure the DC voltage between TP1 and TP GND (or the mounting screw of the 3-terminal regulator). Next, adjust trimpot VR1 for a reading of the mains voltage divided by 100. As previously suggested, if the mains voltage is 230VAC, set test point TP1 to 2.3V. Finally, adjust trimpot VR2 to set test point TP2 to 2.00V. 18 Fig.5: the cutout and drilling diagram for the IEC connector at the end of the box. IEC CONNECTOR MOUNTING PLATE: MATERIAL 1mm SHEET ALUMINIUM OR STEEL A 50 5.5 5 A 27 30 CUTOUT FOR IEC CONNECTOR 6 18 HOLES A: 3.0mm DIAMETER CORNER RADIUS 2.5 A 18 CL 25 5.5 4.0 38 Fig.4: the cutout and drilling diagram for the GPO socket in the case lid. The large cutout can be made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece and carefully filing the job to a smooth finish. Fig.6: follow this diagram to make the mounting plate for the IEC connector. as is or you can mount it on a wall adjacent to the appliance. The case can be secured to the wall using four screws which are accessed via internal channels adjacent to the lid mounting SC screws. December 2008  67