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Power mains appliances from a 12V car battery with this ... 40-WATT INVERTER This compact 40W inverter can drive low power appliances such as shavers from a 12V battery. It is ideal for use when camping in areas where a 240V AC supply is unavailable, or as part of a small solar power installation. · By JOHN CLARKE An inverter which operates from a car battery can be a very useful item to have for powering mains equipment. While some equipment can be powered either from the mains or from a DC supply, there are some appliances which do not have this option and must be operated from 240V AC. This 40-Watt Inverter is suitable for use with appliances which draw 40W or less (eg, fax machines, electric toothbrushes, battery chargers for mobile telephones and incandescent lamps). It is not suitable for fluorescent lights, however, since the start- ing current and peak voltage required are too great for the inverter to supply. The inverter circuitry is housed in a plastic case measuring 155 x 158 x 54mm. This has a panel-mount mains socket attached to the front panel, along with a power switch. The power leads which go to the battery emerge from the rear of the case, just below the fuseholder. To operate the inverter, you simply connect the power leads to the 12V battery, plug in the mains appliance and switch on. The appliance should then operate in its usual manner. The 40-Watt Inverter is housed in a small plastic case & can be used to power all sorts of small appliances (eg, battery chargers & fax machines). The rating can be easily increased to 60W by substituting a larger transformer (see text). 42 SILICON CHIP As can be seen from Table 1, the inverter has quite good voltage regulation and is reasonably efficient at full power. The poor efficiency at lower powers is mainly due to the use of a low-cost transformer to step up the voltage to mains output. Lack of feedback voltage regulation is another contributing factor. This poor efficiency at low output powers is tolerable since the extra circuitry and cost is not warranted in a low-power design such as this. The circuit As the accompanying photographs show, relatively little circuitry is used in the inverter. Apart from the power transformer, it uses two inexpensive !Cs, two Mosfet transistors and a few other sundry bits and pieces. Fig.1 shows the circuit details. At the core of the circuit are Mosfets Ql and Q2 which are used to drive transformer Tl. This transformer is a standard mains transformer with two separate low voltage windings which are connected together as a centretapped primary winding. By alternately switching the 12V supply across each half of the primary winding using Ql and Q2, the transformer produces an approximate 240V AC output across its secondary. Ql and QZ are switched on and off out of phase so that when Ql is on, Q2 is off and vice versa. These Mosfet transistors are Motorola MTP3055E devices which are specifically designed to switch inductive loads (such as a transformer) without the need for external transient protection. Instead, these devices each have an internal avalanche diode for transient protection and for commutating reverse voltages. We can not recommend any alternative devices to the MTP3055E, so do not substitute for this component. F1 POWER +12V~o-------tt-------------------, 100n Vee 100 + 16VW:r OUTPUT SOCKET 02 MTP3055E 1k T1 r,!'~--r---,M2165_ T __,__ 150k 240V IC1 7555 47k D.1I Vee 47k T 10k Vee 100n B 15k ELJC VIEWED FROM BELOW ~ GDS 1.6k 8.2k 47k OEAO TIME COMPARATORS T 40W INVERTER 10k The remaining components in the circuit are there to provide the out-ofphase drive signals for Ql and Q2. ICl is a CMOS 555 timer which is set up as an oscillator operating at 50Hz. This is wired in a somewhat unconventional manner, however. Normally, the astable configuration uses a timing capacitor (on pins 6 & 2) which is charged via a resistor connected to the positive supply rail and then discharged into pin 7. In this circuit though, the timing capacitor (O. lµF) is alternately charged and dis- charged by the pin 3 output via a 150kQ resistor. The circuit works like this: at switch-on, pin 3 of ICl goes high and charges the O. lµF timing capacitor via the 150kQ resistor. When the capacitor voltage reaches 2/3Vcc (ie, 2/3 the supply rail voltage), pin 3 switches low and the capacitor discharges via the 150kQ resistor until it reaches 1/3Vcc. At this point, pin 3 switches high again and so the cycle is repeated indefinitely while ever power is applied. Table 1: Performance Input Voltage Input Current Load Output Voltage Efficiency 13.8V 1A ON 279VAC O"lo 13.8V 2.2A 15W 265VAC 49% 13.8V 4A 40W 248VAC 72% 12.0V · 3.8A 40W 230VAC 88% Fig.1: 555 timer IC1 & transistor Q3 provide antiphase clock signals to comparators IC2a & IC2b. These comparators then drive Mosfet transistors Ql & Q2 which in turn switch the primary of the step-up transformer (Tl). IC2c & IC2d are the dead-time comparators. Note that the 50Hz output from ICl is a genuine square wave with very close to 50% duty cycle since pin 3 swings fully to the supply rails due to the CMOS output. The waveforms of Fig.2 show the square wave output at pin 3 of ICl and the capacitor voltage at pins 2 and 6. The square wave output from pin 3 of ICl is fed to the inverting input of IC2a (pin 10) via a voltage divider consisting of two 47kQ resistors (one in series and the other to the positive supply rail). The resulting waveform at pin 10 is a square wave which swings between the +12V supply rail (Vee) and ½Vee. IC2a is a comparator and its output at pin 13 goes high each time the inverting input (pin 10) goes lower than the non-inverting input (pin 11). If the non-inverting input is at ¾Vee, then IC2a's output will be low when pin 10 is at Vee and high when it is at ½Va: The open collector output at pin 13 FEBR UA RY1992 43 Vee Variations On A Theme PIN3 IC1 ov As this project was being developed, one of our readers enquired about its suitability to drive a telescope. To do this, its output frequency needs to be varied between about 40Hz and 60Hz. ~ 20ms V e e ~ 2/3Vce PINS2,6 1/3Vee IC1 OVt--------------Vee 3/4Vec PIN1 IC2e ov 3/4Vee In the meantime, Altronics has indicated that they will have a kit available for this project shortly after this issue goes on sale. Priced at around $90, it will use an improved and larger version of the transformer specified above and will deliver more power - around 60 watts at 230VAC. See the Altronics catalog in this issue for further details. ~ ~ '"! PIN2 IC2d This can be achieved by substituting a series 120kQ resistor and 5QkQ potentiometer, as shown in Fig.6. However, this will not be the most efficient way of powering a small telescope motor. We hope to present a low power version of the circuit in a coming month. erated to drive QZ. First, the square wave signal at pin 3 of IC1 is inverted Vee using transistor Q3. This inPIN14 verted signal is extracted from IC2b the junction of the two 4 7kQ resistors in Q3's collector cirOV cuit and, as before, swings Vee between Vee and ½Vee. The inverted signal is then fed to PIN13 IC2a the inverting input (pin 8) of ICZb and the output of this ov comparator then drives the Fig.2: this diagram shows the waveforms gate of QZ. generated by the major circuit sections. Note Note that the non-invertparticularly the waveforms generated by the ing inputs (pins 11 & 9) of deadtime comparators (IC2c & IC2d) & how ICZa and ICZb and joined tothey effectively narrow the positive-going pulses from IC2a & IC2b. gether and are nominally at ¾Vee (we'll look more closely at this shortly). However, because the has a lkQ pull-up resistor and drives signal on pin 8 of ICZb is inverted the gate of Ql via a 100Q resistor. compared to the signal on pin 10 of Each time ICZa's output is pulled high, Ql turns on and switches one half of ICZa, the outputs from these two comparators (and thus the drive signals to the transformer primary to ground. Ql & Q2) are 180° out of phase. Tlrnt lakes care of the drive cirThus, Ql & QZ are alternately cuitry to Ql. We now return to IC1 to switched on and off to drive their see how the out-of-phase signal is genov RESISTOR COLOUR CODES D D D D CJ D D D D 44 No. Value 5-Band Code {1%) 1 6 150kQ 47kQ 15~Q 10kQ 8.2kQ 1.6kQ 1kQ 100Q brown green black orange brown yellow purple black red brown brown green black red brown brown black black red brown grey red black brown brown brown blue black brown brown brown black black brown brown brown black black black brown 1 2 2 5 SILICON CHIP respective halves of the transformer primary winding. At least, that's the basic scheme. In practice it's not quite as easy as that. If we simply use out-of-phase waveforms to drive the transistors as described above, both transistors will be on for a short time at the transition points. That's because these devices take some time to change state, which means that the next transistor in the sequence will turn on before the other has had a chance to turn off. This will cause heavy transient currents to flow in the output stage and cause overheating of the Mosfet devices. Dead time comparators To avoid this problem, we have added a "dead-time" circuit to ensure that both transistors are off at the transition point. Essentially, we turn the active transistor off early in the cycle and the other transistor on late. This job is performed by comparators ICZc and ICZd. Let's see how this circuit works. A voltage divider consisting of a series resistor string between the Vcc supply rail and ground is used to provide the reference voltages for ICZc and ICZd. From the Vee rail, we have a l0kQ resistor, then four resistors (100f.!, 1.6kQ, 8.ZkQ & 100Q) which total 10kQ, and finally another 10kQ resistor to ground. Note the 2/3Vcc and 1/3Vcc voltage points shown on the circuit. These correspond to the switching voltages used for oscillator IC1. The 2/3Vcc point is tied to pin 5 of IC1 which is also nominally at 2/3Vcc. Fig.3 (right): here's how to install the parts on the PC board & complete the wiring. Use mains-rated cable for the connections between the transformer & the mains socket & note that Ql & Q2 must be electrically isolated from the rear panel using TO-220 mounting kits. We have connected these two 2/3Vcc points together to remove any slight variation that may exist between these two voltages. The non-inverting input ofICZc (pin 7) connects to the voltage divider at the junction of the 1000 and 1.6kQ resistors. This pqint is at 0.663Vcc, which is just slightly less than 2/3Vcc (0.666Vcc). Similarly, the inverting input of IC2d (pin 4) is connected to the junction of the lO0Q and 8.ZkQ resistors in the bottom half of the divider. This points is at 0.336Vcc, which is slightly higher than 1/3Vcc (0.333Vcc). To complete the dead-time circuit, the inverting input ofICZc (pin 6) and the non-inverting input of ICZd (pin 5) are connected to the timing capacitor on pins 2 & 6 of ICl. As shown in Fig.2, the signal voltage across the timing capacitor takes the form of a triangular waveform which swings between 2/3Vcc and 1/3Vcc. Fig.2 shows the resulting output signals generated by comparators IC2c & IC2d. Note that the output of IC2c (pin 1) swings low just before the voltage across the timing capacitor reaches 2/3Vcc and then swings to ¾Vee again shortly after this point. Similarly, pin 2 of ICZd swings low just before the capacitor discharges.to 1/3Vcc and swings to ¾Vee again a short time later. The open collector outputs of IC2c & ICZd are tied together and connected to a voltage divider consisting of 15kQ and 47kQ resistors (to produce the ¾Vee voltage). Thus, the combined outputs of IC2c & IC2d produce brief low-going pulses every lOms which straddle the transition points of the switching waveform produced by ICl. (Note: the outputs from IC2c & ICZd are shown separately on Fig.2 for clarity). This pulse waveform is applied to the non-inverting inputs of IC2a & IC2b (pins 11 & 9). Each time the outputs of IC2c & IC2d swing low, the outputs of IC2a & IC2b are also forced ► ~ \/5,)J ,\ ( + METAL REAR PANEL FUSE HOLDER INSULATING BUSH MICA WASHER SUPPLY LEADS 'XcoRD GRIP GROMMET I) O 0 01 02 GDS GDS 240V <at> OUTPUT SOCKET 0 FRONT PANEL low and both Ql & QZ are off. For the rest of the time, the outputs of IC2c & ICZd are at ¾Vee and so IC2a & IC2b gate through the respective waveforms WARNING This project produces an output voltage at mains potential. For this reason, exercise care when working on the unit and make sure that any equipment to be used with it is in a safe condition . on their inverting inputs to drive the switching transistors. The resulting outputs from IC2a & ICZb are shown at the bottom ofFig.2. Because, the switching pulses that are applied to the transistors are slightly narrowed, the transistor that's on has time to turn off before the other turns on and so the possibility of contention is eliminated. Power for the circuit is derived from a +12V car battery. This supply connects directly to the centre tap of transformer Tl via a 5A fuse and power switch S1. The remainder of the cirFEBR UA RY 1992 45 Fig.4: here is the full-size etching pattern for the PC board. cuit is powered via a 100Q decoupling resistor and voltage clamping diode ZDl. This zener diode is used to quench any high voltage spikes which could otherwise damage the 7555. Finally, the decoupled supply rail to the ICs is filtered using 100µF and 10µF electrolytic capacitors. Construction Most of the parts for the SILICON CHIP 40-Watt Inverter are mounted on This oscilloscope photograph shows the output waveform produced by the dead-time comparators (IC2c & IC2d) at top and the sawtooth voltage developed across the 0.lµF timing capacitor (bottom). a PC board coded SC11203921 and measuring 125 x 46mm. Fig.3 shows the parts location on the PC board. Begin the construction by installing PC stakes at all external wiring points, then install the resistors. Check each resistor value on your multimeter before installing it on the board, just to be sure that you have the correct value. Now install the two ICs , Q3 and ZD1 as shown on Fig.3 . Make sure that these parts are all correctly oriented (see Fig.1 for Q3 's pin connections). Finally, install the capacitors on the board. The two 0. lµF capacitors can go in either way around but take care with the polarity of the two electrolytics. The completed board assembly can now be mounted on the lid of the case at the rear and secured with four selftapping screws (the board mounting holes align with the integral plastic standoffs on the lid). Use an oversize INSULATING . MICA WASHER -~~jl drill bit to shorten the unused standoffs so that the board sits neatly in position. Once the board is in position, install the metal rear panel and mark out the mounting holes for the two Mosfets. These devices should be mounted directly behind their respective PC stakes (see photo). Drill these mounting holes to 3mm, then mark out and drill mounting holes for the fuseholder and cordgrip grommet. The Dynamark label can now be affixed to the plastic front panel and the cutout made for the power switch. This done, remove the front section of the power socket and use the back section to mark out its mounting and lead access holes. These holes can now drilled to size and the socket secured to the panel. Nylon screws The transformer is mounted towards the front of the lid in an area which is free of ribs, and is secured using two 4BA x 12mm nylon screws and nuts. Do not use metal screws to secure the power transformer, as they could represent a safety hazard if the transformer breaks down to frame. Similarly, for safety reasons, do not use a metal front panel or an aluminium front panel label. Instead, be sure to use a plastic panel and a plastic adhesive label (or a plastic panel with screened lettering) as specified in the parts list. The remaining hardware items can 120k SCREW r lllillD{s 50k IC1 --...._ CASE .L T0220 DEVICE This is the 240VAC output waveform that's delivered when driving a 40W load (obtained used a 20:1 probe). 46 SILICON CHIP Fig.5: mounting details for Mosfet transistors Ql & Q2. Smear all mating surfaces with heatsink compound before bolting the assemblies together, then use you DMM to check that the metal tabs are indeed isolated from the rear panel. Fig.6: this simple modification to the clock circuit based on 555 timer IC1 will let you vary the output frequency from about 4060Hz, so that the unit can be used to drive a small telescope motor. The 50kQ pot should be mounted on the rear panel. PARTS LIST 1 plastic instrument case, 155 x 158 x 64mm, with metal rear panel 1 plastic Dynamark front panel label, 140 x 56mm (note: do note use an aluminium front panel label) 1 PC board, code SC11203921, 125 x 46mm 1 M2165 60VA transformer 1 panel mount mains socket 1 panel mount 3AG fuse holder 1 5A 3AG fuse 1 cord grip grommet 1 panel mount 15A rocker switch 2 TO-220 mounting kits 11 PC stakes 2 4BA x 12mm nylon screws, nuts & washers 1 1-metre length black heavyduty hookup wire 1 1-metre length red heavy-duty hookup wire 2 large alligator clips (or cigarette lighter socket; see text) The PC board is secured to the lid of the case using self-tapping screws , while the transformer is secured using nylon screws & nuts. Use cable ties to bundle the various leads together, to keep the wiring neat & tidy. now be installed on the front and rear panels and the wiring completed. Note that the front panel must be installed upside down on the lid, as shown in the photographs; ie, with the power switch to the left. Follow the wiring diagram (Fig.3) carefully and use 240VAC 10A cable for all wiring to reduce voltage losses. Similarly, use heavy-duty colour coded cable (red for positive, black for negative) for the external battery leads. These leads should be fitted with large alligator clips to make battery connections quick and easy. Alternatively, you can terminate the battery leads in a cigarette lighter socket but make sure you get the polarity right. No earth connection Note that the Earth pin of the mains output socket is not connected to any part of the circuit. It does not have to be and nor should it be in a fully floating supply such as this. The same rule applies to portable 240VAC generators. The two Mosfets must be isolated from the metal rear panel using stand- ard TO-220 insulating kits (ie, mica washers and plastic bushes). Fig.5 shows the mounting details for these two devices. Smear all mating surfaces with heatsink compound before bolting the assemblies together. Finally, check your work carefully before installing the fuse and completing the case assembly. Testing To test the unit, connect it to a 12V car battery (or to some other 12VDC supply capable of 5 amps or more) and plug a 40W lamp into the mains socket. Check that the lamp lights as soon as power is applied and that it delivers about the same light output as it does when plugged into a standard mains outlet. If the inverter does not function, switch it off immediately and check carefully for wiring errors and for bad or missed solder joints. If these checks don't reveal anything, disconnect the transformer from the + 12V rail, then re-apply power and check the voltage on the supply pins oflC1 & ICZ . These pins should be at about 12V, depending on the output from the battery. Semiconductors 1 7555 CMOS timer (IC1) 1 LM339 quad comparator (IC2) 2 MTP3055E 12A, 60V power FETs (01 ,02) 1 BC548 NPN transistor (03) 1 16V 1W zener diode (zo·1) Capacitors 1 100µF 16VW RB electrolytic 1 10µF 16VW RB electrolytic 2 0.1 µF metallised polyester Resistors (0.25W, 1%) 1 150kQ 1 8.2kQ 6 47kQ 1 1.6kQ 1 15kQ 2 1kQ 2 10kQ 5 100Q Miscellaneous Machine screws and nuts, selftapping screws, mains-rated cable, tinned copper wire. Finalfy, if you have access to an oscilloscope, you can check the circuit waveforms against those shown in Fig.2 and the accompanying photographs. Note, however, that the waveform at the outputs of ICZc & ICZd will be a combination of the separate waveforms shown in Fig.2, as indicated previously. SC FEBRUARY1992 47