Fullscreen HTML5Med Res (??MB)HTML4

Build this 15-watt 12-240V inverter This 15W inverter is ideal for operating lowpower AC equipment from a 12V battery. It's based on the popular 40W inverter published in · the February 1992 issue & is ideal for use with camcorder battery rechargers & telescope drives. By JOHN CLARKE & DARREN YATES The 40W inverter published in our February 1992 issue proved to be very popular, mainly because of its simple design and compact size. But, as we very quickly discovered, many people wanted an even smaller design, mainly for driving equipment like camcorder battery rechargers, power telescope drives, electric toothbrushes and the like. As it turned out, it wasn't too difficult to produce a low-power version with an output of around 15W. We did this by replacing the original 60VA transformer with a couple of 7VA PCmount units (wired in parallel). These new transformers are much smaller The completed inverter is shown here with the optional hand-held controller. This controller varies the output frequency so that the unit CiUl he used to control a telescope drive. 82 SILICON CHIP than the 60VA unit and allow the project to fit comfortably into a medium-size zippy box measuring 150 x 90 x 50mm. Apart from that, the new circuit is virtually identical to the old unit, except that we've added a power LED to give on/off indication. This LED sits at one end of the case next to a panelmount mains socket. The other end of the case carries the on/off switch and a 5-pin DIN socket that interfaces to an optional hand-held controller circuit so that the unit can be used with a telescope drive. Basically, the hand-held controller is there to vary the output frequency, to allow speed control for a telescope drive. If you don't want to drive a telescope, just leave the DIN socket & its connecting leads out and ditch the hand-held controller so that the unit functions as a conventional 15W inverter. 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 the usual manner. Note, however, that this device cannot drive fluorescent lamps as the starting current required is too high for the circuit to produce. 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 low-cost transformers 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. Circuit details . As the accompanying photographs show, relatively little circuitry is used in the inverter. Apart from the power transformers, it uses two inexpensive ICs, 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 QZ which are used to drive mains transformers · Tl and TZ. These transformers are wired in parallel and each has two separate low-voltage f1 POWER 5A +12V0----0---0----- S 1 ~ - - - - - - . - - - - - - - - - - - - - - - - - - - - - - - - - - - - , 100n Vee POWER LED1 1 1DD + 16VWJ K OUTPUT SOCKET SPEED VR1 50k LIN IC1 7555 mr .,. Vee HAND CONTROLLER ; ~ 47k DEAD TIME .,. j GDS B EOc VIEWED FROM BELOW STOP S2 COMPARATORS 15W INVERTER .,. Fig.1: the circuit uses 555 timer IC1 & transistor Q3 to provide antiphase clock signals to comparators IC2a & IC2b. These comparators then drive Mosfet transistors Qt & Q2 which in turn switch the paralleled transformer primary windings. IC2c & IC2d are the dead-time comparators (see text). windings which are connected together to form a centre-tapped primary winding. Because Ql and Q2 alternately switch the 12V supply across each half of the primary windings, the transformers produce an approximate 240V AC output across their paralleled secondaries. Ql and Q2 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 Table 1 : Performance of Prototype Input Voltage Input Current Load Output Voltage Efficiency 13.0V 1.4A OW 272VAC 0% 12.0V 1.7A 15W 230VAC 74% 12.3V 1.8A 15W 235VAC 68% 13.0V 2A 15W 244VAC 58% do not substitute for this component. 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 (0. lµF) is alternately charged and discharged by the pin 3 output via a 150kQ r!;)sistor. The circuit works like this: at switch-on, pin 3 of ICl goes high and charges the 0. 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 reJUNE 1992 83 the CMOS output. Fig.2 shows the waveforms genPIN3 erated by the major circuit IC1 sections. ov The square wave output from pin 3 of IC1 is fed to 20ms the inverting input of IC2a (pin 10) via a voltage diV · e e ~ vider consisting of two 4 7k.Q 2/3Vee resistors (one in series and PINS2,6 1/3Vee. IC1 the other to the positive supOV+--------------ply rail). The resulting waveform at pin 10 is a Vee square wave which swings 3/4Vec between the +12V supply PIN1 IC2c rail (Vee) and 1/2Vcc. ov IC2a is a comparator and its output at pin 13 goes high each time the inverting in3/4Vee put (pin 10) goes lower than PIN2 the non-inverting input (pin IC2d 11). If the non-inverting inov put is at 3/4Vcc, then IC2a's Vee output will be low when pin 10 is at Vee and high when PIN14 IC2b it is at 1/2Vcc. The open collector outov put at pin 13 has a lkQ pullup resistor and drives the gate of Ql via a lO0Q PIN13 IC2a resistor. Each time IC2a's output is pulled high, Ql OV turns on and switches the Fig.2: this diagram shows the waveforms bottom half of the transgenerated by the major circuit sections. Note former primary to ground. particularly the waveforms generated by the That takes care of the dead-time comparators (IC2c & IC2d) & how drive circuitry to Ql. We they effectively narrow the positive-going now return to ICl to see how pulses from ICZa & ICZb. the out-of-phase signal is generated to drive Q2. First, the square wave signal at pin peated indefinitely while ever power 3 of ICl is inverted using transistor is applied. Note that the output from ICl is a Q3. This inverted signal is extracted genuine square wave with almost a from the junction of the two 4 7kQ 50% duty cycle. This is because pin 3 resistors in Q3's collector circuit and, swings fully to the supply rails due to as before, swings between Vee and Vee ~ ~I ~ ~ 1/2Vcc. The inverted signal is then fed to the inverting input (pin 8) of IC2b and the output of this comparator then drives the gate of Q2. Note that the non-inverting inputs (pins 11 & 9) of IC2a and IC2b and joined together and are nominally at 3/4Vcc (we'll look more closely at this shortly). However, because the signal on pin 8 of IC2b is inverted compared to the signal on pin 10 of IC2a, the outputs from these two comparators (and thus the drive signals to Ql & Q2) are 180° out of phase. Thus, Ql & Q2 are alternately switched on and off to drive their 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 simultaneously 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 flo w 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 eJlsure 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 IC2c and IC2d. These two comparators act together as a window comparator. Pin 7 of RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 84 No. Value 4-Band Code (1%) 5-Band Code (1%) 1 1 6 150kQ 100kQ 47kQ 15kQ 10kQ 8.2kQ 1.6kQ 1kQ 100Q brown green yellow brown brown black yellow brown yellow purple orange brown brown green orange brown brown black orange brown grey red red brown brown blue red brown brown black red brown brown black brown brown brown green black orange brown brown black 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 3 1 2 5 SILICON CHIP OUTPUT SOCKET 0 5-PIN DIN SOCKET WIRING SIDE :~~-0~ '()/ 4 3 '~ <at> ------8 A~K <. LE01 7 Fig.3: leave the 150kQ resistor (shown dotted) out of circuit & install the 5-pin DIN socket if you intend using the unit to control a telescope drive. If you just want a fixed-frequency inverter to power an appliance, install the resistor & delete the DIN socket. S1 0 IC2c is biased just below 2/3Vcc, while pin 4 of IC2d is biased just above 1/3Vcc. These reference voltages are compared with the 1/3Vcc to 2/3Vcc triangular ·waveform that appears across the 0. lµF timing capacitor on pin 2 of ICl. The result is that pin 1 of IC2c swings low just before the voltage across the timing capacitor reaches 2/3Vcc and then swings to 3/4Vcc again shortly after this point. Similarly, pin 2 of IC2d swings low just before the timing capacitor discharges to 2/3Vcc and swings the 3/4Vcc a short time later (see Fig.2) . The open collector outputs of IC2c & IC2d are tied together and connected to a voltage divider consisting of15kQ and 47kQ resistors (which produce the 3/4Vcc voltage). Thus, the corn- POWER bined outputs ofIC2c & IC2d produce brief low-going pulses which straddle the transition points of the switching waveform produced by ICl. This pulse waveform is applied to the non-inverting inputs of IC2a & IC2b and ensures that both transistors are off at the transition points. Hand controller For the variable speed drive version, the 150kQ timing resistor on pins 3 & 6 of ICl is replaced by two wires which go off to the hand-held control box. This box contains a 50kQ linear pot (VRl), a lOkQ resistor, a lO0kQ resistor which can be bypassed by momentary switch S3, and a SPST toggle switch (S2). By varying VR1, we can vary IC1's output frequency (and hence the 240VAC output frequency) from about 45Hz to 90Hz. Similarly, by pressing S3 to short out the 100kQ resistor, we can increase the frequency to about 150Hz. This makes the unit suitable for use with telescope drives and other low-powered equipment that relies on frequency for speed control. SPST switch S2 is used to start and stop the drive motor. When the switch is closed, the non-inverting inputs of IC2a & IC2b are pulled low, and so the MOSFETs (and thus the output) are switched off. Power supply Power for the circuit is derived from a 12V car battery. This supply connects directly to the centre tap of transformer T1 via a 3A fuse and power switch Sl. LED 1 and its associated 1kQ current limiting resistor provide power on/ off indication. The remainder of the circuit is powered via a lO0Q 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 lOµF electrolytic capacitors. Board assembly The PC board is secured to the bottom of the case using machine screws & nuts. Be sure to use mains-rated cable for the two connections to the output socket. All the parts except those for the optional hand-held controller are mounted on a PC board coded SC11106921 and measuring 123 x 82mm. Fig.3 shows the parts layout on the PC board. Begin construction by soldering in the eight wire links. These links should all be nice and straight, so that they don't short out other components on the board. If necessary, you can JUNE 1992 85 the two ICs, the two MTP3055 MOSFETs (Ql & Q2), zener diode ZD1 and transistor Q3. Make sure that all these parts are correctly oriented. In particular, mount the two MOSFETs with their metal tabs facing away from transformer T2. It is not necessary to fit heatsinks to the MOSFETs in this circuit because of the low power involved. PCB and SCHEMATIC CAD - :- - --- --- - ·-- - -· -- - - - ·':!: · ... i -T~ IF AMPLIFIER ' ,· t !~ Initial testing - I' !i' i, ;J : i ____ _______ ______ _'- -- r·----------~ C•U!Sif.ff.• -~ ~ t'l: 5~1. =~l: -., : H ' 1111 I . ... , Fig.4: here are the wiring details for the optional hand-held controller. Note the link between two of the pot terminals. lll+lllllllllllllfHHllllflll"l'III t . "'°MI o Qg o f'III • IJ EASY-PC • Runs on PC/XT/AT/286/386 with Hercules, CGA, EGA or VGA. • Design Single sided, Double sided and Multilayer boards • Provides Surface Mount support • Standard output includes Dot Matrix/Laser/Inkjet printers, Pen Plotters, Photo-plotters and NC Drill • Award winning EASY-PC is in use in over 12,000 installations in 70 Countries World-Wide • Superbly Easy to use • Not Copy Protected straighten the link wire by clamping one end in a vyce and pulling on the other end with a pair of pliers so that the wire stretches slightly. Once the links are in, solder in the resistors, capacitors and the two M205 fuseclips. Note that each fuseclip has an outer guard to keep the fuse in place, so be sure to install the fuseclips the right way around. The 150kQ resistor shown dotted should be omitted if you intend using the external hand-held controller but otherwise should be included (see below). The semiconductors can now all be installed - see Fig.1 for the pin connection details. These parts include At this stage, the board assembly will be complete apart from-fitting the two power transformers. Before installing these, it's a good idea to check that the circuit is working up to this point. .If you haven't already done so, you will have to fit a 150kQ resistor between pins 3 & 6 of ICl before testing can proceed. This resistor can be temporarily tacked into position if you intend using the hand-lfeld controller. To test the unit, first connect the LED via a couple of flying leads, then connect a 12V power supply (be careful with the polarity). This need only be a 12VDC 300mA plugpack to start with since we aren't driving a load. If you have a CRO handy, switch the circuit on and check the waveforms on pins 13 & 14 of IC2. You should see a switching waveform at about 50Hz on both pins (see Fig.2). If you don't, disconnect the supply and check the PC board for shorts, missed solder joints and parts in the wrong way around. If you don't have a CRO, check the voltages on pins 13 & 14. If the circuit is working correctly, the meter will indicate an average voltage of 6V DC at these two points. Options: • 1000 piece Schematic symbol library • Surface Mount symbol library • Gerber Import facility For full info 'phone, fax or write: BTC PO BOX -432 GARBUTT 4814 QLD. PH (077) 21 5299 FAX (077) 21 5930 86 SILICON CHIP A small plastic zippy case holds all the parts for the hand-held controller. Tie a knot in the connecting cable so that it cannot be pulled out of the case. PARTS LIST 1 PC board, code SC11106921, 123 x 82mm 1 plastic zippy box, 150 x 90 x 50mm 1 SPST toggle switch 2 M205 fuseclips 1 M205 5A fuse 1 flush panel-mount mains socket 2 ?VA mains transformers with dual 9V secondaries (Altronics M-7118 or Jaycar MF-1006) 4 rubber feet SC11106921 Fig.5: check your etched PC board against this full-size artwork & correct any defects before installing the parts. Assuming everything is working correctly, you can now install the two transformers. Don't forget to remove the 150kO resistor you tacked into circuit if you will be using the external hand-held controller. Final assembly The PC board is mounted towards one end of the specified case so that there is plenty of clearance between the transformers and the mains socket. Use the board as a template for marking out its mounting holes, then drill the holes and install machine screws and nuts (to act as spacers) at each location. Next, you'll need to mark out and "cut" the hole for the front of the mains socket. Once the hole has been marked, it can be made by first drilling a series of small holes around the inside circumference, then knocking out the centre piece and filing to a smooth finish. The square cutout for the power switch (S1) at the other end of the case can be made in similar fashion. You will also have to drill holes next to this switch for the DIN socket (if necessary) and power SUP.ply leads, plus a hole next to the mains socket for the power indicator LED. The assembly can now be completed by connecting flying leads to the external wiring points on the board, then mounting the various items inside the case and installing the remammg wumg. Note that 240VAC cable should be used for the connections between the PC board and the mains socket. Remote control The parts for the hand-held controller fit comfortably into a small zippy box measuring 83 x 54 x 28mm. Fig.4 shows the wiring details. The pot (VRl) and the two switches are simply mounted directly on the lid of the case and the two resistors then soldered to the appropriate terminals as shown in Fig.4. The four leads (we used 5-way telephone cable) from the handheld controller emerge through a hole in one end of the case and terminate in a 5pin DIN plug. Make sure that the plug wiring matches the wiring to the DIN socket in the inverter unit. Final testing To test the unit, connect it to a 12V car battery and plug a 15W lamp into the mains socket. The lamp should light as soon as the inverter is switched on and should deliver about the same output as it does when plugged into a standard mains outlet. If the inverter does not function, switch it off immediately and check for wiring errors and for bad or missed solder joints. If these checks don't reveal anything, re-apply power and check that the supply rails to the ICs are correct. You should find +12V on Semiconductors 1 7555 CMOS timer IC (IC1) 1 LM339 quad comparator (IC2) 2 MTP3055 N-channel MOSFETs (01 ,02) 1 BC548 NPN transistor (03) 1 5mm red LED (LED1) 1 15V 1W zener diode (ZD1) Capacitors 1 100µF 16VW RB electrolytic 1 10µF 16VW RB electrolytic 2 0.1 µF 63VW MKT polyester Resistors (0.25W, 1%) 1 150kO 1 8.2kO 6 47kO 1 1.6kO 1 15kO 2 1kO 2 10kO 5 1000 Miscellaneous Insulated hookup wire, machine screws, nuts & washers . Hand-held controller · 1 zippy box, 83 x 54 x 28mm 1 SPST switch (S2) 1 SPST normally open momentary switch (S3) 1 50kn linear potentiometer (VR1) 1 100kn 1% 0.25W resistor 1 5-pin DIN plug 1 5-pin DIN socket 2 metres of 4-core cable 1 knob to suit pins 4 & 8 of ICl and on pin 3 of ICZ. Finally, if you have access to an oscilloscope, you can check the circuit waveforms against those shown in Fig.2. Note, however, that the waveform at the outputs of IC2c & IC2d will be a combination of the separate waveforms shown in Fig.2. SC JUNE 1992 87