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Light, compact & efficient 12-240VAC 200W inverter This light & compact 200W 12V-240VAC inverter can drive mains appliances, including power tools, fluorescent & incandescent lights, TVs, etc from a 12V battery. It is ideal when camping, for use at building sites or as part of a solar power installation. By JOHN CLARKE This 200W inverter covers the medium power range and is suitable for powering household appliances such as power tools, hifi and video equipment and personal computers. It is unsuitable for driving microwave ovens, washing machines and other higher power appliances. While inverters described in electronics magazines in the past have usually employed heavy mains trans26  Silicon Chip formers (apart from our 2kW sinewave inverter), this new design uses a high frequency transformer which is small, light and efficient. To give a comparison, the 40W 50Hz square wave inverter published in the February 1992 issue of SILICON CHIP weighed about 1.25kg. This new design, which puts out five times as much power, weighs 1kg. Because it doesn’t use a mains trans- former, the new design also draws a much smaller current when in the standby condition; ie, when powered up but with no load connected. Its standby current is 55 milliamps which compares very favourably with the 1 amp standby current of the 40W inverter referred to above. Square wave The output waveform of the new inverter is a “modified square wave” with a duty cycle of 35%, the best compromise waveform for a low cost inverter. This is explained by the diagram of Fig.2 which shows the three Top of page: the 200W inverter is fitted with a low-profile 240VAC power point & is suitable for powering many power tools & other domestic applianc­es. ISOLATED VOLTAGE FEEDBACK +340V R1 +12V Q3 ISOLATED GATE DRIVER +12V Q1 T1 Q5 AC X Q2 Q6 ISOLATED GATE DRIVER 100 385VW AC Y Q4 R2 25kHz SWITCHMODE DRIVER ISOLATED GATE DRIVER 240VAC OUTPUT ISOLATED GATE DRIVER 0V OVERCURRENT AMPLIFIER Ri MODIFIED SQUARE WAVE GENERATOR DC-DC CONVERTER SQUARE WAVE 'H' PACK Fig.1: block diagram of the 200W inverter showing the high fre­quency DC-DC step-up stage & H-pack output stage. Fig.2: various 50Hz inverter output waveforms. (a) is the ideal; (b) has low amplitude; and (c) is the modified square wave output used in the 200W inverter. common inverter waveforms. Note that they all have the same RMS value of 240V. The sinewave is the ideal waveform since it has no harmonics and it swings over a range of ±340V peak. Sinewave output is usually reserved for high power inverters because of the extra complexity. The second common inverter waveform is the square wave which, despite having the required 240V RMS value, has a peak swing of only ±240V. This is • • • • • • • • Features often insufficient for correctly power­ ing appliances which rely on the peak voltage of the 50Hz mains waveform. This includes any appliance with a rectifier and filter capacitor power supply such as computers, VCRs, TV sets, hifi systems and so on. Then there is the “modified square wave”. There are many types of modified square wave inverters. Some start off with a low duty cycle and a high peak voltage (as in Fig.2c) on light loads and increase the duty cycle to a full square wave (Fig.2b) when driving a full load. This duty cycle variation is used as a means of Small size (1kg mass) Low standby current Modified square wave output Peak-peak voltage equal to mains sine wave Under voltage shutdown 30A over-current limiting Fuse protection Fully isolated output for safety Specifications Input voltage .......................................11-14.8VDC (12V lead acid battery) Output voltage ............................................ 240VAC modified square wave Power rating ....................................... 200W short term, 150W continuous Surge power .......................................................................................350W Standby current ..................................................................................55mA Full load current .........................................................25A DC (200W load) Output regulation .................................................................................< 8% Efficiency ................................................................ > 70% for loads > 60W 50Hz accuracy .....................................................................................±5% Fig.3: this diagram shows how the gate signals to the H-pack Mosfets are arranged to give the modified square wave output. February 1994  27 output volt­age regulation. However, it also means that the peak voltage will depend upon the load which is less than ideal. 28  Silicon Chip Our new 200W Inverter provides a fixed 35% duty cycle re­gardless of load current so that the peak voltage is maintained. Output regulation is achieved by keeping the peak voltage con­stant. Fig.1 shows the block diagram of the 200W Inverter. It incorporates a high frequency DC-DC converter and an H-pack output stage. The DC-DC converter has a switch­ mode driver to control Mosfets Q1 and Q2. These devices drive transformer T1 in push-pull mode. The step-up ratio is 38:1 and the resulting AC voltage is rectified by a full wave bridge Fig.4: the circuit of the 200W inverter. At left is the 25kHz DC-DC step-up section involving transformer T1. At top right is the H-pack output stage, while at bottom right is the 1MHz burst circuitry. February 1994  29 The pencil in this shot is pointing to Mosfet Q1. Q1 & Q2 are BUK436-100A Mosfets rated at 33 amps, 100 volts & 125 watts. They are mounted on the heatsink as shown in Fig.8. and filtered with a 100µF 385VW capacitor. The isolated feedback circuit adjusts the Mosfet switching so that the DC voltage from the inverter is maintained at +340V regardless of the load current. The Mosfets are protected against overcurrent if, say, an excessive load is connected to the inverter. Over­current protec­ tion is achieved by detecting the voltage drop across resistor Ri. If the voltage exceeds a preset level, the switchmode driver reduces the duty cycle applied to the Mosfets and thus reduces the overall current. You might think that the transformer step-up ratio of 38:1 is far greater than necessary to give the 340V required. This ratio has been made larger to offset inevitable losses in the inverter and to provide good output voltage regulation. The 340V supply rail is fully floating with respect to the 12V battery terminals by virtue of the step-up transformer and the isolated voltage feedback. This will prevent the battery terminals from being at a high potential above ground should a fault occur in any equipment powered by the inverter. Across the 340VDC supply are connected four high voltage Mosfets in an H-pack arrangement. Q3 is in series with Q4 while Q5 is in series with Q6. The junction between Q3 and Q4 is point X, while the junction between Q5 and Q6 is point Y. If Q3 is turned on and Q4 off, point X is pulled up to +340V. Conversely, if Q4 is on and Q3 off, then point X is pulled down to 0V. Similarly, point Y can be pulled down to 0V when Q6 is turned on and pulled up to +340V when Q5 is on. The square wave generator circuitry has four outputs which drive Q3, Q4, Q5 and Q6. This allows the circuitry to pull point X to +340V and point Y to 0V for one half of the 50Hz waveform, then pull X to 0V and Y to +340V for the other half of the 50Hz waveform. Note that the Mosfets are switched on for only 70% of the time so the overall duty cycle of the waveform is 35%. Fig.3 shows the switching process in the H-pack output stage. Each of the output Mosfets is a FRED FET (Fast Recovery Epitaxial Diode Field Effect Transistor), made by Philips. The term “FRED” means that they incorporate a fast recovery reverse diode which protects the device from peak reverse voltages which can be generated when driving inductive loads. Apart from incandescent lamps and heaters, virtually all mains appliances can be regarded as inductive. Circuit description The full circuit for the 200W Inverter is shown in Fig.4 While there is a fair amount of componentry involved, the basic circuit operation is the same as detailed in the block diagram. At the heart of the DC-DC converter is IC1, a TL494 pulse width modulation 30  Silicon Chip ▲ This photo highlights the 1MHz gate drive circuitry for the H-pack Mosfets. Note the tiny toroids which are wound as transform­ers. Fig.5 (facing page): the full wiring diagram of the inverter. Note the differ­ent diameters of enamelled copper wire specified for the links. Be sure to use heavy-duty cables where specified (see text) & take care with the orientation of transformer T1. REAR PANEL CORD-GRIP GROMMETS EARTH BLACK RED 1.25mm ENCU D2 10  10  ZD1 2.2uF 100V 1 2.2uF 100V Q5 ZD2 ZD3 TP2 ZD4 T1 Q7 D10 1 100pF D12 220k TP1 IC2 4050 D9 ZD5 Q8 100pF D14 220k 0.1 T4 1 150k D8 T5 T6 3.3k Q13 Q14 0.1 56k 820  IC4 IL300 220k 560pF 560pF 100uF 385VW ZD7 D23 120  D21 D20 D22 D19 Q12 IC10 4023 1 D18 0.1 Q11 560pF IC9 4013 D17 1 1 1k 2200 Q16 .047 1 0.1 IC6 555 0.1 IC7 555 0.1 VR1 0.1 Q15 10  10k IC3 LM358 .001 IC5 LM358 1 T2 .0047 12k 10k 10k 0.1 390k 10k 1 Q10 100pF D15 0.1 0.1 47k 4.7k 10k 0.1 0.1 1M 1 1M 4.7k RO .001 D16 D4 22k IC1 TL494 K 47k D3 0.1 10uF D7 D6 1.2M 10k 10k D5 100pF D13 560pF 2.2k ZD6 Q9 220k D11 T3 0.1 Q6 10k R1 0.8mm ENCU D1 Q4 0.1 400VDC 1000uF Q3 Q2 Q1 IC8 4017 1 15k 150pF 220pF K S1 A F1 N GPO A LED1 K FRONT PANEL February 1994  31 This interior view of the 200W Inverter highlights the small high frequency transformer & the 100µF high-voltage reservoir capacitor. Note that holes must be drilled in the heatsink flange to clear the mounting screws for the earth lug & Mosfet Q3. (PWM) controller. It contains a sawtooth oscil­lator, two error amplifiers and a pulse width modulation compara­ tor. It also includes a dead time control comparator, a 5V refer­ence and output control options for push-pull or single ended operation. The components at pins 5 and 6 set the operating frequency of the inverter at about 25kHz. This frequency was selected to obtain the maximum power from the transformer. The PWM controller generates variable width pulses at pins 9 and 10 and these are buffered by the triple paralleled buffers of IC2, to drive the gates of Mosfets Q1 and Q2 via 10Ω resistors. 32  Silicon Chip Mosfets Q1 and Q2 drive the primary winding of transformer T1 which has its centre-tap connected to the +12V battery supply. Each Mosfet is driven with a complementary square wave signal so that when Q1 is on, Q2 is off and when Q2 is on, Q1 is off. The resulting waveform on the primary is stepped up by the secondary winding. Zener diodes ZD1 and ZD2 protect Q1 and Q2 from overvol­ tage. They operate at follows: when each Mosfet switches off, the transformer applies a positive voltage transient to the drain. If this exceeds the breakdown voltage of the zener (75V), it con­ducts and turns on the gate of the Mosfet which effectively then clamps the transient. The diodes in series with each zener prevent negative gate voltages. The stepped-up secondary voltage of T1 is rectified by high-speed diodes D3-D6 and filtered by the 100µF 385VDC capaci­tor. Voltage feedback A voltage divider comprising a 1.2MΩ resistor and a 3.3kΩ resistor monitors the high voltage DC from the inverter and drives op amp IC5a. This in turn drives linear optocoupler IC4. This device provides electrical isolation between input and output and drives IC3b, another op amp. Note that IC5b, the second op amp in the LM358 package, is not used. continued on page 37 SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au by transis­tors Q15 and Q16. We shall discuss the 1MHz source later in this article. The secondary output of transformer T2 is rectified using four 1N4148 switching diodes (D20-D23) and filtered with a 0.1µF capacitor. The resulting DC is regulated by 12V zener diode ZD7 and then powers IC5 and part of IC4. Current limiting Fig.6: primary winding details for the 25kHz DC-DC inverter section (T1). The four primary coils are quadrifilar wound with 1.25mm diameter enamelled copper wire. Note that the secondary winding is not shown. Its inputs (pins 2 & 3) are tied to pin 4 on the PC board. Trimpot VR1 is used to adjust the DC error signal from IC4 and thereby sets the high voltage DC rail. The signal from VR1 is amplified by IC3b and applied to the internal error amplifier in IC1 via diode D8 to control the pulse width modulation drive to the Mosfets. If the high DC voltage becomes greater than +340V, the pulse width drive is reduced. Similarly, if the voltage drops below +340V, the pulse width is increased until the correct voltage is achieved. Note that op amp IC5 and the high voltage side of IC4 cannot be powered from the 12V battery since the high voltage circuitry has to be fully floating. Hence they need their own isolated DC supply. This is provided by transformer T2. This transformer is driven at 1MHz via a .0047µF capacitor The current drain of the DC-DC Inverter is kept in check by op amp IC3a. This monitors the voltage drop across the 430µΩ sensing resistor connected between the sources of Q1 and Q2 and the negative supply (ie, 0V). IC3a amplifies the voltage drop across this resistor (which is a set length of specific diameter wire) by 391 so that only a very small voltage need appear across the resistor before overcurrent occurs. IC3a’s output is fed to the pin 16 input of IC1 via diode D7. It effectively overrides the voltage control of IC3b should the current rise above 30 amps. Dead time Dead time mightn’t sound like a good idea but is necessary in pushpull inverters otherwise the transistors or FETs can destroy themselves. This can happen because at the moment of switch-over, both Mosfets can be on. The “dead time” comparator at pin 4 prevents the push-pull outputs at pins 9 and 10 from changing over at the same time. It does this by providing a brief delay between one output going low and the other output going high. The dead time is also increased when power is first applied to achieve a slow start up. Initially, the 10µF capacitor between pins 13 and 14 and pin 4 is discharged. This forces a 100% Fig.7: winding details for the five toroid isolating transform­ers. dead time, with both outputs at pins 9 and 10 off. As the capacitor charges via the 47kΩ resistor to ground, the dead time is reduced slowly until it reaches its minimum value. Under-voltage protection is provided to prevent the battery from being discharged too much. Pin 2 of IC1 monitors the battery voltage via a voltage divider comprising 10kΩ and 12kΩ resistors. When the battery drops to below about 10V, the outputs at pins 9 and 10 switch off to shut down the circuit. H-pack output As discussed previously, four Mos­ fets are connected in an H-configuration across the high voltage supply. Mosfets Q3, Q4, Q5 and Q6 are driven by identical transformer coupled gate driv­ers to provide isolation from the low voltage circuitry. The gate driver for Q3 consists of transformer T3, diodes D9 and D10, transistor Q7, zener diode ZD3 and the 220kΩ resistor and 100pF capacitor. To switch on Q3, we apply a 1MHz signal to the primary side of T3. Its secondary voltage is then rectified by D9 and filtered by the 100pF capacitor. The resulting DC signal is fed via diode D10 to the gate of Q3, while zener diode ZD8 provides gate voltage clamping at 15V. So while the 1MHz signal is applied to T3, Q3 is on. To turn Q3 off, the 1MHz signal to T3 is removed but this does not ensure a sufficiently rapid switch-off. This is where Q7 comes into play. The 100pF capacitor discharges via the 220kΩ resistor until the base of transistor Q7 goes 0.7V below its emitter. Q7 then switches on to quickly discharge the gate ca­pacitance of Q3 and ensure a rapid turn-off. As mentioned in the description of Fig.8: mounting details for the Mosfets. Note that Mosfets Q1 & Q2 are also fitted with a finned heatsink. February 1994  37 PARTS LIST 1 plastic instrument case, 200 x 155 x 65mm 1 aluminium panel, 195 x 63 x 2mm 1 Dynamark front panel label, 195 x 63mm 1 PC board, code 11309931, 171 x 141mm 1 finned heatsink, 55mm long x 105mm wide (Altronics Cat. H-0522 or equivalent) 1 5AG panel mount fuseholder 1 30A, 5AG fuse 1 panel mount SPST rocker switch 1 5mm LED bezel 1 miniature mains power point (Clipsal NO.16N or equivalent) 1 30A red battery clip 1 30A black battery clip 3 cable ties 2 cord-grip grommets for 3.5mm dia. wire 3 ring type crimp lugs (blue, 4mm stud) 4 TO-220 mica washers plus insulating bushes plus screws & nuts 2 TOP-3 mica washers plus insulating bushes plus screws & nuts 2 Philips ETD34 ferrite transformer cores (2 off 4312 020 37202) (T1) 1 Philips ETD34 coil former (1 off 4322 021 33852) 2 Philips ETD34 mounting clips (2 off 4322 021 33892) 5 Philips RCC6.3/3.8/2.5 3F3 ring cores (5 off 4330 030 34971) (T2-T6) 5 3mm dia. machine screws, nuts & star washers 5 6BA nylon screws & nuts Wire & cable 1 1.5m length red heavy duty cable (41 x .32mm, DSE Cat. W-2286 or equivalent) 1 1.5m length black heavy duty cable (41 x .32mm, DSE Cat. W-2288 or equivalent) 1 200mm length blue 10A 240VAC mains wire 1 200mm length brown 10A 240VAC mains wire 1 150mm length red hookup wire 1 150mm length blue hookup wire 1 1m length 1.25mm dia. enamelled copper wire 1 16m length 0.4mm dia. enamelled copper wire 1 300mm length 0.8mm dia. enamelled copper wire 1 500mm length 0.8mm dia. tinned copper wire 1 1m length 0.2mm dia. enamelled copper wire the block diagram, Q3 and Q6 switch on and off together and Q4 and Q5 switch on and off together. Consequently, their respective transformers (T3 and T6 and T4 and T5) are driven together. However, each pair of trans­ formers is connected out of phase on the PC board to provide even loading on the transformer drivers. In order to drive the T3-T6 transformers, we need to produce bursts of 1MHz signal every 10ms but only for 70% of the time; ie, for 7ms. In addition, the bursts need to be directed alter­nately to T3 and T6 for one 10ms period and to T4 and T5 for the second 10ms period. Five ICs produce the requisite 50Hz bursts of 1MHz signal. IC6 is a 7555 timer connected to oscillate at 1kHz and it drives IC8, a 4017 decade counter with 10 decoded outputs. The 5, 38  Silicon Chip Semiconductors 1 TL494 switchmode controller (IC1) 1 4050 CMOS hex buffer (IC2) 2 LM358 dual op amps (IC3,IC5) 1 IL300 linear optocoupler (IC4) 2 7555 CMOS timers (IC6,IC7) 1 4017 CMOS decade counter decoder (IC8) 1 4023 CMOS dual D-flipflop (IC9) 1 4023 CMOS triple 3-input NAND gate (IC10) 2 BUK436-100A N-Channel Mosfets (Q1,Q2) Philips 4 BUK655-500B N-Channel FRED FETs (Q3-Q6) Philips 4 BC557 NPN transistors (Q7-Q10) 3 BC338 NPN transistors (Q11,Q13,Q15) 3 BC328 PNP transistors (Q12,Q14,Q16) 19 1N4148, 1N914 switching diodes (D1,D2,D7-D23) 4 BYW95C 600V 3A fast diodes (D3-D6) Philips 2 75V 400mW zener diodes (ZD1,ZD2) 4 15V 400mW zener diodes (ZD3-ZD6) 1 12V 400mW zener diode (ZD7) 1 5mm red LED (LED1) Capacitors 1 2200µF 16VW PC electrolytic 1 1000µF 25VW PC electrolytic 1 100µF 385VDC electrolytic (Philips 2222 052 58101) 2 10µF 16VW PC electrolytic 2 2.2µF 100V MKT polyester 14 0.1µF MKT polyester 1 0.0047µF MKT polyester 2 0.001µF MKT polyester 4 560pF MKT polyester 1 220pF ceramic 1 150pF ceramic 4 100pF ceramic Resistors (0.25W 1%) 1 1.2MΩ Philips VR37 3 1MΩ 7 10kΩ 1 390kΩ 2 4.7kΩ 4 220kΩ 1 3.3kΩ 1 150kΩ 1 2.2kΩ 1 56kΩ 2 1kΩ 2 47kΩ 1 820Ω 1 22kΩ 1 120Ω 1 15kΩ 3 10Ω 1 12kΩ Miscellaneous Insulating tape, heatsink compound 6 and 7 counts of IC8 are ORed with diodes D17, D18 and D19, so that the input pins to NAND gate IC10a are high whenever pins 1, 5 or 6 of IC8 are high. These three outputs are high for three counts in 10 or for 30% of the time. Consequently, after inver­sion by gate IC10a, the output is high for 70% of the time, which is what we want. IC10a drives pins 8 and 11 of gates IC10b and IC10c. Pins 1 and 13 of IC10b and IC10c respectively connect to the complemen­tary outputs of IC9, a 4013 D-flipflop. This flipflop toggles its Q and Q-bar outputs each time it receives a clock pulse from pin 5 of IC8. This occurs every 10ms. The remaining inputs of IC10b and IC10c connect to a 1MHz oscillator, IC7, another 7555 timer. IC10b and IC10c can only pass the 1MHz signal when their other two inputs are both high. This occurs 70% of the time for each alternate 10ms period. For example, the output of IC10b passes the 1MHz signal during one 10ms period and the IC10c output passes the 1MHz signal for the second 10ms period. The output (pin 9) of IC10b is buffered by complementary transistors Q11 and Q12 to drive T4 and T5 via separate 560pF capacitors. Similarly, the pin 10 output of IC10c is buffered by Q13 and Q14 to drive T3 and T6 via separate 560pF capacitors. Let’s now recap on the circuit operation. Mosfets Q1 and Q2 are driven by IC1 at 25kHz to step up the 12V to 340V DC which is regulated and otherwise current limited. Then the H-pack Mosfets are switched to provide a 50Hz modified square wave with an output close to 240VAC RMS. Power for the circuit is obtained from the 12V battery via a 30-amp fuse which supplies the inverter transformer T1 direct­ly. The low current part of the circuit is then supplied via switch S1 and a 10Ω decoupling resistor. A 2200µF capacitor across the supply ensures that the heavy switching currents to the DC-DC converter do not produce voltage fluctuations. A LED connected across the supply in series with a 2.2kΩ resistor indicates when power is on. Construction The 200W Inverter is housed in a plastic instrument case measuring 200 x 155 x 65mm. Most of the circuit components are mounted on a PC board which measures 171 x 141mm (code 11309931) – see Fig.5. Construction of the inverter involves winding several coils and a transformer, assembling the PC board and a small amount of hole drilling and wiring. Note: we do not recommend this project to inexperienced kit builders. Construction can begin by checking the PC board against the published pattern. Look for any broken tracks or shorts and repair any faults now to avoid problems with the circuit opera­ tion later on. Note that 3mm holes should be drilled for the battery supply connections adjacent to transformer T1. If these are not drilled, drill them now. Solder a 3mm brass nut underneath each of these holes, on the copper side of the board. The PC stakes and links can now be installed. Note that there are three types of links and it is important to install them in the correct positions. 0.8mm enamelled copper wire is used for the high voltage sections of the circuit to help provide greater safety since they present less chance of accidental contact when the circuit is running. Most enamelled copper wire is selfflux­ing, meaning that the enamel will strip under heat from a solder­ing iron. However, make sure that each solder joint is a good one. Now all the ICs, resistors and diodes can be inserted. Note that resistor R0 should not be installed at this stage, as it may not be required. More about this point later. Be careful with the orientation of the ICs and diodes and be sure to insert the correct type of zener diode in each position. Now insert the transistors, noting that there are three different types, so be careful to place them in the correct positions. Insert all the capacitors, taking care with the orien­tation of the electrolytics. This waveform shows the 7.5ms bursts of 1MHz signal from pins 9 & 10 of IC10. These signals are fed to the toroid isolating trans­formers, rectified & used to turn on the H-pack Mosfets. This oscilloscope photo shows the gate drive signals to Mosfets Q1 & Q2. Top trace is gate of Q1; lower trace, gate of Q2. Note the time interval between the respective gate pulses to Q2 & Q2, to ensure “dead time”. Winding the coils Transformer T1 is wound using 1.25mm diameter enamelled copper wire. Fig.6 shows how it is done. Locate pins 1, 2, 3 and 4 of the transformer bobbin and terminate four wire ends to these pins. Wind the four wires together (ie, quadrifilar winding) and make three turns. Terminate the wire ends at pins 14, 13, 12 and 11. Insulate the winding with a layer of paper and a layer of insulating tape. Now the secondary is wound on with 0.4mm enamelled copper wire. Terminate one end of the wire to pin 7 and wind on 115 turns neatly, side by side. Insulate between each layer with insulating tape before winding the next layer and make sure that each layer is wound in the same direction as the last. Finally terminate the wire end on pin 8. That completes the secondary winding. The transformer is assembled by in- This is the 240VAC output waveform from the inverter when driv­ing a 160 watt lamp load. Note that this wave shape changes very little, regardless of the load. serting the ferrite cores into each end of the bobbin and fitting the clips at the ends to hold them in place. Check that the faces of the ferrite cores are absolutely clean before assembling them. Toroids T2 and T3-T6 are each wound using 0.2mm enamelled copper wire, as shown in Fig.7. Each February 1994  39 RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 3 1 4 1 1 2 1 1 1 7 2 1 1 2 1 1 3 Value 1MΩ 390kΩ 220kΩ 150kΩ 56kΩ 47kΩ 22kΩ 15kΩ 12kΩ 10kΩ 4.7kΩ 3.3kΩ 2.2kΩ 1kΩ 820Ω 120Ω 10Ω winding is wound tightly with the wires as close together as possible. Keep the two windings separate to ensure electrical isolation between them. T3, T4, T5 and T6 must be wound identically. Final PC board assembly Transformer T1 and the toroid coils can now be installed. When inserting T1, make sure that it is oriented correctly. The 1.25mm diameter primary winding end must be adjacent to Mosfets Q1 and Q2. The toroids are secured with Nylon screws and nuts. Do not use metal screws since they will reduce the isolation between the primary and secondary windings. Be sure to orient the toroids correctly on the PC board; ie, the 12-turn secondaries should be adjacent to the associated 220kΩ resistors. Mosfets Q1-Q6 can now be inserted into the PC board and sol­dered. The lead length for each Mosfet should be 10mm. Position the PC board in the case and check the four inte­gral standoffs used to support the PC board in place. Use a large drill to shorten all the unused standoffs so that the PC board will sit neatly in position. Secure the PC board in place with self-tapping screws and slide the rear metal panel into its slot. Mark out the positions for the Mosfet mounting 40  Silicon Chip 4-Band Code (1%) brown black green brown orange white yellow brown red red yellow brown brown green yellow brown green blue orange brown yellow violet orange brown red red orange brown brown green orange brown brown red orange brown brown black orange brown yellow violet red brown orange orange red brown red red red brown brown black red brown grey red brown brown brown red brown brown brown black black brown holes on the rear panel. Drill these holes to suit the 3mm mounting screws. While you’re at it, drill and file the two cord grip grommets and the earth lug (3mm). The finned heatsink is also retained with four screws and nuts, two at the top and two at the bottom edge. Drill the necessary holes in both the rear panel and heatsink and the holes in the heatsink for Mosfets Q1 and Q2. The heatsink fins will also need drilling out with holes large enough for the screw heads for Mosfet Q3 and the earth lug. Remove any burrs around the holes, particularly where the Mosfets mount, to prevent punch-through of the mica insulating washers. You will need to secure the earth terminal screw and the screw for Q3 with nuts before attaching the heatsink to the rear panel. This is because these screws cannot be inserted once the heatsink is on. Apply a smear of heatsink compound between the mating faces of the heatsink and rear panel to ensure good heat transfer. Fig.8 shows the mounting details for each of the Mosfets (Q1-Q6). They need to be isolated from the panel with a mica washer and insulating bush. When you have tightened down the screw and nut, set your multi­meter on a high “Ohms” range and check that the metal tab of each device is indeed isolated from the rear panel and heatsink. 5-Band Code (1%) brown black black yellow brown orange white black orange brown red red black orange brown brown green black orange brown green blue black red brown yellow violet black red brown red red black red brown brown green black red brown brown red black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown red red black brown brown brown black black brown brown grey red black black brown brown red black black brown brown black black gold brown Work can now be done on the front panel. Use the front panel label as a guide to positioning the 30-amp fuse holder, switch, LED bezel and mains socket. Drill out the holes for each of these, then affix the label and cut out the holes with a reamer and sharp knife. Secure the fuse holder, switch, LED and LED bezel and the mains socket to the front panel, ready for wiring. Follow the wiring diagram carefully and use the correct wire, as specified. If the two cordgrip grommets do not grip the wires secure­ly, use some heatshrink tubing to increase the wire diameter. Do not use one grommet to secure both wires since there is a pos­ sibility that the wires may short out. The heavy duty hook-up wires (41 x 32mm) from the negative terminal of the battery and the fuseholder are fitted with crimped lugs and then secured with screws to the PC board (these screws go into the nuts previously soldered to the underside of the board). Use cable ties to tidy up the wiring when completed. Fit the battery leads with 30A battery clips. Testing Warning! Exercise extreme caution when doing measurements on this inverter. The voltages can be lethal. Use only one hand and do not touch any part of the circuit, particularly if Fig.9: actual size artwork for the PC board (code 11309931). Check your etched board for defects by comparing it against this pattern & correct any defects before installing the parts. you have connected an oscilloscope earth lead. Always check the voltage between TP1 and TP2 and wait until the voltage dies to a safe level (less than 30V) before touching any part of the circuit. Before applying power, check your work carefully and verify that your wiring and parts layout is the same as the wiring diagram of Fig.5. For the initial tests, it is best to use a 12VDC power sup­ply. Connect the +12V to switch S1, on the same side that LED 1 connects (ie, we don’t want power applied to T1 or to Mosfets Q1 & Q2). With switch S1 off, apply power. Check that +11.4V is present at the supply pins of all the ICs; ie, pins 8,11 &12 of IC1, pin 1 of IC2, pin 8 of IC3 and IC5, pin 6 of IC4, pins 4 & 8 of IC6 and IC7, pin 16 of IC8 and pin 14 of IC9 and IC10. There should also be 12V across ZD7. A DC measurement across ZD3, ZD4, ZD5 and ZD6 should show about 5.4V. Similarly, between ground and the gate of Q1 and ground and the gate of Q2 should show about 5.0V. If you have an oscilloscope, the wave­forms in the accompanying oscilloscope photographs should be compared. If all these tests check out OK, you are ready for a high voltage test. Disconnect the 12V supply used for initial testing although, if it can deliver 8 amps or more, it can be used for the initial high voltage tests too. Rotate trimpot VR1 fully anticlockwise. This will set the high voltage to a minimum. Place the lid on the inverter and connect it to your 12V supply or 12V battery. Switch on S1 brief­l y and then turn off. The reason for having the lid on the in­verter at initial switch-on is that if there is something wrong with the high voltage side of the circuit, one or more components may blow. So the lid on the inverter will protect your eyes! Alternatively, wear eye protection goggles. Now take off the lid and remember that the circuit is now dangerous. Check the DC voltage between test points TP1 and TP2. It should be above 100V DC but falling. Do not touch any part of the circuit until the voltage drops to a safe level (below 30V). Now apply power again and check the voltage between TP1 and TP2. Watch the meter and adjust VR1 slowly until the voltage is set at 340V DC. You can now install the lid and load test the unit. Check that it will drive 240VAC light bulb loads up to 200W. If the fuse blows when powering a 200W load, the R0 (1MΩ) resistor should be installed to slightly increase the dead SC time for IC1. February 1994  41