Fullscreen HTML5Med Res (??MB)HTML4

Build this +5V to ±12V DC converter This low-cost project uses only junkbox components to convert a +5V DC supply to ±12V DC rails (24V total) capable of supplying up to 100mA. What’s more, you can easily change it to provide other output voltages. By DARREN YATES The most convenient way to power most projects is to use a DC plugpack supply. These little “black boxes” provide a single fixed DC rail and they usually have quite a bit of grunt as well. Most plugpacks can supply 300mA or more which is more than ade­quate for most projects. But what if your project requires dual (ie, positive and negative) supply rails? These are unavailable from plugpack sup­plies and you have to resort to using an AC supply, a bridge rectifier, filter capacitors and positive and negative voltage regulators instead. This approach can sometimes be inconvenient and causes unnecessary expense if you already have a DC plugpack or some other DC supply; eg, a car battery or solar panel. Fortunately, there is a way around this problem and that’s where this project will be of use. It’s a simple converter that can produce ±12V supply rails (100mA max.) from any 5-10V DC supply. In addition, you can easily adjust the circuit to produce lower output voltages and each supply rail can be adjusted inde­pendently of the other. The only proviso is that the input vol­tage must be lower than the output voltage. You can also use the circuit to stepup the DC input vol­tage to a much larger single supply rail. For example, you can derive a 24V rail simply by connecting across the ±12V rail, or you can connect between either supply rail and ground. Block diagram Fig.1 shows the block diagram of the ±12V Converter. As you can see, it uses a master oscillator and this produces two anti-phase pulse waveforms. Each anti-phase waveform is then fed to a switching inductor driver circuit. These switching driver circuits step up the input voltage to produce the positive and negative output rails. In addition, each driver circuit is fitted with a supply regulator so that the master oscillator is not disturbed while it is running. Circuit diagram Let’s now take a look at the complete circuit diagram – see Fig.2. Transistors Q1 and Q2 are connected as a standard astable multivibrator and this forms the anti-phase pulse waveform generator. The associated 470pF and .0022µF capacitors determine the duty cycle of the waveform and set the frequency of oscill­ation to approximately 13.3kHz. In practice, the frequency is not all that important, as long as it is somewhere in the vicinity of 1215kHz. The two output sig- The converter uses only low-cost parts & can be powered from any 5-10V DC source. It provides both positive & negative supply rails up to ±15V & the output voltages can be varied by changing two zener diodes. 34  Silicon Chip nals are taken from the collectors of Q1 and Q2 and fed to the supply driver circuits via 22kΩ resistors. The positive supply driver circuit is based on transistors Q3-Q5, while the negative driver circuit uses transistors Q6-Q8. Since these two driver circuits are different, we’ll go through them one at a time. Starting with the positive rail, Q4, Q5 and their associat­ed parts function as a step-up voltage converter. In operation, the pulse waveform from Q1’s collector is fed to the base of Q4. This signal has a duty cycle of approximately 20%; ie, the output is high for 20% of the time and low for the remaining 80%. Q4 acts as an inverter and thus drives Q5 with a high-duty pulse waveform. However, as we’ll see later, this part of the circuit can be disabled by the voltage regulation circuitry. Q5 is a TIP122 Darlington NPN transistor and this switches inductor L1 on and off. When Q5 is on, current flows through L1 and energy is stored in the inductor. During this time, diode D1 is reverse biased since its anode is effectively connected to ground. When Q5 subsequently switches off, the collapsing magnetic field asso­ciated with the inductor tries to maintain the current through it and so the voltage across the inductor rises. D1 now becomes forward biased and so the inductor dumps its stored energy into a 470µF reservoir capacitor. This capacitor is used to smooth the DC output to the load. Voltage regulation As well as supplying the load, the output voltage is also applied to zener diode ZD1 via a 4.7kΩ resistor. This part of the circuit, in conjunction with Q3, forms the voltage regulator for the positive rail step-up converter. The voltage regulation works like this: as the voltage across the 470µF output capacitor rises from 0V, Q3 will initial­ ly be off and ZD1 will be non-conducting. This allows the signal from Q1 to operate the step-up circuitry as normal. However, as the output voltage rises, ZD1 eventually breaks down and clamps Q3’s base to 12V. Q3’s emitter continues to rise though, which it does for about another 0.6V (ie, it rises to about 12.6V). At this point, Q3 turns on and pulls Q4’s base high, thus turning SUPPLY REGULATOR POSITIVE SUPPLY DRIVER SUPPLY INPUT MASTER OSCILLATOR GND NEGATIVE SUPPLY DRIVER Fig.1: block diagram of the ±12V converter. It uses a master oscillator to drive positive & negative step-up converter circuits. SUPPLY REGULATOR +5-10V 4.7k Q3 BC558 10k B 2x1N4004 E Q4 BC558 B C ZD1 12V 400mW 47k D1 FR104 E Q5 TIP122 C 470  47k +12V OUT C B 470 16VW E 1k +5-10V 4.7k L1 D5 D4 4.7k 470pF .0022 22k GND Q2 C BC548 B Q1 BC548 B E C +5-10V 470 16VW E 1k Q7 BC548 22k B B 10k E C VIEWED FROM BELOW B C D3 1N4004 470  B D2 FR104 C E E C L2 -12V OUT 470 16VW E 4.7k ZD2 12V 400mW B CE Q6 BC548 Q8 TIP127 L1-L2 : 60T, 0.4mm DIA ECW ON NEOSID 17-732-22 ñ12VCONVERTER CONVERTER ±12V Fig.2: Q1 & Q2 form the master oscillator, while Q4, Q5 & inductor L1 function as a switching converter to step up the supply for the positive output. Similarly, Q7, Q8 & L2 function as a switching regulator which provides the negative output. Zener diodes ZD1 & ZD2 set the output voltages. Brief Specifications Input supply ............................................................ +5 to +10V DC Maximum output ..................................................... ±15V DC Maximum output current......................................... 100mA at ±12V Efficiency................................................................. 50% (approx). Quiescent current.................................................... 50mA (5V DC supply) September 1993  35 Semiconductors 4 BC548 NPN transistors (Q1,Q2,Q6,Q7) 2 BC558 PNP transistors (Q3,Q4) 1 TIP122 (or BD679, BD681) NPN Darlington transistor (Q5) – see text 1 TIP127 (or BD680, BD682) PNP Darlington transistor (Q8) – see text 2 FR104 fast-recovery 1A diodes (D1-D2) 3 1N4004 silicon diodes (D3-D5) 2 12V 400mW zener diodes (ZD1-ZD2) Capacitors 3 470µF 16VW electrolytics 1 .0022µF MKT polyester 1 470pF MKT polyester Resistors (0.25W, 1%) 2 47kΩ 4 4.7kΩ 2 22kΩ 2 1kΩ 2 10kΩ 2 470Ω GND +OUT -OUT 470uF L1 470  1k Q4 ZD1 4.7k Q3 D1 L2 Q5 D2 Q8 1k 22k .0022 4.7k 47k 4.7k 47k Q2 Q1 470pF 470  Q7 Q6 10k 4.7k 1 PC board, code 11109931, 102 x 57mm 2 14.8mm OD Neosid 17-732-22 toroidal cores 1 3-metre length of 0.5mm diameter enamelled copper wire 5 PC stakes The negative rail is derived in a similar fashion, the main difference being that everything is reversed; ie, NPN transistors are swapped for PNP devices and vice versa. In this case, the drive signal appears at the collector of Q2 and is fed to the base of Q7. Unlike the signal fed to Q4, this signal has a duty cycle of 80%. Q7 in turn drives PNP Dar­lington transistor Q8, while the associated inductor (L2) is connected between Q8’s collector and ground. As before, the inductor tries to maintain the current through it when its associated switching transistor (Q8 in this case) turns off. The difference here is that the voltage on the collector goes negative instead of positive, which is why fast-recovery diode D2 and the 470µF filter capacitor are connected the other way around. Zener diode ZD2 and transistor Q6 make up the voltage regu­lator for the negative rail. Q6 remains off until the output voltage drops below about -12.6V. At this point, Q6 turns on and pulls the base of Q7 to -0.6V, thus turning Q7 and Q8 off. The voltage on the negative rail now rises towards 0V and when it rises above -12.6V, Q6 turns off again and the converter circuit restarts. Diode D3 protects Q7 by preventing its base from going any lower than -0.6V when Q6 turns on. If it wasn’t for 470uF D5 D4 Negative rail PARTS LIST GND +IN 470uF 10k This process is repeated indefinitely while ever power is applied and thus keeps the output regulated to +12.6V, as set by ZD1. Diode D4 protects Q4 by clamping its base to the supply rail when Q3 switches on. Thus, if the supply rail is +5V, Q4’s base will be clamped to +5.6V when Q3 turns on, regardless of the output voltage. D5 ensures that Q4 turns off completely when its base is pulled high. 22k Q4 off and disabling the voltage stepup circuit. The output voltage across the 470µF capacitor now decreases due to the load current. However, as soon as it drops below about 12.6V, Q3 turns off again and releases the high on Q4’s base. This allows the voltage step-up circuit to restart and so the output voltage increases until Q3 turns on again. D3 ZD2 Fig.3: install the parts on the PC board as shown in this diagram. The two inductors are made by winding 60 turns of 0.5mm diameter enamelled copper wire onto a toriodal core. this diode, Q7’s base would be pulled almost to the negative output rail when Q6 turned on and this would destroy the transistor. Construction Building the +5V to ±12V Converter is quite straightfor­ward, since all the parts are mounted on a small PC board. This board is coded 11109931 and measures 102 x 57mm. Before you start any construction work, check the board carefully for any shorts or breaks in the copper tracks. Faults of this kind will be quite rare but it pays to make sure before mounting any of the parts. Fig.3 shows how to install the parts on the PC board. Begin by installing the five PC stakes at the external wiring points, then install the wire link, resistors and diodes. The accompany­ ing table lists the colour codes for the resistors but it’s also a good idea to RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 2 2 2 4 2 2 36  Silicon Chip Value 47kΩ 22kΩ 10kΩ 4.7kΩ 1kΩ 470Ω 4-Band Code (1%) yellow violet orange brown red red orange brown brown black orange brown yellow violet red brown brown black red brown yellow violet brown brown 5-Band Code (1%) yellow violet black red brown red red black red brown brown black black red brown yellow violet black brown brown brown black black brown brown yellow violet black black brown Protect your valuable issues Silicon Chip Binders Fig.4: check your PC board for defects by comparing it with this full size etching pattern before mounting any of the parts. The two inductors can be secured in position by gluing them to the board using epoxy resin or by pouring a little hot wax over them. To test the unit, you will need a power supply with an output of 5-10V DC and this should be connected to the board via your multimeter. Set the meter to the 2A range and make sure that you have the supply polarity correct before switching on. With no load connected, the current should be about 50mA for a 5V supply and about 30mA for a 10V supply. If the current drain is appreciably more than this, switch off immediately and check the board carefully for assembly errors. If everything is OK, disconnect your multimeter, select a suitable voltage range and check the output voltages. You should get a reading of about +12.6V for the positive rail and -12.6V for the negative rail. Changing the output The output voltage for each rail is set by its correspond­ing zener diode. You can alter these as you wish to give voltages other than ±12V, with the proviso that the input voltage must always be less than the desired output voltages. The output voltage is approximately equal to the zener diode voltage plus 0.6V for the positive rail, or the zener diode voltage minus 0.6V for the negative rail. For example, if ZD1 is rated at 13V and ZD2 at 15V, you will end up with +13.6V and -15.6V rails. Footnote: we would like to thank Adilam Electronics for supplying the FR104 fast-recovery diodes used in SC this project. These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. ★ High quality ★ Hold up to 14 issues ★ 80mm internal width ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A11.95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form ➦ check them on a digital multimeter, as some of the colours can be difficult to decipher. The diodes and transistors can be installed next. Make sure that you install these parts correctly. The FR104 fast-recovery diodes and the standard 1N4004 rectifier diodes look very simi­ lar, so make sure that you don’t get them mixed up. Similarly, be sure to use the correct transistor type at each location. Some of the transistors are NPN types while others are PNP types and they don’t take too kindly to being transposed. The TIP122 and TIP127 Darlington transistors (Q5 & Q8) come in TO-220 packages and must be oriented with their metal tabs as shown in Fig.3. The alternative BD679-BD682 Darl­ing­ton power transistors come in TO-126 packages. Take care with the lead connections for these transistors – they must be mounted with their metal surfac­es facing in the opposite direction to the TO-220 types. You have been warned! Finally, install the capacitors and the two inductors (L1 & L2) on the board. The two inductors are identical and are made by winding 60 turns of 0.5mm diameter enamelled copper wire on a 14.8mm outside-diameter Neosid toroidal core. Begin with a 1.5-metre length of wire and thread it half-way through the centre of the core. Now, using one half of the wire, wind on 30 turns as tightly and as neatly as possible. The other half of the wire is then used to wind on the remaining 30 turns. Once each inductor has been wound, strip and tin the wire ends, then solder the leads to the board. Enclosed is my cheque/money order for $________ or please debit my ❏ Bankcard   ❏ Visa   ❏ Mastercard Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ September 1993  37