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Build this 1.5V to 9V DC converter get it going and can be operated from an AA, C or D-size cell. What's more, the TL496 only costs about $2. Sick of how quickly your 9 volt batteries go flat? Then switch over to more cost-efficient 1.5 volt cells with this 1.5V to 9V DC converter. It uses just three components and fits on a small PC board. Block diagram By DARREN YATES There's nothing worse than when you're using a piece of electronic equipment and you're just at a crucial moment when suddenly the rotten 9-volt battery goes flat! They're not cheap either. Don't you just hate paying the prices for a 9-volt alkaline or even just carbon-zinc 9V batteries? And what about the price of 9-volt nicads? Some retailers are charging more than $24 for these things, and when you consider they're only rated for lO0mA.h, that's pretty pricey. Even when you take into consideration the fact that they are rechargeable, you have to wait for up to 16 hours before you can use them again! As the man said, "there has to be a better way"! Well, there is, thanks to the Texas Instruments TL496. The TL496 is a very compact DC step-up switching converter IC which provides a regulated 9V DC output from a 1.5V DC input. It only requires a filter capacitor and an inductor to ~ The block diagram in Fig.1 shows the internal operation of the TL496 IC. An input voltage can be applied to either pins 2, 3 or 4, but each has a separate purpose. If you want to use just one 1.5V battery, then you apply this to pins 2 and 3. Or you may prefer to use two 1.5V cells in series, in which case you just connect them to pin 2 and leave pin 3 floating. Both ways have their benefits. If space is a problem, one cell definitely makes more sense, but the tradeoff is more current drain. Because it's stepping the voltage up six times (from 1.5 to 9V) and because the IC is not perfectly efficient, it uses about eleven times the current we get at the output; eg, we may get lOmA at the output but the circuit consumes 114mA from the battery. If you want to talk about efficiency ratings, the circuit is about 50% efficient. Even so, it is a much cheaper way of powering a circuit from 9V than to use 9V batteries. If you were to use two cells ·in series, the IC needs to step the voltage up three times and so requires only about six times the current. This doesn't change the efficiency of the circuit - it just means that two cells will last about twice as long as one cell. Our circuit uses the single cell option which we think is the most economical overall. High frequency oscillator This 1.5V to 9V DC converter is incredibly simple, thanks to the use of a dedicated switching converter IC from Texas Instruments (the TL496). There are just three components on the board, plus four PC stakes! The circuit can be housed in a separate case or built into the equipment to be powered. 72 SILICON CHIP Returning to the block diagram, the switching voltage regulator control uses a high-frequency oscillator to drive the output transistor, which has an inductor connected between its collector (pin 6) and the supply input. T INPUT (4) 2C INPUT (3V) (2) 1C INPUT (1.5V) (3) ELECTRONICS WORLD 9V SERIES REGULATOR SWITCHING VOLTAGE REGULATOR CONTROL 1----- NOVEMBER SPECIALS (6) SWITCH USED 'AA' 700mAH NICAD BATTERIES $ 0.50 TEMPERATURE CONTROLLED SOLDERING STATION $135.00 GND (5) GND (7) UNIVERSAL NICAD BATTERY CHARGER $ 29.95 Can do 4 at a time (AA, C, D, or 9V Nlcads) Fig.1: block diagram of the TL496 switching converter IC. It uses a high frequency oscillator to drive a switching transistor. D1 REVERSE BIASED 40 CHANNEL AM CB RADIO $ 79.00 + + VIN Fig.2: how a switching regulator works. When S1 is closed, current flows & energy is stored in the inductor. Note that diode D1 is reverse biased during this time. There may be some readers for whom switching regulators are new territory, so let's briefly go over the principles of operation. Basic principles If you take a look at Fig.2, we have an inductor, a switch, a diode and a capacitor. The inductor represents our coil, switch Sl takes the place of the output transistor in the IC, the diode is the zener diode shown in-the block diagram of Fig, 1, and the ea pacitor is our output capacitor. When the switch is closed (corresponding to the transistor being turned on in the IC), a current flows through the inductor as it stores energy. The anode side of the diode is now effectively connected to ground, so it is reverse biased and no current flows through to the load. In Fig.3 , we open the switch again to stop the current flow but the inductor tries to maintain the current. The voltage across the inductor rises sharply as a result of this. The diode now becomes forward biased and the inductor dumps its stored energy into the capacitor. The TL496 has an internal oscillator which drives the output transis- I 1 -0 VOUT 0- Fig.3: when S1 is opened, the voltage across the inductor rises. D1 is now forward biased & so the inductor dumps its stored energy into the capacitor. tor, switching it on and off at a rate which depends on the load current. The higher the load current, the higher the switching frequency. At any particular load current, the switching rate is not an absolutely steady frequency though; it hunts back and forwards. In fact, what actually happens is that the internal transistor is always turned on for roughly the same period of time, around 0.3 milliseconds. Then, depending on the load current, the switching rate can be anywhere from a few Hertz up to around ZkHz. SMOKE DETECTOR $ 49.95 PIEZO SIREN WITH BACKUP BATTERY $ 59.95 Ideal for Car Alarms DIGITAL DISPLAY AM/FM STEREO CAR RADIO $ 49.95 PORTASOL GAS SOLDERING IRON $ 35.00 ARLEC SUPER TOOL KIT $ 69.50 LOGIC PROBE $ 35.00 SCANNER FANATICS FREQUENCY REGISTER OF VIC. $ 24.95 PHILIPS INFRARED REMOTE CONTROL $ 35.00 COMMON CATHODE 7-SEGMENT DISPLAY $ 1.00 VIDEO DUBBING KIT $ 15.95 ARLEC 2 SPEED CORDLESS DRILL AND SCREWDRIVER $ 69.95 2-CHANNEL FM WIRELESS INTERCOM $ 89.95 SINGLE CHANNEL UHF TRANSMITTER KIT $ 18.00 SINGLE CHANNEL UHF RECEIVER KIT $ 34.90 VULTURE CAR ALARM KIT $ 39.90 ,c 2 2C GNO 1.5V T~~4S 8 OUT>-- - - - . . - , +9V GNO 470 16VW OUTPUT .___ _..___ _ _ _ _ _ _ _ _-<lOV * l 1 : 5JT .0.6J mm OIA . ENCU WIRE WOUNO ON A NfO SIO 17 747 10 TOROIO 1.SV TO 9V DC CONVERTER Fig.4: the complete circuit for the 1.5V to 9V converter. Note that you can also use two 1.5V cells in series, in which case the connection to pin 3 is deleted. INFRA RED NIGHT VIEWER KIT $239.00 TALKING ELECTRONICS ULTIMA FM BUG KIT $ 12.50 M ail Orders Welcome 30 Lacey St, Croydon VIC, 3136. Telephone: Fax: (03) 723 3860 (03) 723 3094 (03) 725 9443 NOVEMBER 1990 73 I I 7 uF 0 _... 9V --- Fig.5: inductor L1 consists of 53 turns of 0.6mm enamelled copper wire on a Neosid toroid core. Fig.6 at right shows the full size PC pattern. The maximum output current from the circuit is about 40 milliamps. At this current, a typical 9V battery would not la.st long at all. By contrast, a 1.5V D-cell will last for about 20 hours at this output current. Circuit diagram The circuit diagram in Fig.4 shows how few components are required one IC, one capacitor, one inductor and that's it! The inductor is a Neosid 17-742-10 toroid core, wound with 53 turns of 0.6mm diameter enamelled copper wire. The exact number of turns is not really important since the IC selfregulates. So as long as there are somewhere between 45 and 60 turns, the circuit will work. The efficiency will tend to vary by a small amount depending on the number of turns, but we found a figure of 53 turns to be about optimum. The 470µF filter capacitor is used to smooth the DC output on pin 8. The ripple output is generally around 50mV peak-to-peak, except at very PARTS LIST 1 PCB, code SC11111901, 60 x 38mm 1 TL496CP 9V switching inverter (IC1) 1 470µF 16VW electrolytic capacitor 1 Neosid 17-742-10 toroid core, 28mm OD, 15mm·ID 2 metres of 0.6mm enamelled copper wire 4 PC pins 1 AA, C or D size 1.5V battery 1 holder to suit battery Miscellaneous Solder, hookup wire, etc. 74 SILICON CHIP :::1 ~: -21 .,.... .,.... Constructing the DC Converter should only take about half an hour. We have designed a small board for the job. It is coded SC11111901 and measures 60 x 38mm. Whether you buy or make the PC board, check that there are no shorts or breaks in the tracks. If there are, touch them up now before you do any soldering. Begin the assembly by installing the four PC pins. These are used to connect up the 1.5V supply and the 9V DC output. Next, wind the inductor. This is the most time-consuming step in the assembly process. You will need just over 2 metres of 0.6mm enamelled copper wire. Winding the wire is a matter of threading the wire through for the required number of turns (53). Be careful not to kink the wire as you do the job. Make sure the turns are reasonably tight and spread evenly around the core. Bring both ends of the finished winding to the same spot so they can be easily soldered into position. Clean the two ends of the winding of enamel, by scraping it off with an old razor blade or utility knife blade. This done, tin the ends with solder, position the toroid on the board and solder the leads to the board. You will also need to use an anchor wire to stop the inductor from moving about on the board. The anchor wire is soldered to two unconnected pads on the board. Next, solder in the 470µF capacitor. Remember to check that its polarity is correct - the negative pin should go towards the inductor. Finally, solder in the TL496 IC. Again, make sure of the correct polarity - pin 1 (indi- 0 ,,- ,- .,.... u(f) low load currents. It can be reduced by substituting a larger capacitor. Construction 0 0 O'l TABLE 1 LOAD CURRENT <at>9V OUTPUT INPUT CURRENT <at>1.5V no load 0.1mA 0.5mA 1mA 2mA 5mA 10mA 20mA 40mA 50µA 1.3mA 5.2mA 11.7mA 25mA 57mA 114mA 230mA 460mA cated by the dot on top of the IC) should be closest to the outer edge of the board and furthest away from the capacitor. Finally, wire up the battery holder to the board, making sure that the polarity is correct. Insert the battery and then measure the output voltage from the board. It should be close to 9 volts DC. The completed board and the battery holder can be housed 'in a small plastic zippy case or, in some instances, built into the equipment it is to power. Don't forget to cut the track to pin 3 of the TL496 if you intend using two 1.5V cells in series. Current loads As mentioned before, depending upon your situation you can use either AA, C or D-size batteries with this circuit. Table 1 shows the expected load and input currents. Ideally, if you require large input currents, say more than 100mA, use a C or D- size cell for best economy. And if you use an alkaline cell rather than a carbon zinc type, it will last considerably longer. ~