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A safe, convenient multi-voltage supply for cars POWERPACK At last: a handy little project that will safely power just about any portable device from the lighter socket in your car. It can provide preset voltages of 3V, 6V, 6V. 9V & 12V. You can also use it to provide a well-regulated output from low-cost DC plugpacks. By PETER SMITH P owering electronic equipment of the alternator to respond instanta- transients, occur when the ignition from a vehicle’s electrical sys- neously to load changes. The response switch is turned off while current tem can be a risky business, time of an alternator is bound by the is flowing in inductive loads (windscreen wipers, alterespecially if the equipment nator field coil, etc.) wasn’t originally designed PowerPack Feature These are negative for in-car use. s  3/6/9/12V switcha in direction, with Large positive and negble output at 1A maxim um a similar energy to ative voltage transients  Operates from ca r cigarette lighter sock the positive swing occur regularly during et or DC plugpack  Protects sensitive devices from voltage tra of load dump tran“normal” operation of nsients  Automatic low batte sients. vehicle electrical systems. ry cut-out prevents ba ttery damage  Easy to read volta Switching spikes The alternator is unge selection from inductive loads doubtedly the main cullike windscreen wipprit. Load dump traners and power winsients, which occur when dows generate even heavy loads are switched higher voltages, as much as 200V off, can cause the alternator’s output to forces of mechanical inertia and the swing to as much as 100V for several long time constant of the excitation positive and negative. winding. milliseconds. These transients have much lower Other nasties, called field decay energy in comparison to load dumps This effect is caused by the inability MAY 2001  87 Opened-out view of the supply, immediately before final assembly. The hardest part is probably drilling the holes for the LEDs and cutting the slot for the switch – these must be done very accurately . However, National Semiconductor (and other companies) have developed versions specifically for the automotive market, and we’ve based this project around one of them – the LM2941 low dropout adjustable regulator. The LM2941 provides “out of the box” protection against line transients and reverse battery connection, as well all the familiar regulator features such as thermal and overload protection. How it does its stuff and field decay transients. Automotive regulator We’ve all seen those switchable plastic “regulators” that are either built into a cigarette-lighter plug or are housed in a small plastic case which plugs into the lighter. Believe it or not, some of the cheaper ones we’ve seen simply contain a resistive divider! They take a stab in the dark at the likely output current and assume “near enough is good enough”. It ain’t! Would you really trust your $200 personal CD player to one of these devices? Even the better ones with some form of regulation can’t cut the mustard. Standard linear 3-terminal regulators such as the 78XX series do not provide adequate protection in this environment. As you can see from the circuit diagram of Fig.1, there’s really not a lot to the PowerPack. Input voltage is applied to either CON1, the car input, or CON2, the plugpack input. Diode D1 provides reverse polarity protection on the plugpack input. A Schottky-type diode is used here to minimise forward voltage losses. Although REG1 incorporates reverse polarity protection, we’ve included D1 on the plugpack input Fig.1: the heart of the PowerPack is an LM2941 automotive regulator (REG1). We have combined it with a comparator to shut off the circuit for input voltages below 11.5V, to avoid excessive discharge of the car’s battery. 88  Silicon Chip Zener diode ZD1 forms a simple shunt regulator, powering IC1 with +5V, while the series LED5 gives a “power on” indication without additional current drain on the input. Diode D2 has been included solely to protect the pin 2 input of IC1 when insertion of the plugpack jack causes pin 2 to be pulled high via the 51kΩ resistor. Construction In order to squeeze everything into an easy-to-carry case, we’ve resorted to a rather unconventional mounting method for the PowerPack’s PC board. It simply sits atop the integral slots in the diecast case and is held in place by the lid and four acrylic feet. As the first step, check that the blank PC board rests snugly on top of the integral guides on all four sides 51k to protect the input filter capacitor tery was completely discharged, due as well. After all, you don’t want it to the PowerPack being inadvertently spewing its insides all over the PC left on indefinitely, it could damage board just because you accidentally the battery. got the supply connections wrong. In normal operation, IC1’s outA bi-directional transient suppresput (pin 1) is close to 0V, holding sor, TVS1, clamps all transients to the regulator in the ON state. If the less than ±150V, protecting the input voltage on pin 2 falls below that on capacitor somewhat and extending pin 3, the output at pin 1 swings to the inbuilt protection in the regulator +5V, switching the regulator off. The to well over ±1100V. switching, or “threshold”, point is set by the ratio of the resistors connected On the output side, a 220µF capacto pin 3. The 360kΩ resistor from pin itor provides the required filtering. 1 to pin 3 provides a small amount Unlike most other linear regulators, of hysteresis to prevent the output the ESR (equivalent series resistance) oscillating about the threshold point. of the LM2941 output capacitor is critical for stable regulator operation. Inserting a plug in the plugpack input (CON2) disconnects one end The output voltage is programmed of the 51kΩ resistor from the 0V line, by the ratio of the two resistors conforcing pin 2 of IC1 high and effectivenected to the ADJ pin (see “Getting ly disabling the cut-out circuit. This other output voltages” on page 93). allows use of 6V and 9V DC plugpacks Slide switch S1 allows selection of on the lower voltage selections. four different values for the top leg of the voltage divider, providing outputs of 3V, 6V, 9V and 12V. LEDs1 - LED4 give indication of the PLUGPACK selected voltage range. We’ve used INPUT a different value current limiting CON2 resistor with each of the LEDs so S1 as to keep the brightness roughPOWER ly equal at each setting. REG1 is a “low dropCON1 TVS1 out” regulator, meaning _ + 0.1mF in this case that we only D1 need about 0.5V (at 1A 100k load) more at the input 10k 1.8k D2 10mF than the output to mainF1 + tain regulation. For lower 22k current levels, the drop360k 100k LED1 1000mF out voltage is even less. 1 IC1 For example. with a load LM393 of 100mA, only 12.1V is 8.2k required on the car input A 12V K (CON1) to provide 12V at 680W + LED2 1 the output. 2 470W 3 9V Note that about 12.5V 4 5 S2 LED3 3.6k would be required on the 1k 110W plugpack input for the 270W 6V 1.3k same result, allowing for + LED4 56W the voltage drop across D1 68W and some ripple. 3V 470W 220W 220mF REG1 100mF 5.6k ZD1 1N 4148 + ACRYLIC FEET MOUNTED ON SOLDER SIDE (SEE TEXT) CABLE TIE CASE GROMMET _+ _+ OUTPUT IC1, an LM393 voltage comparator IC, forms the heart of the low battery cut-out circuit. It has been included to prevent discharge of the car’s battery below about 11.5V. If the battery was discharged below this level, there is a fair chance it will not be able to start the motor. And ultimately, if the bat- CON3 _ TO CIG LIGHTER PLUG Low battery cut-out LED5 Fig.2: use this diagram and the photo above as a guide when installing the components onto the PC board. Note the special comments in the text about mounting the 5-terminal regulator. MAY 2001  89 Parts List – PowerPack 1 PC board coded 11305011, 108mm x 59mm 1 115mm x 65mm x 30mm (LxWxH) diecast metal case (Jaycar Cat HB5036) 1 DPDT PC-mount miniature toggle switch (S1) (Jaycar Cat ST-0565) 1 DP4T miniature slide switch (S2) (Altronics Cat S-2040) 1 2.5mm PC-mount DC jack socket (CON1) (Altronics Cat P-0621) 2 2-way 5mm pitch miniature PC-mount terminal blocks (CON2, CON3) 2 M205 PC-mount fuse clips 1 M205 2A slow-blow fuse 1 Plugpack extension cable (DSE Cat M-9601) OR 1 Plugpack cable and 8 adaptor plugs (DSE Cat M-9603) 1 Cigarette lighter plug 4 Clear acrylic feet (DSE Cat H-1740) 1 3/16" x 5/16" rubber grommet (“Zenith” brand, from hardware stores) 1 2m medium duty 3.5A figure-8 cable 1 M3 x 6mm cheese head screw, nut and star washer Semiconductors 1 LM2941CT low dropout voltage regulator (REG1) (DSE Cat Z-6620) 1 LM393 dual comparator (IC1) 1 1N5822 3A 40V Schottky diode (D1) (Altronics Cat Z-0042) 1 1N4148 small signal diode (D2) 1 1.5KE33CA Transient Voltage Suppressor (TVS1) (Farnell Cat 166-492 or 752-307) 1 1N751A 5.1V 0.5W Zener diode (ZD1) (Altronics Cat Z-0314) 5 5mm high brightness red LEDs (LED1-5) (Jaycar Cat ZD-1793) Capacitors 1 1000µF 50VW PC electrolytic 1 220µF 25VW PC electrolytic 1 100µF 25VW PC electrolytic 1 10µF 25VW PC electrolytic 1 0.1µF 100V MKT polyester (Code 104 or 100n) Resistors (0.25W, 1%) 1 360kΩ 1 160kΩ 1 8.2kΩ 1 3.6kΩ 1 270Ω 1 220Ω 2 100kΩ 1 1.8kΩ 1 110Ω 1 51kΩ 1 1.3kΩ 1 68Ω 1 22kΩ 1 680Ω 2 56Ω 1 10kΩ 2 470Ω Miscellaneous 5cm 22AWG (0.71mm) tinned copper wire Cardboard for insulator Non-acidic silicone sealant of the case. Note that the copper side faces up, so the switch and LEDs will protrude through the bottom (which becomes the top!). If the board falls into the case on any side, it is undersized; compare it with the dimensions of the PC board pattern shown in Fig.7. Referring to the board overlay diagram in Fig.2, begin construction by installing the three wire links and all resistors. You will notice from the photographs that some resistors are mounted vertically instead of horizontally. These are identified on the overlay diagram by a circle (the body) and line. Next, install diodes D1, D2 and ZD1 90  Silicon Chip and the transient suppressor TVS1, taking care with their orientation. The three connectors CON1- CON3 should be mounted next, followed by toggle switch S1, slide switch S2 and the fuse clips for F1. Be sure that these components are seated squarely against the PC board before soldering. Note that the cable entry side of CON1 faces the adjacent 10µF capacitor. The five capacitors can be installed next. All the electrolytic types are polarised, so check their orientation carefully. The 1000µF capacitor is mounted horizontally, so bend its legs over at 90° (at about 3mm from the body) and align it as shown on the overlay diagram before soldering. To complete the first part of the assembly, install IC1, aligning pin 1 as shown on the overlay diagram. Now set the board aside for a moment and reach for your trusty drill! Pass the silver cheese, please Altogether, 10 or more holes need to be drilled in the case sides, ends and bottom. All holes should be marked with a sharp centre punch before drilling. For good results, start with a small drill size for the initial hole, then drill with several intermediate sizes before finishing with the indicated size. The easiest way to get everything in the right spot is to photocopy the drilling template (Fig.3) and label (Fig.6), cut the pieces out and tape to the indicated face of the case. The rounded edges of the case make exact alignment of the templates difficult – patience, patience! Centre punch each “hole” directly through the template, remove the template and drill. Make sure that the surface around the internal side of the regulator mounting hole is smooth and free from rough edges after drilling. If necessary, de-burr the hole. The slot for the slide switch (S2) can be made by drilling a series of holes inside the marked outline, then filing out with a fine jeweller’s file. Test-fit the PC board as you go to make sure that the switch is going to line up with your handiwork. Did we mention there is a tricky bit, involving a rabbit, hat and stick? Would you believe a blind screw? Our challenge was to devise a method of mounting the LM2941 regulator The regulator (right side of board) is shown here soldered in place – but DON’T DO IT LIKE THIS JUST YET!! The regulator is bolted to the case, the PC board is slipped over it THEN it is soldered. SWITCH (S1) THIS END REG1 Æ3 ACCESS HOLE Æ8 S1 HOLE SWITCH (S1) THIS END Æ6 Æ8 Fig.4: this diagram and the photo above shows how the 5-terminal regulator has its legs bent and how it is secured inside the case (see text). GROMMET HOLE DC SOCKET HOLE Fig.3: use these diagrams as a guide when drilling the diecast case. Note that the access hole does not line up with the regulator mounting screw (see text). (REG1) in such a confined space. As the PC board mounts “upside-down” in the case, there is no access to the regulator to screw it down once the board is installed. To solve this problem, we placed a screwdriver access hole on the opposite side of the case. Note that this hole is not directly in line with the regulator hole, as one of the LEDs effectively blocks that path. Begin by bending the regulator leads into shape so that it will assume a position like that shown in Fig.5. when inserted in the PC board. This photo, taken before the front panel was applied, shows the “plugpack” input socket and power switch on the top of the box. To reduce the possibility of the leads breaking off, don’t bend them right at the body of the regulator; allow a couple of mm space from the body. Take your time with this step, as radical bends that need to be undone might mean a replacement LM2941... Temporarily screw the regulator to the case as shown in Fig.4 (head of screw goes inside, nut and washer outside), and gently slide the PC board into position, making sure that all five regulator leads enter their correct PC board holes. Adjust the bends in the leads if necessary so that the board rests against the integral guides without applying any pressure at all to the regulator leads. There should also be approximately even spacing between all edges of the PC board and the sides of the case. Once you are happy with the position, solder REG1 in place. From now on, you’ll need a small Philips head screwdriver with a strongly magnetic tip in order to insert and remove the regulator mounting screw though the access hole on the opposite side of the case. Remove the screw now and remove the board from the case. OK, we’re almost there. Insert all the LEDs into the PC board, but don’t solder them or trim the leads just yet. Note the flat (cathode) side on the LED body is aligned as shown in the overlay diagram of Fig.2. Now slip the board back into place in the case and manipulate the LED leads protruding through the back of the PC board so as to place each LED in its hole in the bottom of the case. Adjust each LED so that its tip is flush with the case surface – easily achieved if the case itself is sitting on a flat surface – then solder in place. At this point, you should trim all component leads so that they are no more that 2mm above the surface of the PC board. This is very important, Fig.5: this chart shows the predicted maximum output current at the four selected output voltages, for variations in the input voltage. MAY 2001  91 The PC board really is a snug fit in the case – in fact, the odds are that you will have to file a little off commercial boards to make them fit. You can clearly see the acrylic stick-on feet in this picture. as leads much longer than this will short out on the lid of the case when we put the whole shebang together. Remove the board once more and glue the 1000µF capacitor to the PC board using non-acidic silicone sealant. Now is a good time to check that you’ve inserted the 2A fuse, too. Pre-flight checks It’s a good idea to perform some basic function tests at this point. At a minimum, you will need a digital multimeter and a 12V DC plugpack or similar power source. Before applying power, check for short circuits between the positive (+) and negative (-) terminals of both CON1 and CON2. With no load connected to the output, apply power and check that the “power” LED illuminates. If it doesn’t, check the orientation of D1, ZD1 and LED5. Next, connect your meter across CON3 and measure the output voltages for all four positions of the slide switch. Assuming you have sufficient input voltage, all measurements should be Resistor Colour Codes     Value    4-Band Code (1%)   5-Band Code (1%)     360kΩ orange blue yellow brown orange blue black orange brown     160kΩ brown blue yellow brown brown blue black orange brown     100kΩ brown black yellow brown brown black black orange brown     51kΩ green brown orange brown green brown black red brown     22kΩ red red orange brown red red black red brown     10kΩ brown black orange brown brown black black red brown     8.2kΩ grey red red brown grey red black brown brown     3.6kΩ orange blue red brown orange blue black brown brown     1.8kΩ brown grey red brown brown grey black brown brown     1.3kΩ brown orange red brown brown orange black brown gold   680Ω blue grey brown brown blue grey black black brown     470Ω yellow violet brown brown yellow violet black black brown     270Ω red violet brown brown red violet black black brown     220Ω red red brown brown red red black black brown     110Ω brown brown brown brown brown brown black black brown     68Ω blue grey black brown blue grey black gold gold     56Ω green blue black brown green blue black gold gold   92  Silicon Chip Fig.6: actual size front panel artwork. It goes on the underside of the case which then becomes the top. Use a photocopy of this as a template when drilling the underside of the case. within 0.1V of the advertised value. For example, when the “6V” position is selected, the output should be between 5.9V and 6.1V. If all voltages are incorrect, suspect a problem with the 1kΩ resistor or the associated connection to pin 1 of the regulator. If some voltages are OK but others are not, check that you have the correct resistor values in the feedback circuit associated with the problem voltage; refer to the circuit and overlay diagrams here. If you have a variable power supply, you can also check that the low battery cut-out circuit works. Starting from above 12V, slowly decrease the input voltage. At around 11.5V, REG1 should be switched off by IC1, disconnecting the output. Now increase the voltage slowly. At around 12.2V, REG1 should be switched on again. We won’t do any testing with a load connected just yet. Let’s continue on with the construction... Construction (episode 2) Fit the rubber grommet to the case. Some trimming with a sharp knife Getting other output voltages Setting the output voltage on the LM2941 is a fairly simple matter. Referring to Fig.8, you can see that all we need to do is set the ratio of R1 to R2 according to a simple formula. The PowerPack uses a fixed 1kΩ resistance for R1 and a switchable resistance for R2, selected via switch S2. For example, suppose we would like to produce 4.5V instead of 3V at the bottom-most switch position. We already know R1 (1kΩ), so we calculate R2 by massaging the formula in Fig.8 a little, so that: R2 = 1kΩ x (4.5/1.275 - 1) = 2.529kΩ 2.529kΩ is obviously not a “standard” resistor value, so we select two standard values (to be placed in series) that are the closest to the calculated value, namely 2.4kΩ and 130Ω. To check what the output voltage will be for our selected values: VOUT = 1.275 x (1kΩ + 2.4kΩ + 130Ω/1kΩ) = 4.50075V (Close enough!) The 2.4kΩ and 130Ω resistors are then installed in place of the 1.3kΩ and 56Ω resistors to get 4.5V at the bottom-most switch position. For more detailed information on the LM2941, you can download the data sheet from the National Semiconductor web site at http://www.natsemi.com If you’ve read the datasheet already and want to know how the PowerPack can provide a 3V (or 4.5V, for that matter) output when the data sheet specifies 5V as the minimum voltage, we’ll have to own up – we’ve made some assumptions about the internal workings of the regulator. We recommend keeping the input voltage (measured at the IN pin) above about 6V, and to be conservative with output loading at these low output voltages. Fig.7: the PC board must have the exact dimensions of the pattern shown here in order to be a snug fit into the specified diecast case. will be required to get a neat fit. Next, we’ll prepare and install the two cables. For the output side, we’ve used a plugpack extension cable for the job, as it already has a moulded connector on the end ready to accept all the various plugpack connector tips. The other end of this cable probably has crimped connections; cut these off and pass the end through the grommet and strip and tin it. For the car connection side, simply solder one end of the length of figure-8 cable to the cigarette lighter plug (wire with the white trace goes to the tip), and pass the other end through the same grommet and strip and tin. Hook up the cables to their respective terminal blocks (CON1 and CON3), connecting the wires with the white traces to the positive (+) sides. Secure a cable tie around both cables at the point they exit the grommet (inside the case) to provide strain relief. Apply a thin smear of heatsink compound to the back of the regulator as well as to the area that it will contact in the case. The metal tab of the regulator is connected to ground, so we don’t need to isolate it from the Fig.8: R1 and R2 are used to set the output voltage of the regulator according to the formula shown here. case. This significantly improves heat transfer and makes it much easier to get the board in and out of the case. Slip the assembly into the case, complete with attached cables. You may need to adjust the cable position and length in the case so as to avoid fouling the LEDs and slide switch, etc. Check that the board is correctly seated on the guides and then screw the regulator to the case. Turn the nut on the outside rather than the screwdriver so as to tighten up the works without applying a twisting force to the regulator package. If you wish, you can cut or file the screw flush with the nut for a neater appearance. Ta-Da! The last step is to secure the board inside the case. To do this, stick four acrylic feet onto the PC board (copper side) in positions roughly as shown in blue outline on Fig.2. We had to cut down one foot with a sharp utility knife so that it didn’t sit over the top of component leads. Next, cut out a piece of cardboard (a manilla folder is ideal stock) to fit neatly inside the inner ridge of the case lid. The lid should be an almost “perfect” fit on the case, meaning that it shouldn’t sit proud of the case by any appreciable amount. Don’t install the seal that is supplied with the case. Screw down the lid and proceed to the testing phase! When you’re sure that your Power-Pack is working properly, remove the lid and apply a daub of non-acidic silicone sealant to each corner of the PC board, right at the edge – in effect “gluing” the PC board to the case. This does make it a little harder to remove the board in future, but it is a necessary evil – it prevents the PC board from moving whenever the switch is toggled or the DC plug is inserted. Without this, the regulator leads and solder joints would take all the strain. SC MAY 2001  93