Fullscreen HTML5Med Res (??MB)HTML4

By JIM ROWE Low-Voltage Adjustable Regulator Need to operate a CD, DVD or MP3 player from the cigarette lighter socket in your car? Or perhaps run a digital still or video camera or some powered speakers from the power supply inside your PC? This Low-Voltage Adjustable Regulator will step the voltage down to what’s needed. It has jumper shunts to select one of six common output voltages (from 3-15V) and depending on the input voltage and the heatsink(s) you use, it can deliver an output current of just over 4A. C ONSIDERING THE PRICE of batteries and the ever-growing array of small items of electronic gear designed to run from low-voltage battery power, it’s not surprising that one of the most common requests from SILICON CHIP readers is for an adaptor so this kind of equipment can be run from either the power supply inside a PC or a cigarette lighter socket in a motor vehicle. Most of the battery-operated equipment we’re talking about is designed to operate at 3V, 6V or 9V whereas the voltages available from vehicle batteries or PC power supplies are rather more restricted. For example, there’s usually only either 12V or 24V available from vehicle batteries, while most 76  Silicon Chip PC power supplies only have 5V and 12V supplies readily available. In addition, the voltage available from a vehicle battery can vary over a fairly wide range depending on whether the engine is running, the battery is being charged and whether the lights and/or air conditioning are on. This sort of voltage variation can cause problems for electronic circuits, as these generally perform much better and more reliably when operated from a regulated power supply. This low-voltage adaptor has been designed for use in virtually any of these common DC voltage step-down applications. It can be connected to any convenient source of input voltage up to about 28V and is “programmed” using a push-on jumper shunt to deliver one of six output voltages: 3V, 5V, 6V, 9V, 12V or 15V. In each case the output voltage is well regulated, remaining very close to the selected voltage despite broad changes in both input voltage and load current level. Circuit description The circuit is shown in Fig.1. The heart of the adaptor is an LM317T adjustable 3-terminal regulator which comes in a TO-220 package. The LM317 is designed to maintain the voltage between its output (OUT) and adjustment (ADJ) terminals at close to 1.25V. At the same time, the current level through its ADJ terminal is maintained at a very low level (typisiliconchip.com.au cally 50mA) and varies by less than 5mA over the full rated load current range (10mA - 1.5A) and the input-output voltage range of 3-40V. The LM317’s actual regulated out­ put voltage can be varied over a wide range using a simple resistive voltage divider. As shown in Fig.1, the divider’s top resistor is connected between the OUT and ADJ terminals of REG1, while the bottom resistor is connected between the ADJ terminal and the negative voltage rail. Since the LM317 maintains the voltage across the upper resistor at 1.25V, the total output voltage can be set for virtually any voltage above this level simply by adjusting the value of the lower divider resistor. The value of the lower resistor is found by taking into account that it needs to drop the desired output voltage minus 1.25V, while carrying the current passing through the upper resistor plus an additional 50mA (from the ADJ terminal). In our circuit, the upper divider resistor is 120W, giving a nominal current of 1.25/120 = 10.42mA. Hence the current through the lower divider resistor is 10.42 + 0.05 = 10.47mA. The value of the lower divider resistance is varied using the jumper shunt to link one of the six “voltage select” pin pairs. For example, when the shunt is fitted in the 3V position, the lower divider resistor is 160W. Similarly, when it’s fitted in the 6V position the lower resistor value is set to (160 + 180 + 18 + 91) = 449W. The resistor values selected by each of the jumper shunt positions have been calculated to give LM317 output voltages as close as possible to the marked values, using standard resistor tolerance values. Current boost So that is how we set the output voltage. However, since the LM317 can only cope comfortably with currents up to around 1.5A, it needs a boost if the adaptor is to supply higher currents. In our circuit, this boost is provided by Q1, a BDX54C/BD650 PNP Darlington power transistor. Q1 has its emitter and base connected across the 22W resistor in series with the LM317’s input. As a result, the voltage developed across the 22W resistor when the LM317 draws current provides Q1 with forward bias. When the current drawn by the load through the LM317 rises to about 55mA, siliconchip.com.au Q1 BDX54C/BD650 C E B 22 + REG1 LM317 IN ADJ 300 15V 5.6 12V 270 120 9V 300 INPUT + OUT 6V 470 F 35V 91 10 F 16V 10 F 16V 100nF OUTPUT 5V 18 180 3.0V 160 – – LM317T BDX54C, BD650 C SC 2008 E B OUT C ADJ IN HIGH CURRENT ADJUSTABLE REGULATOR Fig.1: the circuit is based on an LM317 adustable regulator and a PNP Darlington transistor (Q1) to boost the output current capability. The output voltage is set by the resistive voltage divider string on the regulator’s OUT and ADJ terminals and depends on the jumper shunt installed. the voltage drop across the 22W resistor will be around 1.2V. This is enough to forward bias Q1 into conduction. As the load current rises above this 55mA level, Q1 gradually takes over from the LM317 and handles more and more of the load current. The higher the load current, the greater the proportion that’s handled by Q1. The current boost provided by Q1 doesn’t degrade the voltage regulation performance of the LM317. The regulator still controls the output voltage level closely in the normal way and varies the current passing through Q1 by varying its own current. In effect, Q1 acts purely as a slave to REG1, boosting the total output current capacity. The function of the 470mF capacitor across the adaptor’s input is to provide a degree of smoothing and filtering, to minimise the effect of any alternator noise or power supply ripple which may be present on the input voltage. Further filtering is provided by the 10mF capacitor which is connected between the LM317’s ADJ terminal and Specifications • • Selectable output voltage: 3V, 5V, 6V, 9V, 12V or 15V DC within ±3% • • DC Input voltage: up to 24V battery Output voltage regulation: typically better than 0.5% up to 500mA; better than 1% for output currents up to 1A Output current: up to 4.25A – see Table 1. May 2008  77 load current that the adaptor can handle. Just how hot Q1 and REG1 actually get for a given amount of power dissipation depends on the heatsink size. To be precise, the temperature rise for each device is determined by the power being dissipated and the ‘thermal resistance’ between its internal junction and the surrounding “ambient”, as follows: Table 1: Voltage Adaptor Output Current Ratings Maximum output current Input Volts Output Volts Vin – Vout 6V 3V 3V 830mA 2A 2.8A 3V 9V 275mA 660mA 940mA 5V 7V 350mA 850mA 1.2A 6V 6V 415mA 1A 1.4A 9V 3V 830mA 2A 2.8A 3V 21V 115mA 280mA 400mA 5V 19V 130mA 310mA 440mA 6V 18V 135mA 330mA 470mA 12V 24V With HH-8502 heatsink (on board) With HH-8511 With Q1 on HH-8566 heatsink (on board) heatsink, off board 9V 15V 160mA 400mA 560mA 12V 12V 200mA 500mA 700mA 15V 9V 275mA 660mA 940mA T(case - ambient) = P(tot) x R(j-a) where T(case - ambient) is the case temperature rise above ambient and R(j-a) is the thermal resistance between the junction and ambient. The latter is made up from two thermal resistances in series; the junction to case thermal resistance and the thermal resistance from case to ambient: Table 1: use this table to select the heatsink necessary to suit the required output current from the regulator board. Note that you also have to consider the difference between the input and output voltages when making this selection. the negative voltage rail, and also by the 100nF and 10mF capacitors across the output. Current, power & heatsinking Before we turn to the construction of the adaptor, it’s important to understand how the amount of load current is determined by two factors: (1) the difference between the input voltage and the selected output voltage; and (2) the amount of heatsinking fitted to current booster Q1 (and to a lesser extent, regulator REG1). These things are all linked together because the main limitation on the adaptor’s maximum output current is the heat dissipation in both Q1 and REG1. Q1 can only dissipate a little over 20W for case temperatures up to 100°C, while REG1 has internal overcurrent and over-temperature protection which limits its power dissipation R(j-a) = R(j-c) + R(c-a) where R(j-c) is the internal thermal resistance from junction to case, which is around 4°C per watt for TO-220 devices like Q1 or REG1. R(c-a) is the thermal resistance from case to ambient, which we can lower by fitting the device with a heatsink. For example, the thermal resistance R(c-a) of a TO-220 device like Q1 without any heatsink at all is around 46°C/watt, so its temperature will rise above ambient by about (4 + 46) = 50°C for every watt of power it must dissipate. If we fit it with even a small heatsink like the Jaycar HH-8502, this drops R(c-a) to 20°C/watt, lowering the total temperature rise above ambient to (4 + 20) = 24°C for each watt of power dissipated. So fitting this small heatsink on Q1 will roughly double the adaptor’s power dissipation ability. We can do much better if we fit Q1 with a larger heatsink like the Jaycar to less than about 15W. These limits control the adaptor’s output current because the case temperatures of Q1 and REG1 are proportional to the power they have to dissipate, and their power dissipation is determined in turn by the voltage they have to drop (ie, the difference between the adaptor’s output and input voltages) multiplied by the output current. We can express this mathematically using the following equation: P(tot) = I(load) x (Vin - Vout) where P(tot) is the total power dissipation in watts, I(load) is the load current in amps and Vin and Vout are the adaptor input and output voltages respectively. So the important point to grasp is that the larger the voltage difference (Vin - Vout), the smaller the maximum Table 1: Resistor Colour Codes o o o o o o o o o o No.   2   1   1   1   1   1   1   1   1 78  Silicon Chip Value 300W 270W 180W 160W 120W 91W 22W 18W 5.6W 4-Band Code (1%) orange black brown brown red violet brown brown brown grey brown brown brown blue brown brown brown red brown brown white brown black brown red red black brown brown grey black brown green blue gold brown 5-Band Code (1%) orange black black black brown red violet black black brown brown grey black black brown brown blue black black brown brown red black black brown white brown black gold brown red red black gold brown brown grey black gold brown green blue black silver brown siliconchip.com.au – – OUTPUT 100nF rent capability is to provide Q1 with a larger heatsink, as just discussed. + OUTPUT + + Example 10 F (HH-8511 SHARED HEATSINK) USE SMALL HEATSINK FOR LOWER CURRENT USE, LARGER SHARED HEATSINK FOR HIGHER CURRENT USE 8002 © HS1 (HH-8502) Q1 BDX54C REG1 LM317T 300 5.6 270 300 91 18 180 160 15V 12V 9.0V 6.0V 5.0V 3.0V – INPUT 470 F + POSITION JUMPER SHUNT FOR DESIRED OUTPUT VOLTS 22 18050111 120 10 F + + + – INPUT FROM PC POWER SUPPLY OR VEHICLE BATTERY Fig.2: install the parts on the PC board as shown here. The output voltage is set by installing a jumper shunt in one of the link positions. HH-8511 (which can be shared with REG1 as the latter doesn’t dissipate much power). The larger heatsink reduces R(c-a) to 6°C/watt, resulting in a total temperature rise of only (4 + 6) = 10°C for each watt dissipated. It is possible to reduce the value of R(c-a) even further by fitting Q1 with an even larger heatsink, to allow it to dissipate even more power. However this involves mounting Q1 off the adaptor’s PC board. To summarise, if you want the adaptor to supply as much current as possible, you must limit (Vin - Vout) by reducing Vin. However, Vin must be at least 3V higher than Vout for the adaptor to work correctly. If you’re stuck with a particular input voltage (say 12V), the only way to increase the adaptor’s output cursiliconchip.com.au Let’s say you want to use the adaptor to power a portable CD player from the cigarette lighter socket in your car and the CD player needs 3V DC. So Vin is 12V and the adaptor will have to drop 12 - 3 = 9V. Now let’s assume that Q1 is fitted with just a small HH-8502 heatsink. What current will it be able to deliver to the CD player at ambient temperatures up to 40°C? From what we’ve seen above, the total R(j-a) for Q1 with this small heatsink is around 24°C/watt, so if we want its temperature to rise by no more than 60°C above an ambient of 40°C (ie, to 100°C maximum), the maximum power that Q1 should be called upon to dissipate is 60/24 = 2.5W. If the adaptor will be dropping 9V, this corresponds to a maximum load current of 2.5/9, or about 275mA (power = voltage x current, so current = power/voltage). If the CD player needs to draw more current than this, you’ll have to fit Q1 with a larger heatsink like the HH-8511 which allow it to deliver 6/9 amps, or about 660mA. If this current rating seems pretty low, consider that this example is for a very demanding situation where it is being called upon to deliver the lowest selectable output voltage but from a fairly high input voltage. For an easier example, let’s say you want to provide a radio or some other equipment with 9V but still want to run the adaptor from 12V. This will mean that Q1 will only have to drop (12 - 9 = 3V). So with the smaller HH-8502 heatsink it would be able to deliver up to 2.5/3 or 830mA. Alternatively, with the larger HH-8511 heatsink, it would be able to supply 6/3 or 2A. To make it easier to choose which size of heatsink you need for your application, refer to Table 1 for the most likely combinations of input voltage and output voltage. Note that only practical combinations are shown – ie, where the input is at least 3V higher than the output, so that the unit can operate correctly. Construction All the parts used in the adaptor mount on a small PC board measur- JOIN THE TECHNOLOGY AGE NOW with PICAXE Developed as a teaching tool, the PICAXE is a low-cost “brain” for almost any project Easy to use and understand, professionals & hobbyists can be productive within minutes. Free software development system and low-cost in-circuit programming. Variety of hardware, project boards and kits to suit your application. Digital, analog, serial RS232, 1-Wire™, and I2C facilities. PC connectivity. Applications include: Datalogging Robotics Measurement & instruments Motor & lighting control Farming & agriculture Internet server Wireless links Colour sensing Fun games Distributed in Australia by Microzed Computers Pty Limited Phone 1300 735 420 Fax 1300 735 421 www.microzed.com.au May 2008  79 Parts List 1 PC board, code 11105081, 107 x 39mm 1 HH-8502 19mm square TO-220 heatsink, OR 1 HH-8511 61 x 36 x 30mm ‘U’ heatsink 2 TO-220 silicone washers 1 6x2 length of DIL jumper strip 1 jumper shunt 2 M3 x 6mm machine screws 2 M3 nuts 4 PC board terminal pins, 1mm diameter Semiconductors 1 LM317T regulator (REG1) 1 BDX54C or BD650 PNP power Darlington (Q1) Capacitors 1 470mF 35V RB electrolytic 2 10mF 16V RB electrolytic 1 100nF MKT metallised polyester Resistors (0.25W 1%) 2 300W 1 91W 1 270W 1 22W 1 180W 1 18W 1 160W 1 5.6W 1 120W Where To Buy A Kit This project was developed by Jaycar Electronics and they own the copyright on the PC board. Kits will be available exclusively from Jaycar retail outlets and dealers (Cat. KC-5463) and will be supplied with the HH-8502 heatsink. ing 107 x 39mm. The component overlay diagram is shown in Fig.2. Begin assembly by fitting the four PC board terminal pins (to the external wiring points) and the 6x2 length of DIL jumper strip used for the output voltage programming. Follow these with the single wire link that goes just below the 120W resistor. Next, fit the 10 resistors to the board, taking care to place each one in its correct position. Table 1 shows the resistor colour codes but you should also check each resistor using a DMM before soldering it in, as some of the colours can be difficult to decipher. After the resistors you can install the capacitors, starting with the unpolarised 100nF MKT capacitor up at 80  Silicon Chip the top/output end. Follow this with the three electrolytic capacitors, taking care to fit each of these the correct way around because they are polarised. The next step is to fit the heatsink (either the HH-8502 or the larger HH8511 – see Table 1), along with REG1 and Q1. Each of the latter two devices is mounted “flat” with its leads bent down by 90° about 6mm from its case, so they pass through the relevant holes in the PC board. If you’re just using the small HH8502 heatsink for Q1, REG1 can be fitted directly to the board (ie, no heatsink) and its metal tab secured using an M3 x 6mm machine screw and nut. The machine screw and nut also provide REG1 with a small amount of incidental heatsinking, in conjunction with the copper square underneath. Once you’ve secured its tab, its leads can be soldered to the copper pads under the board. Don’t solder the leads before bolting down the tab – you could stress and crack the solder joints if you do. Q1 is mounted on the top of its heatsink, using a thermal conducting washer or a smear of thermal compound to ensure a good thermal bond. An M3 x 6mm screw and nut are then used to secure the assembly in place, before soldering Q1’s leads to their pads underneath. Alternatively, both Q1 and REG1 can be mounted on the larger HH-8511 heatsink, again using either thermal conducting washers or smears of thermal compound to ensure good thermal bonds. As before, bolt the assembly to the PC board before soldering the device leads. Voltage on the heatsink It is not really necessary to electrically isolate the metal tabs of Q1 and REG1 from each other (or from the heatsink), since they both sit at the output voltage (ie, Q1’s tab is its collector and REG1’s tab is its output terminal). It does mean, however, that the heatsink also operates at the output voltage when power is applied, so make sure it doesn’t short against other equipment. This is also an important consideration if you mount Q1 off the board on a large external heatsink. In that case, you might want to electrically isolate Q1 from the heatsink using a TO220 insulation kit (ie, thermal insulation washer plus insulating bush for HD DRIVE POWER PLUG +12V GND 21 4 3 USE WIRES TO PINS 1 & 2 FOR Vin = 12V Fig.3: a hard disk drive power connector (eg, Jaycar PP-0743) can be used to connect the input of the regulator board to the 12V output from a PC power supply. the mounting screw). That way, the heatsink can then be earthed to other equipment. Voltage selection The next step is to fit the voltage selection jumper shunt to select the required output voltage. That done, connect a DC power source (it must provide at least 3V more than the output voltage you want), then check the output voltage with your digital multimeter. It should be within ±3%. If you are going to be sourcing the adaptor input voltage from your car or truck battery, the input lead can be fitted with a cigarette lighter plug at the far end to mate with the vehicle’s cigarette lighter socket. Jaycar sells two such plugs – the low-cost PP-2000 and the PP-2001 which has an internal 3A fuse. Similarly, if you intend sourcing the adaptor’s input voltage from a PC power supply, the input lead can be fitted with a 4-way plug (as used on the rear of hard disk drives), to mate with a spare power connector inside the PC. Again Jaycar can provide two versions of these plugs: the PP-0743 or the PP-0744. Fig.3 shows the connections for using this type of plug to provide a 12V supply for the regulator board. Note, however, that this input voltage will only be suitable for output voltages up to 9V. Output connector The adaptor’s output lead can be fitted with a power connector to suit the device or devices you’re going to be powering. In many cases this is likely to be a concentric low-voltage DC connector. Finally, when mounting the adaptor inside a case, make sure it has adequate ventilation to dissipate the likely heat SC it will produce. siliconchip.com.au