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Universal voltage regulator By NICHOLAS VINEN For any time you need low-voltage regulated supply rails M OST PROJECTS REQUIRE some form of voltage regulator. The Universal Power Supply project from August 1988 has been so popular, the kit is still on sale some 23 years later! Basically, that design allowed you to build one of four different voltage regulator configurations on a single PC board. It could be configured in both split (positive & negative) rail and single rail versions and could be used with a variety of power transformers, with or without centre-taps. It could also be set up for a variety of output voltages, depending on the regulator(s) used. In view of its popularity, we thought it was time to make some improvements. Accordingly, we have made the following tweaks to improve the original design: (1) Designed a smaller PC board; (2) Added terminal blocks for inputs and outputs; (3) Made it easier to build; (4) Made better provision for regulator heatsinks; (5) Added LED indicators/bleeders for both rails; (6) Added reverse-biased diodes at the output for regulator protection; and (7) Made provision for a wider range of electrolytic capacitor sizes. Universal regulator This project is called a “Universal Regulator” because it’s so flexible. Most commonly, it will be used to convert the AC output from a transformer 38  Silicon Chip (or an AC plugpack) to a regulated DC output. It can also be used to regulate an unregulated DC input voltage or it can be used to step-down a DC input voltage to a lower (regulated) output voltage. As with the original design, the unit can be built in both dual-rail and single-rail versions. The output voltages can range from ±5V to ±24V at currents of up to 1A per rail. It all depends on the transformer and the regulators used. Because this board can generate split (ie, positive and negative) rails, it is ideal for powering op amp circuits. It is also very handy for circuits which only require a positive supply (eg, +12V), in which case some components can be omitted. Transformer labelling Before going further, let’s take a closer look at how transformers are marked. Sometimes you will see a transformer labelled as “9 + 9” or “2 x 9”. This usually means that it has two 9V separate windings which can be connected in series or parallel. If you connect them in series, you have an 18V transformer with a centre tap. If you connect them in parallel and in phase, you have a 9V transformer with twice the current rating of the centre-tap configuration (if they are in anti-phase, you will get no output). A “9-0-9” label implies two 9V secondary windings with a fixed centre tap. These can not be connected in parallel because they will be in antiphase and so there will be no output. If a transformer has a VA rating (and most do), you can calculate the maximum theoretical output current by dividing the secondary voltage into that figure. So for example, a 60VA transformer can provide 2A if its secondary is 30V (60 ÷ 30) or 5A if its secondary is 12V (60 ÷ 12). Some transformers have multiple secondary taps so you can select the best combination for your circuit. Different configurations As with the previous design, the PC board can be built in any of four different configurations according to which parts are installed. The circuit diagrams for these configurations are shown in Figs.1-4. To generate split rails (eg, ±15V), it is a good idea to use a centre-tap configuration, as shown in Fig.1. The transformer secondary windings are connected to a bridge rectifier with the centre tap to ground. The peak rectified DC voltage is the transformer secondary voltage multiplied by 1.414, minus one diode drop (about 0.7V). In the example shown, a 15V-0-15V transformer results in about 20.5V across each filter capacitor, which is then regulated to ±15V using 7815 and 7915 3-terminal regulators (REG1 & REG2). The average filtered voltage will probably be slightly higher than this for light loads on the outputs and lower under heavy load. In this siliconchip.com.au REG1 7815 D1 A T1 INPUT 1 15V 230V K IN D4 A A K K D2 0V K A A A GND C1 2200 F 25V 20.5V 100nF A R1 D3 3 R2 CON1 C2 2200 F 25V 20.5V IN 100 F 25V UNIVERSAL REGULATOR OUTPUT 3 +15V 2 0V 1 –15V CON2 D6  LED2 A K OUT K A K 7815 7915 LEDS D1-D6: 1N4004 A K A 100nF REG2 7915 SC D5 K GND 2011 K  LED1 100 F 25V 2 15V N OUT IN GND IN OUT GND IN GND OUT TAPPED TRANSFORMER SECONDARY, DUAL OUTPUT CONFIGURATION Fig.1: the circuit for use with a centre-tapped transformer to generate split rails. Diodes D1-D4 form a bridge rectifier, while capacitors C1 & C2 filter the rectified AC. Regulators REG1 & REG2 provide a steady output voltage while LED1 and LED2 indicate operation. Different output voltages are obtained by changing the transformer and regulators. REG1 7812 D1 A T1 INPUT 12V 230V 0V 1 K K IN D4 A A K K OUT A GND 15.5V C1 2200 F 25V  LED1 100 F 25V 100nF OUTPUT 3 +12V 2 2 0V 3 1 D2 A A R1 D3 A CON2 A UNIVERSAL REGULATOR K 7812 LED D1-D5: 1N4004 SC D5 K CON1 N 2011 K K A GND IN GND OUT UNTAPPED TRANSFORMER SECONDARY, SINGLE OUTPUT CONFIGURATION Fig.2: this version of the circuit is used to derive a single, positive output voltage from a transformer with no centre tap. As in Fig.1, it uses a bridge rectifier but in this case ground is connected to its negative end and the negative regulator components are omitted. configuration, each filter capacitor is charged at twice the mains frequency (ie, at 100Hz). Note that while the circuit diagrams show a specific transformer and regulator combination, along with the expected filtered DC voltage, these are just examples and other combinations can also be used, as explained later. If a negative output voltage is not required, the centre-tap configuration is no longer necessary. Fig.2 also uses a bridge rectifier for full-wave rectification but the components for the negative output are removed. There is no centre tap connection from the siliconchip.com.au transformer but otherwise the circuit is identical to that of Fig.1. It is also possible to derive a positive single-rail output using a transformer with a centre tap – see Fig.3. In this case, only two rectifier diodes are needed. Note that the rectified output voltage is a little over half that which would be achieved by using the same transformer in the circuit of Fig.2 and ignoring the centre tap (ie, leaving the centre tap disconnected). Finally, in Fig.4, we show how it is possible to derive split rails from a transformer with no centre tap. This circuit is mainly used with AC plugpacks as they generally lack a centre-tap connection. The diodes are arranged to form a full-wave voltage doubler, which essentially consists of two half-wave rectifiers with opposite polarity. Because of this alternating halfwave rectification, the filter capacitors (C1 & C2) are each charged at 50Hz. This means that the ripple voltage on C1 and C2 is roughly twice that of the circuit shown in Fig.1. As a result, the ripple current through the capacitors is also doubled and that means that less current is available (see Fig.6). However, it is still possible to get a March 2011  39 A REG1 7812 K IN K D1 A T1 12V 0V 230V 12V C1 2200 F 25V 15.5V A 1 A GND D4 INPUT OUT K  LED1 100 F 25V 100nF D5 K 3 +12V 2 2 0V 3 1 CON1 N CON2 7812 LEDS D1, D4, D5: 1N4004 A SC 2011 OUTPUT A R1 UNIVERSAL REGULATOR K GND IN K A GND OUT TAPPED TRANSFORMER SECONDARY, SINGLE OUTPUT CONFIGURATION Fig.3: as with Fig.2 this version is used when a single, positive output voltage is required but this time the transformer has a centre tap. As a result, only two diodes (D1 & D4) are required to form a full-wave rectifier. K A T1 15V 230V 0V N INPUT 1 REG1 7815 K A IN D1 D2 A OUT A GND C1 2200 F 25V 20.5V 100nF OUTPUT +15V 2 2 0V 3 1 –15V A R1 R2 C2 2200 F 25V 20.5V 100 F 25V 100nF UNIVERSAL REGULATOR D6 A K K A 7815 7915 LEDS D1-D2, D5-D6: 1N4004 K CON2 OUT REG2 7915 A K A  LED2 GND IN 2011 D5 K 3 CON1 SC  K  LED1 100 F 25V IN GND IN OUT GND IN GND OUT UNTAPPED TRANSFORMER SECONDARY, DUAL OUTPUT CONFIGURATION Fig.4: this version allows a split rail output to be derived from a transformer without a centre tap. This circuit is often used with AC plugpacks, with diodes D1 & D2 used as a full-wave voltage doubler. The circuit of Fig.1 is preferred for use with chassis-mount transformers. full 1A output using this configuration, depending on the particular transformer and output voltage combination. Obtaining other voltages Note that it is possible to use the circuit shown in Fig.4 to generate a single output voltage which is twice that of the circuit shown in Fig.2. This is achieved by using pin 1 of the output connector as ground for the load. The voltage across pins 1 & 3 is then double the usual output voltage. That is why the circuit is known as a “voltage doubler”. As mentioned, the centre tap of a 40  Silicon Chip transformer may be ignored and the transformer is then treated as having a single secondary winding with a voltage that is the sum of the two individual windings. This means that you can derive three different positive DC voltages from a centre-tapped transformer: about 1.4 times the secondary voltage (as shown in Fig.2), half that figure (as shown in Fig.3) or twice that figure (as shown in Fig.4). Dual-output configuration Now that we have had a look at the various circuit configurations, let’s take a closer look at how they work. Fig.1 shows a dual-output (±15V) configuration based on a centre-tapped transformer, a bridge rectifier (D1-D4) and a couple of 3-terminal regulators. As shown, a 15V AC sinewave is applied to pin 1 of CON1 by the transformer. At the same time an identical sinewave is applied to pin 3 but is 180° out of phase. In other words, the voltage at pin 3 is inverted compared to the voltage at pin 1. When the voltage at pin 1 is rising, the voltage at pin 3 is falling. As the voltage at pin 1 approaches its positive peak, diode D1 becomes forward siliconchip.com.au Fig.5: this scope grab shows the operation of the circuit depicted in Fig.1 but with an 18V-0-18V transformer and a 150Ω load on each output (drawing 100mA from each). Channels 1 and 2 (yellow and green traces) show the secondary voltages while channels 3 and 4 (blue and pink) show the voltages across C1 and C2. With a 50Hz mains voltage, the ripple voltage for each capacitor is at 100Hz. The average rectified voltage is 25.38V, close to what we would expect (19V x 1.414 - 2 x 0.7 = 25.47V). biased and so capacitor C1 is charged to this peak voltage (or close to it). Similarly, as the voltage at pin 3 of CON1 approaches its negative peak, diode D3 becomes forward biased, charging capacitor C2 to the peak negative voltage. Ten milliseconds later, the voltages are reversed. Diodes D2 and D4 are now forward biased and both capacitors are recharged but from the opposite winding. This process repeats 100 times a second since the mains frequency is 50Hz (in some countries, 60Hz). The resulting filtered supply rails then supply positive and negative regulators REG1 and REG2. These vary their transconductance so as to maintain a steady voltage at their output pins, as determined by an internal voltage reference and divider network. In this case, we are using 7815 and 7915 regulators to derive +15V and -15V outputs respectively. Want ±12V output rails instead? No problem, just substitute 12V regulators (eg, 7812 & 7912) instead, although for a given current drain, their dissipation will be somewhat higher. If this is a problem, substitute a transformer with a 24V centre-tapped (CT) secondary for the 30V CT unit shown. Similarly, by changing the transformer and the regulators, you can get ±5V or ±9V outputs instead. siliconchip.com.au Fig.6: now we are using the circuit of Fig.4, with a single 18V secondary winding and the same 100mA drain on each output. Channel 2 (green trace) now shows the current through the transformer’s secondary. The capacitors are recharged alternately at 50Hz and the ripple voltage has more than doubled compared to the configuration of Fig.1. The average rectified voltage is lower as well (24.85V). The diodes only conduct about 20% of the time, resulting in a low power factor. The 100µF capacitors on their outputs are not strictly necessary but they result in lower noise voltages at the outputs. They also improve the regulators’ load transient response – if a sudden change in load impedance results in a change in the output voltages, current flows into or out of these capacitors as necessary to compensate, thus reducing the voltage variation. The 100nF capacitors in parallel do the same but they have lower impedance at higher frequencies (due mainly to their lower dissipation or power factor) and so help with more rapid load transients. LED1 and LED2, in combination with their current-limiting resistors R1 & R2, serve three purposes: (1) they provide a visual indication that the circuit is operating; (2) they provide the regulators with a minimum load; and (3) they help to discharge all the capacitors when the AC supply is removed. Finally, diodes D5 & D6 protect the circuit in case of asymmetric loads. Such loads can pull the positive rail negative or the negative rail positive during switch-off or over-current conditions. D5 & D6 clip these transient voltages and prevent damage to the regulators and filter capacitors under such conditions. D5 & D6 also overcome the bootstrapping problems that can occur with certain brands of regulators (mainly L78xx types). Single rail configurations The circuit of Fig.2 is similar in many ways to Fig.1 but lacks the negative regulator and its corresponding negative output rail. It also uses a transformer without a centre-tap but retains the bridge rectifier. Note that in this case, we are using a 7812 3-terminal regulator to derive a +12V output rail. Accordingly, a transformer with a 12V secondary has been specified. If you wanted a +15V output, then its just a matter of using a 15V transformer and substituting a 7815 regulator. Fig.3 also has a +12V output but uses a 24V centre-tapped transformer (12V-0V-12V) and a full-wave rectifier (D1 & D4). As before, it’s easy to get a +15V output – substitute a 30V centre-tapped transformer and a 7815 regulator. Half-wave rectifier As with Fig.1, Fig.4 provides dual (±15V) outputs. In this case though, an untapped transformer is used and diodes D3 and D4 are removed, since there is no transformer secondary winding to drive them. As a result, diodes D1 & D2 function as half-wave rectifiers for their respective positive and negative rails. March 2011  41 4004 R2 SC + 100 F R1 + 4004 2 G 1 + 0V – D4 1102 © SC 3 - rotalug eR lasr evinU Fig.7: this PC board overlay diagram corresponds with the circuit of Fig.3. All the negative regulator components may be omitted, along with diodes D2 & D3. The other way of regarding Fig.4 is as a conventional half-wave voltage doubler circuit which has been “centre-tapped” at the junction of the two 2200µF capacitors. Either way, the result is the same. Because D1 & D2 function as halfwave rectifiers, the ripple voltage superimposed on the DC supply rails will be 50Hz. As a result, for a given current drain, the ripple voltage will be slightly more than twice the 100Hz ripple obtained if the bridge rectifier circuit of Fig.1 is used. This may (or may not) be a problem, depending on the application (see Fig.6). Selecting a transformer Either a chassis-mount mains transformer or a plugpack can be used, as long as it has the correct voltage and current ratings. AC plugpacks are typically available with 9V, 12V, 15V, 16V or 24V output and power ratings up to about 24VA. These are suitable for regulated output currents of about 350mA for a split rail output or 700mA for a single voltage output. If you want to use a chassis-mount mains transformer, you must take proper precautions to make your project safe and to avoid getting an elec42  Silicon Chip G 4004 + DC OUTPUT 2 0V 1 – rotalug eR lasr evinU 1 4004 4004 2 3 D1 11130181 + + C1 2200 F + 100 F D2 + 1102 © SC LED1 REG 1 100nF C2 2200 F - rotalug eR lasr evinU + 100 F REG 2 R1 + 100nF 3 2 – + G 0V 1 – R2 LED2 Fig.8: this PC board overlay diagram corresponds with the circuit of Fig.4. All components are installed except for diodes D3 & D4 although it won’t hurt to put them in. tric shock. These include but are not limited to: earthing the transformer frame, the metal case and any exposed metal (eg, screw heads), proper colour coding for the wiring, an appropriate fuse, insulating mains connections within the case and so on. If you are uncertain as to what precautions to take or don’t have the necessary experience, don’t mess with mains power! It’s quite easy to calculate the appropriate transformer voltage to use for a given output voltage or voltages. However, to save time, we have provided some tables to help you select a transformer. It’s just a matter of using Table 2 to select a transformer for Fig.1 (tapped secondary) or Fig.4 (untapped secondary). Similarly, use Table 3 to select a transformer for Fig.2 (untapped secondary) or Fig.3 (tapped secondary). Note that you may use a transformer with a higher voltage rating than suggested but this will increase regulator dissipation and may require larger heatsinks (which will be discussed later). In some cases, where the output current is moderate (say <250mA), it is possible to use a transformer with DC OUTPUT C1 2200 F CS n© I 2011 DC OUTPUT 3 D1 18103111 CON2 2 4004 - 3 Fig.6: this PC board overlay diagram corresponds with the circuit of Fig.2. This is the only version for which a wire link is necessary. Note that quite a few parts are omitted for this version as there is no negative output voltage rail. LED1 REG 1 100nF CON1 AC INPUT 1 11130181 + + 4004 CS © D5 4004 LED2 D5 18103111 D4 1102 Fig.5: this PC board overlay diagram corresponds with the circuit of Fig.1. All components are installed. Refer to Table 2 for the values of resistors R1 & R2. Capacitors C1 & C2 are typically 25V types but a higher rating is sometimes necessary (see text). n© I 2011 D3 4004 + CON2 – 3 4004 - rotalug eR lasr evinU – 0V D2 R1 D5 © REG 2 100nF 1 + 100 F CON2 C2 2200 F G 2 REG 1 100nF D6 D4 + 100 F 4004 C1 2200 F LED1 4004 + 2 + D1 4004 1 CON1 D3 3 11130181 + + (LINK) SC 1102 D2 + CON1 4004 100 F R1 AC INPUT 4004 + AC INPUT 3 REG 1 100nF CS n© I 2011 DC OUTPUT 2 C1 2200 F 18103111 CON2 4004 D1 LED1 D5 4004 CON1 AC INPUT 1 11130181 + + D6 CS 4004 18103111 n© I 2011 a slightly lower voltage rating than is indicated in these tables. For example, a 12V AC plugpack can be used to obtain ±15V regulated outputs at low current. This is because a transformer typically provides more than its rated voltage when it is lightly loaded. Plugpacks tend to have worse voltage regulation than stand-alone transformers so this comment particularly applies to them. In other words, their output voltage will be even higher when they are lightly loaded. Note that for transformers with secondary voltages above 16VAC (or above 30V AC with a centre tap), you must increase the voltage rating of the large input filter capacitors to at least 35V. If you can’t get 35V capacitors, use 50V types instead. There is enough space on the PC board to fit most brands of 2200µF 50V capacitors but if necessary, use a smaller value (say 1500µF). A multi-tapped transformer like the Jaycar MM2005 is a good choice for powering the Universal Regulator board because it can be configured with a single secondary winding of 9V, 12V, 15V, 18V, 21V, 24V or 30V or alternatively with a centre-tapped secondary winding of 18V (9-0-9), siliconchip.com.au These two photos show the fully-assembled PC board for the version shown in Fig.1 (circuit) and Fig.5 (parts layout). The other three versions use fewer parts. 24V (12-0-12) or 30V (15-0-15). Its secondary current rating (2A) is sufficient for virtually any configuration shown here. As stated previously, this board can also be used to regulate DC voltages. In this case, use the circuit of Fig.1 or Fig.3 depending on whether a negative voltage input is required. The transformer is, of course, deleted. In either case, connect the supply ground to pin 2 of CON1. The supply rail(s) to be regulated then go to pin 1 (positive) and, in the case of Fig.1, to pin 3 (negative). When used in this manner, the maximum regulated output voltage is the minimum input voltage minus 3V. So to obtain a regulated 12V output, the input must be at least 15V. Of course, you would have to use 7812 and 7912 regulators in Fig.1. If the supply is an unregulated DC plugpack, its output will probably be several volts higher than nominal with light loads. So for applications which don’t require a lot of current, you may find that a 12V DC plugpack supplies a high enough voltage for a regulated 12V output but you will have to check. driving a handful of op amps), heatsinks will not be necessary. Having said that, it’s always a good idea to do the calculations for your application to be sure. If the regulators overheat they will shut down and the output voltage will drop dramatically. As a result, damage is unlikely but the circuit will not work correctly. First, calculate the dissipation in each regulator. To do this you need to know the average input voltage to the regulators, which we shall call “Vin”. A reasonable estimate can be calculated as: (secondary winding voltage) x 1.414 - 0.7V. You also need to know the peak current drawn from each output which we will designate as “Iout”. If we designate the regulator’s output voltage as “Vout”, then the dissipation in the regulator is simply (Vin - Vout) x Iout. For example, if a 15-0-15 (30V centre-tapped) transformer is used to provide a regulated ±15V at 100mA, the dissipation in each regulator will be roughly (20.5 - 15) x 0.1 = 0.55W. This is below 0.6W so no heatsinking is necessary. Conversely, if the dissipation is over 0.6W, refer to Table 4 as a guide for heatsink selection. Heatsinks Construction Regulating DC For low-current applications (eg, Parts List 1 PC board, code 18103111, 71 x 35.5 2 3-way terminal blocks, 5.08mm pitch 4 M3 x 15mm tapped Nylon spacers 4 M3 x 6mm machine screws 2 TO-220 heatsinks (optional) 2 M3 x 10mm machine screws, nuts and shake-proof washers for heatsinks (optional) 10mm length of 0.71mm tinned copper wire Semiconductors 1 78xx positive linear regulator (REG1) 1 79xx negative linear regulator (REG2 – optional) 6 1N4004 diodes D1-D6) 1 5mm red (LED1) 1 5mm green LED (LED2) Capacitors 2 2200µF 25V* electrolytics 2 100µF 25V electrolytics 2 100nF MKT (code 100n or 104) Resistors (0.25W, 1%) 2 2.2kΩ 2 680Ω 2 1.5kΩ R1 & R2 – see Tables 1 & 2 * Note: a higher voltage rating is necessary for transformers with secondaries over 16V Building the PC board is easy. The Table 1: Resistor Colour Codes o o o o siliconchip.com.au No.   2   2   2 Value 2.2kΩ 1.5kΩ 680Ω 4-Band Code (1%) red red red brown brown green red brown blue grey brown brown 5-Band Code (1%) red red black brown brown brown green black brown brown blue grey black black brown March 2011  43 Table 2 – Selecting A Transformer For Dual Rail Outputs Output Voltage Tapped Secondary Untapped Secondary Regulator(s) R1 & R2 ±5V 12V AC (6-0-6) 6-9V AC 7805, 7905 680Ω ±6V 15V AC (7.5-0-7.5) 9V AC 7806, 7906 680Ω ±8V 15V AC (7.5-0-7.5) 9V AC 7808, 7908 680Ω ±9V 18V AC (9-0-9) 9V AC 7809, 7909 680Ω ±12V 24V AC (12-0-12) 12V AC 7812, 7912 1.5kΩ ±15V 30V AC (15-0-15) 15V AC 7815, 7915 1.5kΩ ±18V 30V AC (15-0-15) 15V AC 7818, 7918 1.5kΩ ±20V* 36V AC (18-0-18) 18V AC 7820, 7920 2.2kΩ ±24V* 40V AC (20-0-20) 21V AC 7824, 7924 2.2kΩ * Increase voltage rating of 2200µF capacitors to 35V or higher Table 3 – Selecting A Transformer For A Single Output Voltage Output Voltage Untapped Secondary Tapped Secondary Regulator Resistor R1 5V 6V 6-9V AC 12V AC (6-0-6) 7805 680Ω 9V AC 15V AC (7.5-0-7.5) 7806 680Ω 8V 9V AC 15V AC (7.5-0.7.5) 7808 680Ω 9V 9V AC 18V AC (9-0-9) 7809 680Ω 12V 12V AC 24V AC (12-0-12) 7812 1.5kΩ 15V 15V AC 30V AC (15-0-15) 7815 1.5kΩ 18V 15V AC 30V AC (15-0-15) 7818 1.5kΩ 20V* 18V AC 36V AC (18-0-18) 7820 2.2kΩ 24V* 21V AC 40V AC (20-0-20) 7824 2.2kΩ Table 4: Heatsink Selection Guide Dissipation Maximum Thermal Resistance Suggested heatsink <0.6W 45°C/W None 0.6-2W 20°C/W Micro/mini flag (Jaycar HH5502, Altronics H0630) 2-4W 12°C/W Large flag (Jaycar HH8504, Altronics H0637) 4-8W 6°C/W U-shaped (Jaycar HH8511, Altronics H0620) >8W 48 ÷ dissipation in Watts board is coded 18103111 and measures 71 x 35.5mm. Before starting the assembly, it should be checked for hairline cracks or under-etched areas in the copper and repaired if necessary. Figs.5-8 show the various configurations, so choose the one that’s relevant for your application. If you are using the configuration shown in Fig.2 (positive output only, no centre tap), start Finned diecast aluminium heatsink by installing a wire link in place of C2. Do not install this link for any other configurations though. Now install the resistors. Use Table 2 or Table 3 to select the correct values for resistors R1 & R2. If in doubt, use 1.5kΩ for both. R2 may be omitted if the negative output is not used. Follow with the 1N4004 diodes. These must all be correctly orientated, Issues Getting Dog-Eared? as shown on the parts layout diagrams. If some are not used for your chosen configuration you may omit them, although it doesn’t hurt to install all six. Next, fit the two 100nF MKT capacitors. They can go in either way around. Follow with the LEDs, ensuring that the flat sides are orientated as shown on the relevant overlay diagram. After that, mount the two 3-way screw terminal blocks with their entry holes facing outwards. The electrolytic capacitors can now be soldered in place, starting with the two smaller ones. They must all be correctly orientated. The stripe on the body indicates the negative side and these all face towards the bottom of the board. Make sure that the voltage ratings of C1 and C2 are sufficient for your application (see above). If the regulators require heatsinks, it is best to fit them before the regulators are mounted (if possible). For larger heatsinks which may interfere with the PC board, crank the regulator legs slightly with small pliers so that the tabs line up with the edge of the board. The regulator packages can then be pushed down onto the board, with the tab facing the edge, and soldered into place. Finally, complete the board assembly by fitting tapped spacers to the four corner mounting holes. These can be secured using M3 machine screws. Smoke test If you are using a chassis-mount transformer, check that it has been correctly installed and that there are no exposed mains terminals before applying power. To test the unit, connect the transformer secondary leads to CON1, switch on and check that LED1 & LED2 light. Assuming the LEDs do light, use a DMM to check the voltage(s) at CON2 to ensure that they are correct. If not, switch off and check that the correct regulators have been used. If there is no output at all, check that the regulators and diodes are orientated correctly. Once you have confirmed that the output voltages are correct you can SC wire up the outputs. Keep your copies safe with our handy binders Available Aust, only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue or ring (02) 9939 3295 and quote your credit card number. 44  Silicon Chip siliconchip.com.au