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Build this low cost Dual tracking ±18.5V po"7er supply Take a squiz at this: a dual tracking power supply of modest cost giving up to + 18.5 volts DC. It has voltage metering, a LED dropout indicator and short circuit protection. By JOHN CLARKE & LEO SIMPSON Sooner or later, every electronics enthusiast needs a DC power supply. They used to get by with a variable supply giving up to 15 volts or so at around 500 milliamps but today's circuits using op amps, memory and logic devices need a lot more than that. For op amps you need balanced positive and negative supplies of ± 15V while some memory chips such as EPROMs need ± 5V. The problem with designing a power supply for the enthusiast or technican is that it is easy to get carried away with fancy features that are seldom used. The end result is an expensive supply that no one can afford. So we at SILICON CHIP have put our heads together on this project to produce a supply which has good performance and features while keeping the cost within bounds. What are the big cost items in a power supply? That was the question we asked ourselves as we set out to design this supply. The big cost items are the transformer, meter, case, filter capacitors and printed circuit board. We could not eliminate any of these components in a self-contained power supply so we selected them very carefully to optimise the performance versus price ratio. For example, we selected a transformer with a centre-tapped 30V winding rated at one amp. This was much cheaper than a centretapped 44V 1.5-amp transformer that we would have selected as first choice if price was not so important. But we had to admit that the times when enthusiasts want high currents are fairly rare. By picking the smaller transformer, we greatly cut down the power dissipation in the circuit and thereby reduced the heatsinking requirements, the size of the case and the cost of the filter capacitors and regulatprs, all for very little reduction in overall utility of the supply. We also saved money by using a smaller meter, smaller rectifier diodes and so on. The end result is a compact power supply which will serve the needs of the vast majority of electronic enthusiasts and technicians. It will become another in a growing list of SILICON CHIP test equipment. JANUARY 1988 37 D7 1N4002 POWER LOAD S2a 0--0+ E • 2.7k 2200 + 25VW _ 100 + 25VW D10 1N4002 -14V 47k 1200 LOAD OUT S2b o-o- 2.7k 08 2.2M LED2 DROPDUT D11 DUAL TRACKING POWER SUPPLY D41-188 ... ..,. 4x1N4148 Fig.2: the circuit uses a 30V 1A transformer to drive a bridge rectifier and two adjustable 3-terminal regulators. ICl inverts the control voltage provided by VR1 to drive the LM337. IC2 monitors the output ripple to provide drop-out indication. The SILICON CHIP power supply has tracking positive and negative DC outputs adjustable from ± 1.2V to ± 18.5V. Both supply rails are protected against short circuits and 2.Dr x~ - - - . - - - - . - - - - . -- --, ~ ::E 5. ... ~ 1.011--f'----+--- = :::, c., -+-----+----t c:, ; D._._ _...,__ __.__ __.__ _ D 1.2 Fip, 1 10 ~ 15 SUPPLY VOLTAGE (VOLTS) Fig.1: this graph plots the maximum output current for voltage settings between ± 1.2V and ± 18V. 38 SILICON CHIP 20 voltages generated by external loads. Maximum load current is 1. 7A between ± 3V and ± 10V. When the supply stops regulating, a LED indicator lights. You can use the power supply in the conventional way to provide balanced positive and negative rails, or you can take the output from between the positive and negative output teminals and thereby get more than 36 volts DC output. The circuit is fully floating (ie, not tied to mains earth) and so the output can be referenced to earth via the positive, negative or 0V rail. What will it do? Fig.1 shows the maximum output current available for voltage settings between ± 1. 2 volts and ± 18 volts DC with the positive and negative rails loaded. Up to 1. 7 amps is available for settings between ± 3 and ± 10V. Above 10V the available current reduces, to 200 milliamps at ± 18 volts. Remember that this performance applies with both the positive and negative rails loaded, so that by taking the output between the positive and negative rails, you get get up to 1. 7 amps at 20 volts and up to 200 milliamps at 36 volts. Line regulation is within ± 5mV of a given output voltage setting for mains input variation between 220V AC and 260VAC. Load regulation at 1.7 amps is within 100mV at a setting of 9 volts; ie, close to 1 % . Ripple output (ie, 100Hz hum and noise superimposed on the DC rails) is less than lmV peak-to-peak for load currents up to one amp. These are excellent figures. Dinkum. Note that the actual maximum The supply is very easy to wire but you should take extra care with the mains wiring. Use a cord-grip grommet to secure the mains cord. available current from the power supply will depend on the temperature of the heatsink and the amount of power being dissipated in the regulator(s) for a given output setting. Circuit details Fig.2 shows the complete circuit. As already noted, it is based on a 30V centre-tapped 1A power transformer, Arlec 6672A or equivalent. Diodes Dl to D4 are connected as a bridge rectifier which, combined with the two 22001,lF filter capacitors, give plus and minus DC rails of about 21 volts. These unregulated DC rails are fed to LM317 and LM337 3-terminal regulators to provide the adjustable plus and minus supply outputs respectively. We'll briefly explain how these regulators work before going on with the rest of the circuit description. · The regulators are designed to give 1.25V between their output and adjust terminals. With this in mind, and the fact that the current flowing out of their ADJ (stands for ADJust) terminal is negligible, it is easy to design a variable regulated Fig.3: operating principle of the LM317 3-terminal regulator. Rt and R2 set the output voltage (see text). · supply. The circuit of Fig.3 demonstrates their operating principle. Two resistors are used to set the output voltage in the circuit of Fig.3. Rl is fixed while R2 is variable. Since the voltage be~ween the OUT and ADJ terminals is fixed at 1.25V, the current through Rl and R2 is also fixed. This gives a simple formula for the output voltage as follows: Vout = 1.25(1 + R2/R1) In our circuit Rl is 1200 while R2 is made up of of a 2.7k0 resistor in parallel with VRl, a 5k0 potentiometer. The maximum effective value of R2 is thus 1.75k0 and the theoretical output voltage range is therefore between 1.25 volts and 19.5 volts. However, the unregulated DC voltage fed into the regulators is normally not quite high enough to enable 19.5 volts output to be delivered. That explains the circuit as far as the positive regulator (LM317) is concerned but what about the negative regulator'? It has an operational amplifier connected to its ADJ terminal instead of a variable resistor. What giveth'? The idea of the op amp is to provide a mirror of the voltage at the ADJ terminal of the positive regulator. So if the ADJ voltage at the positive regulator is + 10 volts, the op amp will produce an output of - 10 volts by virtue of the fact that it is connected as a unity gain inverting amplifier. So ICl ensures that the negative regulator always tracks with the positive regulator. The 1200 resistor between the ADJ terminal and output of the LM337 is there for two reasons: first, to give the required minimum load for the regulator, and second to set a load current flow into ICl. This load current of 10.4 milliamps impresses a voltage drop of 10.4V across the lkO resistor at the output of ICl. This allows the op amp to drive the ADJ terminal of the LM3 3 7 regulator to - 17. 3 volts in spite of the fact that the negative supply rail to ICl is only - 14 volts. The supply rails for ICl are provided by zener diodes D5 for the positive line and D6 in series with LED 1 for the negative line. Diodes D7, DB, D9 and DlO protect the regulators from reverse voltages which may be generated by capacitive or inductive loads connected across the outputs. Drop-out indicator When the regulators are working as designed, the ripple voltage superimposed on the DC rails will be very low. However, if the current drain is higher than the regulator can supply while still maintaining about 2 volts between its input and output terminals, the ripple voltage will suddenly become quite high. The output voltage will fall rapidly if even more current is called for and the ripple will go even higher. When this condition is beginning to occur you may have no idea that it is happening. You need a visible JANUARY 1988 39 PARTS LIST 1 plastic instrument case, 205 x 159 x 68mm 1 PCB, code SC041-188, 112 x 92mm 1 . Scotchcal front panel, 1 90 x 60mm 1 meter scale display, 52 x • 43mm 1 6672 30V, 1 A transformer 1 single-pole pushbutton mains switch 1 DPDT mini toggle . switch 4 banana panel terminals (blue, white, red and green) 1 5k0 potentiometer 1 knob 1 mains cord and plug 1 cord clamp grommet 2 solder lugs 1 aluminium panel, 196 x 64mm x 1.5mm 2 T0-220 insulating kits (mica washer and bush) 1 MU45 panel meter, 0-1mA movement Semiconductors 1 LM31 7T positive adjustable 3-terminal regulator 1 LM337T negative adjustable 3-terminal regulator 1 TL071, LF351 FETinputop amp 1 741 op amp 9 1N4002 or equivalent 1A diodes 6 1N914, 1N4148 small signal diodes 1 12V 1W zener 1 15V 1W zener 2 5mm red LEDs Capacitors 2 2200µF 25VW PC electrolytic 2 1 OOµF 25VW PC electrolytic 4 1µF 25VW PC electrolytic 1 0.1 µF metallised polyester Resistors (5%, 0.25W) 1 x 2.2MO, 2 x 47k0, 1 x.39k0, 1 X X 22k0, 3 X 2.7k0, 3 X 1k0, 1 2200, 1 X 1800, 2 X 1200 Miscellaneous Solder, hookup wire, insulating sleeving, screws, nuts, selftapping screws etc. Putting it together Close-up view showing how the 3-terminal regulators are mounted (see also Fig.5). Use your multimeter to check that the metal tabs are isolated from chassis. indicator. Hence, we have designed a drop-out indicator using IC2. ICZ is connected as an inverting amplifier with a gain of about 800. It monitors both the positive and negative regulators via 2.7k0 resistors and a O.lµF capacitor. Diodes Dl 1 and Dl 2 limit any noise or ripple signal level to a maximum of ± 0.7V. The amplified ripple at the output of IC2 is fed to a full wave rectifier 40 SILICON CHIP consisting of D13 to D16 via a lkO limiting resistor, to feed a light emitting diode, LED 2. The LED begins to glow when the ripple at one of the regulator outputs becomes greater than about 4mV peak-to-peak. At about 19mV p-p ripple the LED is fully alight. A lmA meter monitors the output voltage via the lkn and 39k0 resistors. This gives it a full-scale reading of 20 volts. The supply is housed in a standard plastic instrument case measuring 205 x 159 x 68mm (Altronics Cat. No. H-0480 or equivalent). All the circuit with the exception of LEDs, switches and the pot, is accommodated on a printed circuit board measuring 112 x 92mm (coded SC041-188). Both 3-terminal regulators are bolted to the rear metal panel of the case for hea tsinking. You can start assembly by checking the copper pattern of the board for any breaks or shorts in the tracks. Compare it with the pattern published in this article. With that done, you can install all the small parts on the printed board. These include the resistors, diodes, links, small capacitors and the two op amps. Make sure that the ICs and diodes are correctly oriented before soldering them into place. Note that the two ICs face in the same direction. Use the wiring diagram of Fig. 4 to check each stage of assembly. Next, install the two 2500µF capacitors and the two 3-terminal regulators. The regulators should be mounted so that their bodies are about 10mm clear of the board, to allow them to be easily bolted to the back panel of the case. We recommend the use of PC pins for all external wiring from the POWER TRANSFORMER CLAMP GROMMET \ MAINS CORD Fig.4: follow this wiring diagram carefully and your supply should work first time. Use medium-duty 24 x 0.2mm cable for connections between the PCB and transformer, and to the output terminals and Load switch (see text). board. They simplify connecting it up and give easy test points when checking voltages. The completed printed board is supported on four of the integral plastic standoffs on the base of the case and secured with self-tapping screws. The transformer must be mounted directly onto the base of the case. To do this, two of the standoffs will have to be removed or drilled out and holes drilled for 3mm roundhead or countersunk screws. Use lockwashers under the two nuts. Note that the mains earth wire is terminated to a solder lug on the rear metal panel of the case and thence to a solder lug secured by one of the transformer mounting screws. The earth wire also goes to the green GND terminal on the front panel. When the printed board has been installed, slide the metal rear panel into the case and mark the mounINSULATING BUSH \ ~ Ml .I WASHER iI SCREW Deburr de burrs HEATSINK (REAR OF CASE) NUT ting hole positions for the two regulators. The mounting holes should be drilled for 2.5mm screws. Fig.5 shows the mounting details for the two regulators. Note that a mica washer and insulating bush must be used to isolate each device from the metal panel. / T0220 DEVICE Fig.5: mounting details for the two 3-terminal regulators. Before securing the regulators, make sure that the mounting holes are free of burrs. Lightly smear heatsink compound on the regulator heatsink surfaces and the mating areas on the metal panel. Then screw the two regulators to the panel as shown in Fig.5. You should then switch your multimeter to a low Ohms range and use it to check that the metal JANUARY 1988 41 tabs of regulators are both isolated from the metal panel. You can then work on the front panel. Kitset buyers can expect that they will be supplied with a screen-printed precut panel but if you're working from scratch you will probably have to make or purchase a Scotchcal panel. The artwork can be used as a drilling template for the front panel. The meter is supplied with its own template for the four mounting screws and 46mm diameter cutout. This latter hole can be made by drilling a series of small holes just inside the circumference of the marked circle and then filing the resulting cutout to a smooth circle. Having drilled all the holes, you can affix the artwork to the front panel. The material covering the holes is then removed using a Stanley utility knife. Now the front panel hardware can be mounted. In complete kits, a new scale should be supplied for the meter. This is easily fitted. Just unclip the meter bezel, undo two screws, remove the old panel and replace it with the new and then reassemble. Alternatively, you can remove the existing scale, erase the numbering and re-do it with Letraset. Complete the wiring by following What's a dual supply? "Wotsa dual tracking power supply anyhow and why would I want one?" we hear you ask, in your ardent quest for knowledge. The word dual refers to the fact that this power supply has two supply rails , one positive and the other negative. The word tracking refers to the fact that when you adjust the positive supply, the negative supply automatically follows so that it has the same absolute value. So if you set the positive output to plus 1 0 volts DC, the negative rail will be very close to minus 10 volts. That's what you'd expect, isn't it? Fig.4. Connecting wires to the potentiometer, the two LEDs and the meter can be light-duty hook-up wire but the remaining wiring should use heavier wire, such as 24 x 0.2mm insulated cable. The 3-core mains cable should have its outer insulation layer removed for a length of about 10cm so that the active lead can reach the mains switch on the front panel. The mains cord can then be secured to the rear panel using a cord-grip grommet. The neutral lead is terminated directly at the transformer, as is the other lead from the mains switch. Both the mains termination on the transformer and the mains switch itself should be sleeved with plastic tubing to avoid the possibility of accidental shock. When all the wiring is complete you should check your work carefully against Fig.4 and Fig.2 (the circuit diagram}. With that done, you can apply power and check the voltages. The unregulated voltages to the input of the two regulators should be about ± 21 volts, while supplies to the two op amps should be + 15V at pin 7 and - 14V at pin 4. Now check that the positive and negative supply rails can be varied over the range from below 1.5V to above 18V and that the two supplies track each other within ± lOOmV. The dropout indicator can be checked for correct operation by connecting a 220 resistor across either the positive or negative supply. Now, when the output voltage is wound up above 15 volts, the LED should light. All that remains is to secure the lid of the case and your power supply is ready for work. lb ~ _, _ .i. CLASS-2.5 • MU -45 • Fig.7: this full-size artwork should be used to replace the existing meter scale. The old artwork is removed by unclipping the meter bezel and undoing two small screws. Fig.6 (left): this full-size reproduction of the PC pattern can be copied and used to etch your own PC board. 42 SILICON CHIP