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13.5V 25A power supply for amateur transceivers This massive power supply has been designed especially for running big amateur transceivers and RF linear power amplifiers. It is much more efficient than typical transistor regulated power supplies and is fully protected with fuses, current limiting and overvoltage crowbar protection. By JOHN CLARKE & LEO SIMPSON Many of the 100-150W amateur transceivers available today require an external 13.5V supply for their power. This can be either in the form of a commercial power supply unit or an automotive battery together with suitable charging facilities. Both of these methods have their drawbacks. Ready-built power supplies are often not up to the task of supplying their full power output for more than a few minutes at a time because of their duty cycle limitations. In other words, they have been de58 SILICON CHIP signed to cope with the power demands of transceivers on speech mode. When long periods of continuous transmitter operation are required, as on RTTY (radioteletype) signals, these duty cycle limited power supplies are likely to be embarrassed. That's not to say that big, continuously rated power supplies are not available. They are, but at a considerable price. They also have the drawback of considerable heat dissipation, which requires big heatsinks and perhaps even fan cooling. The new SILICON CHIP 13.5V 25A Power Supply can supply continuous loads , does not get hot and does not have big heatsinks or need fan cooling. Why? Because it is not designed along conventional regulated power supply lines. The secret of its operation is Triac control and therefore little power loss in the system ofregulation. We'll talk more about this later in the article. Features The new power supply is built into a 3-unit high rack mounting case measuring 483mm (19 inches) wide), 140mm high and 340mm from front to back. On the front panel is the power switch, the output terminals, · the 30A fuse holder and the crowbar and regulated output indicator LEDs. At the rear is the mains input lead, mains fuse and heatsinks for the stud type rectifier diodes. Specifications for the power supply are listed in the accompanying panel and are very good considering the amount of current that the unit can supply. The load regulation var- ies by about 0.26V for every lO0W change in load. This means that for a no-load output voltage of 13.5V, the full load voltage will be 12.56V at 350W. For a change in mains voltage from 240V to 220V AC , the output changes by only 9m V. The figures for regulation and hum and noise output may not seem all that marvellous when compared with a conventional regulated power supply. However, they are quite good enough for use with amateur transceivers and RF linear power amplifiers. The crowbar protection is a plus feature for any transceiver. It works to short the output if it happens to rise above 15V DC. This might occur if there is a large transient on the mains supply, if someone fiddles with the output connections from the supply while it is delivering heavy current, or (horror of horrors) the power supply has a catastrophic failure. If the crowbar protection does operate it may blow the 30-amp output fuse but a more likely result is that the power supply will merely shut down. Normal operation is then restored by switching it off, waiting a minute or so, and then switching on again. The crowbar circuit can be adjusted so that it crowbars at 15V. Similarly, the output voltage can be adjusted for Specifications Rated output voltage ........................................ .... ...... ...... .... .... 13.5V DC Continuous output current ............ ..... .. .. .. ..... .. ...... .... .. ............... 25 amps Peak output current ..... ... .. .. ... ...... .. .. .. ... .. .... .... ........... ... .. ... ...... .. 35 amps Crowbar protection voltage ...... ... .. .. .. ... .. .. ...... .. ................. .... .. .. .. 15V DC Load regulation ..... ..... .. ................... .. .... .. ... .. ... .. .. .. . <7% for 25 amp load Line regulation (260VAC to 220VAC) ... .... ...... .. .. ........ ... .... .. .. . <20mV DC Hum and noise at rated output ....... .... ... 100mV P-P (no switching hash) 13.5V no load output and the overcurrent protection adjusted for a peak current of 35A. Method of regulation One of the main problems associated with conventional regulated highcurrent power supplies is heat dissipation. This is because the circuitry usually requires an input voltage about 10-12V higher than the specified output voltage. This is needed to allow for the inevitable voltage drop across the regulating transistors and to leave some margin so that the power supply can still deliver its rated output current even if the mains supply voltage drops to 220V AC or below. And there lies the problem. If the input to output voltage drop is 12V and the output current is 25 amps, the heat which must be dissipated by the regulators is 300 watts! After you allow for the additional power losses which occur in the transformer, the rectifiers , filter capacitors and internal wiring, a 13.5V power supply rated for 25 amps (ie, around 340 watts) is likely to dissipate a maximum of 380400 watts! In other words, it will waste more power than it can deliver and its efficiency will be less than half. No wonder big heatsinks and cooling fans are Below: the 13.8V 25A power supply is built into a 3-unit high rack-mounting case. Because it features Triac control, it does not get hot & does not require big heatsinks or fan cooling. Other features include foldback current limiting & overvoltage crowbar protection. MAY1991 59 'REGULATED OUTPUT. INDICATOR 15A FUSE CROWBAR INDICATOR POWER Sl ./ A ~ I ,. PHASE CONTROLLER- I I I 240VAC MAINS I I 7kV MAINS ISOLATION 30A FUSE PRI 240V + N~D----------1--+--+---___,, CURRENT SENSE E~ CASE OUTPUT 13.5V 25A OUTPUT SENSE GROUND REFERENCE CURRENT SENSE CROWBAR Fig.1: the general arrangement for the power supply. Regulation is achieved by using a Triac (part of the phase controller) to switch the primary of power transformer Tl. The output from the bridge rectifier is then filtered using two chokes & two 80,000µF banks of capacitors. losses are reduced by other circuit measures. Block diagram a must for these conventional power supplies. When you consider the above points against conventional power supplies it is no wonder that personal computers come with much more compact and efficient switching power supplies. These are now well proven and very reliable but they present a problem when used with sensitive transceivers - interference from the switching hash. The method of regulation used in the n ew SILICON CHIP power supply reduces dissipation to a minimum and produces very little in the way of radio interferen ce. It does this by controlling the mains voltage supplied to the primary of the power transformer. This avoids the power losses in regulating transistors although there are still losses in the transformer, rectifiers and filter components. However, as we shall see, transformer and filter Most of the circuitry for the power supply is mounted on this PC board, while a second PC board holds the two 80,000µF capacitor banks. 60 SILICON CHIP Fig. l shows th e gen eral arrangement for the pow er supply. The 240VAC mains input to the transformer (Tl) is driven by a phase controller which uses a Triac as the switching element. The phase controller circuitry monitors the output voltage and current and then adjusts the amount of mains voltage which is supplied to Tl so that th e output voltage remains within sp ecified limits. If the controller sens es excess output current, then the transform er voltage is reduced to limit the current to safe levels. The secondary winding of transformer Tl drives a bridge rectifier consisting of four 70A stud mounting diodes. These really rugged diodes are mandatory in a big supply of this nature - smaller components quickly snuff it. Instead of.directly feeding a bank of filter capacitors, the output of the rectifier is fed via a 50µH smoothing choke which is quite a substantial component and then to a bank of capacitors totalling 80,000µF. Following these is another smoothing choke, this time of lmH , and then another 80,000µF capacitor bank. The use of smoothing chokes has several big benefits. First, it greatly reduces the huge peak charging currents which would otherwise need to be supplied by the transformer and rectifier diodes. Normally, these charging currents can be expected to be as much as 10 times the average output Because heatsinking requirements are minimised by the design, construction is fairly straightforward. The phase controller switches the 625VA toroidal transformer at upper left to provide output voltage regulation, while a second smaller transformer supplies all the low-voltage control circuitry. current. With a 25 amp rated output, charging currents of this order (250 amps peak) would cause very high heat dissipation in the transformer secondary, in the rectifiers and in the connecting wiring. In effect, the use of smoothing chokes in this power supply is <!throwback to the power supplies of valve amplifiers. The chokes give another advantage too - residual hum with an almost pure lO0Hz sinewave rather than the more troublesome l00Hz sawtooth hum waveform of conventional capacitor-input power supplies. So by using chokes in the smoothing network (also known as pi-section filters) and a phase-controlled Triac in the transformer primary, this power supply completely avoids the use of conventional regulators. power supply, considerable isolation is required to ensure protection of both the user and the transceiver equipment. This is provided by an optocoupler which is rated for 7.5kV isolation. Current sensing presents a problem too, at the high current ouptuts of this supply. In this circuit, the current sense resistor is only 2 milliohms so that the voltage drop across it at a current of 25 amps is only 50mV. The crowbar protection trips an SCR if the DC output voltage exceeds 15 volts. Under normal circumstances, the crowbar SCR will discharge the capacitors and the power supply will shut down. The shutdown is due to the crowbar feedback signal which tells the phase controller to stop supplying power to the transformer. High voltage isolation The complete circuit for the new power supply is shown in Fig.2. The lower section of the circuit is really quite similar to the block diagram Since there is a feedback connection between the 240V AC phase controller and the 13.5V output of the Main circuit while the rest of it is mainly taken up by the circuitry which controls the Triac. Notice that there are two power transformers in the circuit. First, there is the big 625VA job which is controlled by the Triac (down in the lefthand corner of the circuit). The second transformer, T2, is in the top lefthand corner of the circuit and supplies all the low voltage control circuitry. T2 is a 12.6V 150mA transformer which is connected to a half-wave voltage doubler circuit consisting of diodes D5 and D6 and two l000µF capacitors. This provides about 17V across each l000µF capacitor and feeds 7805 positive and 7905 negative 5V regulators. The overall voltage across the two regulators is lOV. The output from the 7905 is designated the GND reference for the circuit, while the output of the 7805 becomes the +10V rail. The reason for producing this fairly complex supply rail is so that we have a precise +5V reference or centre tap for the control circuitry and for the zero crossing detector. The AC waveform from T2 is apMA Y 1991 61 '-:I ::r:: ..... n :z: n 0 [=:: Cl) N Cl) OUT POWER GND t IN I I I : CASE .,. 1, .Iii 16VWI POWER O_N RESET AND DELAY E~ N ~ -- 240VAC I OUT 0.1 25p~AC _J •I I I I I --~--7 ~]: A GND 5 ffi' ffl IN 5 10 .,. 114 --....., !!.D RESET AT ZERO CROSSING T2 2851 I I 33on 2.2k J"lJl_ 10 ~ .,. ~ '"'I I VOLTS SET VR11QO~ 01 BC327 +lOV 10k t *0.1l cu~~Wo~ET :Sl 470k t I CURRENT LIMIT • -1 013 . ~ ~ .,. 33 .J.:" 16VWI SLOW TURN ON CROWBAR INDICATOR LE02 1k~ 4701 470k~ 470k 1N4148 47k .,~. ZERO CROSSING DETECT!)R I .022 1N4148 DlO 1N414~ +10v-~...- - - - - - - - - - - - - - - - - - , 100k +l QV -:- 2.2~ I I m I -;- +10V 2.2 BP 100k .,. I .027! ------II 470k! <at> D11 1N4148 RAMP f\.j\J 2.2k .,. *il.1I I Fig.2: the bottom section of the circuit diagram is quite similar to the block diagram (Fig.1). Triac 1 is used to switch the mains input to transformer Tl, while IC6 (MOC3021) provides the 7kV isolation for the control circuitry. In operation, ICla compares an error voltage from IC3a with a ramp voltage generated by ICld & IClc, & switches the 'Iriac via IC4b & Ql. IC5 & SCRl provide the overvoltage crowbar protection while IClb monitors the voltage across the 2mQ current sense resistor. ~ ~~~ + t-f'r.iN g- ... "'~ ... 0 > ffi _..,"' > 0 ..... 0::<0 c..,O:: ::E i ~ ~ + I c::i~ !;;~ .. ., Ei"' "' ~:,; -u ..,c ., ::E ·c plied to pin 8 ofICld (a comparator) via a lkQ resistor and is clamped to the +5V supply by diodes D7 and D8. ICld is connected as an inverting Schmitt trigger so that its output at pin 14 swings high when the AC waveform falls just below the +5V reference supply rail, and low when the AC waveform goes just above the +5V reference. ~Ii! -oca: a:<> c., ~ ·ffl ~c ~~ •..,i;~ . I· .c:3~"i ~ ~$!c: C: c. c. <O <O + .,.~1 I e"' :::, C g~g CJ) ="' eM a: w cc- ~,§ 3: :~§: 0 C:3: i.n an c. ~~£!~ !:?."' + <( I It) N C .8c~8 c- > U"! ="'M e M ,- Q==--;. ~~ g;: ·'..! -> :is- ~-~ -~ C ' a:~ <"' > .... > I GIJ=;~ < ~ .., =(J ~· ~3:: :is.,:; ~~ -g ::E ,. 3:: ~ m Ramp waveform Thus, ICld's output is a square wave which changes from low to high and high to low at the zero crossing points of the 50Hz mains AC waveform. This square wave is buffered by Schmitt NAND gate IC2c and inverted by gate IC2d. At the output of each of these two NAND gates is a differentiating network consisting of a .022µF capacitor and a 47kQ resistor tied to the +lOV rail. The differentiating networks convert the NAND gate output square waves into spike waveforms - every time the output goes high or low, a positive or negative spike occurs. Diodes D8 and D9 act as an OR gate so that their output is a train of negative spikes with a repetition rate of lO0Hz, locked to the zero crossing points of the 50Hz mains waveform. This waveform is fed to IClc which is an LM339 comparator connected as a Schmitt trigger. The positive pulse output ofIClc is used for two purposes: (1) as a reset line for IC4b (more about this later); and (2) to provide a sawtooth ramp voltage via diode D11 and its associated .027µF capacitor. The resulting waveform is synchronised to the zero crossing points of the mains waveform. It is depicted in the series of waveforms shown in Fig.3. The ramp voltage is applied to cornparator ICla at pin 6 and is compared with the error output of op amp IC3a at pin 7. Error amplifier IC3a is the error amplifier which monitors the DC voltage at the first 80,000µF capacitor bank. This amplifier has a gain of -10 due to its lOkQ input resistor and lO0kQ feedback resistor. A 2.2µFbipolar capacitor across the lO0kQ resistor sets the response time of the amplifier. IC3a amplifies the difference between the voltage across the first 80,000µF capacitor bank and the reference voltage set at its pin 12. This amplified voltage is called the error voltage and is applied to pin 7 of comparator ICla. The waveforms of Fig.3 show the error voltage superimposed on the ramp voltage. Each time the ramp voltage at pin 6 falls below the error voltage at pin 7, the output of IC la goes high. This waveform could ostensibly be used to drive the MOC3021 optocoupler and hence trigger the Triac (in the lefthand corner of the circuit). But we send the waveform through a little more jiggery-pokery before that happens. The output of comparator ICla is applied to the clock input of flipflop IC4b. This merely inverts the waveform and drives transistor Ql which inverts the waveform again and then drives optoptocoupler IC6. The optocoupler drive is shown as the third waveform in Fig.3. Power on delay Now have another look at IC4b because there is a little more to it than we've just described. It is reset by the pulse waveform from IClc (as mentioned earlier) but not before that waveform passes through NAND gates IC2a and IC2b. IC2a functions merely as an inverter but IC2a does a little more since it has a lµF capacitor connected to its pin 6. This provides the power on delay for the Triac circuitry. What happens is that when the circuit is first turned on, the lµF capacitor at pin 6 of IC2b is discharged and therefore IC2b does not gate any reset pulses through to IC4b. Hence, IC4b sits there doing nothing and Ql cannot turn on to drive IC6 and the Triac. So the big transformer gets no voltage applied to it. After a few seconds, when all the voltages for the control circuitry have MAY 1991 63 PARTS LIST 1 3-unit high rackmount case (Altronics H-0417 or equivalent) 1 AT96 18V 625VA toroidal transformer (Harbuch Electronics) 1 50µH 25A choke (L 1) (Harbuch Electronics) 1 1mH 25A choke (L2) (Harbuch Electronics) 1 Neosid 17/742/22 iron powdered toroid (L3) 1 2851 12.6V CT transformer 1 PC board, code SC14105911, 165 x 125mm 1 PC board , code SC14105912, 180 x 176mm 1 Dynamark front panel label, 120 x80mm 1 Dynamark front panel label, 100 x25mm 2 heatsinks, 75 x 105 x 25mm (DSE H-3422) 1 30A panel mount 5AG fuse holder (Altronics S-6030) 1 30A 5AG fuse (Altronics S5977) 1 3AG panel mount 240VAC fuse holder 1 15A 3AG fuse 1 large red binding post 1 large black binding post 1 neon illuminated DPDT rocket mains switch (DSE P-7706) 1 cordgrip grommet 1 mains cord with moulded 3-pin plug 2 Clipsal 563K16 insulated connectors 2 8mm cable clamps 1 2-way mains terminal strip 2 5mm LED bezels 14 PC stakes stabilised, the voltage at pin 6 of IC2b rises sufficiently to allow the reset pulses through to flipflop IC4b and so the Triac starts getting trigger pulses on every hal f cycle of the m ains waveform. But even th en there i s some trickery involved. Slow turn on Big toroidal transformers such as the 62 5VA job use d here do not like being hit with the full m ains voltage when they're first switched on. If that 64 SILICON CHIP 1 1-metre length of 3.2mm squared dual cable (DSE W2015) 1 1-metre length of 7 .5A mains rated cable 1 150mm length of 1.25mm enamelled copper wire 1 2-metre length of 0.63mm enamelled copper wire 8 6mm PC standoffs 5 cable ties 27 machine screws and nuts 4 6mm ID rubber grommets 4 insulating kits for 1/4-28 threaded stud diodes 5 10mm Utilux eyelet lugs 2 4mm Utilux eyelet lugs 1 solder lug 1 100kn miniature horizontal trimpot (VR1) 2 20kn miniature horizontal trimpots (VR2, VR3) Semiconductors 2 70HFR20 or 70HFR40 70A stud diodes (from NSD) , (01 ,02) 2 70HF20 or 70HF40 stud diodes (from NSD), (03,04) 3 1N4002 1A diodes (05,06 ,014) 7 IN4148 signal diodes (07-013) 1 MAC320A8FP insulated tab 20A mains Triac (from VSI), (Triac 1) 1 MCR69-2 25A SCR (from VSI), (SCR1) 1 BC327 PNP transistor (01 ) 1 BC337 NPN transistor (02) 1 7805 +5V 3-terminal regulator (REG 1) 1 7905 -5V 3-terminal regulator (REG 2) 2 5mm red LEDS (LED 1, LED2) 1 LM339 quad comparator (IC1) happens , they draw heaps of curren t and they blow fus es an d dim your house lights. To avoid that little pr obl em , we have a slow turn on feature . This starts out by triggering the Triac very late in each half cycle and so the transformer gets only a small portion of the mains w aveform to nibble at. After that, the Triac trigger pul ses are allowed to arrive earlier in each half cycle and the circuit stabilises at its specified output voltage. This slow turn on feature is pro- 1 4093 quad NANO gate (IC2) 1 LM324 quad op amp (IC3) 1 40 13 dual O-flipflop (IC4) 1 MC3423 overvoltage crowbar (IC5) 1 MOC3021 Triac driver (IC6) 1 V275LA20 Varistor Capacitors 16 10,000µF 50VW PC electrolytic 2 1000µF 25VW PC electrolytic 1 470µF 16VW PC electrolytic 1 33µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 1 2.2µF 50VW bipolar electrolytic 2 1µF 16VW PC electrolytic 1 0.1µF 250VAC mains capacitor 3 0.1 µF monolithic 1 .027µF metallised polyester 2 .022µF metallised polyester · 1 .01 µF metallised polyester 1 .01 µF 250VAC mains capacitor Resistors (0.25W, 5%) 9 470kn 1 680n 1 150kn 2 680n 1W 2 100kn 1 560n 3 47kn 1 390n 9 10kn 1 330n 2 4. ?kn 1 100n 1 3.3kn 4 56n 5W 5 2.2kn 1 47n 1W 5 1kn Miscellaneous Hookup wire, heatshrink tubing (for mains switch & fuseholder), heatsink compound , solder. vi ded by the 33µF capacitor at pin 12 of IC3 and the 150kn/10kn voltage divider on pin 13. The voltage divider supplies a small vol tage to pin 13 in the absence of any voltage across the first 80,000µF capacitor bank, w hile the 33µF capacitor provides a slow ri se in the reference voltage at pin 12. The ramp, the error voltage and the Triac drive thus provide a control loop which keeps the DC output of the supply at a constant voltage, as set by VR1. If, for example, the DC output rises above · its set voltage, the error voltage drops and, as a result of comparator action by ICla, the Triac is fired later in the mains cycle. Thus, the DC output voltage will fall. On the other hand, if the output voltage falls below its set voltage, the error voltage rises higher up the ramp. Thus, the Triac fires earlier in the mains waveform to increase the DC output voltage. RAMP PIN6, IC1a Current limiting Comparator stage IClb provides the current limiting feature. This stage monitors the voltage across the 2 milliohm (2mQ) current sense resistor via a lOkQ resistor, while VR2 provides a reference voltage on pin 5. If this reference voltage is exceeded by the voltage across the 2mQ sense resistor, IClb's output goes low. This low output discharges the 33µF capacitor 1.3-t pin 12 of IC3a via diode D12. This pulls the reference voltage on pin 12 of IC3a low and consequently the DC output is reduced to a very low voltage. At the same time, because of the hysteresis effect of the 470kQ resistor between pins 2 and 5 of IClb, the reference voltage at pin 5 is greatly reduced. This has the effect of lowering the current sense threshold even further so that the trigger pulses delivered from ICla (and IC4b, Ql & IC6) to the Triac come even later in each mains halfcyle. Thus the output current is "folded back" to quite a low value which the supply can deliver without getting hot. Once the current overload is removed, the power supply voltage returns to normal. Crowbar protection The crowbar circuit operates inde- This CRO photograph shows the ramp waveform synchronised with the 50Hz AC mains sinewave. MOC3021 Pli4f!C6 LEO DRll!E Fig.3: these waveforms show how the mains input to transformer Tl is switched to achieve regulation. ICla compares the ramp voltage on its pin 6 input with the error output from IC3a. Each time the ramp voltage falls below the error voltage, ICla's output goes high & the MOC3021 switches on to drive the mains-switching Triac. pendently of the rest of the control circuit but is linked in with it, as we shall see. It is in the righthand bottom corner of the circuit. IC5 is a Motorola MC3423 overvoltage crowbar IC designed specifically for this task. It monitors the output voltage via a lOkQ resistor and trimpot VR3 which is normally set to trip at 15V DC. When the supply's output voltage reaches 15V, IC5's output at pin 8 goes high and turns on SCRl. SCRl "crowbars" (ie, short circuits) the output voltage and discharges the 80,000µF banks of capacitors. If the voltage went high as a result of a circuit defect, SCRl will also blow the 30 amp output fuse so that no further damage can result. Normally though , an over-voltage condition may not be due to a circuit defect and so the crowbar IC can shut down the control circuitry. It does The lower trace in this photo is the chopped mains waveform applied to the transformer at light loading. This photo shows the chopped mains waveform applied to the transformer with a 350W load. MAY1991 65 . The big 70A stud-mounting diodes in the rectifier (D1-D4) are bolted to heatsinks on the rear panel of the case. Because of the efficient technique used to regulate the supply, heatsinking requirements are modest compared to more conventional units which have large losses in the regulator transistors. this as follows: when the trip condition occurs, pin 8 goes high and pin 6 goes low and turns off Q2 which is associated with flipflop IC4a. Q2 causes IC4a to change state so that its Q-bar output (pin 2) goes low. This causes the 33µF capacitor at pin 12 of IC3a to be discharged via diode D13. As we have seen before, when this capacitor is discharged, the supply is shut down and so very little current passes through SCRl. When IC4a's Q-bar output goes low, its Q output (pin 1) goes high and this lights LED 2 to indicate that the crowbar circuit is on. The supply must be turned off and then on again after a minute or so, to resume normal operation. Normal supply indicator IC3b and IC3c are connected together as a window comparator to indicate when the output voltage of the power supply is within the range +11.8 to +14.4V DC. It does this as follows. The inverting inputs (pins 6 &.9) of IC3b and IC3c monitor the output of 66 SILICON CHIP the supply via a voltage divider consisting of two l0kQ resistors. Their non-inverting inputs (pins 5 & 10) are connected to the +10V rail via a vo ltage divider consisting of three resistors. This establishes a reference voltage of +7.2V at pin 5 and +5.9V at pin 10. Thus, provided the voltage at pins 6 & 9 lies between these two voltages, LED 1 will be lit, indicating that the supply output voltage is between 14.4V and 11.SV (ie, twice the reference voltages). If the voltage at pins 6 & 9 goes below +5.9V, the output of comparator IC3c will go high and so LED 1 will go out. Similarly, if the voltage at pins 6 & 9 goes above +7.2V, the output ofIC3b will go low, again causing LED 1 to extinguish. That just about concludes the circuit operation so let's recap on the story. Basically, this unit is a big brute force power supply with filtering performed by chokes and two 80,000µF banks of capacitors. The regulation process is achieved by a Triac in the primary of the power transformer so that very little power is wasted. The control circuit includes turnon delay, slow turn-on, foldback current limiting, crowbar over-voltage protection and output voltage indication, all of which we have just described. Despite all this circuit complexity the unit is very easy to build as most of the parts are mounted on two PC boards. One of these boards holds all the control circuitry while the second board holds the two 80,000µF capacitor banks. But that's all we have space for this month. Next month, we will complete the description of the power supply by giving the construction and setting up details. Acknowledgements Our thanks to the following corn panies for their assistance with this project: Harbuch Transformers, for the design and supply of the 625VA toroidal power transformer and the two iron cored filter chokes; Altronics Distributors, for the supply of the rack mounting case (Cat No. H-0418) and the electrolytic filter capacitors; NSD, for the 70HF20 stud diodes; and VSI Electronics Australia Pty Ltd, for various Motorola ICs and semiconductors. SC