Fullscreen HTML5Med Res (??MB)HTML4

DC VOLTS DC AMPERES Adjustable 0-45V 8-amp power supply Do you need a really big power supply? One that's big enough to allow you to do .away with the car batteries on your workbench? Well this is for you. Its output is adjustable from 0-45V DC & it can deliver currents up to 8 amps. Design by JOHN CLARKE In the past, a power supply rated at 0-45V and 8A would have been much bigger than this unit. It would have needed really big heatsinks and possibly fans as well to keep it cool. It would also have needed a much bigger transformer and more filter capacitors, and would generally have been a much more expensive unit. 64 SILICON CHIP So why is this unit not as big as those older designs? Because it is a switchmode power supply, using the same principle as the millions of power supplies used in personal computers. These are light in weight, compact in size and highly efficient. Because it uses the switchmode principle, this new supply has no large heatsinks yet always runs cool or only slightly warm to the touch. The new supply is housed in a large plastic instrument case and has meters for voltage and current. There are two knobs to adjust the output: one for voltage and one for the current. Just below the voltage knob is a toggle switch which allows the supply to deliver a fixed 13.8V output - handy if you are working on any automotive device. There is also a pushbutton switch to allow the maximum output current to be set an d a toggle switch to connect the supply output to the front panel terminals. The supply has three binding post terminals on the front panel: red for positive, black for negative (OV) and green for Earth. Neither POWER A o--o"""a---.-S1 240VAC 35V ►-----r--------------1t-----O+ + Nu----,--+--~ Cl-CS DC OUTPUT 50VDC ~ARTH OUT +12V CURRENT CONTROL CIRCUITRY VOLTAGE Fig.1: this simplified block diagram shows how the circuit works. The control circuit monitors the output voltage & current & drives an optical fibre link. This optical fibre link in turn controls FET Q1 which functions as a switching regulator. D2, L1 & C6/C7 filter the pulsed waveform from Q1 to produce a steady DC output voltage. the positive or negative terminals are internally connected to the power supply chassis so it may be used as a fully floating supply or with one side connected to Earth. There is a red LED above both adjusting knobs and these indicate Regulator Dropout and Current Overload. These will normally not come on unless the supply is overloaded or for a brief moment at switch-on. Inside, the new supply has a large PC board and two power transformers (one a large toroidal unit and a smaller unit). On the rear panel there ~s a finned heatsink and two semiconductor devices. These devices are a fast recovery diode and, at the heart of whole circuit, a 33-amp 100V FET (field effect transistor). The supply dimensions are 352 x 129 x 280mm (W x H x D), including knobs and rear projections. It weighs 5.5kg, which is surprisingly light for a supply of this rating. Now let's have a look at the circuit details. Simplified circuit While switchmode supplies have the advantage of lightness and efficiency, they tend to be more compli- cated in circuitry than conventional linear power supplies . In the case of this power supply, the situation is more complicated because we have used a FET as the main switching device rather than paralleled bipolar transistors. The FET is much more rugged but requires more complicated drive circuitry. Never fear though, because the basic concept is relatively simple, as we shall now describe. Fig.1 shows the simplified circuit for the new power supply. Transformer T1 (the big toroidal type) has a 35VAC secondary which is rectified by bridge 1 and filtered by capacitors C1 -C5 (a total of 23,500µF). This results in an unregulated 50VDC source for the switchmode supply. Switching transistor Q1, diode D2, inductor L1 and the output capacitors C6 & C7 comprise the stepdown switching regulator circuit. Q1 is switched on and off at about 20kHz. If the pulse switching waveform has a short duty cycle (ie, Q1 is off most of the time), very short pulses of current will be fed to L1 and the resultant DC voltage across C6/C7 will be low. Conversely, if the pulse duty cycle is high, Q1 will be on for most of the time and the DC voltage across C6/C7 will be high. By varying the pulse duty cycle from zero to 100%, the output voltage can be varied from zero to close to 50V; ie, around 45V or a little higher. Control circuit The 20kHz pulse switching waveform is produced by the control circuitry. This monitors the output of the switching circuit and therefore needs its own supply voltage which is provided by the small transformer Specifications Of Prototype Output Voltage ...................................................... .. . 0-45V Output Current .. .. .................... .................... ..... ........ 8A below 35V 6A at 40V Load Regulation ...... ........................ .... .......... .... ........ 1% Ripple and Noise .. .......... .. ........................... ...... ....... 5mVp-p at 6A 13V 10mVp-p at 8A 18V 40mVp-p at 8A 35V Current Limit ...... .... .... ................ ............................ .. 800mA to 8.6A Overcurrent Limit ............................... .. .................... 9A Fold back Current ........ .... .. .............. ...... .... ...... .. ..... .. < 2A JA NUA RY 1992 65 Despite the apparent circuit complexity, the supply is easy to build as most of the parts are mounted on a single large PC board. The circuit employs switchmode regulation, so relatively little heatsinking is required. T2, the bridge rectifier comprising diodes D3-D6, and the associated 7812 regulator. The control circuitry feeds its pulses to Ql via a light link. This consists of a LED and photodiode detector pair which are coupled via a short length of optical fibre cable. Both the detector and the LED are contained in neat little packages which attach to each end of the short optical cable. Made by Siemens, they provide complete electrical isolation between the control circuitry and Ql. Ql is a Philips 33A Mosfet which has a very low on resistance of about 0.052Q. This means that its dissipation is very low, even when the supply is delivering currents of 8 amps. Now let's have a look at the complete circuit as shown in Fig.2. Circuit description The whole circuit of Fig.2 looks 66 SILICON CHIP pretty daunting but if we deal with it in sections it becomes easier to understand. First, let's recognise the similarities between the whole circuit of Fig.2 and the skeleton circuit introduced in Fig.1. In the top lefthand corner ofFig.2 is the transformer Tl and its associated bridge rectifier and filter capacitors. It provides the 50V DC which feeds Ql (in the centre top section of the circuit). Ql is driven by a bank of paralleled inverters (IC2), driven in turn by Q2 and the optical fibre link comprising ICl and LEDL The light path is depicted by a dotted line between the ICl and LEDL LEDl is controlled by IC3, Q3 & Q4 and, together with all the other ICs shown on Fig.2, these comprise the control circuit shown on Fig.1. Ql is connected in series with the negative supply rail, with D2, inductor Ll and capacitors C6 & C7 forming the switchmode arrangement shown in Fig.2. The switching action of Ql can cause large voltage transients at its drain electrode and to protect against excessive voltages here, a 75V zener diode (ZD3) connects from the drain of Ql to its gate, via diode D1. Thus, if the voltage at the drain of Ql exceeds 75V, Ql is turned on again to shunt the transient. As noted above, the gate signal for Ql comes via IC2, Q2 and the light link, ICl . IC2 and Q2 are powered from a 15V rail derived by 15V zener diode ZDl from the 50V supply via Fig.2 (following pages): FET Ql & the optical fibre link (ICl & LED 1) form the heart of the circuit, while oscillator IC3a, comparator IC3b & error amplifier IC5d form the control circuit depicted in Fig.1. VRl, IC5b & IC5c provide the adjustable current limit feature while IC3c provides foldback current limiting protection. CJ-118A 20MHz Dual Trace C,s~UJ♦ ♦S~c:>pe SPECIFICATIONS: Operating modes Yl, Y2, Yl and Y2 added are swltchable alternately and Intermittently. Bandwidth, MHz: ■ DC coupled input: at -3dB .. 0-20 ■ DC coupled input: at -6dB .. 0-35 ■ AC coupled input: at -3dB .. 3xl0·'-20 ■ AC coupled input: at -6dB .. 2xl0·'-35 ■ input with external divider in position 1:1 , .. 0-7.0 Transient response built-up time, ns, max.: ■ ■ DC coupled input: .. 17.5 with external divider in position 1:1 .. 50 Deflection factor (11 calibrated steps with 1-2-5 seq.) ■ ■ ■ mV /DIV: Accuracy,%: Max. input (AC/DC) V: .. 5-10' .. +/- 4 .. 250 Input Impedance, Mohm/pf: ■ ■ ■ DC coupled input: with external divider in position 1:10 with external divider in position 1:1 .. 1/20 .. 10/15 Dimensions: .. 1/100 212 x 133 x 336mm Sweep speed (21 calibrated steps with 1-2-5 seq.) ■ ■ ■ .. 20-50xl0 .. +/- 4 .. +/- 8 ns/DIV: Accuracy, %: 50ns/DIV+ Accuracy,%: 20ns/DIV Screen Size: 4" 3 Trigger modes: ■ Auto, turning to triggered upon application of trigger signals HOPM (NORM). Trigger source: ■ Includes 2 x 1:1 10:1 probes, and screen protection cover. Internal (from channel Yl or Y2), external. Sensitivity for Internal synchronization, V: ■ ■ 50Hz to 4MHz, DIV., Max.: .. 0.8 4MHz to 20MHz, DIV, Max.: .. 2 Sensitivity for external synchronization, V, Max.: ■ lOHz to 20MHz .. 0.2 Typical external synchronization ■ Input imp. kohm/pf .... , Note: for Government Dept's, Schools, Sales Tax No. holders, etc only $430.43 ea. (ex-tax) - official order must be produced .. 70/15 . Available from: ··· ····.·.• · David Reid Electronics >/127 York Street,> Universe Computers 127 Melbourne St, .· ....... ... •···· ·.· . SYDNEY "-cS.W.2000 Nth ADELAIDE S.A. 5007 Ph:(02)2671385 Fax: (02)2618905 Ph: (08) 2391577Fax: (08) 239 1221 {Mai/orders ·welcome} A.C.N. 001-372-372 {Mai/orders welcome} +50V +15V 3900 0.5W +4.7V 0.1 1k C1-C5 5x4700 50VW + - Z01 15V 3W Z02 4.7V 1W 0.1 3 j 2 \ \ \ LEVEL SHIFTER \ \ o~rb~\L i~:E \ \ \ OUT \ 6.8k \ 100k 100k \ \ 100k 03 BC338 C11 1000 25VW + - + 1 16VW \ 10k 2.2k \ \ \ 10 16VW 100k B 04 BC328 820pf 10k 470k B_ _ _ _ _ ___..__-4----___.__ _.___ _ __..._ RAMP OSCILLATOR _._4_.______J...__--l..._L--l--L--COMPARATOR VOLTAGE ERROR INPUT 09 1N4148 +12V CURRENT 08 1N4148 S2: 1: CURRENT 2: SET CURRENT 47k 2.2k 270k 10 16VW CURRENT M2 1mA CURRENT 100k 10~~1N~--4---+------'.___J LE03 CURRENT LIMIT 0.1 - 0.1.I. 1k 10 + C 47n 68 SILICON CHIP >-. K 1.5k 16VW CURRENT LIMIT AMPLIFIER A OVERCURRENT LIMIT LOAD S3 ~+ .001 A +12V 18k 100k S4: 1: ADJUSTABLE 2: 13.BV FIXED 02 MUR1515 K C6,C7 2x1000 50VW 47k +2.5V + 0.1 250VA1 CB-C10 3x1000 50VW - + - A 13.BV VR6 50k AOJ - VOLTAGE;;i,,,.,.._---l'""""<□ S4 VR4 10k LIN 1k + 1 16VW L1 VOLTAGE ERROR AMPLIFIER FILTER =]J= VOLTAGE REFERENCE 0.1 250VA1 EARTH Jn A - - - - - e - - - -- -- - - - - - - - ~ . . -- - - - -- ---4----e- - -- -- - -..---+---+----+----+--+--.-+12v 2.2k 4.7k 22k 100k 1 18k 100k 16VW _ 13 10!2 5W 07 1N4148 1 16VW + - 1k 5W 2.2k 2.2k 470!2 05 BC639 10k 0.1 14 100k 8 C LE02 OROPOUT K 1k 470n DROPOUT DETECTOR MINIMUM LOAD SWITCHING 10V OFF 5V OFF L1: 10T 1.2mm DIA ENCU ON NEOSIO 17-745-22 IRON POWDER CORE L2: 6T 1.2mm DIA ENCU ON NEOSIO 17-742-22 OR PHILIPS 4330 030 60210 CORE . . .,,.1 C a<at>c BF199 BC328,338 aOE 8C639 VIEWED FROM BELOW 1 23 m,.,,.~ AK 40V BA POWER SUPPLY JA NUARY 1992 69 Vp (a) Vp (b) Fig.3(a): the triangle waveform VT from IC3a is compared with the error voltage VE from IC5d to produce the gate switching pulses for Qt. When VE is high, comparator IC3b delivers wide pulses as shown in (a). Conversely, when VE is low, the comparator delivers narrow pulses as shown at (b). two lkQ resistors in parallel. ICl, the light detector, is powered from a 4.7V rail derived by zener diode ZDZ from the 15V rail via a 390Q resistor. Light detector ICl, the light detector, has an integral photodiode and an amplifier with an open collector output at pin 3. This is loaded with a lkQ resistor and drives transistor QZ, a voltage level shifter which drives ICZ. Five inverters in ICZ are paralleled so that they have sufficient drive for the gate of Ql. The inverters are connected to the gate via a 470Q resistor which effectively slows down the turn-on and turn-off times for Ql by a slight amount. This has been done deliberately to reduce the amount of high frequency filtering needed for the final DC output. Control circuitry The core of the control circuitry comprises the triangle waveform generator IC3a, the error voltage amplifier IC5d and comparator IC3b. IC3a is a comparator which is connected as a Schmitt trigger oscillator running at Z0kHz. The output waveform, a triangle (or sawtooth), is taken 70 SILICON CHIP from across the 820pF capacitor and fed to the inverting input of comparator IC3b. IC3b then compares the triangle voltage with the error voltage fro m IC5d and generates switching pulses to drive LEDl and, ultimately, the gate of Ql. This process is illustrated by the waveforms of Fig.3. Have a look at how the triangle waveform VT is compared with the error voltage waveform VE in the comparator IC3b. When the error voltage is relatively high as in Fig.3(a), the comparator delivers wide pulses from its output at pin 1. On the other hand, when the error voltage is relatively low, the comparator delivers narrow pulses as shown in Fig.3(b). The output of IC3b feeds transistors Q3 & Q4 which form a buffer stage to drive LED 1 via a 270Q resistor. LED 1 is the transmitting end of the optical link which drives ICl and, ultimately, the gate of Ql. We now flick across to the top righthand corner of the circuit (Fig.2) to have look at the section involving IC5d, the error voltage amplifier. This op amp monitors the output voltage of the power supply to make sure that it is controlled within close limits. IC5d compares a portion of the output voltage, applied to its inverting (-) input at pin 6, with a reference voltage applied to its non-inverting (+) input at pin 5. The reference voltage at pin 5 is tapped off ZD4 by VR4 or VR6 (depending on the setting of switch S4). ZD4 is an LM336Z -2.5V precision reference diode. IC5d amplifies the difference between the voltage at its Main Features • 0-40V adjustable output • 800mA to 8A adjustable current limit • Short circuit proof with foldback cu rrent limit • Separate voltage and current metering • Regulator dropout and current overload indication • Output load switch • 13.8V output switch setting • Switchmode design • Minimal heatsinking pin 6 (representing the output voltage) and the voltage at pin 5 (representing what the output voltage should be). This voltage is then applied to pin 7 of IC3b (as discussed previously) via diode D9. Current monitoring If you have a look just to the left of ZD4, you will see two arrows pointing down, marked B and C. This break in the negative supply line from Ql goes to the current monitoring section, shown in the bottom lefthand corner of the circuit. IC5b, IC5c & IC3c provide the current monitoring functions of the circuit. IC5c detects the current flowing by monitoring the voltage developed across the two paralleled 0. lQ 5W resistors (Rl & RZ). But there's quite a bit more to it than that, brought about by the need to adjust the current limit value, which is done by VRl. One of the problems with a large supply is how do you set the current limit? You can't just bung a temporary load across the output and then twiddle a knob to set a current which may be as much as 8 amps. If you did so, there would be enormous heat produced in your temporary load and also in the supply itself. There is also the voltage setting to consider. While it may relatively easy to set a current limit value when the supply is set for a relatively high voltage, it becomes more difficult to do so when the supply is set for a low voltage because the temporary load must then have a very low resistance - and then that would not suit if a high voltage output was required. Clearly, the current setting cannot be done by connecting a temporary load on the supply. We have to arrange to have the ammeter show the current limit being set at up to 8 amps (by VRl) without having any large current flowing. This is achieved by pushbutton switch SZ. In the normal setting oI SZ (position 1), the ammeter (MZ) is effectively connected across the two eurrent monitoring resistors Rl and RZ. When SZ is pushed (position 2), the ammeter is connected to the output of op amp IC5b, a voltage follower connected to VRl, the current adjust control. So when SZ is pushed, IC5b feeds a current through the ammeter which is identical to what it would get for a given current from the supply. So PARTS LIST 1 instrument case, 355 x 250 x 122mm (Altronics Cat. H-0492) 2 aluminium front & rear panels to suit above case 1 steel baseplate to suit case 1 PC board , code SC04201921, 189 x 167mm 1 Dynamark front panel label, 340 x 117mm 1 0-50V meter scale 1 0-1 0A meter scale 1 M-3092 35V+35V, 300VA toroidal mains transformer 1 2851 12.6V 150mA mains transformer 1 72mm long heatsink (Altronics Cat. H-0522) 2 MU65 1mA meters 1 panel-mount mains 3AG fuse holder 1 3AG 7.5A fuse 1 7.5A mains cord & plug 1 cord grip grommet to suit mains cord 1 3-way mains terminal block 1 2-way mains terminal block 2 21 mm diameter collett knobs 1 panel-mount illuminated mains switch (S1) (Altronics Cat. S3218) 1 15A toggle switch (S3) (Altronics Cat. S-1057) 1 SPOT or DPDT momentary action pushbutton switch (S2) (Altronics Cat. S-1092) 1 SPOT toggle switch (S4) (Altronics Cat. S-1210) 1 green binding post 1 red binding post 1 black binding post 2 5mm LED bezels 1 TO-220 mica washer & in~ulating bush 1 SOT-93 mica washer & insulating bush 1 Neosid iron-powdered core, 17745-22 1 Neosid iron-powdered core, 17742-22; or Philips 4330 030 60210 1 50mm-length of 2.2mm OD plastic optical fibre pushing the "Set Current" switch S2 doesn't actually cause any load current to flow. Neat, huh? IC5b's output also goes to IC5c, th e current limit amplifier. It amplifies 7 solder lugs 33 PC stakes 1 length of insulating sleeving for fuse holder and mains switch 10 plastic cable ties Wire & cable 1 500mm-length blue mains-rated wire 1 750mm-length brown mainsrated wire 1 250mm-length green/yellow mains-rated wire 1 2m-length yellow hookup wire 1 2m-length red hookup wire 1 2m-length blue hookup wire 1 2m-length orange hookup wire 1 400mm-length 0.8mm tinned copper wire 1 1m-length 1.2mm enamelled copper wire Screws & nuts 4 6mm untapped brass spacers 4 screws & nuts to mount PCB 5 screws, nuts & star washers for earth term inals 3 screws & nuts for terminal block 4 screws & nuts for mounting 2851 transformer, bridge rectifier & heatsink 2 screws & nuts for mounting 01 & D2 7 self-tapping screws (for mounting metal baseplate) Semiconductors 1 BUK436-1 00A 32A N-channel Mosfet (01) 1 BF199 NPN RF transistor (02) 1 BC338 NPN transistor (03) 1 BC328 PNP transistor (04) 2 BC639 NPN transistors (05,06) 1 SFH551V Siemens light link receiver (IC1) 1 74C14, 40106 CMOS hex Schmitt trigger (IC2) 2 LM339 quad comparators (IC3, IC4) 1 LM324 quad op amp (IC5) 1 SFH750V Siemens light link transmitter (LED1) the difference between the voltage at its non-inverting input, which is the current setting voltage, and the voltage across the paralleled 0.1Q resistors , Rl & RZ. 2 5mm red LEDs (LED2,LED3) 1 FB3502 35A rectifier bridge 1 15V 3W zener diode (ZD1) 1 4.7V 1W zener diode (ZD2) 1 75V 1W zener diode (ZD3) 1 LM336Z-2.5 zener reference (ZD4) 5 1N4004 1A diodes (D1 ,D3-D6) 1 MUR1515 15A, 150V ultrafast recovery diode (D2) 5 1N4148 switching diodes (D7-D11) 1781212V regulator (REG 1) Capacitors 5 4700µF 50VW PC electrolytic (C1-C5) 5 1000µF 50VW PC electrolytic (C6-C10) 1 1000µF 25VW PC electrolytic (C11) 3 10µF 16VW PC electrolytic 4 1µF 16VW PC electrolytic 6 0.1 µF monolithic 1 0.1 µF 50VW ceramic or polyester 2 0.1 µF 250VAC metallised polycarbonate 1 .001 µF metallised polyester 1 820pF ceramic Resistors (1 % 0.25W) 1 470kQ 6 1kQ 1 270kQ 4 1kQ 5W 9 100kQ 1 820Q 3 47kQ 3 470Q 1 27kQ 1 390Q 0.5W 5% 2 22kQ 1 270Q 1W 5% 2 18kQ 1 220Q 410kQ 2 100Q 2 6.8kQ 1 47Q 1 4.7kQ 1 39Q 5W 6 2.2kQ 1 10Q 5W 1 1.5kQ 2 0.1Q 5W 1 1.1 kQ Trimpots 2 50kQ enclosed horizontal trimpots (VR5,VR6) 2 10kQ linear pots (VR 1, VR4) 1 ?00Q enclosed horizontal trimpot (VR2) When the power supply delivers current to a load, there is a voltage developed across the. 0. lQ resistors. If this voltage exceeds the setting ofVRl , then the output of IC5c will go low JANUARY 1992 71 IC5a's output goes high to turn on LED 2, indicating that the supply is out of regulation. Minimum loading resistors This view show~ the fibre-optic light link that's used to isolate the control circuitry from the switching circuitry. It uses a LED & a photodiode/amplifier in separate packages, with the two joined by a short length of optical cable. and shunt the error voltage signal from the input of comparator IC3b via diod e DB. This will throttle back the duty cycle of Mosfet Ql until the current delivered equals the current set. Also, when IC5c's output goes low, it causes comparator IC4d to turn on LED3 , the current overload indicator. Foldback current mode · In addition to the current limit mode, this power supply has current foldback. This is a very worthwhile feature and acts to limit the current to a safe value in the event of a short circuit. It acts independently of the adjustable current limit mode so that even if you have the current limit set at maximum (nominally B amps), the fo ldback mode still works. What happ ens is that if the output current rises to a figure of 9 amps, because of excessive loading or a short circuit , the current is quickly reduced to a much safer value of less than 2A. Co mparator IC3c provides the foldback mode. Its non-inverting input is connected to a voltage divider string consisting of 22kQ, 2 7kQ, BZOQ an d 1. lkQ resistors. The voltage input corresponds to a 9A current. IC3c monitors the difference between the voltage at its non-inverting input and that developed across the current sensing resistors Rl & RZ. When the 9A current figure is exceeded (corresponding to 0.45V), pin 72 SILICON CHIP 13 of IC3c goes low and pulls the error input ofIC3b low via diode D10. This throttles Ql right back and so the output current is greatly reduced. At the same time, diode Dl 1 pulls the junction of the 22k0 and 27k0 resistors low which effectively reduces the overcurrent limit input at pin 11 of IC3c to a figure well below the initial 9A setting. This condition is maintained until the cause of the overload is removed. Regulator drop out Comparator IC4a and op amp IC5a provide a visible indication that the supply is not regulating; ie, not delivering the voltage which it is supposed to . IC4a monitors the output of Q3 & Q4 (the stage which drives LED1, the light link transmitter). Whenever LED 1 is driven, the output of IC4a is low and when LED 1 is off, the output of IC4a goes high. When it's high , the O. lµF capacitor at pin 2 of IC5a is charged via diode D7. When the output ofIC4a is low, the O. lµF capacitor can discharge via the 10k0 resistor connected across it. Since the 0. lµF capacitor is kept charged while ever pulses are sent to LED 1, IC5a has a low output. If LED 1 is on continuously, then Ql is fully on and the supply is out of regulation since there is no more means of control. When this happens , the 0.1µF capacitor discharges and Comparators IC4b & IC4c and transistors Q5 & Q6 provide minimum current loading for Ql , the switching Mosfet. This is necessary because a switching regulator such as this does not work reliably at very low values of load current; the switching pulses become extremely narrow and they tend to become irregular as the circuit tries to throttle itself back sufficiently to maintain a given output voltage. The solution to that is to have a certain minimum load current at all. times. This is achieved with three sets of resistors. Firstly, the two lkO 5W resistors in parallel are permanently connected across the supply. These provide sufficient current drain at voltages above 10V. For voltages between 5V and 10V, Q6 is used to switch in a 390 5W resistor and for voltages below 5V, an additional 100 5W resistor is switched in by Q5. Comparators IC4b & IC4c control the switching of Q6 & Q5. The noninverting inputs (pins 9 & 11) are tied to a divider string consisting of a 22k0 resistor and two 4700 resistors. The inverting inputs (pins B & 10) of each comparator monitor the supply output voltage via a voltage divider oonsisting of an 1BkQ resistor and a lkO resistor. Thus, when the output voltage is reduced below 10V, IC4b's output goes high and switches on Q6. Similarly, when the supply voltage is reduced below 5V, IC4c's output goes high and switches on Q5. Final filter The filter network consisting of toroidal inductor Ll and capacitors C6 & C7 removes most of the switching spikes from the output voltage but an extra stage of filtering is required to obtain the low output noise and ripple quoted in the specification panel. This is provided by another toroidal inductor (LZ) and capacitors CB, C9 & C10. This is augmented by the 0. lµF capacitors connected between earth and the positive and negative rails of the supply. Next month we will describe the construction and setting up of the supply. SC