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PROJECTS AND CIRCUITS

All reasonable precautions are taken to ensure that the advice and data given to readers is reliable. We cannot, however, guarantee it and we cannot accept legal responsibility for it. A number of projects and circuits published in EPE employ voltages that can be lethal. You should not build, test, modify or renovate any item of mains‐powered equipment unless you fully understand the safety aspects involved and you use an RCD adaptor.

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ONE of the most useful pieces ofequipment an electronics experi-menter can possess is a good work-

bench power supply. Indeed even for thosewho already have one, a second will oftenprove useful for projects requiring morethan one source of supply or for theoccasional design that requires long-termtesting.

Ideally a workbench supply should have

an output voltage that can be adjusted rightdown to zero as it is occasionally useful tobe able to power a circuit gradually fromthis when fault-finding. There should alsobe a fast and effective current limitingfacility, again adjustable from zero as thisalso provides valuable protection whentesting circuits.

If the supply can be set to deliver a con-stant output current instead of a voltage itcan also be used as a charger for the manytypes of re-chargeable battery that areavailable nowadays, both alkaline andsmall sealed lead-acid types. The latter arenormally charged to a constant voltage butuntil this is reached a current limit is oftenrequired to prevent an excessive chargerate.

Preferably the supply should have twometers so that the voltage and current sup-plied to the load can be seen at a glance,and most users would probably prefer ana-logue meters to the digital type for this.

Power supplies meeting these specifica-tions can be expensive, but this project canbe constructed at reasonable cost, especial-ly if some of the parts such as the case andmeters are already to hand, or are pur-chased from inexpensive sources such assurplus stores or amateur radio rallies.

The power supply described here pro-vides an output of up to 20V with currentin two ranges of 100mA or 1A, whichshould be more than sufficient for most ofthe projects which appear in EPE.

The design of a good power supply is

more complex than might be supposed. Itmust provide a constant voltage or current

whilst being able to react rapidly to anychanges in the load it is supplying. Thismeans that the circuit must have a fastresponse and remain stable for a widerange of output loads, voltages andcurrents, so the circuit is inevitably a com-promise between speed and stability.

The design used for this project shouldbe fast enough for most purposes and hasgood output stability.

The method of voltage control used maybe a little difficult to follow so a simplifiedexplanation is in order before a descriptionof the full circuit. Some of the more com-mon methods of controlling voltage areshown in Fig.1.

In Fig.1a the controlling device is a sim-ple transistor. A reference voltage isapplied to the base and this is reproducedat the emitter to supply the load, minusabout half a volt for the base-emitter dropof the transistor. This can suffice for sim-ple designs but suffers from small changeswith variations in load, partly because itdraws a small current from the referencesource, which is usually a variable resistorconnected to a fixed voltage.

Use of a power MOSFET in place of thetransistor would cure this, but these have amuch larger and less predictable gate-source voltage drop.

An op.amp can be used to partly over-come these problems, as shown in Fig.1b,but there are still some disadvantages. Asthe required output voltage rises, the inputvoltage to the transistor must also rise sothere is effectively less control voltageavailable as the output approaches the sup-ply voltage. Also, this simple circuit can-not supply an output voltage greater thanthe reference.

Everyday Practical Electronics, January 2002 33

g

d

s

bc

e

bc

e

VREF

VREF

VREF

V+

V+

V+

0V

TR1

0V

TR1

TR1

0V

R1

R2

A)

B)

C)

+IC1

LOAD

LOAD

LOAD

+IC1

Fig.1.Various methods of voltage control:(a) transistor, (b) op.amp and transistorand (c) op.amp and power MOSFET.

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A somewhat better solution is shown inFig.1c. A power MOSFET is controlled byan op.amp and a pair of resistors causesthe op.amp to multiply the reference volt-age. But the problem of low drive voltageat high output voltages remains and it isdifficult to sense the output current fromany of these circuits.

An arrangement which overcomes these

difficulties is shown in Fig.2. It uses twopower supplies, a “main” one of about 25Vcapable of supplying up to 1A of currentfor the output, and an auxiliary “split” sup-ply of +12V for the controlling circuit. The“0V” rail of this is the “ground” for thecontrol section. It operates as follows:

A reference voltage is produced usingregulator IC1 and applied to resistor R1.During normal operation op.amp IC2 usesfeedback to maintain equal voltages at itsinputs, so a constant current flows into R1.This current also flows through resistorR2, developing a voltage across it directlyproportional to its value. If a linear vari-able resistor (potentiometer) is used for R2it will provide linear control of the output.

Perhaps the easiest way to understandthe action of the circuit is to consider whatwould happen if the voltage at the junctionof R1 and R2 were to rise slightly. The out-put of IC2 would start to rise and thus turnon the MOSFET TR1 a little more. Thiswould cause the 0V rail to rise with respectto the main negative supply rail so thatmore current would flow through R2 tobring the input to IC2 down again, restor-ing the balance.

The 0V rail, of course, is the positiveoutput so the voltage developed across R2is what is delivered to the load. Naturally,as the 0V rail rises with respect to the mainsupply, so do the positive and negative12V rails, so that even when the outputvoltage is close to the maximum of themain supply there will still be a full 12Vavailable for IC2 to use in controlling thegate of power MOSFET TR1.

It is also simple to measure the output

current in this arrangement using thesensing resistor R3, which develops a cur-rent-dependent voltage with respect to the

34 Everyday Practical Electronics, January 2002

control circuit “ground”. The only disad-vantage of this arrangement is that thereturn path for the reference current fromresistor R1 is via the main supply, TR1 andR3, so this current should be kept small toprevent any noticeable effect on the mea-sured output current.

This also applies to the voltmeter whichis connected directly across the output.

The full circuit diagram for the Versatile

Bench Power Supply is shown in Fig.3.The incoming 230V a.c. mains passesthrough a 2-pole isolating switch S1 to thetwo transformers T1 and T2, whilst thetransient suppressor VDR1 removes anybrief high voltage spikes that may occur.

No supply fuses are fitted to the proto-type, a 3A fuse in the mains plug beingconsidered adequate.

Transformer T1 is a 20V 20VA (1A)type and, with bridge rectifier REC1 andreservoir capacitor C2, produces a no-loadvoltage of about 30V. Under full-load con-ditions this drops to about 24V which isstill sufficient to maintain the 20V output.Capacitor C1 and resistor R1 also help toeliminate transient voltage spikes.

Transformer T2 is a smaller 100mAtype with a centre-tapped 9V-0V-9V out-put. Arranged as shown with bridge recti-fier REC2, it develops both positive andnegative supplies of about 12V each. A 5Vreference voltage is generated by IC1, astandard 78L05 regulator.

The earthing arrangements used in this

project are slightly unusual and requiresome explanation. Earthing of a mains-powered project is essential both for safe-ty and because capacitive couplingbetween windings in the transformer cantransfer potentially damaging a.c. voltagesto the output, even though from a veryhigh impedance.

However, the output of a bench powersupply is often required to have d.c. isola-tion from earth so that other earthed equip-ment, such as oscilloscopes, can be safelyconnected to any part of the circuit on test.

Commercially produced bench suppliesoften overcome this problem by using trans-formers with internal foil screens between

the primary and sec-ondary windingswhich are earthed toeliminate the capaci-tive coupling. Theseare not readily avail-able to the home con-structor, so a differentmethod must be used.

Switch S2 allowsthe mains earth to beconnected directly toeither the positive ornegative output termi-nal or left “floating”,where capacitivelycoupled a.c. voltage isgrounded throughcapacitors C3, C4 andresistor R2. Althoughnot ideal, this systemworks well in practiceand reduces noise anda.c. voltage at the out-put to a few millivolts

ResistorsR1, R2 10 (2 off)R3, R6,R8, R9 4k7 (4off)R4 22kR5 220kR7, R15,

R19 1k (3 off)R10 1 (7W wirewound)R11 180kR12 1k8R13 220R14 2k2R16 27kR17, R18 10k (2 off)R20 820R21 68kVDR1 250V a.c. transient supp.

All 0·6W 1% metal film, except wherestated.

PotentiometersVR1 10k 18-turn cermet presetVR2 100k, rotary carbon, lin.VR3, VR4 50k 18-turn cermet

preset (2 off)VR5 1k rotary carbon, lin.VR6 500 18-turn cermet

preset

CapacitorsC1, C3, C4 470n 250V, class X2

suppression (3 off)C2 2200 radial elect. 63VC5, C7,

C9, C11, 100n ceramic,C14 to C16 resin-dipped (7 off)

C6, C8 470 radial elect. 16V(2 off)

C10 10 radial elect. 50VC12 22p ceramic,

resin-dippedC13 100 radial elect. 63V

SemiconductorsREC1 PW01, 6A 100V bridge

rectifierREC2 W005 1·5A 50V bridge

rectifierD1 to D5 1N4148 signal diode

(5 off)D6 BZX61C30V 30V 1·3W

Zener diodeD7 red l.e.d. 3mmTR1 BUZ11 power MOSFETTR2 BC214L pnp transistorIC1 78L05 +5V 100mA

voltage regulatorIC2 to IC4 TL071 op.amp (3 off)

MiscellaneousME1, ME2 100A moving coil

meter (2 off)S1, S3, S4 d.p.d.t. toggle switch,

250V a.c. (3 off)S2 1-pole 3-way rotary switchT1 20V 20VA transformerT2 9V-0V-9V 100mA trans.SK1, SK2 4mm screw terminal

post/socket (1 red,1 black)

Printed circuit board, available fromthe EPE PCB Service, code 333; casewith metal front and rear panels or metalcase, 205mm x 140mm x 110mm; plasticfeet (4 off); cable ties (see text); knob (2off); 8-pin d.i.l. socket (3 off); heatsink,100mm x 65mm; fixings; connectingwire; solder, etc.

SeeSSHHOOPPTTAALLKKppaaggee

Approx. CostGuidance Only ££3399

excluding meters & case

g

d

s

+25V

+12V

-12V

0V

5V

IN IC1

MAINPSU

CONTROLPSU

OUT

COM

REFERENCE

CURRENTMEASURING

CIRCUIT

LOAD

0V

R1

R2

R3

TR1

5V

SETVOLTS

OUTPUT

+

VOLTMETER

+IC2

Fig.2. Simplified schematic of the voltage control methodused.

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Everyday Practical Electronics, January 2002 35

g

d s

b

ce

+

- +

-

IC1

78L0

5

INO

UT

CO

M

NE

GG

ND

PO

SG

NDFLO

AT

VD

R1

1A20V

T1

230V

0V

20V

0V

R1

10Ω

470n

C1

RE

C1

10Ω

R2

S2

470n

C3

470n

C4

2200

µC

2

4k7

R3

230V

A.C

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AIN

S

L NE

INP

UT

S1a

ON

/OF

F

250V

230V

0V0V9V0V

9VRE

C2

100n

C5

470µ

C6

100n

C7

470µ

C8

100n

C9

10µ

C10

100n

C11

100k

VR

2S

ET

VO

LTS

4k7

R9

1N41

48

D1

1N41

48

D2

4k7

R8

1Ω 7WR10

VR

110

k

SE

TM

AX

VO

LTS

22k

220k

R4

R5

4k7

R6

3

3

3

2

2

2

6

6

61kR

7

BU

Z11

TR

1

22p

C12

+5V

1k8

R12

220Ω

R13

2k2

R14

180k

R11

50k

VR

3S

ET

VO

LTM

ET

ER

100

AµM

E1

VO

LTS

100µ

C13

100n

C14

30V

D6

1N41

48

1N41

48

D4

D5

100n

C15

1kR15

1kVR

51kR19

SE

TC

UR

RE

NT

100n

C16

68k

R21

500Ω

VR

6

SE

TC

UR

RE

NT

ME

TE

R

820Ω

R20

100

AµM

E2 C

UR

RE

NT

10k

10k

R17

R18

27k

R16

50k

VR

4S

ET

MA

XC

UR

RE

NT

1N41

48D

3

1A100m

A

S3

CU

RR

EN

TR

AN

GE

OU

TP

UT

ON

/OF

F

+V

E

-VE

S4a S4b

S1b

+12

V

+25

V

0V -VE

-12V

OU

TP

UT

TR

2B

C21

4L

+

+

PW

016A

/100

V

W00

51

5A/5

0V

+

+

++

+

7 4

4

47

7

a

a

a

k

k

k

ak

a

a

k

kD

7

a kC

UR

RE

NT

LIM

IT

T2

9V-0

V-9

V10

0mAP

P

+

+

+

SK

1

SK

2

TL0

71IC

2

TL0

71IC

3

TL0

71IC

4

BZ

X61

C30

V1

3W

*

*

*

*

SE

E T

EX

T

Fig.3. Complete circuit diagram for the Versatile Bench Power Supply.

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at most. For safety, capacitors C3 and C4must be 250V class X2 suppression types.

The voltage controlling part of the cir-

cuit is constructed around op.amp IC2 andoperates as described earlier. The resis-tance to which the reference voltage isapplied includes the preset VR1 so that themaximum output voltage may be set toexactly 20V.

The input to IC2 has a pair of protectiondiodes, D1 and D2. Capacitor C12 ensuresstability but is small enough to allow a fastresponse to output load changes. PowerMOSFET TR1 handles the full output cur-rent. Output voltage is indicated by meterME1 with VR3 allowing preset adjustmentfor calibration to 20V full-scale.

Load current is sensed by the one ohmresistor R10. At full output this dissipates awatt of heat, so a 7W type is used to dissi-pate this safely. A potential divider acrossR10, consisting of R12 with R13 and R14in parallel, allows selection of either thefull potential across it or one tenth throughswitch S3, giving the 100mA and 1A cur-rent ranges.

The voltage from S3 is compared byop.amp IC3 with an adjustable voltagederived from the reference by divider net-work VR4, R16 and the current controlVR5. When the S3 voltage exceeds thissecondary reference voltage, the output ofIC3 goes low, pulling the gate voltage ofTR1 down through diode D3 to limit thecurrent flow through this transistor.

It also turns on transistor TR2, whichilluminates light emitting diode (l.e.d.) D7to indicate that current limiting is takingplace. Preset VR4 allows the current ranges

to be set precisely to 100mA and 1A,whilst VR5 gives control from zero to theselected maximum.

Op.amp IC4 also receives the voltagefrom S3, and converts it into a 1-100Adrive for the current indication meter ME2.Preset VR6 allows calibration of this meterfor full-scale corresponding to the 100mAand 1A signal inputs. The high value ofresistor R21 prevents any possibility ofmeter damage through overdriving.

Double-pole switch S4 allows the outputto be switched off independently and alsoprovides instant isolation from the circuitconnected to it at any time.

Zener diode D6 protects TR1 from anyhigh voltage pulses arising from the con-nected load. This can happen with sometypes of inductive load and, whilst normal-ly tolerant of brief high currents, semicon-ductors are easily damaged by excessivevoltage.

As this unit is mains powered, its con-

struction should only be carried out bythose who are suitably experienced orsupervised. All mains connectionsshould be fully insulated using heat-shrink or other suitable materials.

36 Everyday Practical Electronics, January 2002

Internal view of the prototype showing the a.c. supply components.

C1

S1

R1

R3

REC1

230V

0V

0V

0V

20V

+

+

+

R2

9V 9V

T2 C3C4

S2

C2

CONTROLCIRCUITSUPPLY

MAINSUPPLY

EARTHSELECT

ON/OFF

230VA.C.

MAINS

L EN

S1 AND S2 MOUNTED ONFRONT PANEL. VIEWEDFROM REAR

ALUMINIUM PANEL

VDR1

POSNEG

T1

0V9V 9V

230V 0V

P

EARTH FOR FRONTAND REAR PANELS

Fig.4. Layout and interwiring of power supply components mounted on analuminium base plate. The plate also acts as the heatsink for the 6A bridgerectifier REC1. S1 and S2 are mounted on the front panel.

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Construction should begin with thetransformers, the rectifier REC1 and earth-ing arrangements. These can be assembledin any manner preferred by the construc-tor. The layout used in the prototype isshown in Fig.4.

These components are attached to a190mm × 100mm metal plate drilled to fitthe threaded mounting pillars provided inthe plastic enclosure. The two transform-ers and rectifier REC1 are bolted in placeso that the plate acts as the heatsink for therectifier.

The large electrolytic capacitor C2 isfastened with a cable tie passed throughholes to each side of it. The three 470nFcapacitors, C1, C3 and C4 are glued inplace. It is essential that you ensure thatgood adhesion occurs. Preferably also usecable ties for these components as well toprevent them from contacting mains con-nections should they become detached.

Note the arrangement of the earthwiring, which uses one of the main trans-former mounting bolts as a “common”point for all earth connections. Any othermetal parts of the case used, such as frontand rear panels, must also be connected tothis point.

This part of the circuit can now be fittedinto the case and tested. When connectedup and powered, the output of the “main”supply should be about 30V d.c. acrosscapacitor C2 and 9V-0V-9V a.c. should beavailable from the 100mA transformer.These supplies will be needed for testingthe remainder of the circuit.

The front panel should be drilled as

shown in Fig.5 and assembled next. Theearthing selector switch S2 should be fittedand tested with a continuity testing meter.Mains switch S1 can also be connected up.The mains input is taken to the upper two

Everyday Practical Electronics, January 2002 37

Completed power supply with the top-half of the plastic case removed to reveal thecircuit board and power MOSFET TR1 bolted to the metal rear panel.

33mm

198mm

104mm

14mm 14mm

65mm 65mm

27mm

31mm

33mm

18mm 23mm

77mm

27mm

27mm

30mm

108mm

D7

M2 M1

S1

S2

VR5 VR2

S4

S3

OUTPUT+

30mm

19mm

Fig.5. Front panel drilling details and dimensions. The general layout of compo-nents and front panel lettering are shown in the photograph below.www.ep

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connections on this. Transient suppressorVDR1 is fitted directly to the lower twoconnections on the output side.

The rear metal panel of the case has fourmounting bolts for the printed circuit board(p.c.b.) which can be used as a template tomark out for drilling. It also has an externalheatsink to aid dissipation of heat fromTR1, which is secured to the inside with aninsulating mounting washer and a boltpassing right through the heatsink. Fournuts are used as spacers for the p.c.b. whenit is fitted.

Most of the rest of the circuit is con-

structed on the p.c.b. whose details are

38 Everyday Practical Electronics, January 2002

CURRENT

VOLTS

333

REC2

IC1

C6

C10

C8

C11

C7

C5

C9

D2D1

R8

R7

IC2

R4

R5

R6

VR1 VR4R16

D4D5C15

D3

IC3 R18

R17

R19

TR2

IC4

VR6 VR3

R21

C16

R11

C13

R10

R12

R13

R14

R9

C14 D6

R20

R15

INCOMOUT

++

+

+

+

e

bc

aa

a

a

a

a

kk

k

k

k

k

C12

FLAT ME1

ME2

+

+

+

+

FROM MAIN

SUPPLY 25V+

g sd

TR1

a k

SETCURRENT

METER

SETVOLT

METER

SETMAX

CURRENT

SETMAX

VOLTS

1A

100mA

9V

9V

0VFROM 9V-0V-9VTRANSFORMER

OUTPUT ON/OFF

MAIN SUPPLY VE

VR2 VR5

D7

CURRENTLIMIT

SETOUTPUTVOLTAGE

SETOUTPUT

CURRENT

OUTPUTS4

OUTPUT

S3

SK1 SK2

5in (127mm)

2 6in (67mm

)

CURRENTRANGE

INSULATINGKIT WASHER

Fig.6. Topside printed circuit boardcomponent layout, interwiring to frontand rear panel mounted componentsand full-size underside copper foilmaster. Note the MOSFET (TR1)bolted to the rear panel using an insu-lating kit, in the photograph above.www.ep

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shown in Fig.6. This board is availablefrom the EPE PCB Service, code 333.

After ensuring that the board will fit onthe mounting bolts, the pins for externalconnections can be fitted, there are 23 ofthese, and then the links, of which there areseven. Next fit the 0·6W resistors, diodes,dual-in-line (d.i.l.) i.c. sockets, the smallceramic capacitors, presets VR1, VR3,VR4 and VR6 , transistor TR2, regulatorIC1, rectifier REC2, the four remainingelectrolytic capacitors and 7W resistorR10, preferably in this order for ease ofassembly.

Fully check your assembly for correct-ness of the soldering, componentpositioning and polarity. The board is thenready for testing.

The aim of using separate op.amps for

the various functions is partly to make test-ing and trouble shooting easier, so hopeful-ly this will be the case.

The first test is to connect the 9V-0V-9Vtransformer as shown in Fig.6. Using 0V (thecentre-tap connection) as a reference, the cir-cuit should be powered up and the presenceof +12V and –12V checked at pins 7 and 4respectively of the three d.i.l. sockets.

The presence of +5V from regulator IC1can also be checked at its output pin. Next,the MOSFET TR1 and VR2, the Voltagecontrol, should be connected, along withthe two leads from the main power supply,from REC1 and C2. IC2 should now beinserted into its socket.

Whilst monitoring the output with avoltmeter, the circuit should be poweredup, and it should now be possible to varythe output from zero to around 20V withVoltage control VR2. If so, VR2 can beturned right up and the voltage set to exact-ly 20V with preset VR1. If the voltmeterME1 is now connected this can be adjustedfor an indication of 20V (full scale) withpreset VR3.

Next the current range switch S3 should

be connected and set to 100mA, and the cur-rent control VR5 should be connected. Thecurrent control op.amp IC3 should be insert-ed into its socket, and a meter set to read acurrent of about 100mA connected in serieswith a 100 ohm resistor across the output.The circuit should again be powered and thevoltage control turned right up.

Potentiometer VR5 should now control thecurrent, from zero to around 100mA. Preset

VR4 can be adjusted for a maximum outputof exactly 100mA. The 1A range shouldautomatically be correct following this.

The l.e.d. D7 can be connected, thisshould light whenever current limiting isactive.

Finally, the current meter ME2 shouldbe connected, op.amp IC4 inserted, andpreset VR6 set for full scale at 100mA. Asbefore, the 1A range should now automati-cally be correct following this adjustment.

Once these tests are concluded, the

board can be attached to the rear panel tocomplete the assembly. It might be worth-while re-checking the adjustments, the pre-sets have been arranged for easy accesswith the case top half removed to make thissimple.

The unit will normally be used to supplya constant voltage to circuits on test, but itmay also be operated continuously in con-stant current mode which is very useful forcharging NiCad batteries of various types.

Although the heat-sinking will be ade-quate for most purposes, the fact should beborne in mind that it could dissipate over20W and therefore get rather hot. The sup-ply of low voltages at high currents forlong periods is not recommended.However, most loads of the kind that arelikely to be found in the average workshopshould present no problems.

If in doubt, just place a hand on theheatsink now and again to check tempera-ture. The prototype has been used continu-ally for weeks on end to power circuitsused with electric clocks, and is also thefavourite for programming PICs in theworkshop, leaving another supply free foroperating the PlC-driven circuitry.

It should be noted that the current limit

takes a finite time to operate so there aresome applications where external limitingis needed. The output of IC3 has to comeout of saturation and slew through abouteight volts before limiting begins, and thiswill take a microsecond or two!

The limit protects the supply itselfagainst short circuits, and in almost everypractical fault situation it is fast enoughto prevent damage to faulty circuits ontest as these are rarely total short circuitsanyway.

However, it should be remembered thatthe MOSFET output device can handlevery high currents and has a large capacitorbehind it so, for the brief time it takes forthe limit to operate, many amperes can bedelivered into a short! An obvious exampleis the testing of l.e.d.s.

The limit should never be relied upon onits own for this, a suitable series resistorshould always be used.

Everyday Practical Electronics, January 2002 39

Rear of the completed power supply unit showing the finned heatsink bolted to therear panel to aid dissipation of heat from the power MOSFET.

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