e01c098.PDF

POWER SUPPLY

Power Supply (2)

Part 2: soldering, sawing and drilling

Design by R. Pagel

The construction of the Digital PSU requires extra care and precision.

Just like the electronics inside, the mechanical construction and overall

ﬁnish of the instrument should meet professional requirements.

VOLTAGE

CURRENT

POWER

OFF

DIGITAL POWER SUPPLY

0 - 25 V

0 - 2.5 A

000166 - F

Figure 1. Front panel layout for an elegant ﬁnal appearance.

As indicated at the outset, there are two ver-

sions of the power supply that you can build.

However, we have decided not to produce

special circuit board and front panel layouts

for the smaller power supply and concen-

trated on the larger 2.5 A unit.

For the 2.5 A power supply a metal enclo-

sure 200 mm (width) by 180 mm

(depth) by 100 mm (height) and a

24 V 80 VA toroidal transformer are

used, with all the parts of the enclo-

sure securely connected to earth. A

mains filter must also be fitted, or

else the microcontroller may reset in

the event of mains interference. The

circuit board is fixed to the front

panel with bolts, while the base sim-

ply carries the toroidal transformer

(and possibly also, as shown in the

photograph, the mains filter). The

heatsink, RS232 connector and next

Elektor Electronics

12/2001

Digital Bechtop

POWER SUPPLY

to that the mains cable with strain

relief are fitted to the rear panel.

Alternatively, a ‘cold condition’ IEC

mains inlet with fuse carrier can be

fitted. Ideally a mains inlet with

built-in ﬁlter can be used. The spec-

ification of the filter we used was

2L=2.4 mH, Y=2200 pF, X2=0.1

, and the ﬁlter must of

course have a suitable maximum

load speciﬁcation.

Next we get our hands dirty with

the construction of the enclosure.

Preparing the front panel in particu-

lar demands a lot of work and high

accuracy since it constitutes an

advertisement for home-built test equipment.

The unpopulated circuit board can be used

as a drilling template for the front panel. Take

care to allow an adequate gap (at least

6 mm) between the mains switch connec-

tions and the circuit board! The rectangular

hole for the LCD can be carefully cut out

using an electric jigsaw or a fretsaw and the

000166-1

Figure 2a. Layout of the double sided circuit board for the power supply.

12/2001

Elektor Electronics

and R=1.0 M

POWER SUPPLY

R39

IC6

R38

C20

IC5

R41

C16

C10

C11

C18

R15

R18

R34

R11

R21

R22

R23

R16

R17

C13

R19

R20

C14

R13

R14

R10

C17

C19

IC7

4A/T 250V

2A/T 250V

C12

C1’

Figure 2b. Component mounting plan of the double sided circuit board for the power supply.

cuts can then be ﬁled straight. If you are not

conﬁdent of doing a professional job, you can

use a display with a bezel to cover the rough

edges. Cutting the hole for the mains switch

proceeds similarly. Use a taper drill for a cir-

cular mains switch. The holes for the push

buttons can be drilled out in stages using var-

ious twist drills until they have a diameter

0.5 mm bigger than the buttons, and then

filed smooth. Then four more holes are

required for the mounting screws, which

should be countersunk so that the heads of

the 3 mm countersunk screws are perfectly

ﬂush with the panel. On the rear side

fit nuts to the screws as tightly as

possible. Since the nuts should not

(and indeed, without damaging the

front panel, cannot) be undone again

it is a good idea to ﬁx the nuts with

superglue. The thread should pro-

trude a few millimetres to ensure

that the screws have adequate grip.

The ﬁxing holes for the two terminal

posts must also be suitably drilled.

If the anodised or brushed front

panel is presentable (free of

scratches and other faults) it can be

annotated using rub-down lettering

followed with a few coats of protec-

tive spray. Alternatively you can

make a front panel using the sug-

gested layout in Figure 2 . Photocopy

the layout onto paper at 1:1 scale

and apply a few coats of protective

spray. Spray from a distance of at

least 40 cm to ensure that the toner

does not run. When completely dry

apply general-purpose glue to the

back of the paper and glue it to a

sheet of white self-adhesive labels.

Thus we have a robust front panel

foil which is easy to work with and

which, thanks to the self-adhesive

labels, will stick ﬁrmly to almost any

surface.

The foil should not be attached to

the front panel until after the power

supply has been fully tested. In order

to avoid bubbles when attaching the

foil, it should always be smoothed

out from the centre to the edge.

Finally, go over the foil with a rubber

roller or a round pencil. The cutouts

and holes can now be carefully cut

through using a knife. Work only

from the front of the foil making

many small cuts, especially when

making the holes for the push but-

tons. An alternative way of making

front panel foils using a PC and a

printer was described in the

July/August 2000 issue of Elektor

Electronics .

Component mounting

After the double-sided printed cir-

cuit board has served its purpose as

a drilling template, the components

can be ﬁtted according to the plan in

Figure 3 . This should proceed with-

out difficulty since there are no sur-

face-mount components, wire links,

hairline tracks or tightly spaced sol-

der pads to worry about. The board

should be populated in the usual

fashion. Before starting to solder,

000166 - 12

DB9

Figure 3. Connections between K1 and the

sub-D socket.

100

Elektor Electronics

12/2001

POWER SUPPLY

COMPONENTS LIST

2.5 A version

(1 A version in brackets)

D3,D4,D5,D6 = 1N4148

B1 = B80C5000 (B80C1500)

T1 = BC557B

T2,T3,T4 = TIP142 (1 only)

T5,T6,T7 = BC547B (not ﬁtted)

IC1 = LT1491

IC2 = TLC272

IC3 = PIC16F84A-04P, programmed,

order code 000166-42

IC4 = 4066

IC5, IC6 = 74HC164

IC7 = 78L05

IC8 = MAX232

the board. Ensure that any solder flux is

removed from the area around IC1, IC2 and

IC4 after soldering, to avoid the effect of par-

asitic resistance.

After checking all the soldered connec-

tions the display can be suitably ﬁxed using

long bolts. The electrical connections are

made using bare wire. Now the whole assem-

bly can be fixed to the front panel, and the

transformer and PC interface wired up as

shown in Figure 4 .

The power transistors are fitted with an

insulating mica washer and fixed to the

heatsink, the module being connected to the

main circuit board using three 10 cm long

0.75 mm 2 wires.

Resistors:

(* Metal ﬁlm, 1% tolerance)

R1 = 220

, 5W

R2 = 330 Ω

, 0.5W

Ω

R4 = 100k Ω

R5 = 330

(1k

R6 = 1k Ω , 0.5 W

R7,R18,R23 = 1k

R8,R17 = 39k Ω *

R9,R10,R13,R14,R40,R41,R43 =

10k Ω

R11,R12 = 47k

Miscellaneous:

F1 = Fuse, 2A, time lag

F2 = Fuse, 4A, time lag (2A, time lag)

S1-S8 = pushbutton, C&K 3FTL6 +

1S09 22.5, or general-purpose

10 × 10 × 20mm, lead pitch 5mm

TR1 = mains transformer, toroid, 24V,

80VA, e.g. UI 39/1721 V, 2571 mA

X1 = ceramic resonator, 4MHz, 3

pins

LCD module 16

R15,R16 = 10k Ω

R20,R39 = 1k Ω

R21 = 1k

Initial operation

Once everything has been assembled and

wired together, you will not want to wait a

month for the description of the software and

operation of the unit; you will want to try your

power supply out immediately. With IC1 and

the microcontroller not ﬁtted, the mains can

be switched on for the first test. As long as

no smoke signals are emitted, the unit can be

left switched on.

Between pins 4 and 11 of IC1 a voltage of

27 V (for the 1 A power supply) or 32 V (for

the 2.5 A power supply) should be measured.

There should be a voltage of 12 V across

Zener diode D2 and a voltage of 5.1 V

between pins 5 and 14 of the microcontroller.

If all is well, turn off the power supply and ﬁt

the remaining ICs. Turn P2 fully to the left.

When the unit is turned on again the display

should read:

* (0

)

R22 = 18k Ω

* (39k Ω

* 0.6W

R35-R38 = 100k Ω

Ω

R44 = 3k Ω 3

R45,R46,R47 = 0

1, with backlight

Fuse holder, PCB mount

Fuse holder, chassis mount, (alterna-

tively IEC mains appliance socket

with integral fuse)

Two wander sockets (red, black)

Fl1 = mains ﬁlter, 2A (not ﬁtted)

K1= 9-way SIL pinheader, with 9-way

sub-D socket (female), chassis

mount

K2 = 5-way pinheader

K4,K5 = 2-way PCB terminal block,

lead pitch 5mm

14-way IC socket with turned pins

18- way IC socket with turned pins

12 solder pins

Heatsink, size 180

51, 0.5 W (not ﬁt-

ted)

P1 = 10

multiturn preset

P2 = 500 Ω

preset H

P3 = 2k

5 preset H

Capacitors:

C1 = 10,000µF 50V (4,700µF 35V),

axial

C2 = 470µF 50V, radial

C3,C13,C14,C15,C16,C17 = 10µF

25V, radial

C4,C7,C11 = 100nF, ceramic, 5mm

lead pitch

C5 = 10nF, ceramic, 5mm lead pitch

C6 = 1nF, ceramic, 2.5mm lead pitch

C8,C9 = 10µF, tantalum bead

C10 = 4nF7, FKS-2

C12 = 4 µ F7 63V, MKS-4

48 mm, spec

0.6 K/W (100 × 50 × 31mm, spec

2.4 K/W)

Enclosure (w × d × h) 200 × 180 × 100

mm, e.g. Telet type LC950

Mains switch, mains cord

PCB, order code 000166-1

00.0V 0.00A

When a push button is pressed (or held

down) the display should switch to ‘set

mode’ and show the values as they are

adjusted either in milliamps or millivolts. To

adjust the offset voltage, turn P2 gradually to

the right, checking whether the display

changes. When the display stops changing,

you have gone too far and you should back

the potentiometer off a little.

Alternatively, connect a PC to the interface

and start up HyperTerminal with the follow-

ing interface settings

Semiconductors:

D1 = P600D (1N4007)

D2 = zener diode 12V 1.3W

make sure that the push buttons

stand a good 10 mm higher than the

electrolytic capacitors. A few com-

ponents are soldered to the rear of

the board: the bridge rectifier (for

reasons of space), the trim poten-

tiometers, the connectors, the solder

tags for the transistors and the fuse

(so that it can be changed easily).

The circuit board connector for the

banana sockets is mounted on the

component side. The components

under the display, such as C19,

must, where possible, be mounted

ﬂat.

If you know that you will never

want to reprogram the microcon-

troller on the board, you can of

course dispense with the program-

ming connector K2. High-quality

sockets should be used, at least for

the two expensive ICs, IC1 and IC3.

R1, R2 and R6 can get hot and

should be ﬁtted raised slightly above

9600, 8, N, 1, hardware handshake

Now adjust the trimmer so that the value rep-

resented by the ﬁrst ﬁve digits after the D is

as small as possible, but not zero. A value of

2 is ideal, since the unit will then continue to

operate correctly even if the value should

change slightly as a result of ageing, mechan-

12/2001

Elektor Electronics

101

R3 = 150

R19 = 8k

R24-R34 = 1

R42 = 47k

POWER SUPPLY

Figure 4. The main circuit board.

ical shock or temperature drift.

After this adjustment the output voltage

should be measured with the voltage setting

at 0.00 V. It should be 30 mV (±15 mV). Next

set the voltage to 19.0 V and adjust P1 for an

actual output voltage of 19.03 V. The

output current adjustment and its

displayed values will then automati-

cally be correct. If the values are

awry, or the above values cannot be

achieved, the first thing to check is

that all the components are correctly

fitted and that there is no solder

residue between the tracks.

(000166-2)

102

Elektor Electronics

12/2001

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