e001032.pdf

POWER SUPPLIES

transformer board

for the Audio DAC 2000

and many other circuits

The general-purpose

transformer board

described in this

article has space

for two small

mains transform-

ers, two mains fil-

ters and two fuse

holders. As such, it

will undoubtedly

prove a boon and a

practical aid in the

construction of not only

the Audio DAC 2000, but

in that of many other cir-

cuits as well.

8 V transformer of

which only one of the sec-

ondary windings is used —

the second remains available

for any other purposes as

yet unforeseen.

The transformers

are flanked on the

board by mains filters

L1-C2 and L2-C3 and

an additional decou-

pling capacitor, C1 –

see diagram in Figure 1. Also,

since not all mains transformers are

short-circuit-proof, two fuse holders

have been incorporated.

The component layout and

track side of the board are shown

in Figure 2 .

So as to enable the finished board to

be fitted in a compact enclosure, low-

profile transformers are used in the

prototype. These are available in a 4 VA

or 6 VA format from manufacturers

such as Hahn, Block, and

Monacor/Monarch. Hahn also has

3 VA, 10 VA, and 16 VA models in their

catalogue. Although these models dif-

fer in height, their width and depth are

the same. The board is suitable for use

with all these. The components list

gives model numbers for transformers

suitable for use with the Audio

DAC 2000.

The specified ratings for the fuses

(32 mA, delayed action) apply to the

non-short-circuit-proof 4 VA trans-

formers from Hahn and

Monacor/Monarch. When other trans-

formers are used, these ratings should

be altered accordingly.

The enclosure should be chosen

with a view to the appropriate safety

regulations. For instance, since the

Design by T. Giesberts

on the mains PCB. One

reason for this is that it ensures that the

board does not become unduly large

— and expensive — and another is

that many constructors like to solve

their mains power supply in their own

way.

Another important reason for not

placing the mains transformer on the

mother board is that it is frequently

desirable to keep the transformer as far

away from the signal circuits as possi-

ble. This prevents problems with hum

and other interference.

Many readers will have noted (and

some have indeed commented) that

over the past few years the projects

published in this magazine seldom

provide space on the associated

printed-circuit board for a mains trans-

former. One of the reasons for this is

safety. Working with mains voltages is

and remains a risky undertaking and

not-too-careful constructors, especially

the younger ones, run the risk that the

potentially lethal mains voltage

appears at places where it should not.

Much of this risk is obviated by the use

of discrete mains adaptors, as has been

the case in most recent mains-operated

projects.

Where, for one reason or another,

the use of a mains adaptor is pre-

cluded, modern practice is nonetheless

to avoid placing the mains transformer

T WO TRANSFORMERS

Although the reasoning in the intro-

duction is sound and practical, it is an

unfortunate fact that many suitable

mains transformers, particularly the

smaller ones, are available only with

terminals for soldering on to a board —

they have no other fixing facilities.

It is for this reason that the present

transformer board has been designed.

Although this is intended primarily

for use with the Audio DAC 2000, it

Elektor Electronics

1/2000

can also be used for many other appli-

cations.

In line with many modern circuits,

the Audio DAC 2000 works from two

supply voltages: +5 V and ±12 V.

The requisite transformer (sec-

ondary) voltages are 8 V (or 9 V)

and ±15 V respectively. For the

single supply, the prototype

uses a 2

Figure 1. The board

has space for not only

two mains transform-

ers, but also two

mains filters, and two

fuse holders.

Tr1

32mA T

B82721-K2401-N21

100n

275V

mains-carrying fuses are located at the

edge of the board, this should be at

least 6 mm away from the metal enclo-

sure.

Also, the regulations require the

minimum distance between mains-car-

rying tracks and ground to be at least

10 mm. This means that the board

must be mounted on 10 mm long

nylon or polythene spacers.

Finally, if the board is used in con-

junction with the Audio DAC 2000 and

a 2

100n

275V

Tr2

100n

275V

B82721-K2401-N21

zie tekst

* see text

siehe Text

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32mA T

000001 - 11

8 V) trans-

former is used, the dissipation of IC17

in the DAC rises sufficiently to make it

desirable for the device to be mounted

on a small heat sink.

9 V (instead of a 2

32mA/T

[000001]

Figure 2. The board

layout is designed pri-

marily for use with

low-profile transform-

ers, although taller

ones can also be

used.

COMPONENTS LIST

Capacitors:

C1,C2,C3 = 100nF 275V AC class X2,

raster 15mm

32mA/T

000001-1

Inductors:

L1,L2 = 2 x 27mH/0.4 A e.g.,

Siemens type B82721-K2401-N21

Miscellaneous:

K1 = 2-way PCB terminal block,

raster 7.5mm

K2 = 3- way PCB terminal block,

raster 5mm

K3,K4 = 2- way PCB terminal block,

raster 5mm

F1,F2 = fuse 32 mAT (slow), with

PCB mount holder

Tr1 = PCB mount mains transformer

2 x 15 V/4 VA, e.g., Block FLD4/15;

Hahn BV UI 302 0165; Monacor

FTR-415. *

Tr2 = PCB mount mains transformer

2 x 8 (or 9)V/4 VA, e.g., Block

FLD4/8 or FLD4/9; Hahn BV UI 302

0161; Monacor FTR-49.

PCB, order code 000001-1 (see

Readers Services pages)

*) Hahn and Monacor series not short-cir-

cuit resistant. Block series short-circuit

resistant.

Elektor Electronics

1/2000

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