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This is only a preview of the April 1993 issue of Silicon Chip. You can view 48 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Build An Audio Power Meter":
Items relevant to "Three-Function Home Weather Station":
Articles in this series:
Items relevant to "12VDC To 70VDC Step-Up Voltage Converter":
Articles in this series:
Items relevant to "A Digital Clock With Battery Back-Up":
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Special circuit for vintage radio restorers!
Build this step-up
voltage converter
This little unit converts 9-12V DC from a
mains plugpack up to a maximum of 70V DC
at 40mA. It can be used with battery operated
valve radios or any device that requires a
supply voltage of more than 12V DC.
rate intercom system, this project can
also be tailored to provide the 50V
required to operate them.
In general, this circuit can be made
to generate any DC voltage up to about
70VDC at modest output current levels.
Circuit details
By DARREN YATES
If you're into vintage radio then
you'll know that 'B' batteries are impossible to get for those battery-operated valve receivers. It would be a
pity if melodic sounds were never to
be heard again from these grand old
sets for lack of a power source.
In a recent Vintage Radio column
(December 1991), John Hill suggested
a 'B' battery eliminator using five 9V
batteries. This provides a 45V supply
but some old valve radios need a
+6 7. 5V DC rail. Stacking seven or eight
9V batteries to provide around +7DV
DC is not really practical and it would
be quite expensive too.
As a result, many old battery powered radios have been left to gather
dust because the batteries to drive
them are no longer available or the
alternatives are just too expensive.
This is where our little high voltage
supply comes to the rescue. It allows
you to run an old battery powered
radio direct from the mains and it is
quite cheap.
If you have a couple of old Telecom
phones that you wish to run as a sepa-
The circuit for the High Voltage
Converter is based on a Motorola
MC34063A DC-DC converter IC
(Fig.2). This was previously featured
in the Portable SLA Battery Charger
published in the July 1992 issue of
SILICON CHIP.
The MC34063A was designed to
convert a DC voltage up to a level of
about 40V maximum but by using a
high voltage external pass transistor,
the IC can be made to produce at least
7DVDC.
The input voltage source can be
just about anything from dry cells to a
plugpack or your car battery but it
170 µH
L
r------------ ---
8I
i
I1
--+--◄
180
I
I
I
I
I
12
Rsc
1N5819
0.22
V1n 0 - - - - - - l
I
12 V
1.25 V
Ref
Reg
1100
jcrJ
I 1soo
:
pF
14
I
L-- --- -- ------------ -~
R2
Vout
- - ---.,1t1,------------0 28 V/175 mA
47k
The High Voltage Converter is ideal for generating the B+ rail
for a battery powered vintage radio receiver, or for any
application requiring up to 70VDC at 40mA. The output
voltage can be adjusted by changing two resistor values.
56
SrucoN CHIP
Fig.1: block diagram of the Motorola MC34063
DC-DC controller IC. It uses an internal oscillator to
drive an RS flipflop & this in turn drives a Darlington
transistor pair to switch an external inductor.
S1
+
0.221l
SW
F1
2A
o-/
Vin
-+
1801l
7
8
1
~
IC1
MC34083A
B
4.7k
H~
K
A
BCE
.0047+
470 +
~
~
63VW+
~
L1 : TWO LAYERS OF 0.63mm DIA ENCU
ON NEOSID TOROIDAL CORE 17/732/22
RB
1k
~
HIGH VOLT AGE DC-DC CONVERTER
Fig.2: the final circuit uses external pass transistor Qt to switch inductor Ll,
so that the output voltage can be boosted to 70V. Resistors RA 1 & RA2 in the
negative feedback network set the output voltage to the required value.
must be able to supply around 250300mA.
To understand how the circuit of
Fig.2 works, it is useful to have a look
at Motorola's own step-up circuit for
the MC34063A, as shown in Fig, 1.
Here there is no external transistor, as
internal transistor Q1 takes care of all
the load current. Briefly, the circuit
works as follows.
An input voltage of 12V is applied
to pin 6 of the IC and also to the
170µH inductor L via resistor Rsc· This
resistor provides current monitoring
and the IC shuts down if the voltage
across Rsc exceeds 0.3V.
An internal oscillator, with its operating frequency set by the capacitor
at pin 3, .drives Q1 and QZ and thus
switches current through the 17DµH
inductor. Each time transistor Q1
switches off, the collapsing magnetic
field associated with the inductor will
try to maintain the current through it
but since Q1 is off, the only available
current path is via the 1N5819 diode
to the 150µF capacitor C0 • Thus, the
capacitor charges to a considerably
higher voltage than the input of 12V.
Negative feedback around the circuit is used to set the output voltage
to a predetermined value. This is set
by a voltage divider network consisting of Rl and R2. This feeds the inverting input (pin 5) of an internal
comparator, while a 1.25V reference
feeds the non-inverting(+) input.
Thus, when the voltage at pin 5 is
just above 1.25V, Ql is not driven and
when it is below 1.25V, Ql is driven
at a high frequency. Typically, the circuit of Fig.2 can regulate the output
voltage to within about ±50mV.
Now take another look at our circuit of Fig.2 and note the differences
between it and Fig.1 which we have
just discussed.
In the original circuit of Fig.1, pin 1
was connected to the junction of the
inductor and the diode but this presented a problem for our application.
Pin 1 is actually the collector of Ql
inside the IC and it has a collectoremitter voltage (Vee) rating of 40V.
This would be far exceeded if we
pushed the circuit to produce the 70V
or so we require.
The solution is to use Ql inside the
PARTS LIST
1 PC board, code 11102931,
100x 55mm
1 self-adhesive front panel label,
54 x 99 mm
1 plastic zippy case, 130 x 67 x
42mm
1 SPST miniature toggle switch
1 33mm OD toriodal core
(Altronics Cat. L-5120)
1 3.5mm socket
1 2-pin DIN panel socket
1 2-pin DIN plug
3 metres of 0.63mm enamelled
copper wire
2 M205 (2AG) fuse clips
1 2A M205 fuse
Semiconductors
1 MC34063A DC-DC converter
(IC1)
1 TIP31 C NPN transistor (01)
1 BY229-400 fast recovery diode
(D1)
1 33V 1W zener diode (ZD1)
Capacitors
2 470µF 63VW electrolytics
1 .0047µF 63VW MKT polyester
Resistors (1 %, 0.25W)
2 27kO
1 1800
1 4.7kO
1 0.470 SW
1 1kO
A small heatsink must be fiUed to power transistor Qt to aid heat dissipation.
There's no need to isolate Qt 's tab from the heatsink but make sure that the
latter does not touch any other components.
·
Miscellaneous
Solder, screws, washers, nuts,
hook-up wire .
APRIL
1993
57
DC INPUT
SC11102931
~
_Qj
Fig.4: check the PC board for defects
against this full-size artwork before
mounting any of the parts.
Fig.3: make sure that all polarised parts are correctly oriented
when installing them on the PC board & don't forget the wire
link that sits directly under the back of the heatsink.
IC as an emitter follower which then
drives external transistor Ql (a bit
confusing, that) . This external transistor has more generous voltage and
current ratings. Pin 1 is now moved to
the supply rail side of the inductor
and so the internal transistor sees no
more than about +12V on its collector.
Apart from the use of an external
transistor, the circuit of Fig.2 works
in an almost identical fashion to the
original circuit of Fig, 1. However,
there are a few other differences which
we will explain.
Note the two 470µF capacitors connected in series across the output. We
would have preferred to use just one
output capacitor rated at 100V but
63V capacitors are much more read-
ily available; ergo , we have used two
in series.
Connecting two capacitors of nominally the same capacitance and voltage rating does not mean that they
will equally share the voltage; the actual voltage across each capacitor will
mainly depend on their leakage resistance and we have no control over
this factor. We solved that this problem by "swamping" the leakage
resistances of the capacitors with parallel connected 27kQ resistors (RA 1 &
RAz) and these do double duty by
forming part of the voltage feedback
network to pin 5.
Output voltage
As we said before, the output voltage can be adjusted to any value up to
70VDC. This is achieved simply by
RESISTOR COLOUR CODES
0
0
0
0
0
0
58
No.
Value
4-Band Code (1%)
5-Band Code 1%
2
27kQ
4.7kQ
1kQ
180Q
0.47Q 5W
red violet orange brown
yellow violet orange brown
brown black red brown
brown grey brown brown
not applicable
red violet black red brown
yellow violet black red brown
brown black black brown brown
brown grey black black brown
not applicable
1
SILICON CHIP
Once the IC is in, install the
5W resistor, the fuse clips and
the electrolytic capacitors. The
HIGH VOLTAGE
fast recovery diode (D1) and
CONVERTER
power transistor Ql can then be
fitted. Note that Ql must be fitted with a small heatsink and
this is best attached before the
HVOUT
DCIN
transistor is installed.
The last component to be installed is the toroidal inductor.
ON
This is wound in two layers using 70 turns of 0.63mm enamPOWER
elled copper wire. Begin with a
3-metre length of wire and thread
it half-way through the centre of
the toroid. Now, using one half
Fig.5: this full-size front panel artwork can be used as a drilling template for the
of the wire, wind on 35 turns.
on/off switch. Spray the finished label with a hard-setting clear lacquer before
Make sure that you wind the
attaching it to the lid of the case.
turns as neatly as possible and
keep the turns tight and close together.
adjusting resistors RA 1 and RAz in the the power switch and the two power The other half of the wire is then used
divider string. The equation to derive sockets are installed on a PC board to wind on the other 35 turns.
Once the inductor has been wound,
the correct values for resistors RA 1 measuring 100 x 55mm and coded
strip and tin the wire ends, then sol11102931.
and RAz is as follows: RA1 + RAz =
(Vout/1.25 -1) x 1000
Before beginning any construction der the complete unit to the board.
You can secure the inductor in a
This is the total value for RA 1 and work, check the board carefully for
RAz• You need to then divide this any shorts or breaks in the tracks by number of ways: by using a small
value by two to get the value for RA 1 comparing it with the published pat- nylon cable tie that passes through
and RAz• These resistors should be tern. Repair any defects that you do the toroid and two holes in the board;
equal in value if possible, so that equal find, then start the assembly by in- by pouring a little hot wax over the
toroid; or by using an adhesive like
voltages are developed across the ca- stalling the resistors and ·wire links Blu-Tac®.
see Fig.3.
pacitors.
The .0047µF capacitor on pin 3 sets
Next, install the 33V zener diode,
Switching on
the switching frequency, while the the .0047µF MKT capacitor and the
33V zener diode (ZDl) protects the IC IC. It is usual to install ICs last but, in
Check that all the wiring is correct
this project, it's best to install it before before applying power to the board.
from over-voltage.
some of the bigger components go in.
You will need a power supply capaConstruction
Be sure to orient the IC exactly as ble of delivering 12V. This should be
shown on the wiring diagram.
connected via your multimeter which
All of the components except for
should be set to the 2A range.
Make sure that you have the supply
polarity correct before switching on.
After an initial surge, the current
should drop back to a few milliamps
at most. If the current drain is appreciably more (with no output load connected), switch off immediately and
check carefully for assembly errors
on the board.
If all is OK, disconnect your multimeter, select the 200V DC range, reconnect the power supply and measure the output voltage. You should
get a reading of about 70VDC (RA 1 =
RAz = 27kQ), depending on the tolerance of your resistors.
Finally, the board can be mounted
on the bottom of a plastic utility case
and appropriate sockets fitted to accept the input and output connecThe PC board is secured to the bottom of the case using machine screws & nuts,
tions. The on/off switch can be fitted
with additional nuts used as spacers. A 3.5mm socket accepts the input from
the plugpack, while the output is fed to a 2-pin DIN socket.
to the lid of the case.
SC
=flW
=Ui/Jl!J+
APRIL
1993
59
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