Build this 1.5V
to 9V DC converter
get it going and can be operated from
an AA, C or D-size cell. What's more,
the TL496 only costs about $2.
Sick of how quickly your 9 volt batteries go
flat? Then switch over to more cost-efficient
1.5 volt cells with this 1.5V to 9V DC
converter. It uses just three components and
fits on a small PC board.
Block diagram
By DARREN YATES
There's nothing worse than when
you're using a piece of electronic
equipment and you're just at a crucial moment when suddenly the rotten 9-volt battery goes flat!
They're not cheap either. Don't you
just hate paying the prices for a 9-volt
alkaline or even just carbon-zinc 9V
batteries? And what about the price
of 9-volt nicads? Some retailers are
charging more than $24 for these
things, and when you consider they're
only rated for lO0mA.h, that's pretty
pricey. Even when you take into consideration the fact that they are rechargeable, you have to wait for up to
16 hours before you can use them
again!
As the man said, "there has to be a
better way"! Well, there is, thanks to
the Texas Instruments TL496.
The TL496 is a very compact DC
step-up switching converter IC which
provides a regulated 9V DC output
from a 1.5V DC input. It only requires
a filter capacitor and an inductor to
~
The block diagram in Fig.1 shows
the internal operation of the TL496
IC. An input voltage can be applied
to either pins 2, 3 or 4, but each has a
separate purpose.
If you want to use just one 1.5V
battery, then you apply this to pins 2
and 3. Or you may prefer to use two
1.5V cells in series, in which case
you just connect them to pin 2 and
leave pin 3 floating.
Both ways have their benefits. If
space is a problem, one cell definitely
makes more sense, but the tradeoff is
more current drain.
Because it's stepping the voltage
up six times (from 1.5 to 9V) and
because the IC is not perfectly efficient, it uses about eleven times the
current we get at the output; eg, we
may get lOmA at the output but the
circuit consumes 114mA from the
battery. If you want to talk about efficiency ratings, the circuit is about
50% efficient. Even so, it is a much
cheaper way of powering a circuit
from 9V than to use 9V batteries.
If you were to use two cells ·in series, the IC needs to step the voltage
up three times and so requires only
about six times the current. This
doesn't change the efficiency of the
circuit - it just means that two cells
will last about twice as long as one
cell. Our circuit uses the single cell
option which we think is the most
economical overall.
High frequency oscillator
This 1.5V to 9V DC converter is incredibly simple, thanks to the use of a
dedicated switching converter IC from Texas Instruments (the TL496). There
are just three components on the board, plus four PC stakes! The circuit can be
housed in a separate case or built into the equipment to be powered.
72
SILICON CHIP
Returning to the block diagram, the
switching voltage regulator control
uses a high-frequency oscillator to
drive the output transistor, which has
an inductor connected between its
collector (pin 6) and the supply input.
T INPUT (4)
2C INPUT (3V) (2)
1C INPUT (1.5V) (3)
ELECTRONICS
WORLD
9V SERIES
REGULATOR
SWITCHING
VOLTAGE
REGULATOR
CONTROL
1-----
NOVEMBER
SPECIALS
(6) SWITCH
USED 'AA' 700mAH NICAD
BATTERIES
$
0.50
TEMPERATURE CONTROLLED
SOLDERING STATION
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GND (5)
GND (7)
UNIVERSAL NICAD BATTERY
CHARGER
$ 29.95
Can do 4 at a time (AA, C, D, or 9V
Nlcads)
Fig.1: block diagram of the TL496 switching converter IC. It uses a
high frequency oscillator to drive a switching transistor.
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Fig.2: how a switching regulator
works. When S1 is closed, current
flows & energy is stored in the
inductor. Note that diode D1 is
reverse biased during this time.
There may be some readers for
whom switching regulators are new
territory, so let's briefly go over the
principles of operation.
Basic principles
If you take a look at Fig.2, we have
an inductor, a switch, a diode and a
capacitor. The inductor represents our
coil, switch Sl takes the place of the
output transistor in the IC, the diode
is the zener diode shown in-the block
diagram of Fig, 1, and the ea pacitor is
our output capacitor.
When the switch is closed (corresponding to the transistor being
turned on in the IC), a current flows
through the inductor as it stores energy. The anode side of the diode is
now effectively connected to ground,
so it is reverse biased and no current
flows through to the load.
In Fig.3 , we open the switch again
to stop the current flow but the inductor tries to maintain the current.
The voltage across the inductor rises
sharply as a result of this. The diode
now becomes forward biased and the
inductor dumps its stored energy into
the capacitor.
The TL496 has an internal oscillator which drives the output transis-
I
1
-0
VOUT
0-
Fig.3: when S1 is opened, the
voltage across the inductor rises.
D1 is now forward biased & so
the inductor dumps its stored
energy into the capacitor.
tor, switching it on and off at a rate
which depends on the load current.
The higher the load current, the higher
the switching frequency. At any particular load current, the switching rate
is not an absolutely steady frequency
though; it hunts back and forwards.
In fact, what actually happens is
that the internal transistor is always
turned on for roughly the same period of time, around 0.3 milliseconds.
Then, depending on the load current,
the switching rate can be anywhere
from a few Hertz up to around ZkHz.
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,c
2
2C
GNO
1.5V
T~~4S
8
OUT>-- - - - . . - , +9V
GNO
470
16VW
OUTPUT
.___ _..___ _ _ _ _ _ _ _ _-<lOV
*
l 1 : 5JT .0.6J mm OIA . ENCU WIRE
WOUNO ON A NfO SIO 17 747 10 TOROIO
1.SV TO 9V DC CONVERTER
Fig.4: the complete circuit for the 1.5V
to 9V converter. Note that you can
also use two 1.5V cells in series, in
which case the connection to pin 3 is
deleted.
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M ail Orders Welcome
30 Lacey St, Croydon
VIC, 3136.
Telephone:
Fax:
(03) 723 3860
(03) 723 3094
(03) 725 9443
NOVEMBER
1990
73
I
I
7 uF
0
_...
9V
---
Fig.5: inductor L1 consists of 53 turns of 0.6mm enamelled
copper wire on a Neosid toroid core. Fig.6 at right shows the
full size PC pattern.
The maximum output current from
the circuit is about 40 milliamps. At
this current, a typical 9V battery
would not la.st long at all. By contrast,
a 1.5V D-cell will last for about 20
hours at this output current.
Circuit diagram
The circuit diagram in Fig.4 shows
how few components are required one IC, one capacitor, one inductor
and that's it!
The inductor is a Neosid 17-742-10
toroid core, wound with 53 turns of
0.6mm diameter enamelled copper
wire. The exact number of turns is
not really important since the IC selfregulates. So as long as there are somewhere between 45 and 60 turns, the
circuit will work. The efficiency will
tend to vary by a small amount depending on the number of turns, but
we found a figure of 53 turns to be
about optimum.
The 470µF filter capacitor is used
to smooth the DC output on pin 8.
The ripple output is generally around
50mV peak-to-peak, except at very
PARTS LIST
1 PCB, code SC11111901, 60 x
38mm
1 TL496CP 9V switching
inverter (IC1)
1 470µF 16VW electrolytic
capacitor
1 Neosid 17-742-10 toroid core,
28mm OD, 15mm·ID
2 metres of 0.6mm enamelled
copper wire
4 PC pins
1 AA, C or D size 1.5V battery
1 holder to suit battery
Miscellaneous
Solder, hookup wire, etc.
74
SILICON CHIP
:::1
~:
-21
.,....
.,....
Constructing the DC Converter
should only take about half an hour.
We have designed a small board for
the job. It is coded SC11111901 and
measures 60 x 38mm. Whether you
buy or make the PC board, check that
there are no shorts or breaks in the
tracks. If there are, touch them up
now before you do any soldering.
Begin the assembly by installing
the four PC pins. These are used to
connect up the 1.5V supply and the
9V DC output.
Next, wind the inductor. This is
the most time-consuming step in the
assembly process. You will need just
over 2 metres of 0.6mm enamelled
copper wire. Winding the wire is a
matter of threading the wire through
for the required number of turns (53).
Be careful not to kink the wire as you
do the job. Make sure the turns are
reasonably tight and spread evenly
around the core.
Bring both ends of the finished
winding to the same spot so they can
be easily soldered into position. Clean
the two ends of the winding of
enamel, by scraping it off with an old
razor blade or utility knife blade. This
done, tin the ends with solder, position the toroid on the board and solder the leads to the board.
You will also need to use an anchor
wire to stop the inductor from moving about on the board. The anchor
wire is soldered to two unconnected
pads on the board.
Next, solder in the 470µF capacitor. Remember to check that its polarity is correct - the negative pin should
go towards the inductor. Finally, solder in the TL496 IC. Again, make sure
of the correct polarity - pin 1 (indi-
0
,,-
,-
.,....
u(f)
low load currents. It can be reduced
by substituting a larger capacitor.
Construction
0
0
O'l
TABLE 1
LOAD CURRENT
<at>9V OUTPUT
INPUT CURRENT
<at>1.5V
no load
0.1mA
0.5mA
1mA
2mA
5mA
10mA
20mA
40mA
50µA
1.3mA
5.2mA
11.7mA
25mA
57mA
114mA
230mA
460mA
cated by the dot on top of the IC)
should be closest to the outer edge of
the board and furthest away from the
capacitor.
Finally, wire up the battery holder
to the board, making sure that the
polarity is correct. Insert the battery
and then measure the output voltage
from the board. It should be close to 9
volts DC.
The completed board and the battery holder can be housed 'in a small
plastic zippy case or, in some instances, built into the equipment it is
to power. Don't forget to cut the track
to pin 3 of the TL496 if you intend
using two 1.5V cells in series.
Current loads
As mentioned before, depending
upon your situation you can use either AA, C or D-size batteries with
this circuit. Table 1 shows the expected load and input currents. Ideally, if you require large input currents, say more than 100mA, use a C
or D- size cell for best economy. And
if you use an alkaline cell rather than
a carbon zinc type, it will last considerably longer.
~
