AMATEUR RADIO
By GARRY CHATT, VK2YBX
DC-to-DC battery charger
for 12V gel batteries
Want to recharge a 12V gel cell from your car
battery? This simple circuit will do the job. Based
on an LM3524 switching regulator and a BUZ71
Fet power transistor, it provides a 14.4V output at
currents up to two amps.
A significant problem encountered by amateur operators in
the field is that of battery life. If
equipment is operated from sealed
lead acid cells, there's usually no
way to recharge a flat battery
without access to a 240 volt
generator and a suitable charger.
Attempting to charge a spent 12
volt gel battery by connecting it
directly to a 12 volt car battery is
not a good idea. If the car battery is
fully charged, it may deliver very
high currents to the gel battery and
if it is not fully charged, it will be a
futile exercise.
But there is a way of charging a
12V gel battery from a source of
lower potential. It can be done by
using a step-up switching regulator.
The circuit described here is
capable of supplying 14.4V DC at 2
amps, which is quite ample for
charging sealed gel cells. It is
capable of operating at around
80% efficiency from input voltages
as low as lOV.
How it works
Basically, the circuit comprises a
pulse width modulator (PWM)
which drives inductor Ll via switching transistor Ql, as in Fig.1.
In this circuit, Ql is used to
switch Vin across inductor L1 at a
pulse rate determined by the PWM
generator. For the duration of each
pulse, Ql is turned on and energy is
drawn from the supply and stored
in Ll. Diode D1 is reverse biased,
and output current is supplied by
the charge stored in capacitor Cout•
When Qt turns off, Vl will rise to
the point where D1 turns on. The
Vout
-JONr
Lfl
--1 OFF 1--
+
PWM
Cout+
-=
RL
i
lout
.,.
Fig.1: basic scheme for the switchmode charger. It uses a pulse
width modulator (PWM) to drive inductor L1 via switching
transistor Qt. When Ql is on, current is supplied to the load by
Cout; when Ql is off, the current is supplied via L1 and D1.
78
SILICON CHIP
output current is then supplied
through L1 and D1 to the load and
any charge lost from Cout during
the previous cycle is replenished.
The complete circuit is shown in
Fig.2 and uses an LM3524 Pulse
Width Modulator chip which is
available from several manufacturers. An RC network connected
from pins 6 and 7 of the LM3524
sets the frequency of operation to
about 50kHz. Resistors Rl and R2
set the voltage on pin 2 to 2.5V by
dividing down the internallygenerated 5V reference (VREF) at
pin 16.
R3 and R7 form a second voltage
divider. Under no load conditions,
this also sets the voltage at pin 1 of
the chip to 2.5V. This divider acts
as a voltage sensing circuit connected to the output of the
regulator.
As the output of the circuit is
loaded, the output voltage drops, as
does the voltage at pin 1 of the chip.
When this happens, ICl automatically increases its output pulse
width, thus allowing higher output
current to be supplied at the preset
output voltage.
In our circuit the output voltage
is set by R3 and R7 to about 14.4
volts, which is sufficient to safely
charge a typical sealed gel battery.
If you wish, you can increase the
output voltage to about 15.2V by increasing R7 to 1 lkO.
The pulsed output from IC1 appears at pin 12 and drives NPN
transistor Ql via diode D1. Qt, in
turn, drives Q2 which is a BUZ71
Fet power transistor. Each time pin
12 ofICl goes high, Ql and Q2 turn
87
10k
+v~o------+-------....------.....- - + - - - - - - t - - - - - - - - ,
ea +
4.7
16VWJ
89
1k
15
1&ao
16VW
+
-
C2
IC1
LM3524
.Ol
C6
.01
R10
1k
16
81
4.7k
VR1
83
2.2k
86
120k
C3
.0022
10k
C4
470pF
).. LED1
CHARGE
...
~ ~
B
EQc
SWITCHMODE CHARGER
VIEWED FROM
BELOW
GOS
K A
Fig.2: the PWM output from the LM3524 IC appears on pin 12 and drives Fet power transistor Q2 via D1 and
Qt. R7 and R3 set the output voltage to about 14.4V but this can be increased by increasing R7 to 11kn.
PARTS LIST
1 PCB, code SC 141-0988-1 ,
88 x 52mm
1 small heatsink, DSE Cat.
H-3490
4 alligator clips to suit (for
battery connections)
1 panel-mount fuseholder
1 3A fuse
1 T0-220 mica washer and
insulating bush
A small heatsink must be fitted to the BUZ71 power Fet to aid cooling. Isolate
the Fet from the heatsink using a mica washer and insulating bush. (Note:
the PCB was modified slightly after this photo was taken).
off and current is supplied to the
load and to C5 via 11 and D2. When
pin 12 is low, Ql and Q2 are on and
the output current is supplied by
C5.
D1, R4, R5, RB and Ql ensure
that the Fet is turned on hard, thus
ensuring clean switching and
minimising power dissipation in the
Fet. The main advantage of the Fet
over bipolar transistors is its small
dynamic resistance, typically less
than rn. This allows the device to
switch relatively high currents
without the need for substantial
heatsinking.
Diode D2 is a high speed fast
recovery diode capable of handling
7 amps, which is more than enough
for this application. R6 presents a
light load to the circuit when no external load is connected, thus ensuring good regulation from no-load
to full load conditions.
Semiconductors
1 LM3524 voltage regulator IC
(Geoff Wood Electronics)
1 BUZ71 power FET transistor
(Geoff Wood Electronics)
1 BC54 7 NPN transistor
1 BY229 or MUR1550 fast
recovery diode
1 1N914 silicon diode
1 3mm red LED
Capacitors
2
2
2
1
1
1OOOµF 16VW electrolytic
4. 7.µF 16VW electrolytic
.01µF ceramic
.0022µF greencap
470pF ceramic
~
Resistors (0.25W, 5%)
1 x 120k0, 1 x 1OkO, 3 x 4 .7k0,
1 X 2.2k0, 2 X 1k0, 1 X 2700, 1
x 4 70, 1 x 1OkO 5mm trimpot
(horizontal mount)
SEPTEMBER 1988
79
Fig.3: install the parts on the PCB as shown here.
The spare pad adjacent to pin 14 of ICl can be
used to terminate the anode lead for the optional
charge indicator LED.
Charge indication is provided by
LED 1 which is wired between pin
14 of IC1 and ground. Note that the
LM3524 has two output devices
(see Fig.4). In this circuit, we are using one to drive Qt and the Fet, and
the other to drive the charge indicator LED.
As the pulse width narrows, as
Fig.4: you can use this pattern to etch your own printed
circuit board or you can buy a ready-etched board
from the usual suppliers (see address panel, page
104).
occurs when the circuit is lightly
loaded, the brilliance of the LED
drops. Under high load conditions,
the pulse width will be longer and
so the LED will be driven harder.
Construction
A small printed circuit board
(PCB) has been designed to simplify
Further notes on the
RF sniffer probe & preamplifier
FERRITE
1~!1
,on
.Di
OUTPUT
l--:-0
The circuit diagram for the RF
sniffer probe published in the
June issue was for an initial prototype, rather than tor the final
version which was shown in the
coded photograph. The above
diagram shows the correct version, the main differences being
the connections for the 4 7 on
base bias resistors on transistor
stages Q 1 , 02 and Q3.
80
SILICON CHIP
The supply feedpoint was
also shown incorrectly - it
should go to the midpoint of the
two 1on resistors as shown
above. The coded photograph
published on page 7 3 of the
June issue is correct as are
other constructional details.
Our thanks to those readers
who took the trouble to point
out the above discrepancies.
construction. Fig.3 shows where all
the parts go.
You can install the parts in any
order but watch the polarity of the
IC, transistors and electrolytic
capacitors. Inductor 11 is made
from 20 turns of 1mm enamelled
copper wire wound on a 30mm
length of ferrite rod (available from
retailers as "antenna rod").
A small heatsink must be fitted to
the power Fet. The type recommended is from Dick Smith Electronics (Cat. H-3490) and comes
with two small pegs which allow it
to be soldered directly to the PCB.
Use an insulating bush and mica
washer to isolate the metal tab of
the Fet from the heatsink.
The PCB can be housed in any
suitable enclosure, either plastic or
metal, as the RFI (radio frequency
interference) generated by the
regulator is minimal. The unit
should include fuse protection to
guard against output shorts, and
this can easily be implemented using a chassis mount fuse holder on
the enclosure. A 3A fuse should do
the job.
Before using the switching
regulator, VR1 should be adjusted
to give optimum efficiency. To do
this, connect a 12V 18-watt (or
thereabouts) lamp across the output and adjust VR1 for minimum
current drain from the battery.
Finally, don't charge a battery
that is still connected to other
equipment. The circuit has no high
voltage, spike or current limiting
protection, so damage could result
if you try.
~
