This is only a preview of the July 1996 issue of Silicon Chip. You can view 25 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. Items relevant to "Build A VGA Digital Oscilloscope; Pt.1":
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Items relevant to "Remote Control Extender For VCRs":
Items relevant to "Build A 2A SLA Battery Charger":
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Charge SLA batteries
away from the mains
Want to charge a sealed lead acid (SLA) battery
away from home? This simple project lets you
use your car or boat battery to automatically –
and safely – charge 12V SLA batteries.
By JOHN CLARKE
Sealed Lead Acid (SLA) batteries
are used in a host of devices: cam
corders, spotlights, toys, portable TVs,
communications equipment and even
go-anywhere vacuum cleaners . . .
and these are only a few applications
which spring to mind.
SLA batteries are great when you’re
at home or close to a power outlet
and charger. But charging them when
you’re miles (or even kilometres) from
home? – that’s a different matter!
54 Silicon Chip
Despite what many people believe,
you cannot simply connect a 12V SLA
battery to your car or boat battery via a
current limiting resistor and expect it
to charge properly. The reason for this
is that there is insufficient potential
difference between the two batteries
to fully charge the SLA battery.
What’s more, even when such a system is used to partially charge an SLA
battery, it requires constant monitoring
to ensure that the battery isn’t cooked
by too high a charging current.
Main Features
• Powered from 12V battery
• 2A average current limit
• Suitable for 12V 6.5A.h &
greater
capacity SLA batteries
• Efficient switchmode desig
n
• Fuse protected
• Reverse polarity protectio
n
• Power indication
Many readers have asked for a safe,
reliable means of charging SLA batteries from vehicle or boat batteries,
which is the reason this project was
developed. Whether camping, boating,
travelling or off-roading, this charger
will come in handy.
Fig.2: inside
the Motorola
MC34063 DC-DC
converter IC
which forms
the heart of the
circuit.
Fig.1: the basic operation of the
DC-DC converter.
It connects to the car or boat battery
and directly charges any SLA battery
with a capacity of 6.5Ah or more to an
endpoint of 13.8V, without the need
for constant monitoring. The charger
will initially supply over 2A to a
discharged battery and this current
will gradually decrease as the battery
voltage reaches 13.8V.
Another application is as a solar battery charger, using 12V panels. The circuit will step-up the voltage from the
panels when it drops below 12V and
thereby improve overall effic
iency.
This design effectively supercedes the
design published in November 1991.
Basic operation
In operation, the SLA battery
charger steps up the voltage from the
battery using a DC-to-DC converter.
Fig.1 shows the basic principle of the
step-up circuit.
When switch S1 is closed, current
I1 flows through inductor L1. When
S1 opens, the field collapses and an
induced current, I2, flows through the
load via D2. C1 is also charged at this
time and discharges through the load
when S1 is closed.
By using a transistor or Mosfet in
place of S1 and by monitoring the
output voltage across the load, we
can adjust the on and off times for the
switching so as to provide a constant
voltage output.
A Motorola MC34063 DC-DC converter IC has been used to control this
operation. This IC contains all the
necessary circuitry to produce either
step-up, step down or an inverting
DC converter. Its principal sections
are a 1.25V reference, comparator,
oscillator, RS flipflop and a Darlington
transistor pair (Q1 and Q2) – see Fig.2.
The frequency of operation is set by
a capacitor on pin 3. A 0.001µF cap
acitor, for example, will set it running
at about 30kHz. The oscillator drives
the flipflop which in turn drives the
Darlington transistor.
Excess current is sensed at the
current peak input (pin 7) and this
switches off the flipflop and Darlington
transistor, to bring the current under
control.
The on time for the Darlington
transistor is set by the comparator.
This is used to monitor the output
voltage. When the pin 5 comparator
input exceeds the 1.25V reference,
the comparator goes low to keep the
flipflop from setting and thus holds
the Darlington off.
Conversely, if the output voltage
is too low, the inverting input of the
comparator will be below the 1.25V
reference and so the Darlington can be
toggled by the RS flipflop at the rate
set by the oscillator.
The complete circuit
Fig.3 shows the full circuit diagram.
The Darlington emitter at pin 2 drives
the gate of Mosfet Q1 while the 120Ω
resistor turns off the gate whenever
the Darlington is off. Note that the
Darlington collectors at pins 1 and 8
are connected to the positive supply
(pin 8 via a 47Ω resistor).
A 0.1Ω resistor between pins 6 & 7
sets the peak current delivered to the
Fig.3: The similarities between the complete circuit and that of Fig. 1 are obvious, with the IC
and Q1 effectively replacing the switch and its functions.
July 1996 55
This view inside the assembled SLA charger shows how the PC board clips
into place using the integral side pillars. Note that inductor L1 is secured to the
board with cable ties – don't rely on its leads to hold it in place.
inductor to 0.3V/0.1Ω, or 3A peak.
The average current supplied to the
load via D2 is limited to a little under
2A. A 0.68µF capacitor at the output
filters the voltage before it is applied
to the SLA battery.
Voltage regulation is provided by
the 22kΩ and 2.2kΩ voltage divider
resistors connected to pin 5. When
the output voltage is 13.8V, the voltage at pin 5 is 1.25V. Since this is the
reference voltage on the internal comparator (see Fig.2) the IC will maintain
13.8V at the output.
The 0.1µF capacitor at pin 5 removes
transient voltages which could cause
the IC to behave erratically.
Diode D1 has two purposes. First,
it provides reverse polarity protection for the circuit. This may be of no
consequence if a cigarette-lighter plug
is used to obtain the battery voltage.
However, if clip leads are used, then
reverse polarity is a distinct possibility
and protection is useful.
The second purpose for D1 is to prevent overcharging. This can happen
with a step-up circuit when the input
voltage rises above about 14V. At this
point, the FET will be permanently
turned off and so there is a direct path
to the batter via D1, L1 and D2.
However, the resulting drop across
56 Silicon Chip
D1 and D2 (approx. 1.2V) will limit
the voltage applied to the SLA battery.
Finally, fuse and zener diode protection has been included to limit
the short circuit current and prevent
transient voltages damaging IC1. LED1
provides power indication.
Construction
The Silicon Chip 2A SLA Battery
Charger is housed in a plastic case
measuring 130 x 68 x 42mm. The components are mounted onto a PC board
coded 04305961 and measuring 103 x
60mm. A front panel label measuring
62 x 126mm affixes to the top lid.
Begin construction by checking the
Both power diodes and the Mosfet are
mounted on finned heatsinks. There
is no need for insulating bushes or
washers but make sure that the leads
do not contact the heatsink.
PC board for shorted tracks or small
breaks, then insert all the PC stakes.
These are located at the four external
wiring points on the PC board.
Next, insert and solder in all the resistors, using the accompanying table
as a guide to the colour codes. This
done, insert the IC and zener diode.
The capacitors are next: there are no
polarity-conscious capacitors in this
circuit so their orientation is unimportant. However, the fuseholder clips
must be inserted correctly, otherwise
the fuse will not clip in. It is best to fit
the fuse into the clips before inserting
them into the PC board.
D1, D2 and Q1 are each mounted
horizontally on the PC board with a
heatsink and secured with a screw
and nut. Bend the leads for each
component at right angles before
mounting these devices. LED1 is
mounted on the end of its leads so
that it will later protrude through the
front panel.
Inductor L1 is wound with 1mm
enamelled copper wire on a ferrite
toroid. Draw half the length of wire
through the centre of the core and
wind on 22 turns neatly side by side.
The direction is unimportant. Now,
using the other end of the wire, wind
on another 22 turns so that the toroid
has 44 turns neatly wound around the
core. The windings are terminated
into the PC board holes as shown
on Fig.4.
Make sure that the wire ends are
stripped of insulation before soldering. The insulation can be scraped off
with a knife or melted off with a hot
soldering iron. L1 is secured in place
with two cable ties which loop through
holes in the PC board and around opposite sides of the toroid.
Final assembly
The assembled PC board can now
be installed in the case. First, affix the
label to the front panel and drill out
the holes for the LED and switch S1.
You will also need to drill out holes
in the top and bottom of the base of
the case to accept the cable grommets.
This done, place the PC board in
the case and test the lid to check that
the LED passes through its front panel
hole. Adjust the height of the LED if
necessary, so that it just protrudes
through the lid.
Next, connect the lighter plug to
a length of twin automotive wire or
heavy-duty figure-8 cable. This done,
Fig.4: install the parts on the
PC board as shown on this
wiring diagram.
Fig.5: check your board against this fullsize pattern before installing the parts.
pass the other end of the lead through a grommet
and terminate the wires to the PC board and S1
as shown. Similarly, connect the battery clips to
one end of the second length of twin automotive
wire, pass this wire through the second grommet
and connect the leads to the output terminals on
the PC board.
You are now ready to test the unit. Apply
power from a 12V supply (or battery) and check
that the LED lights. If it doesn’t, check that the
LED is oriented correctly and that the supply is
connected with the correct polarity.
Now measure the voltages on IC1 with a multimeter. There should be about 12V across pins
4 and 6. Now connect a 470Ω resistor (or there
abouts) in parallel with a 100µF 16VW (or larger)
electrolytic capacitor across the output terminals
(positive of capacitor to positive terminal, negative
to negative) and check that the output voltage is
about 13.8V.
Note that some power supplies will not cope
well with the battery charger since it draws high
RESISTOR COLOUR CODES
❏
❏
❏
❏
❏
No.
1
2
1
1
Value
22kΩ
2.2kΩ
120Ω
47Ω
4-Band Code (1%)
red red orange brown
red red red brown
brown red brown brown
yellow violet black brown
5-Band Code (1%)
red red black red brown
red red black brown brown
brown red black black brown
yellow violet black gold brown
July 1996 57
PARTS LIST
SILICON CHIP SOFTWARE
Now available: the complete index to all
SILICON CHIP articles since the first issue
in November 1987. The Floppy Index
comes with a handy file viewer that lets
you look at the index line by line or page
by page for quick browsing, or you can
use the search function. All commands
are listed on the screen, so you’ll always
know what to do next.
Notes & Errata also now available:
this file lets you quickly check out the
Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index
but a complete copy of all Notes & Errata text (diagrams not included). The file
viewer is included in the price, so that you can quickly locate the item of interest.
The Floppy Index and Notes & Errata files are supplied in ASCII format on a
3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File
Viewer requires MSDOS 3.3 or above.
ORDER FORM
Semiconductors
1 MC34063 DC-DC controller
(IC1)
1 MTP3055E, BUZ71 60V
Mosfet (Q1)
2 BY229 fast recovery diodes
(D1,D2)
1 16V 1W zener diode (ZD1)
1 5mm red LED (LED1)
PRICE
❏
Floppy Index (incl. file viewer): $A7
❏
Notes & Errata (incl. file viewer): $A7
❏
Alphanumeric LCD Demo Board Software (May 1993): $A7
❏
Stepper Motor Controller Software (January 1994): $A7
❏
Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7
❏
Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7
❏
Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7
❏
Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7
❏
I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7
Capacitors
2 100µF 16VW electrolytic (for
testing)
2 0.68µF 250VDC MKT
polyester
1 0.1µF 63VW MKT polyester
1 .001µF 63VW MKT polyester
Resistors (0.25W, 1%)
1 22kΩ1
120Ω 1W
2 2.2kΩ
1 47Ω
1 470Ω (for testing) 1 0.1Ω 5W
POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5
Disc size required: ❏ 3.5-inch disc
❏ 5.25-inch disc
1 PC board, code 14305961,
103 x 60mm
1 plastic case, 130 x 68 x 42mm
1 front-panel label, 62 x 126mm
1 SPDT toggle switch (S1)
3 TO-220 mini heatsinks (19 x
19 x 9.5mm)
3 3mm screws and nuts
2 M205 PC board fuse clips
1 3A M205 fuse
1 battery clip, red
1 battery clip, black
1 cigarette lighter plug (or two
more battery clips)
2 cable ties
2 cord grip grommets
1 Neosid 17-742-22 iron powder
ring core
4 PC stakes
1 1.5-metre length of 1mm
enamelled copper wire
1 3-metre length of 5A twin red/
black automotive cable
TOTAL $A
Enclosed is my cheque/money order for $A__________ or please debit my
Bankcard ❏ Visa Card ❏ MasterCard
❏
Card No.
Signature_______________________________ Card expiry date______/______
Name ___________________________________________________________
PLEASE PRINT
Suburb/town ________________________________ Postcode______________
Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your
order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number
(Bankcard, Visa Card or MasterCard).
58 Silicon Chip
✂
✂
Street ___________________________________________________________
current pulses. However, a 100µF
capacitor connected to the power
supply terminals will usually prevent
the supply from going into overload.
If this fails, use a 12V battery instead.
Now you can test the charger on
an SLA battery. Connect the charger
to the lighter socket in your car and
to the SLA battery and check that the
battery charges to 13.8V. Check the
temperature of D1, D2 and Q1. They
SC
should run warm.
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