siliconchip.com.au Australia’s electronics magazine July 2019 29
underside of the board. The easiest way to do this is to
plug CON2 into CON1 on the control board, attach the
two boards using the four corner mounting holes, 12mm
tapped spacers and short machine screws and then solder
CON2 to the Mosfet board. That ensures the two headers
line up properly.
The M8 brass screws that will be used as the battery
terminals can now be fed through the Mosfet PCB, with a
shakeproof or crinkle washer under the screw head (which
goes on the bottom side of the board) and another under the
nut which is done up tightly on the top side of the board.
This should give good electrical contact to the PCB and
means that you don’t need to solder the screws and nuts to
the boards, which is difficult and makes disassembly im-
possible. (You can see that this was done on the prototype
in the photos below.)
While the Mosfet board is now complete, you could con-
sider adding some tinned copper wires
in parallel with the copper on the board.
This will reduce the voltage drop across
the device, as well as its dissipation, and
make it more robust.
However, we do not feel that this is
strictly necessary due to the use of extra-
thick 2oz copper on this board.
If you do want to run some extra wire,
you can solder lengths of tinned copper
wire from between pins 2 & 3 of each Mos-
fet to between pins 5 & 6 on the Mosfet on
the other side of the board. You can then
solder wires from the tabs of each Mosfet
to the nearby battery terminal.
You may be able to solder these to the
exposed copper around the nuts, or directly
to the nuts themselves, with a very hot iron.
Make sure any added wires do not project above the board
any higher than the bodies of the Mosfets; otherwise, they
could potentially short to the metal lid of the case later.Testing
Ideally, you should use an adjustable bench supply
with current limiting for testing. Set it to 12V and
around 50mA, then apply power between the main
battery terminal and the ground pad on the Mosfet
PCB (or pin 5 of CON1 or CON2). You should ob-
serve a current flow which settles at around 8mA.
LED1 should remain off.
Measure the voltage at the auxiliary battery termi-
nal relative to GND. It should be low, close to 0V. Now
increase the supply to around 14V.
You should observe LED1 switch on. The current draw
should increase slightly. The voltage at the auxiliary bat-
tery terminal should now have risen to the supply voltage.
Reduce the supply voltage back to 12V and confirm that
LED1 switches off and the voltage at the aux battery termi-
nal drops back to 0V within a few seconds. This verifies
that everything is working as intended and you can now
proceed to finish construction.Adding a bypass switch
There may be times where the vehicle battery is low, but
you still want to connect it to the auxiliary battery. One
example would be if the vehicle battery is flat but the aux-
iliary battery is charged, and you want to ‘jump start’ the
vehicle using the aux battery.
While you could do this with a screwdriver across the
terminals, it’s much nicer to have a switch which forces
the unit to operate.
This is quite easy to do, but it does have one limitation
in that this won’t work if the vehicle battery is dead flat,
since the unit is powered from it. But it should work down
to at least 10V, or possibly even less.
The easiest way to achieve this is to connect a switch be-
tween pin 7 of IC1b and GND. When this switch is closed,The two
PCBs are stacked
as shown, with the
8mm brass battery connection posts fitted firmly in place
with washers ensuring good contact with the PCB tracks.
Fig.4: the front panel can either be
photocopied or even better, downloaded
from siliconchip.com.au/shop/11/5059
Ideally, it should be laminated before
glueing in place.