siliconchip.com.au Australia’s electronics magazine July 2019 31
Parts list –
Solid State Dual Battery Isolator
1 double-sided PCB coded 05106191, 98 x 71mm
1 double-sided PCB with 2oz copper, coded 05106192,
98 x 71mm
1 IP65 diecast aluminium box, 115 x 90 x 55mm
[Jaycar HB5042/HB5044, Altronics H0423] OR
1 IP65 polycarbonate box, 115 x 90 x 55mm
[Jaycar HB6216/HB6217]
1 panel label, 115 x 90mm
2 35mm long M8 brass screws
6 M8 brass hex nuts
6 8mm ID brass flat washers
4 8mm ID brass or beryllium copper star/crinkle washers
4 8mm ID Nylon screw insulators (if using a metal case)
4 12mm long M3 tapped Nylon spacers
8 M3 x 6mm panhead machine screws
2 small eyelet quick connectors
1 M3 x 10mm panhead machine screw, shakeproof washer
and two hex nuts
Semiconductors
1 LM339 quad comparator, DIP-14 (IC1)
1 LP2950ACZ-5.0 5V low-dropout linear regulator, TO-92
(REG1)
6 40V 100A+ N-channel Mosfets, TO-263-7 (Q1-Q6)
[eg Infineon IRFS7434TRL7PP*]
1 5mm LED (LED1)
2 15V 1W zener diodes (ZD1,ZD2)
2 5kW 15-18V transient voltage suppressors, DO-214AB/
SMC (TVS1,TVS2) [eg, Bourns 5.0SMDJ15CA-H*]
4 1N4148 small signal diodes (D1-D4)
1 5-pin SIL socket (CON1)
1 5-pin header (CON2)Capacitors
1 4.7μF 50V electrolytic
4 100nF 50V ceramic or MKT
1 3.3nF 50V ceramic or MKT
Resistors (all 1/4W 1% metal film)
1 22kW 3 10kW 2 6.8kW# 3 4.7kW#
1 2.7kW 1 1.5kW 2 100W
# use ±0.1% tolerance resistors for the tighter threshold
ranges mentioned in the text* available from
Mouser or Digi-Keyit will pull that pin down to 0V, which means that the volt-
age at pin 6 will always be higher than pin 7, so output pin
1 will go low, switching on Mosfets Q1-Q6. This switch is
shown with dotted connections in Fig.1.
We’ve also shown the most convenient points to sol-
der wires to go to the switch in Figs.2 & 3. Simply solder
a wire here, to the COM terminal of an SPST switch, then
a wire from the NO terminal of that switch to a conveni-
ent ground point.
When you activate this switch, you need to remember
to switch it back into its normal position later, for the unit
to go back to doing its job!
Case assembly
There are only four holes to drill: two in the lid for the
battery terminals (main and auxiliary), plus one for the
LED, and one 3mm hole in the side of the case for the
ground eyelet.
If you’re installing the optional bypass switch (S1), then
you may wish to mount it on the lid, in which case you
will need to drill an extra hole. Make sure that the switch
won’t foul the Mosfet board once it’s mounted.
You will probably find that you have more room if you
mount it low on the side of the case, and that may also
make it harder to trigger the bypass function accidentally.
If you’re using a metal case, ground is connected to the
case internally and then externally, to the vehicle chassis
or one of the battery terminals. You will also need to find
a way insulate the two 8mm bolts from the lid of the case.
With a plastic case, the easiest way to provide a GND ter-
minal is to feed a long M3 screw through the GND termi-
nal on the Mosfet board, attaching it to the PCB in a simi-
lar manner as the two large 8mm screws (ie, with shake-
proof washers and nuts). This can then project up through
a fourth hole in the lid.
Or you could connect the ground eyelet to a screw which
is externally accessible elsewhere.
There’s no need to provide any insulation for the 8mm
screws when using a plastic case; however, you will need
to seal all the exit holes with neutral cure clear silicone,
to ensure that the case remains watertight.
Download the panel label artwork from the SILICON CHIP
website and print it at actual size. You can then cut it out
and use it to mark out the hole positions in the lid. Drill
them all to 3mm, then enlarge the two battery terminal
holes to 8mm with larger drills, a stepped drill bit or a ta-
pered reamer.
Laminate the label and cut out the holes using a sharp
hobby knife. You can then stick it to the lid using contact
adhesive or a thin smear of neutral-cure silicone.
Other options for creating adhesive panel labels are de-
scribed on our website at siliconchip.com.au/Help/Front-
Panels
Now plug the two boards together and join them using
Nylon tapped spacers and machine screws. Mount the
whole assembly on the underside of the lid, remembering
to use insulators for the 8mm screw shafts if the lid is metal.
Attach the assembly to the lid using a flat washer and
nut, then another flat washer and nut, which can later be
used to clamp the battery wires or terminals.
Seal any possible water entry points (eg, around the LED
lens) with neutral cure silicone, then, if using a metal case,
drill a hole in the side of the case for the ground eyelet
and attach it using a machine screw, shakeproof washer
and two nuts.
You can then insert the sealing gasket into the channel
in the underside of the lid, cutting it to size so that it fits
around the full circumference.
With that in place, lower the lid onto the case and attach
it using the supplied screws.
Don’t forget to attach the case (if metal) or ground screw
to the vehicle’s ground, either via the chassis or to one of
the battery negative terminals.
You can then wire up the two battery positive wires to
the unit and verify that LED1 lights and the auxiliary bat-
tery begins to charge when you switch on the engine.
Don’t forget to use heavy automotive cable with a suffi-
ciently high current rating (25A+) to handle the high charg-
ing currents which can occur. The prototype used 35mm^2
automotive starter motor cable. SC