siliconchip.com.au Australia’s electronics magazine July 2019 25
with a large mounting tab, which serves as both the drain
and thermal contact for the device, allowing heat to dis-
sipate into the PCB.
Despite the impressive specifications, these devices cost
under $4 each.
Circuit description
The circuit is shown in Fig.1. You can see the six power
Mosfets (Q1-Q6) at the top, between the two battery posi-
tive terminals. They are not all connected in parallel, for
an important reason.
All power Mosfets have an internal ‘body diode’ (also
known as a parasitic diode or internal diode) which is an
inherent part of their construction, and this allows current
to flow in one direction even when the FET is switched off.
So to prevent unwanted current flow in either direction,
the six Mosfets are arranged as three pairs (Q1-Q3 & Q4-
Q6), which are connected in ‘inverse series’.
This way, the body diodes of each set of three Mosfets
are connected anode-to-anode and so block current flow in
both directions, unless both sets of Mosfets are switched on.
In this case, all the body diodes are effectively shorted out.
Despite the FETs having very high current ratings, three
have been paralleled in each set as cheap insurance against
failure.
For example, the isolator could happen to be switched
on during engine starting and starter motor currents can be
very high, and high currents can also flow when the auxil-
iary battery is first connected to the vehicle electrical sys-
tem after being fully discharged.
A single LM339 quad comparator (IC1) is used for all
control functions. This contains four standard compara-
tors with open collector outputs, which go low when the
voltage at the inverting (-) input is higher than the voltage
at the non-inverting (+) input, and are high impedance the
rest of the time.
That turns out to be quite useful in this circuit.
I chose a switch-on threshold of 13.4V and a switch-off
threshold of 12.6V. The main battery voltage is applied to
pin 4 of CON1 and to a string of resistors to ground, which
forms a voltage divider. The top part of the divider is 11.5kΩ
[4.7kΩ + 6.8kΩ] and the bottom part is 6.8kΩ. This gives a
division ratio of 2.69 [(11.5kΩ + 6.8kΩ) ÷ 6.8kΩ].
So at the switch-on battery voltage threshold of 13.4V,
that means that 4.98V is applied to pin 6 of comparator
IC1b (very close to 5V), and at the switch-off threshold of
12.6V, pin 6 of IC1b sees 4.68V [12.6 ÷ 2.69].
A 5V reference voltage is supplied by linear regulator REG1,
powered from the main battery via a 100Ω resistor, and this
voltage is applied to pin 7 of IC1b, the non-inverting input.
Initially, output pin 1 of IC1b is high but once the main bat-
tery voltage rises above about 13.4V, the pin 6 input voltage
exceeds that of in 7 (ie, 5V) and so output pin 1 goes low.
This pulls current through the 4.7kΩ resistor and LED1,Shown here without its connecting leads (with their insulating covers, they’d hide half the panel!) use of the isolator is
simplicity itself: connect the “main” terminal to the “main” battery positive and the “aux” terminal to the “aux” battery
positive, with a chassis connection provided through the diecast metal case secured to the vehicle. That’s it! The LED will
glow when the main battery voltage is high enough to charge the auxiliary battery.