26 Silicon chip Australia’s electronics magazine siliconchip.com.au
so LED1 lights up.
In this condition, diode D4 is forward-biased and so the
voltage divider formed by the 100Ω and 1.5kΩ resistors
comes into play, reducing the voltage at pin 7 of IC1b from
5V down to about 4.69V (ie, 5V x 1.5kΩ ÷ [1.5kΩ + 100Ω]).
That has the effect of reducing the switch-off threshold
to 12.6V (4.69V x 2.69) as desired.
That prevents the unit from switching on and off rapidly
if the battery voltage is near either threshold.
The output voltage from pin 1 of IC1b is also fed to the
pin 8 inverting input of IC1c, which has its pin 9 non-invert-
ing input connected to the 5V rail, so it acts as an inverter.
So when the main battery voltage rises and IC1b’s output
goes low, IC1c’s output goes high allowing the gates of the
FET’s to be pulled up via the 10kΩ resistor, switching them
on (as described below) and connecting the two batteries.
REG1 is a micropower regulator, both to minimise the qui-
escent current but also (and most importantly) because it
has an excellent initial tolerance of ±0.5%.
This, along with the 1% resistor tolerances, determines
how accurate the switch-on and switch-off voltage thresh-
olds will be.
Note that if you change the battery sense voltage divider
resistors, you can calculate the new switching thresholds
by calculating the divider ratio, then multiplying 5V and
4.7V by this ratio.
To change the hysteresis (ie, the spread of these two
thresholds), you would need to change the value of the
1.5kΩ resistor at pin 7 of IC1b; a lower value gives more
hysteresis, and a higher value, less hysteresis.Fig.1: the circuit is basically a comparator which senses when the main battery voltage is high enough to charge the
auxiliary battery and turns Mosfets 1-6 (or 1-12) on to do so. When the main battery voltage drops the Mosfets turn off.