9V DC Adapter With Battery Backup

With just a low cost DC
adapter and the circuit described here it is possible to build a low
cost stabilized, uninterruptable 9V supply. On the grounds of safety and
economy, a simple unstabilized 12V D.C. adapter is used as the power
source, a universal adapter with its output set to 12 V will do equally
well. The output voltage of an adapter under low load conditions (up to
approximately 1/3 of the rated output current) is over 15 V, even at the
rated output current, there will be sufficient voltage to supply a 9 V
voltage regulator. The rating of the DC adapter should be chosen
according to the output current required at 9V. Common values are 300mA,
500mA and 1A.

The 9V voltage regulator used in this circuit has a built in thermal
shutdown mechanism so that if too much current is drawn from the
device, it simply turns off as it overheats and will not supply any
current until the case temperature returns to normal. If the unit is
intended to supply more than say 150-200mA then to prevent thermal
shutdown it will be necessary to fit a heatsink to the voltage
regulator. The rule of thumb used to calculate the size of heatsink is
that you should be able to touch it during operation at maximum load,
without burning you finger. When choosing the DC adapter, it is always
better to select one with a higher current rating than is needed this
will ensure that its output voltage is high enough to be able to also
charge the 12V cells.

As long as mains voltage is on the DC adapter, the voltage across C1
will be higher than the voltage of the cells. Charging current will flow
through R1 and D1 to the cells. Current also flows to the voltage
regulator and out to the load connected at the output. Diode D2 in this
situation will not conduct because the voltage at its cathode is greater
than that at its anode When the mains voltage fails or is turned off,
diode D2 conducts and current will now flow from the Nickel Cadmium
cells to the voltage regulator, thereby automatically keeping the output
voltage at 9V. The value of resistor R1 is chosen so that a charging
current to the cells is not greater than 1/10th of the cells capacity
(if the cells are rated at 1100mAh, the charging current must not exceed

From the point of view of cell longevity it is better to reduce this
charging current even further (1/20 or 1/50 C). When calculating this
resistor, the value of the no-load voltage should be used. This will
give the highest charging current. To calculate the charging current
using R1 with a value of 180 Ω. The cells measure 13.8 V when fully
charged and the no-load output voltage of the DC adapter is 17V.
Charging current is given by the formula: (17V – 13.8V – 0.7V) / 180 =
13.9mA. Substituting the actual measured values in this formula will
enable you to calculate the value of R1 to give the correct charging
current for the cells.

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