9V DC Adapter With Battery Backup

Posted on Feb 5, 2014

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 und

9V DC Adapter With Battery Backup
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er 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nger. 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‚ow through R1 and D1 to the cells. Current also‚ows 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...

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