Backup Power System with PIC16F870

Posted on Jan 29, 2013

The PIC16F870 keeps track of battery voltage as well as both charging and discharging currents. It also drives the 4 digit display, and switches the AC and load relays. The basic operation is as follows: While AC power is available, the battery voltage is constantly monitored and topped off as needed. When the AC power fails, the load relay engages the 12-volt system. This connects to lights, fans, and other 12 volt devices as needed. Should the battery voltage fall too low, the load relay is disengaged, preventing the acid batteries from being damaged by over discharging.

Backup Power System with PIC16F870
Click here to download the full size of the above Circuit.

Refering to the SCHEMATIC, which differs some from the LAYOUT, the planned SCR that switched power from the power rectifiers to the battery was eliminated from the design. (This left a redunant diode feeding power to the 7805). Instead, a 110 volt solid-state relay was added to the transformer primary circuit. This eliminated the rather large idling current from the scrounged transformer. In order to measure currents up to 10 amps, .01 ohm resistors were made out of about 5 inches of #22 solid hookup wire. A simple resistor devider is used to monitor the voltage. Since this is a Pulsed charging system (note lack of humongus filter capacitors), the battery voltage is only measured between the charge pulses. The ac connection to RC1 is used to synchronize this operation. Also, makeing a reasonable current measurement is done by summing several samples spaced over one complete charge pulse interval. In other words, the actual charge is going on only about 20 percent of the time. So taking samples over the entire interval automatically 'does the math'.

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