Posted on Mar 7, 2013

The sensing circuit rapidly disconnects the battery voltage and load whenever the voltage drops below a preset threshold. One-way operation prevents the circuit from reconnecting the load if the voltage should then rise above the threshold. Cl ensures that the circuit doesn`t activate while making connections to the battery; if you accidentally reverse these connections, Dl will block the turn on the relay. After you connect the battery, nothing happens until you depress pushbutton switch Sl, which allows relay Kl to energize.

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When you release Sl, the relay remains on only if the battery voltage is above the minimum level. You preset this threshold-to 31.5 V when testing 36-V batteries, for example-using R4. Ql begins to turn off as the battery voltage drops. Once the threshold level is reached, Q2 also begins to turn off, and its rising collector voltage provides positive feedback to the base of Ql, accelerating the turn off. When Q2 turns off, the relay drops out, disconnecting the battery from its load.

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