The circuit has an internal power supply – based on the ubiquitous 7805 three The fixed resistors in the bridge must be close tolerance types mounted such that RR they are in thermal contact with each other. If necessary, R9 may be made smaller to make the ‘idle’ (neutral) range smaller. The two positive feedback networks associated with IC2b and IC2, are decoupled at the output of IC2a by R6 and R7 to ensure that the hysteresis of the window comparator does not affect the reference voltages supplied by R8-R9-R1o. The LEDs at the opamp outputs indicate whether the battery is charged (D4 lights), discharged (D6 lights), or is it a ‘neutral’ state (D5 lights). The output voltage of IC2a controls a window comparator built around IC21 and IC9c. In practice, input voltages as small 2 -+2.5 mV or -2.5 mV are detected reliably by the balanced bridge. The small voltage unbalances the bridge and is amplified 100 times by opamp IC2, which s wired as a noninverting amplifier. The voltage drop that exists across the chassis cable of the battery is fed to resistor R1, which forms part of a measuring bridge consisting of Ry-R2-R3-R4-Pi. The circuit is essentially a window comparator based on opamps. A third LED is provided to indicate a kind of neutral area in which the battery is only lightly charged or discharged. Two LEDs fitted in the car interior indicate whether the battery is charged or discharged with a significant current, providing a reassuring check on the generator function.
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