Thursday, February 23, 2012

Test a car battery voltage using LM3914

This circuit uses the popular and easy to find LM3914 IC. This IC is very simple to drive, needs no voltage regulators (it has a built in voltage regulator) and can be powered from almost every source.

This circuit is very easy to explain:
When the test button is pressed, the Car battery voltage is feed into a high impedance voltage divider. His purpose is to divide 12V to 1,25V (or lower values to lower values). This solution is better than letting the internal voltage regulator set the 12V sample voltage to be feed into the internal voltage divider simply because it cannot regulate 12V when the voltage drops lower (linear regulators only step down). Simply wiring with no adjust, the regulator provides stable 1,25V which is fed into the precision internal resistor cascade to generate sample voltages for the internal comparators. Anyway the default setting let you to measure voltages betw1een 8 and 12V but you can measure even from 0V to 12V setting the offset trimmer to 0 (but i think that under 9 volt your car would not start). There is a smoothing capacitor (4700uF 16V) it is used to adsorb EMF noise produced from the ignition coil if you are measuring the battery during the engine working. Diesel engines would not need it, but i`m not sure. If you like more a point graph rather than a bar graph simply disconnect pin 9 on the IC (MODE) from power. The calculations are simple (default)
For the first comparator the voltage is : 0,833 V corresponding to 8 V
* * * * * voltage is : 0,875 V corresponding to 8,4 V
for the last comparator the voltage is : 1,25 V corresponding to 12 V

2 comments:

  • Sesilia says:
    April 10, 2013 at 12:49 PM

    Testing a car battery is very superb blog,keep it up,troubleshooting of a battery is much essential to know
    car batteries tend to last longer if located inside the vehicle
    rather than under the hood and you will rarely ever see one corrode.
    I also agree that factory installed batteries will only last 2-3 years.

  • christina says:
    April 12, 2013 at 1:32 PM

    Testing a car battery is very superb blog,keep it up,troubleshooting of a battery is much essential to know
    Car batteries tend to last longer if located inside the vehicle
    rather than under the hood and you will rarely ever see one corrode.
    I also agree that factory installed batteries will only last 2-3 years.

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