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Wednesday, February 27, 2013

LM3914 Battery Gas Gauge SLA

Hi all,

Here is a new circuit devolped for Owen (KB1CGP). This is a sealed lead acid battery monitor that uses the venerable LM3914. The design criteria was to have a multicolor indication of a specified voltage on a battery. I decided to go with all off the shelf with fixed value resistor that did not need any calibration required. Some other circuits I have seen online use a pot to change the ratio of the measured input voltage or change reference voltage of the ten internal op-amps.




The LM3914 Dot/Bar Display Driver Description (datasheet)
Much of the display flexibility derives from the fact that all Display outputs are individual, DC regulated currents. Various effects The LM3914 is a monolithic integrated circuit that senses can be achieved by modulating these currents. The analog voltage levels and drives 10 LEDs, providing a linear individual outputs can drive a transistor as well as a LED at the analog display. A single pin changes the display from a same time, so controller functions including "staging" control moving dot to a bar graph. Current drive to the LEDs is can be performed. The LM3914 can also act as a program- regulated and programmable, eliminating the need for resis- mer, or sequencer. Driver tors. This feature is one that allows operation of the whole The LM3914 is rated for operation from 0°C to +70°C.

There is also a LM3915 for Log measurement in 3db step, this IC is great for VU meters I've designed in the past.

Circuit Design Criteria
Lead acid batteries are 12V nominal but fully charge can range up to 14 volts, and discharged can not drop below 10.7 volts unless damage occurs to the cells, So a full bar graph would have to indicated a voltage of 14 volts all green LED on, yellow led would indicated cautionary voltage condition from 10 – 12 volts and red LED would indicate below 10.7 volts, see example table below:
I decided to go with the 1.25 volt precision reference voltage regulator that was internal to the IC (connect pin 6,7 and 8 to GND). I also decided to change the internal op amp voltages so that the first op amp would trigger at .8V and step up .05V/Opamp, this would give me a range of .8v to 1.25 volts with 10 steps to do this I added a 15 K resistor to RLO (pin4). This made the voltage reference resistance network total to be 25K, this gave a volatge drop of .05V/1K. If I divided the signal voltage by 10 this would give me the following:


Using a 90K for R1 and a 10K for R2 would give me exactly a divide by 10, but 90K are not a standard resistor, also the max voltage would 12.5V and the red LED would not turn on until we dropped below 10V. So I used a 100K for R1 and 10K for R2 and calculated the following:


This was a better result 13.75 max voltage and the red LED would turn on at 11V, the compromise was now the input voltages were not a nice round numbers, this is not to big of a deal it's being used at a gas gauge, but a table can be printed on the side panel of the meter with the voltage level to the corresponding LED. This worked out great RED = Dead, YELLOW = Almost dead and GREEN = OK plus no calibration just 1% tolerance resistor which I have on hand.