LM3914 Battery monitor

[geditor]

I am a student (14 years old) and I am trying to make a battery monitor using the LM3914 IC. However, at the moment in my circuit diagram the signal source and reference voltage come from the same power supply (the reference voltage is supply by a voltage divider as display in the datasheet and the signal voltage comes directly from the battery as in the datasheet). Is this correct or do I need the voltages to comes come from two separate supplies, the signal voltage from the battery I am measuring the voltage of and the reference voltage from a constant power supply?
Thank you.

 

[(*steve*)]

You will need to use a zener diode or a voltage regulator (or a proper voltage reference) for your reference source. It can come from the same power supply, but remember that the voltage reference will have to be substantially lower than the lowest voltage your battery will get to . 1.25, 2.5, and 5 volt references are fairly easily available.

If you use a simple voltage divider then it will vary with your battery voltage and the output of the LM3914 will not change as the battery voltage varies.

Remember that your circuit will draw some power, so it may be clever to operate it with a momentary contact switch to minimise power usage to the times when you actually want it.

 

[electro_pa]

Hi geditor,
Steve, (above reply) has not got it quite right. (Sorry Steve)
LM3914 already contains a reference voltage generator available on 'Ref. Out', pin 7.
Please read LM3914 Application Notes at:
http://www.national.com/profile/snip.cgi/openDS=LM3914
You will be able to measure your battery voltage and supply LM3914 from the same battery, as long as it is more than 3 volts, since that is minimum for LM3914.
All you need is to use the 1.25volt reference option from the inbuilt source and connect a voltage-divider from the battery supply to 'Sig In', pin 5. Two, high-value resistors or a 500Kohm potentiometer could be adjusted to make the indication work over a very wide range. LM3914 allows supply from 3 volts to 25 volts. Watch out that your supply is not coming along with spikes above 25 volts, or the LM3914 will kark it!
12 volt car batteries, connected to an operational car, will always have nasty transient voltages on them sometimes, that will certainly damage a LM3914.
If a car is the case, protect the 'LED V+ and Pin 3 V+ with 20 volt Zener diode an some small resistor, depends on your zener diode max current but 100 ohms would probably be OK.

 

[(*steve*)]

Silly me. I assumed he was using an external reference voltage.

(Which is what he said, but maybe not what he meant)

Perhaps geditor should post the circuit he is planning to use...