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LM3914 Help - Battery Monitor Circuit Needed!

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Skyone

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Hey guys,

I'm in need of an 8.4v battery monitor circuit, using a LM3914 from National. The battery is NiMH (though it shouldn't make a difference...).

Also, if I wanted to change the battery from 8.4v to 7.2v, what would I have to change on the diagram (I'm guessing some resistor values)?

How would I calculate the resistor values if I wanted the circuit to monitor a different size battery?

Thanks in advanced!

-Skyone
 
Skyone said:
Hey guys,

I'm in need of an 8.4v battery monitor circuit, using a LM3914 from National. The battery is NiMH (though it shouldn't make a difference...).

Also, if I wanted to change the battery from 8.4v to 7.2v, what would I have to change on the diagram (I'm guessing some resistor values)?

How would I calculate the resistor values if I wanted the circuit to monitor a different size battery?

Thanks in advanced!

-Skyone
I just posted one on another forum for a lead-acid. The monitor had a range of 11.8 to 13.8V. I could change it for you if you tell me the range you want to cover.
 
Skyone said:
Atleast a 7.2v-8.4v would be awesome, maybe 7.2-9.6v, if it's not any trouble.

Thanks again!
Do you mean you want it to be adjustable? Keep in mind that the high end indicates fully charged, and the low end means you need to recharge.
 
Ron H said:
Skyone said:
Atleast a 7.2v-8.4v would be awesome, maybe 7.2-9.6v, if it's not any trouble.

Thanks again!
Do you mean you want it to be adjustable? Keep in mind that the high end indicates fully charged, and the low end means you need to recharge.
It would be nice to be adjustable, yes.
 
Each fully charged cell is about 1.5V fresh from the charger. Then an 8.4V battery with 7 cells will be 10.5V.

Each discharged cell will be about 1.0V. Then a 7.2V battery with 6 cells will be 6.0V.

Do you want the voltmeter to accurately show this entire range?
 
audioguru said:
Each fully charged cell is about 1.5V fresh from the charger. Then an 8.4V battery with 7 cells will be 10.5V.

Each discharged cell will be about 1.0V. Then a 7.2V battery with 6 cells will be 6.0V.

Do you want the voltmeter to accurately show this entire range?
Yeah, Skyone, you need to specify the adjustment range and the full-scale range(s).
 
Ok, I need a 7 Cell (8v-10.5v) battery monitor. I have no more need for the 6 Cell 7.2v one. Does this help?

EDIT: Meaning that it doesn't need to be adjustable, just set to measure a regular 8.4 NiMH battery.
 
I think the schematic is on my home computer. I'll look for it when I get home.
 
What does the power supply's voltage have to be? I would like to use the battery I'm monitoring as a supply, with a voltage regulator to cut down the voltage to however much the 3194 needs.

Thanks!
 
OK, here's the schematic. I haven't tested it, or even simulated it, but I think it will work.
 

Attachments

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Thanks, Ron.

The only thing I'm confused on is the TL071 (U2). What are the pins that are shown in the schematic? I'm guessing IN+, IN-, OFFSET 1 and OFFSET 2, while OFFSET 2 is attached to GND? Or is IN- attached to GND? Also, on R5, and R8 - are those linear pots?

EDIT: What do the LEDs have to be rated at, in voltage?

-Thanks!
 
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Skyone said:
Thanks, Ron.

The only thing I'm confused on is the TL071 (U2). What are the pins that are shown in the schematic? I'm guessing IN+, IN-, OFFSET 1 and OFFSET 2, while OFFSET 2 is attached to GND? Or is IN- attached to GND? Also, on R5, and R8 - are those linear pots?

EDIT: What do the LEDs have to be rated at, in voltage?

-Thanks!
I've edited the schematic to show op amp pin numbers. You can actually use almost any op amp.
The pots are linear. Adjust R8 first, and then R5.
Red LEDs drop about 2 volts, but you can use other colors. Keep in mind that you will only get about 10mA per LED as designed. Let me know if you need more current.
 

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Thanks again.

I'll purchase all the needed parts this weekend, I'll keep you updated on how it works out in this thread.

-Skyler
 
Hey guys,

I have a schematic setup question simlair to the opening posters.

I need a 7.2V 6-Cell battery monitor, doesn't need to be adjustable, that will be supplied by the battery it is monitoring, the red LED would light up when it drops 2V or so. I would really appreciate it if someone could tell me how the previously posted schematic can be modified.

Thanks!
 
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interplex said:
I need a 7.2V 6-Cell battery monitor, doesn't need to be adjustable, that will be supplied by the battery it is monitoring, the red LED would light up when it drops 2V or so.
An LM10 combines a 200mV reference and an opamp and both are low power. It is in an 8-pins case and is perfect for a voltage monitor.
 
The Opamp is acting as a Non Inverting Buffer. LEDs can be regular LEDs you buy at the store, I think Blue or white would also work, you would have to adjust the resistor for more or less current.

interplex said:
Hey guys,

I have a schematic setup question similar to the opening posters.

I need a 7.2V 6-Cell battery monitor, doesn't need to be adjustable, that will be supplied by the battery it is monitoring, the red LED would light up when it drops 2V or so. I would really appreciate it if someone could tell me how the previously posted schematic can be modified.

Thanks!

Voltage Comparator.
 
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