Author Topic: How to use LM3914 bar graph to indicate 7S Li-ion  (Read 2582 times)

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Offline anishkgtTopic starter

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How to use LM3914 bar graph to indicate 7S Li-ion
« on: April 19, 2019, 01:51:14 pm »
Hello All,

I have the attached schematic please advise the correct way to wire these up to indicate the voltage of each cell. The voltage is 7 cells in series at 4.2v and the lower voltage is 3 and highest is 4.2. I saw Dave's video on how he uses the IC with 8.4v but i did not know how it would be in a 7 cell configuration.

 

Offline Audioguru

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #1 on: April 19, 2019, 07:46:56 pm »
You might be too late because the LM3914 in the DIP through holes package is obsolete and not available anymore. Tiny surface-mount ones are still made and available if you can use them. The datasheet explains how to calculate and wire resistors to show the voltage range you want but I am sorry that I am too busy to do it for you.

If you use surface-mount ICs then calculate that they do not overheat if you use them in the bar mode, they will be much cooler in the dot mode.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #2 on: April 19, 2019, 09:17:40 pm »
You might be too late because the LM3914 in the DIP through holes package is obsolete and not available anymore. Tiny surface-mount ones are still made and available if you can use them. The datasheet explains how to calculate and wire resistors to show the voltage range you want but I am sorry that I am too busy to do it for you.

If you use surface-mount ICs then calculate that they do not overheat if you use them in the bar mode, they will be much cooler in the dot mode.
Thank you for the comments. Still searching for the values of the resistors.


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Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #3 on: April 20, 2019, 09:52:53 am »
"lower voltage is 3 and highest is 4.2."

As you're dividing the cell voltage by 2 with the 10ks, SIG will be 3V/2 to 4.2V/2, so you need to arrange RLO = 1.5V and RHI = 2.1V.
.  That took much longer than I thought it would.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #4 on: April 20, 2019, 11:38:02 am »
"lower voltage is 3 and highest is 4.2."

As you're dividing the cell voltage by 2 with the 10ks, SIG will be 3V/2 to 4.2V/2, so you need to arrange RLO = 1.5V and RHI = 2.1V.
Thanks

My battery voltage is 7s. So is there is max voltage that the SIG can withstand.
I need keep my RHI = 4.2v and RLO = 3. So in a 7s that would translate as 29.4v and 21v.


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Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #5 on: April 20, 2019, 12:47:30 pm »
"I need keep my RHI = 4.2v and RLO = 3."

There's only the battery voltage of 3V - 4.2V across each LM3914, so RHI has to be always a bit lower than 3V.

Due to variations in the 1.28V Ref, the internal 12k divider, and the ADJ current, I think you'd have to do something like that where you adjust VR1 for an accurate 2.1V on RHI, and not worry if the 1.5V on RLO is off a bit.
... or something.
« Last Edit: April 20, 2019, 03:06:25 pm by StillTrying »
.  That took much longer than I thought it would.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #6 on: April 20, 2019, 06:54:03 pm »
Quote from: StillTrying
There's only the battery voltage of 3V - 4.2V across each LM3914, so RHI has to be always a bit lower than 3V.

[/quote
In the schematic I had posted earlier, I am trying to read the voltage per cell. With the current drawing it’s impossible as the series resistance alters the reading.


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Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #7 on: April 20, 2019, 07:08:16 pm »

There's only the battery voltage of 3V - 4.2V across each LM3914, so RHI has to be always a bit lower than 3V.

I did not get what you meant here.


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Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #8 on: April 20, 2019, 10:42:57 pm »
There's only the battery voltage of 3V - 4.2V across each LM3914, so RHI has to be always a bit lower than 3V.

I did not get what you meant here.

Why not!
.  That took much longer than I thought it would.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #9 on: April 21, 2019, 12:32:54 am »
I mean why does RHI have to be lower than 3v when it should be 4.2v.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #10 on: April 21, 2019, 12:41:13 am »
This is what Dave does in his video ()
 

Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #11 on: April 21, 2019, 12:50:33 am »
"I mean why does RHI have to be lower than 3v"

The whole LM3914 is only powered by 3V when it's cell is only 3V, so the Refs have to be less than that.

"when it should be 4.2v"

The 2 10ks on SIG divide the 4.2V down to 2.1V, so RHI should be 2.1V.
.  That took much longer than I thought it would.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #12 on: April 21, 2019, 09:57:36 am »
Well i guess a 500K would be just fine i get about 1.21v
 

Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #13 on: April 23, 2019, 07:54:53 pm »
Get anywhere?  :)
There's an LTspice LM3915, which might help a little bit.
.  That took much longer than I thought it would.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #14 on: April 23, 2019, 08:09:43 pm »
Yea figured it out. R1 and R2 remains as dave suggested. Just removed the lower resistor R7 and connected R5 between BAT and pin 5.
 

Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #15 on: April 23, 2019, 08:45:33 pm »
Just removed the lower resistor R7 and connected R5 between BAT and pin 5.

That won't work because Vsig = Vbat = V+ = RHI

and Page 7, Common-Mode Limits says the inputs can't go within 1.4V of V+.

www.ti.com/lit/ds/symlink/lm3914.pdf
.  That took much longer than I thought it would.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #16 on: April 23, 2019, 08:50:46 pm »
The LM3914 is set up for Rlo =3V and Rhi at 4.2V. It is measuring one nominal 4V cell, hence removed R7 making the resistor divider.

In the video Dave used a 10K/10K divider as he was measuring two cell in series, ie: 6v thru 8.4v, the divider was used to drop pin#5 to 3V thru 4.2V
 

Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #17 on: April 23, 2019, 09:18:57 pm »
"as he was measuring two cell in series, ie: 6v thru 8.4v, the divider was used to drop pin#5 to 3V thru 4.2V"

I'd bet his V+ was 8.4V.
.  That took much longer than I thought it would.
 

Offline anishkgtTopic starter

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #18 on: April 23, 2019, 09:20:25 pm »
"as he was measuring two cell in series, ie: 6v thru 8.4v, the divider was used to drop pin#5 to 3V thru 4.2V"

I'd bet his V+ was 8.4V.
Yes he was that was why the 10k/10k voltage divider was used to drop it down for SIG(pin 5).


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Offline StillTrying

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Re: How to use LM3914 bar graph to indicate 7S Li-ion
« Reply #19 on: April 24, 2019, 07:39:01 am »
"10k/10k voltage divider was used to drop it down for SIG(pin 5)."

Some divider is need to keep SIG 1.4V below V+.

I changed the resistors in LTs LM3915 to work as an LM3914, and using the the values I posted in Reply#5 it's not too bad with V+ 3 to 4.2V and SIG 1.5 to 2.1V. Bottom of the plot is the 10 LED currents, they're probably too high 4 or 5mA would do.

Edity.
If I was using this to monitor 18650 charging I'd make to top 3 LEDs 4.1V, 4.2V, 4.3V, I've seen 18650 get very warm very quick at 4.3V.
« Last Edit: April 25, 2019, 05:52:52 am by StillTrying »
.  That took much longer than I thought it would.
 


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