How to use LM3914 bar graph to indicate 7S Li-ion - Page 1
Forum section: electronics / beginners

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 19, 2019, 01:51:14 pm
content: 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.

author: Audioguru
author_rank: Super Contributor
timestamp: April 19, 2019, 07:46:56 pm
content: You might be too late because the LM3914 in the DIP through holes package is obsolete and not available anymore.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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 19, 2019, 09:17:40 pm
content: Quote from: Audioguru on April 19, 2019, 07:46:56 pmThank you for the comments. Still searching for the values of the resistors.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: StillTrying
author_rank: Super Contributor
timestamp: April 20, 2019, 09:52:53 am
content: "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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 20, 2019, 11:38:02 am
content: Quote from: StillTrying on April 20, 2019, 09:52:53 amThanksMy 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.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: StillTrying
author_rank: Super Contributor
timestamp: April 20, 2019, 12:47:30 pm
content: "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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 20, 2019, 06:54:03 pm
content: Quote from: StillTryingThere's only the battery voltage of 3V - 4.2V across each LM3914, so RHI has to be always a bit lower than 3V.[/quote

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 20, 2019, 07:08:16 pm
content: Quote from: StillTrying on April 20, 2019, 12:47:30 pmI did not get what you meant here.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: StillTrying
author_rank: Super Contributor
timestamp: April 20, 2019, 10:42:57 pm
content: Quote from: anishkgt on April 20, 2019, 07:08:16 pmWhy not!

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 21, 2019, 12:32:54 am
content: I mean why does RHI have to be lower than 3v when it should be 4.2v.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 21, 2019, 12:41:13 am
content: This is what Dave does in his video ()

author: StillTrying
author_rank: Super Contributor
timestamp: April 21, 2019, 12:50:33 am
content: "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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 21, 2019, 09:57:36 am
content: Well i guess a 500K would be just fine i get about 1.21v

author: StillTrying
author_rank: Super Contributor
timestamp: April 23, 2019, 07:54:53 pm
content: Get anywhere?There's an LTspice LM3915, which might help a little bit.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 23, 2019, 08:09:43 pm
content: 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.

author: StillTrying
author_rank: Super Contributor
timestamp: April 23, 2019, 08:45:33 pm
content: Just removed the lower resistor R7 and connected R5 between BAT and pin 5.That won't work because Vsig = Vbat = V+ = RHIand Page 7,Common-Mode Limitssays the inputs can't go within 1.4V of V+.www.ti.com/lit/ds/symlink/lm3914.pdf

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 23, 2019, 08:50:46 pm
content: 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

author: StillTrying
author_rank: Super Contributor
timestamp: April 23, 2019, 09:18:57 pm
content: "as he was measuring two cell in series, ie: 6v thru 8.4v, the divider was used to droppin#5 to 3V thru 4.2V"I'd bet his V+ was 8.4V.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 23, 2019, 09:20:25 pm
content: Quote from: StillTrying on April 23, 2019, 09:18:57 pmYes he was that was why the 10k/10k voltage divider was used to drop it down for SIG(pin 5).Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: StillTrying
author_rank: Super Contributor
timestamp: April 24, 2019, 07:39:01 am
content: "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.