Confused on LM3914 resistor selection

Thread Starter

bigjoncoop

Joined Feb 1, 2019
113
I'm trying to build a 10 segment LED bar graph using a LM3914 voltmeter for a single cell 18650... 2.8v/3v - 4.2v Every place I go for information on what value resistors to use for the voltage divider all have different info.... One place said the voltage has to be 1.25v below VCC. If that's the case that is straight forward.. But I'm trying to clarify....

Im in the middle of Designing the pcb and do have the components yet in order to test my self. And I would like to put the resistor value on the pcb silkscreen layer.

Thank you
 

Dodgydave

Joined Jun 22, 2012
9,206
Post your circuit for better results.

Have a look at the datasheet for more information, the signal input is on pin 5, but the upper and lower Voltage levels can bet set individually on pins 6 and 4, by using the internal voltage regulator on pin 7.
 

ElectricSpidey

Joined Dec 2, 2017
1,175
I never try to use the default configuration in the datasheet...way too confusing. (for me)

I always use a voltage divider at the upper set and another at the lower set. (pins 4 & 6)

Then reference out simply ties to ground through a resistor, and reference adjust ties directly to ground.
 
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MisterBill2

Joined Jan 23, 2018
6,348
I think that the 3914 is linear in dB, not volts. At least one of the series was, and so it might not be a good voltmeter. OR that may have been the LM3915. One was linear and one was logrithmic.
 

Audioguru again

Joined Oct 21, 2019
1,601
I think that the 3914 is linear in dB, not volts. At least one of the series was, and so it might not be a good voltmeter. OR that may have been the LM3915. One was linear and one was logrithmic.
No. The LM3914 is a linear voltmeter and still available but only in surface mount packages. The LM3915 had 3dB between each step and is obsolete.
 

Thread Starter

bigjoncoop

Joined Feb 1, 2019
113
hi big,
This is what my simulator shows for 2.8V thru 4.2v.
Can you follow the image OK.?
EView attachment 210369
I think this the same lm3914 calc / sim i found on google, but was just web based. didn't understand 100% of the the variables.

so it looks like 3.5k and 6.45k is the voltage divider resistor values ...

also the 10 seg bar graphs im using are multi color.... anything need to be changed so each bar in the graph is as brite as it can be? so that way one colors isnt brighter or dimmer....

i will post my schematic asap. almost finished
 

ericgibbs

Joined Jan 29, 2010
10,205
hi big,
The voltage label near pin #1 and pin #10 are showing the user the voltage range of the LED's, they are not voltage input pins.
E
 

Thread Starter

bigjoncoop

Joined Feb 1, 2019
113
@ericgibbs


here is a quick schematic i made instead my entire project.

Just to clarify . Im using 9v input to power circuit since datasheet shows 6.8v-18v. Just plan on using a LM7809. these are going on Battery packs anywhere from 3s-7s . (12.8v-29.4v) these led graphs are to show the voltage of each of the large packs in series. So ifs the battery is 3s ill have 3 graphs, 4s will have 4, etc... I've heard that these Lm3914 will run on 5v so I could use a LM7805 instead...

Here is basically what it is / where I got idea. Except I want my make it with just 3 meters per pcb...
maxresdefault.jpg

My Schematic :
Lm3914_Circuit .JPG
 
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ericgibbs

Joined Jan 29, 2010
10,205
hi big,
You said you wanted a LED voltage Range of 2.8V thru 4.2V.
If you ground pin #4 RLO in your circuit diagram that will make the Range from 0V thru say 4.2V, you don't want that.

The resistor between pins 7 and 8, is 3k5 [ that sets the LED current at~7mA] the resistor from pin 8 to 0V is 6.45k

Re look at the pin 4 RLO resistor value, post what you think it should be for 2.92v.
E

BTW: your Battery is the wrong way around..;)
 
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Thread Starter

bigjoncoop

Joined Feb 1, 2019
113
Just to clarify . Im using 9v input to power circuit since datasheet shows 6.8v-18v. Just plan on using a LM7809. these are going on Battery packs anywhere from 3s-7s . (12.8v-29.4v) these led graphs are to show the voltage of each of the large packs in series. So ifs the battery is 3s ill have 3 graphs, 4s will have 4, etc... I've heard that these Lm3914 will run on 5v so I could use a LM7805 instead...
hi big,
You said you wanted a LED voltage Range of 2.8V thru 4.2V.
If you ground pin #4 RLO in your circuit diagram that will make the Range from 0V thru say 4.2V, you don't want that.

The resistor between pins 7 and 8, is 3k5 [ that sets the LED current at~7mA] the resistor from pin 8 to 0V is 6.45k

Re look at the pin 4 RLO resistor value, post what you think it should be for 2.92v.
E

BTW: your Battery is the wrong way around..;)
AHH... I UNDERSTAND . .. RLO pin sets the "LOWEST"graph voltage. so i made a voltage divider using my 9v vcc to make +/- 2.8v
Lm3914_Circuit .JPG
 

Thread Starter

bigjoncoop

Joined Feb 1, 2019
113
@ericgibbs

thx for helping me figure that out. now one more question .... what's the easiest way to add 2 more Lm3914 and bar graphs to this circuit? just replicate it or can i use both existing voltage dividers to set the range of the other two?
 

ericgibbs

Joined Jan 29, 2010
10,205
hi big,
Normally you would not use VCC as the source of the voltage for the LM3914, RLO, use the internal reference voltage.

Do you have the datasheet for the LM3914.?
E

Update:
You will find that using trim pots for the resistors will enable the exact setting of the 2.8V thru 4.2V range.

This image shows what is inside the IC.
AAA 398 10.15.gif
 
Last edited:

Thread Starter

bigjoncoop

Joined Feb 1, 2019
113
hi big,
Normally you would not use VCC as the source of the voltage for the LM3914, RLO, use the internal reference voltage.

Do you have the datasheet for the LM3914.?
E

Update:
You will find that using trim pots for the resistors will enable the exact setting of the 2.8V thru 4.2V range.

This image shows what is inside the IC.
View attachment 210422
I Don't quite understand ? The data sheet shows input voltage of 6.8v-18v input going to pins 9 and 3

Capture.PNG
 

ericgibbs

Joined Jan 29, 2010
10,205
hi big,
My advice is that you build on a bread board only one LM3914 circuit, debug that and then consider adding more LM3914's.

Are you referring to the LM3914 datasheet for guidance.?

E
 

eetech00

Joined Jun 8, 2013
1,946
Hi

Here's a circuit I made a quite a while ago. It was originally intended to monitor a battery where the supply was the battery being monitored. I've modified it to provide 20mA for each LED and separated the supply from the monitored battery.
The IC is capable of 30mA per LED but wouldn't recommend operating it that high.

1592839818236.png
 
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