LM3914 Power Level Indicator - Assistance Needed

[Cire Yeldarb]

Hello,

So first off, here is what I am trying to do: I want to make a Battery Monitor for a "Panasonic Li-Ion 18650 3.7V 3400mAh PCB Protected Rechargeable Battery" using the LM3914 and 10 (20mA) LEDs.

The PCB Shuts the Battery off at 2.75 Volts and at full charge, the Battery's peak is 4.25 Volts, so I would like to make the range of the Power Level Indicator 2.78V Low to 4V High (steps of .14V).

I have been reading through the Datasheet and posts on this and other websistes. Using mostly the 0V-5V Example from the data sheet (Figure 1) and the LM3914.exe program from here (Figure 2), I have created a visual representation (Figure 3) of how I believe everything should be connected (I work much better visually).

Basically what I need help with is determining what Resistors I need for R1-4, or if I need to change anything else in my wiring setup. From Figure 2, it looks like it gives me R1 and R2 (.75k?), but I do not know how to figure out R3 and R4 from that program...

Any help you can give will be greatly appreciated!

(Figure 1)

(Figure 2)

(Figure 3)

 

[ericgibbs]

hi,

To measure a 4 Volt signal on a LM3914 requires a 5.5V power supply, 3.7V is not enough

E

 

[ronv]

Eric, why is that?

 

[ericgibbs]

Eric, why is that?

hi Ron,
If I read the d/s correctly it states Vin range is Vsup -1.5V

Eric

 

[ronv]

Ahh, didn't see that one.
So could he make Vh 1.25 and scale the 4 volt input to be 1.2 and just make it work?

 

[ericgibbs]

Ahh, didn't see that one.
So could he make Vh 1.25 and scale the 4 volt input to be 1.2 and just make it work?

hi Ron,
Not sure how the OP intends to use this indicator.?

If he powers the indicator from the charger he will draw upto 10*.02A = 0.2A from the charger.

It would be possible to divide down the actual battery voltage for Vin and set the Rhi and Rlo voltages to suit.

 

[Cire Yeldarb]

To measure a 4 Volt signal on a LM3914 requires a 5.5V power supply, 3.7V is not enough

Hmm, alright... so is there any Voltage at which I can power the LM3914 off of the Battery I am monitoring? Like would doubling my battery solution to create a 7.4V pack make any difference (and then having to up the low and high voltages being monitored)?

With what I am building, I am very limited on space, So I cannot just have another battery in there unfortunately...

If that will not work, is there anything that will get me "close enough" to what I want to do?

 

[Cire Yeldarb]

It would be possible to divide down the actual battery voltage for Vin and set the Rhi and Rlo voltages to suit.

Sorry, Started my post above before you posted this (am at work, so am typing between work items)

But this sounds like it might be a work around for me... How would I go about doing that though, I am an electronics newbie as you can probably tell...

If he powers the indicator from the charger he will draw upto 10*.02A = 0.2A from the charger.

I am not concerned about the draw though, I have enough wiggle room for this

 

[ronv]

I think this is okay, but you might want to have someone else check it out.

Follow the data sheet for decoupling capacitors, etc.

 

[Cire Yeldarb]

Okay, so I re-worked my diagram to match yours to make sure I understood everything correctly.... Does this look like it matches?



And stupid question since I am new to all this.... what are the "resistors" with the arrows coming into their middles (like the 2K off pin 5 and the 10K second inline from pin 4)? Cause I can tell they are not resistors, but do not know what they are to have something connect in the middle like that...

I assume those are what the notes refer to when you say "Set to .88V" and "Adjust to 4 Volts"?

 

[ronv]

The resistor going to Ref out (pin 7) should be 1.8K.
The resistors with the arrow in the middle are called trimmers. The "wiper" in the middle can move back and forth across the resistive element making it adjustable. Here are several.
https://www.mouser.com/Passive-Comp...tors-Through-Hole/_/N-9q0yt?P=1yzs2l9Z1yzmq85

 

[ronv]

Cire, Lets make the 10K trimmer a 25K trimmer.

 

[Cire Yeldarb]

Huh, fascinating! Are the trimmers that much bigger than resistors? And when selecting one, does the Wattage/Voltage values matter for what I am doing?

And I have made the adjustments to my diagram... I assume then that when something connects to the middle of the "trimmer" in your picture implies that the middle leg of the actual trimmer is the one connecting to each element.


So from your diagram it sounds like once I have it all wired up like this, I just need to attach the battery when it is full, and adjust the signal trimmer til all the LEDs light up...but what do I do to adjust it for the low-end trimmer? Wouldn't that one's "wiper" already have to have been set?

 

[ronv]

Huh, fascinating! Are the trimmers that much bigger than resistors? And when selecting one, does the Wattage/Voltage values matter for what I am doing?

They are not to large. I think 5 or 6mm square is easy. The mechanics makes them larger.

And I have made the adjustments to my diagram... I assume then that when something connects to the middle of the "trimmer" in your picture implies that the middle leg of the actual trimmer is the one connecting to each element.

Right you are the center pin will be the one with the arrow in the diagram.
View attachment 74997

So from your diagram it sounds like once I have it all wired up like this, I just need to attach the battery when it is full, and adjust the signal trimmer til all the LEDs light up...but what do I do to adjust it for the low-end trimmer? Wouldn't that one's "wiper" already have to have been set?

If you have a voltmeter you can use it to set the bottom one to .88 volts. Do you have a voltmeter?

The way it is set up now only one led will light at a time. It is called dot mode. You can change it to bar mode but keep in mind all the power for the leds comes from your battery so it will go dead sooner if you leave it on.

 

[Cire Yeldarb]

If you have a voltmeter you can use it to set the bottom one to .88 volts. Do you have a voltmeter?

I do not have one of those....

The way it is set up now only one led will light at a time. It is called dot mode. You can change it to bar mode but keep in mind all the power for the leds comes from your battery so it will go dead sooner if you leave it on.

How do I switch it to Bar mode? I think that would look better

I looked again in the datasheet and it says "Bar Graph Display: Wire Mode Select (pin 9) directly to pin 3 (V+ pin)."

 

[ronv]

If you have a Harbor Freight where you live I bet you could get one there for very little and since your already becoming an expert at reading data sheets it might come in handy. Otherwise you would have no way to know the battery was almost dead to set the last led off or just barely on.
Don't forget to pick up a 10Ufd. capacitor to put from V+ to V- pins.

 

[Cire Yeldarb]

never heard of that store so there probably isn't one near me, but I shall check...

oh yeah, and the capacitor was that dotted line in the datasheet that went from Vled to the V- pin right? I'll have to add that to my diagram too, looks like I forgot it...

 

[ronv]

That's it. Try to make all the ground connections close to pin 2 as well.

You could probably find a meter on e-bay.

**broken link removed**

 

[Cire Yeldarb]

Okay, so I went to Fry's Electronics and found all the resistors, trimmers, and capacitors... And then I realized I never figured out how to pick the Wattage on the Resistors...
The Trimmers and Capacitors did not seem to have a listed wattage so I just picked up the ones you said to get for those, the 2k and 25k trimmers and the 10ufd Capacitor. But what wattage of resistors should I get? I was able to find all the ones listed for 1/2 watt and 1/4 watt, but at 1 watt and up a few of them I was unable to find... I figure they should all be the same wattage at least?

and one more thing... which end of the capacitor is + and which is negative? I assume it matters because it is connecting to one of each wire? but the capacitor I got does not say, in fact both ends have a negative symbol "- > -"

 

[Cire Yeldarb]

and is this the proper placement of the Capacitor in the diagram?