Linear LM3914 bargraph from 9v to 12v help

[JMACgyver]

Hi all,

I've perused the TI datasheet for the LM3914 driver typical setup and applications, and have fooled around with the values a bit to try to get it to do what I want. So far, no luck, so I turn to this forum.

What I Have:

A 12-volt supply, made up of 8 C-cell batteries, wired in series.
Devices to be hooked up to this power source are one that prefers a 12V source, but can work ok down to around 9v
and one that is rated at 9V nominal, but may work ok down to 8.4 or so.

What I Am Looking For:

A modification to the Typical application cicuit (0-5V source, full range graph) that I can power with the same voltage that is being measured.
*However,* I only need to measure from the top (12V) down to about 9V.
So, I can either wire from a 9v to 12v swing with 0v as ground, or;
wire for a 0v to 3v swing using the 9v as ground, but powering the circuit with the voltage from 0v to 9v (or 0v to 12v)

I hope this isnt too unclear. What is needed though, is to use standard component values, and have the graph go from one (or none) LED lit when measuring just below 9v, to all of them lit when measuring 12v or above, and a linear representative difference shown for voltages between those two. My attempts have resulted in 2 LEDs at 9V, 8 at 10.5V and all at 12V+, and various other non-linear or non-full range swings.

Any questions, please ask. Like I said, the substitutions I've tried have not had the desired effect, so if anyone know the right values for what I ned to do, and where to wire them to, I'd be much obliged.

Regards,
--Electro--
aka The Other David

[pebe]

Hi David. Will this circuit do it for you?

The LM3914 is powered from the full 12V and the input sensitivity is set for 0V to 3V. The 9.1V zener feeds a 10K resistor from the 12V supply, leaving 2.9V at the junction. The 1M pot would then be adjusted to give 3V at its slider. So the chip would always read 9V less than the supply voltage.

The data sheet for a BZX55-91 zener shows that its voltage tolerance is anything between 8.5V and 9.6V, so it may be better to go for a 10V zener.

I hope that may help.

[JMACgyver]

Hi David. Will this circuit do it for you?

The LM3914 is powered from the full 12V and the input sensitivity is set for 0V to 3V. The 9.1V zener feeds a 10K resistor from the 12V supply, leaving 2.9V at the junction. The 1M pot would then be adjusted to give 3V at its slider. So the chip would always read 9V less than the supply voltage.

The data sheet for a BZX55-91 zener shows that its voltage tolerance is anything between 8.5V and 9.6V, so it may be better to go for a 10V zener.

I hope that may help.

Thank you for your response.

I don't normally stock/have zeners in my kit; is there a way to do this project with other components? and would it be possible to use say, a 5, 10, 50 or 100K pot? 1Meg potentiometers are harder to come by, especially in the small package and with the precise control i'd need.
I forgot to mention, I've got very little room in this particular battery box, so I'd probably be transferring as much as I can to surface mount.

If not, I can work your circuit into the original, and see what happens.

Thank you,
David

[pebe]

There is another way round the problem. But it would mean a continuous extra current of about 10mA. But I assume that because of the current drain of the bar display, you will not have this connected all the time so that would not matter.
Is that correct?

[pebe]

Hi David,
I have studied the data sheet of the LM3914 in more depth now, and it looks like the solution you need is quite simple.

In the National Semiconductors datasheet for the LM3914, there is a circuit on page9 entitled ‘Expanded Scale Meter, Dot or Bar’ for testing 5V zeners. Use that circuit as it is and feed it from the battery supply. Adjust R1 to give 1.2V as Vd.

Connect a potential divider to your battery, with 15K as the top arm and 10K as the bottom arm. When the battery is down to 9V there will be 3.6V at the junction of the resistors. Feed that as an input to pin5 of the LM3914. Then adjust R4 so the bottom LED segment just comes on at 9V (you may have to reduce R3 slightly). When the battery is at 12V the voltage at the input will go up to 4.8V and the top LED should then light.