Help using LM339 as voltage indicator.

[alparent]

I'm making a 8 LED voltage indicator using 2 LM339.
It's working perfectly up to the last LED.
The reference voltage for the last LED is 8.88 volts but it comes on when the input voltage is only at 8.5 volts?
Like I said, all the other stages works perfectly.
What's up with the last LED?

Any help is greatly appreciated.
Thans

[armdnrdy]

Schematic please.

[PRR]

> Schematic please.

yes yes yes

> input voltage is only at 8.5 volts?

Read LM399 datasheet.

"The upper end of the common-mode voltage range is V+ -1.5V"

If you run a 10V supply, then it fails to function correctly if either input is brought above 8.5V.

The reason is easy to see in the Schematic Diagram page 21. The inputs are PNP Darlingtons. They need 1.2V from base (input) to emitters, plus a small voltage across the "100uA" current source from the V+ line. If an input is closer to V+ than 1.5V, one or more of Q1-Q4 input devices turns-off, leaving Q5 Q6 in doubt what to do. (Actually you can work-out the most likely "wrong" output, or experiment.)

Divide-down your 8.88V to 4.44V and cut your input voltage to half.

[alparent]

It works.....and it works!

Thanks Paul.

I tried dividing the reverence and input voltage in half and it work great.

But since this will go in a power supply. I will also have a source of 12v. So I tried leaving the reference and input as original but I powered the 2 LM339 with 12v. That also worked great.

Thanks for the lesson.

Having a site like this is an amazing source for learning. I'm always flabbergasted (BIG word) by the knowledge and willingness to help of people on this forum.