About the LM3914

[fsr]

I was playing with this IC after watching [eevblog #204], but when the display didn't turn on exactly at the Vrlow voltage, i noticed somethig curious: if you take a look at the block diagram, there is no comparator attached directly to the Rlo pin, so that means that the first led turns on at Vrlo + (Vrhi - Vrlo) / 10. That seems a little bit messy to me, like they needed to add an additional comparator, or remove the lowest resistor in the ladder.

[Benta]

What's messy about it? It's the intended function. If you like, you can add a passive LED that's on all the time at the bottom of the ladder.

[T3sl4co1l]

It becomes clearer when you think of it as a 10-bit unary analog-to-digital converter.  The bit transitions fall at the high side of one bit, becoming the low side of the next bit.

Unary: a positional number system like any other, except, the positions are all 1, so, the more 1s you have, the more the result is.  A thermometer code (such as the '3914 produces, with the mode pin strapped appropriately) can be read this way.  The dot mode is the same thing but the active pin represents the numerical value.

Tim

[fsr]

What's messy about it? It's the intended function. If you like, you can add a passive LED that's on all the time at the bottom of the ladder.

Not on all the time. One that will turn on at the vrlo voltage, just as the higher led will turn on at the vrhi voltage. But for that, it cannot be passive, another comparator would be needed.

[floobydust]

When you cascade LM3914's, it's so the top LED and (next IC) bottom LED don't have the same threshold. There is no "zero" LED =vref_low.

[fsr]

When you cascade LM3914's, it's so the top LED and (next IC) bottom LED don't have the same threshold. There is no "zero" LED =vref_low.

Yes, it does also make a lot of sense if Vrlo is at GND, because the lowest led won't ever turn off if there wasn't that first resistor there. But it would be nice to have an extra comparator for getting a led exactly at Vrlo for an expanded scale, when you don't have Vrlo at GND. Or even an extra pin that bypasses that first resistor. After all, if you chosen that Vrlo voltage threshold, it would be useful to know if the input went under that value.
Probably doable, anyways, just need to take a look into it, and choose a new, lower Vrlo, so that the led turns off at the desired voltage.

EDIT: i think i have the answer. Tomorrow is breadboarding time

[fsr]

Sucess! I had to lower R2B, however. The most likely explanation for that, is that the internal divider is probably closer to the minimum value than the typical. Around 8k1 according to measurements of voltage and current.



The Aneng meter seems to have a certain lag on the measurements. I had to move the PSU's potentiometer really slow and several times to detect the right voltage at which the first led turns on. I need to change that potentiometer for one with multi-turn.

The idea is to make a car battery/alternator meter. If the battery goes below 9.5v when starting the car, it has issues. Also useful to check that the voltage never goes over 15v (last led).