lm3914 first led stays on and cascade questions

Thread Starter

jeff hulbert

Joined Feb 11, 2016
6
I assembled a 3914 voltmeter circuit. everything scales as intended from led 1 to 10, bar/dot modes work as they should.

the issue I'm having is with led #1. in bar mode theres no issue as they all stay on anyways. but, dot mode starts at led 1, then starts running up as should yet leaves led 1 on/active

as for the second part. I'm looking for a way to cause a smooth spike during turn on to cause the led's to sweep from 1 to 10 and back like the gauges in a vehicle do.

I was thinking the ref hi resistance could be temporarily modded to allow high to be set a lot lower or create an inductance like spike with cap to slowly drain it off. any thoughts on this would be great

thanks, jeff
 

Sensacell

Joined Jun 19, 2012
2,582
I think I remember something about that chip having a high leakage on LED 1, the fix is a resistor across the LED, sized such that the leakage current across the resistor is below the VF of the LED- keeping it dark.

Then again, maybe it's something else...
 

Dodgydave

Joined Jun 22, 2012
9,305
Don't see the point of R2, your ref hi is set at 1.25V so each led will come on at 125mV intervals, asfor the sweep at switch on, you need to pulse the signal input to ref out or to the supply, using a fet and time delay.
 

Thread Starter

jeff hulbert

Joined Feb 11, 2016
6
I think I remember something about that chip having a high leakage on LED 1, the fix is a resistor across the LED, sized such that the leakage current across the resistor is below the VF of the LED- keeping it dark.

Then again, maybe it's something else...
that's what I had to do

Don't see the point of R2, your ref hi is set at 1.25V so each led will come on at 125mV intervals, asfor the sweep at switch on, you need to pulse the signal input to ref out or to the supply, using a fet and time delay.
the meter read 15.5 at the top exact and 11 at the bottom in simulation. I assumed it would then divide the in between into 10 even points (if that makes sense)

as for the signal pulse with fet, do you have an example circuit for that or an article on that implementation
 

Dodgydave

Joined Jun 22, 2012
9,305
Yes thats what Actually happens in the real world, if you set the refhi at 15V, but your diagram has the adj pin shorted to ground and therefore ref out will only give out 1.25V.
 

ROTiree

Joined Jan 27, 2018
11
Thanks, Sensacell, for the tip - that issue was driving me mad for a couple of hours.

That said, it only appears to be an issue when you have a high Vf LED in that position (blue or purple, for example). I wanted a dot-display with the bottom 2 LEDs in blue, then a bunch of greens, 2 yellows and a red in LED10, so it's "Cold... cold... getting warmer... keep going... OK, nearly there... STOP!" I took the 2 blues out and put greens in those two positions and there was no problem at all. Since your top-tip, I've put the blues back in and strapped a 1k in parallel with the first blue LED - sweet as a nut, now. I guess I'll have to do the same with the other blue, because it's King bright compared with LED1 (no, really... it IS made by Kingbright :D )
 
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