# LM3916 LED bar graph display wiring?

#### wingers

Nov 17, 2009
18
Hi,

I’m new to this forum and have joined up because I’ve been making some circuit boards at home and am struggling to get one of them working properly. I’m by no means an expert but can wield a soldering iron and have had some success with these kind of boards in the past…
Anyway, onto the point at hand - I’ve got four 0-10V outputs from a bank of thermocouples and I’m trying to use these to light four bar graph displays of 10 LEDs each.

I’ve sourced some LM3916 chips which seem to be what I need but the circuit I’ve created doesn’t seem to work

I guess I’m really looking for someone who has used these chips before and could help explain what I’m doing wrong. Failing that then if anyone wouldn’t mind looking through the datasheet I’d be very appreciative…

The part I don’t fully understand is the internal voltage regulation so I’m guessing that that is where I’ve gone wrong.

Thanks,

Wingers

#### Resqueline

Jul 31, 2009
2,848
Now, that's a pretty comprehensive & explanatory data & application sheet.
Can you measure some voltages around your circuit?

I don't know what resistor values you used but with regards to the formulas, remember that divisions precedes additions & subtractions (when on the same line).

#### wingers

Nov 17, 2009
18
I have measured voltages around the circuit but haven’t found anything to suggest why the LEDs are lighting. Everything seemed to be as I intended.

I've left my camera at work today but tmrw I should be able to post a picture of my circuit design if it would help...

With regards to the datasheet, the part that confuses me is the internal current regulation on page 8.

I understand that the current out of Ref(out) dictates the LED current but I couldn’t tell if I should link Ref(adj) and Ref(out) with a resistor as shown in the diagram on page 8. I thought that that was only done as one example and that I could connect R(ref) to ground via a pot and use that to set the current.

The difference with my circuit is that I've set up a potential divider to provide the R(Lo) & R(Hi) voltages so I didn't see the need to use Ref(out) to supply any voltage. I then decided to use the internal current regulation to set the LED current. To do so I assumed that Ref(out) would have 1.25V hence I could control the current simply with a resistor to ground. On closer reading it does state that the 1.25V would be between Ref(out) and Ref(adj) so now I’m wondering if I need to set up a potential divider as shown on page 8. Or at least connect pin 8 to ground.

The other thing I've thought about is that I'm running 4 of these chips in parallel so don't think that the 1.25V would be affected but I just wanted to make sure. I've got all four joined together then running through a resistor to ground.

Does all that make sense?

Wingers

#### Resqueline

Jul 31, 2009
2,848
All your theory makes sense exept;
You don't need to set up a potential divider, just a resistor from Ref(out) to ground.
"connect R(ref) to ground via a pot and use that to set the current" : same as above?

But a highly suspect part is if you joined the four Ref(out)'s together with a common resistor to ground. That won't work, you have to keep them separate with each their own resistors.

#### wingers

Nov 17, 2009
18
OK, I’ll switch to one resistor per chip then. Other than that, it seems that I have the correct circuit though which is odd as it’s not working. I can’t remember exactly what I tried now but I’m fairly sure that I tried running the circuit with only one chip in place. This would remove any effects of joining the four Ref(out) pins together and even like that it never worked.

With regards to the output from Ref(out) I should be able to check that this gives 1.25V even when Ref(adj) is left open. Assuming it does then I’m back to square one really with a circuit that appears to be correct.

Unfortunately my car is off the road for now so I can’t recheck this atm. I’m planning to make the changes I can to the board and get the car running again so that I can get this testing again some time next week.

Wingers

#### Resqueline

Jul 31, 2009
2,848
Ref(adj) has to be grounded for Ref(out) to give 1.25V, or else Ref(out) will soar up to supply voltage. If you left it open then that could explain why all LED's are lighting with no input.

It should be ok to use a common drive for the four R(Lo) & R(Hi) voltages though.

#### wingers

Nov 17, 2009
18
OK that makes sense to me now. I’ll re-work the Ref(out) so that each one has a single resistor and I’ll ground each of the Ref(adj) pins. Hopefully that's all that I need to get it working.

I guess that without grounding Ref(adj) I could well be limiting the current out of Ref(out) and therefore the LED current to zero. That would explain why none of the LEDs light up…

I’ll post a pic of my circuit when I get home.

Wingers

#### wingers

Nov 17, 2009
18
Here's my circuit board:

Wingers

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