When the LED current is only 20mA each, the output drivers on the LM3915 have a dropout voltage of 1.5V max plus a saturation voltage of 0.4V max. Then you must be using 1.1V Infrared LEDs with a 3V supply. They are invisible.

 

Well Audioguru, I'll take your word on that. No, I'm not using IRs they are 3mm low power leds. I am not sure what current I am running through them, although I have added the pot for intensity adjustment. I have only tested the circuits for a couple of minutes and they worked just fine. I didn't think to test the temperature of the chip though. I do have quite a few of the 3915s and I put them in sockets so replacing burned out ones will not be a problem. So, I'll just have to give it a long term test and see what happens. If I burn them out I'll be back for some more help with dropper resistor selection.

What about the power supply? Can I test it without loads?

Thanks again.

Larry

 

OK Audioguru, I have just finished a two hour soak test of music (I used music because of the variations and I wanted a real world kind of test) at full scale with the intensity and reference voltage up full. My chips are still at room temperature. So, whatever you are doing it must be extreme or in error. My VU meters are working fine with no discernible heating at all. Also, the 317 regulator at 3volts does not even need a heatsink (it is barely getting warm) so it cannot be drawing much current.

I think you had better take a closer look at your circuit and work out why yours is not working as it should.

Still waiting on the answer about the power supply!

Thanks

Larry

 

No, I'm not using IRs they are 3mm low power leds.

All LEDs except yours have a colour and have a certan range of forward voltage drop.
Are your LEDs 1.7V to 2.0V red ones? Are they 3.3V to 3.7V blue or white ones?

I am not sure what current I am running through them, although I have added the pot for intensity adjustment.

You did not post your schematic so I don't know how you set their current with a pot.

What about the power supply? Can I test it without loads?

A power supply with its parts calculated properly works fine. If something is overloaded then it will probably fail soon.

OK Audioguru, I have just finished a two hour soak test of music (I used music because of the variations and I wanted a real world kind of test) at full scale with the intensity and reference voltage up full. My chips are still at room temperature. So, whatever you are doing it must be extreme or in error. My VU meters are working fine with no discernible heating at all.

My LEDs are very bright with a current of 26mA each. I usually have the LM3915 set to the BAR mode so frequently all LEDs are lighted. Is that extreme?
Maybe you have a low current and maybe yours is in the DOT mode with only a single LED lighted. Usually only a few LEDs light on a VU meter set to the BAR mode when music plays.

Also, the 317 regulator at 3volts does not even need a heatsink (it is barely getting warm) so it cannot be drawing much current.

I don't know why you use an LM317 voltage regulator. LEDs set their own voltage. The LM3915 sets the LED current with one or two resistors from its pin 7 to ground.

Here is my schematic:

 

I'm sorry, I have forgotten the voltage of the LEDs I bought. I usually write that kind of stuff on the package but this time I forgot. I have contacted the company to get that info from them.

I am using one of the standard variations for a VU meter I.E. 7 x green 1 x yellow and 2 x red for the peak.

The pot for intensity is exactly as suggested in the datasheet with the replacement of the pot with a multi-turn pot. Page 10 figure 8. I.E. a 50k pot through a 4.7k from pin 6 to ground.

Good, testing without load should be OK as long as I haven't muffed something. Thanks for letting me know.

No, that is not extreme. However, as I said, you are running more than 7volts through your LEDs and that is why you need a dropping resistor, according to the first page of the datasheet(figure 1 notes).

No, bar mode all the way. I purposely maximised the volume to get all LEDs illuminated pretty much for the whole of each musical piece. Yes there was a 10 second or so gap of no illumination between each piece but as I was playing 80's pop (with the speakers removed from the circuit obviously) it was pretty much full volume noise all the way through.

I used an LM317 regulator for reasons I have already given. I wanted more adaptability in my circuit. Also, I read the datasheet several times before embarking on design and came to the conclusion that a separately controlled power supply for the LEDs was the best way to give this adaptability and minimise any heating in the 3915s.

OK, I have just measured the current through the whole lot I.E. LM317 and 2 X 3915 and 20 x 3mm LEDs through a 1A 12V bench supply. At idle I am drawing only 15mA and with a generated 250hz sine signal not noise or music this time at full scale I am barely drawing 55mA, I.E. not enough to heat the chip or the 317 by much. Granted, My red LEDs are not as bright as I would like and I guess I will have to do something about that. There is not much difference between the red and green LEDs but it is just noticeable. Maybe up the current in the first green and yellow LEDs, add limiting resistors and maybe a dropper as you have done. However, I think for this project it will suffice as is.

Thanks again.

Larry

 

Darn, I have just heard from the supplier of the LEDs and they do not know any details about them. They apparently came from a company foreclosure of a manufacturer of LED signs and large video displays. So, I guess they were state of the art LEDs but other than that I have no idea. All I do know is they are working fine in my VU meter at 3 volts and the whole thing is drawing upto 55mA at full scale.

Thanks

Larry

 

55mA for the ICs plus 10 LEDs is only 5mA or less for each LED which will not be bright.

 

That is two ICs plus 20 leds! 10 per channel! They are quite bright enough for the purpose. It can be seen across the room in full daylight. I wish I could tell you what type/manufacturer the leds are but I cannot.

Larry

 

Your LEDs use only 2.5mA each?

I always buy Name-Brand parts that have an honest detailed datasheet.

My LEDs are Fairchild and Philips Luxeon. They are very bright with a wide angle.
Some cheap LEDs are old dim ones in a case that focusses the beam into a narrow but bright angle that cannot be seen if it does not shine at you.