Hello

I want to build a bar graph LED mode with Lm3914N.

How do I regulate the current for the LED to 30mA?

 

hi,
Iled = 12.5/R1 where R1 is across pins 7 and 8. approx 430R nearest preferred value.
Do you have more details or circuit diagram.?
E

 

As you can see in my circuit I took the resistance with 416Ohm, but you can see the Iled is just 10mA.

Karan

 

hi,
Sorry, the circuit so small I cannot read it.
E

 

Oh maybe you just click on the pic, it should zoom in.
Anyway i'm sending you another pic, hope you can read it now

 

hi,
I make close to 30mA on my simulator, how are you measuring the current.?
I would recommend that you use a max current of 25mA
E

 

hey
I don't know which program you use, but I am using circuit wizard and if you start the simulation with this circuit you can move the cursor to the wire and you see the current which is 10mA.
Karan

 

Hi Karan,
The datasheet for the LM3914 states 12.5/R1, which in my calculation gives approx 30mA and my LM3914 also shows ~30mA, which it should be.

I see you have a voltage source of 0.7V a 2KHz into Vsig, try a DC voltage without modulation and recheck the current

 

yap according to the calculation 12.5/R1 --> R1~615Ohm the current should be 30mA.

I think the source of the audio-output of a laptop or a smartpone would be around 0.7V/1kHZ sine or something like that i am not sure.

Although I use 5V DC, the current remains 10mA. I am just confused and trying to solve the problems for weeks.

Karan

 

hi Karen,
It could be a problem with your simulator.?
I would suggest you build the project on a bread board and measure the 'real' LED current.
Take care not to exceed the total dissipation of the LM3914, which is approx 1Watt, if the LED current is ~30mA and Vsupply of 9V.

E
EDIT:
Try this LM3914 sim.
http://www.electro-tech-online.com/tools/LM3914V2.php
This image shows your values in the sim [its Imax of 25mA], in Bar Mode you will require a 15R 1Watt resistor in series with the LED supply.

Added:
If you are using a 9V battery say a PP3 type and all 10 LED's are trying to draw 30mA each, in Bar Mode, thats 300mA, which may result in the nominal 9V battery voltage dropping well below 9V, so its possible that you will not be able to maintain 30mA per LED.?

 

Hi
I will try this and thanks for your suggestion and help

Karan

 

@ericgibbs
Click not on the image. Click on the words, "circuit.png". New tab opens and THAT image will zoom.

 

hi 12,
Many thanks...
E