# Driving 6 x 3.2 V Blue LEDs with 5V input

Discussion in 'The Projects Forum' started by krishnanrnair, Jul 28, 2011.

1. ### krishnanrnair Thread Starter New Member

Aug 20, 2010
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0
Hi

I am working on one of the projects where I need to light up 6 x 3.2V Blue LEDs as one Segment. There are about 25 segments. I am tired of soldering individual leds with 82 Ohm resister (about 150 in nos)

Can anyone suggest an alternative with transistor to drive each segment, connecting LEDs either in series or parallel

Thanks for

Krish

2. ### krishnanrnair Thread Starter New Member

Aug 20, 2010
5
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If not possible with transistors, even driver ICs could be of great help rather than soldering resisters at every LED leg

Apr 30, 2011
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You don't have enough voltage for any other option except a boost converter. Solder away.

The LED array wizard (http://led.linear1.org/led.wiz) says:
* each 82 ohm resistor dissipates 39.7mW
* together, all resistors dissipate 6W
* together, the diodes dissipate 10.6W
* total power dissipated by the array is 16.5W
* the array draws current of 3.3A from the source

Last edited: Jul 28, 2011
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4. ### Wendy Moderator

Mar 24, 2008
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This is a super simple boost converter. I doubt it is what you want though. Voltages can be less than 5V and still work.

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5. ### SgtWookie Expert

Jul 17, 2007
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What is your LED's rated current?
Right now, you're averaging almost 22mA current flow; 10% over 20mA
If they are rated 20mA, you should be using a 91 Ohm resistor.
Rlimit >= (Vsupply - Vf_LED) / Desired_Current = (5v-3.2v)/20mA = 1.8/0.02 = 90 Ohms

6. ### krishnanrnair Thread Starter New Member

Aug 20, 2010
5
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Hi Bill,

I still doubt if this simple booster will work, reason I have about 20 outputs coming from AT89S52 microcontroller connected to the -ve of the LED through resistance (as of now) which I would like to enhance.
Do you mean to say that I have to build this booster for every output..

Last edited: Jul 28, 2011
7. ### krishnanrnair Thread Starter New Member

Aug 20, 2010
5
0
You are right I should have used 90 Ohm resistor instead thought of using 82. otherwise also I am not in mood to solder 150 resistors, that is why I am looking for alternatives which could be used to drive 6 LEDs as there are 25 segments of 6 led each.

Thanks

8. ### SgtWookie Expert

Jul 17, 2007
22,183
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If you are going to try to drive them directly from your microcontroller, you will have problems.
Datasheet for your microcontroller is here: http://www.atmel.com/dyn/resources/prod_documents/doc1919.pdf
Electrical specifications are on page 29.
Absolute Maximum output current is 15mA, but you need to stay well below that.
Iol is 10mA, and you are limited to 71mA current maximum for ALL pins of the microcontroller.

You will need to use either some kind of driver IC, or many discrete MOSFETs to drive the LEDs.

9. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
You will note that the 555 driver has digital inputs. I didn't really think you would want to use them, but you're going to have to use something from what I can tell, even if it is a logic MOSFET.

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10. ### krishnanrnair Thread Starter New Member

Aug 20, 2010
5
0
Thanks a lot Bill

Can any one guide me how to build MOSFET in to my project...

11. ### Wendy Moderator

Mar 24, 2008
20,772
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The key is to get a logic level MOSFET, a very special breed of transistor. It may work with 5V, if you get the right part.

As to how to use MOSFET, this is not meant to be a comprehensive guide, but something just to get you started...

LEDs, 555s, Flashers, and Light Chasers

Chapter 10 - Transistor Drivers

Maybe some one can recommend a specific part and source, what part of the world are you?