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Topic: not sure about resistors (Read 813 times) previous topic - next topic

Arjan901

Jan 09, 2013, 05:51 pm Last Edit: Jan 09, 2013, 07:16 pm by Arjan901 Reason: 1
Hello,

For my led cube i still need resistors. I'm going to make an 5x5x5 led cube with an arduino mega 1280.
Specifications:

Code: [Select]
Microcontroller ATmega1280
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
Digital I/O Pins 54 (of which 15 provide PWM output)
Analog Input Pins 16
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 128 KB of which 4 KB used by bootloader
SRAM 8 KB
EEPROM 4 KB
Clock Speed 16 MHz


I bought 5mm leds without specifications.
based on some information found on the internet, a white led uses 3.2v, 32a, see: http://ledcalculator.net/

when i search on websites which resistor i have to use, i found a lot of different values. Can anyone give me the correct resistor value to be sure that it will work for the cube?

many many thanx

codlink

A regular white LED runs about 1.8-2.2v.  And wants around 20mA.   

I use a site that gives me the resistor values if you know the LEDs specs.

http://ledcalculator.net/

At 2v, I would use around 150Ohm.  But you could use anything that is a little higher than that.  Shouldn't see much difference in light output.
//LiNK

Hippynerd

5v seems a bit high, but that maybe correct, im not sure. I used some while LEDs from cheapo flashlights, and they were running at 4.5v (3 AAA batteries).

Do you have any info in the LEDs? How much current do they support? most smaller LEDs (like 5mm) seem to be 20ma. You will need to know how much current you want to send them, and what voltage you are using to calculate the proper resistor size.

How are you driving the LEDs. If you are driving them with constant current drivers, you wont need resistors on the LEDs (but you will need to set the current with a resistor).

Are you running this off the power from the arduino, or do you have a separate power supply? if your power supply is running higher than 5v, you may need resistors based on the voltage from your power supply.

Search for LED calculator, and you will find websites that will help you figure out the proper resistor size.

To find out your forward voltage, you may need to hook up a resistor, and measure the volatage to calculate the current, then try the right size resistor and measure the current (to make sure its actually doing what you want).
https://sites.google.com/site/rgbledcubes

fungus


5v seems a bit high, but that maybe correct, im not sure. I used some while LEDs from cheapo flashlights, and they were running at 4.5v (3 AAA batteries).


What on earth are you on about?

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

retrolefty

Quote
I bought 5mm leds without specifications.
based on some information found on the internet, a white led uses 5v, 32a.


Wow, a 160 watt white led, I want one, got a link?  ;)

Lefty

CrossRoads

I believe Vf for White LEDs is generally in the 3.2V to 3.6V range, similar to a Blue LED.
Example:
http://www.dipmicro.com/store/LED3W 3mm
http://www.dipmicro.com/store/LED5W 5mm
http://www.mpja.com/5mm-White-LED-10000mcd-Non-Diffused/productinfo/17145+OP/ 5mm

Some may be higher:
http://www.mpja.com/10mm-White-LED-15000-mcd-Non-Diffused/productinfo/15415+OP/ 10mm


Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Arjan901

the items i have:

I normally connect the arduino to my computer.
But i have a 9V, 1A adapter: http://dx.com/p/arduino-9v-1a-power-adapter-black-2-flat-pin-plug-100cm-cable-126288
these leds i use: http://dx.com/p/14000mcd-25packs-white-5mm-led-1106

so what will it be ?

Hippynerd

Maybe you can get a datasheet from the vendor. I looked, and didnt find any specs on the website, and I read a bunch (but not all 56 reviews), and found nothing of value.

If you get nothing from the vendor, you can still figure it out yourself, but it may take an hour out of your life, to learn, and apply the information you learn, to calculate the specs for yourself.

Nobody here will be able to do that for you. The only thing you could hope for is that someone reading your thread bought the exact same LEDs from the same vendor, and did the calculations themselves, and even then you would have to trust a stranger. It seems to me, you will be better off just doing it yourself.
https://sites.google.com/site/rgbledcubes

CrossRoads

Hook up an LED with a 220 ohm resistor using +5 & Gnd.
Measuse the voltage across the LED when it is on, that will tell you Vf.
Measure the voltage across the resistor (= 5V - Vf) and divide by the resistor value, that will tell you  much current was going thru:
V/R = current
so for example (5 - 3.2)/220 = ~8.2mA

Do not power the LED from 9V and try to control it from an arduino pin.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

MarkT


I believe Vf for White LEDs is generally in the 3.2V to 3.6V range, similar to a Blue LED.



Unsurprising if you know that white LEDs are actually blue LEDs with yellow fluorescent compound in front of them...
[ I won't respond to messages, use the forum please ]

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