LED's

[Tresguey]

ok electronic guru's. as i am an amatuer i am haveing a difficult time figuring out this one. i am going to make a sign out of LED's that just spells out a word. nothing special just turns on and stays on. well as i have been testing things out on a breadboard i run like 5 LED's in parallel an power them off the same source..... why is it that they start out bright then get dimmer? i thought by putting them in a parallel system they would all get the same power.

 

[Joel Rainville]

i run like 5 LED's in parallel an power them off the same source..... why is it that they start out bright then get dimmer?

What exactly is the power source?

 

[Tresguey]

a 9volt battery

 

[Joel Rainville]

a 9volt battery

That's it? No regulation of any kind?

Do you have a multimeter or some way to check your battery's life?

Are you using current limiting resistors for each LED?

 

[Tresguey]

i checked the battery and it check ok. as far as resistors....no i didn't use any. i figured that the leds could take the extra 0.3 volts.

 

[Gorgon]

i checked the battery and it check ok. as far as resistors....no i didn't use any. i figured that the leds could take the extra 0.3 volts.

Hi,
You didn't tell the type of LEDs you use, but connecting them in parallel is not the way to do it. You connect them in series, normally with a current limiting resistor. (From what you say I suspect that the 'parallel' is a typo)

If you don't use an external resistor you use the batterys internal resistance as a current limiter, until the battery voltage drops to the combined LED forward voltage. Reaching this voltage the current drops and the light goes off.

You'll also need more headroom than 0.3 volts to make the battery last longer.

If you have a lot of LEDs to drive, I would think that some form of power supply is better than batteries. The higher the voltage the more LEDs can be connected in each row.

If you use 9V, use 4 LEDs in each row with a resistor of 220ohm. This gives you around 10mA through the LEDs. (I suppose the LEDs are standard red ones, with 1.7Vf.)
If you need more rows, use one resistor and 4 LEDs in each row.

TOK

 

[Tresguey]

LOL...yeah i had a brain fart there. your right. they are in series. my project will use about 100 led's and be power by an ac source. i'm just making a sign for my company's SEMA booth. i like taking baby steps. i was just trying out how the leds would look. but couldn't figure out why they were brighter closer to the positive lead and dimmer near the negative lead.

 

[audioguru]

You have 2 problems:
1) The LEDs started bright then dimmed because you didn't use a current-limiting resistor. So the current through the LEDs was way beyond their max rating which caused them to overheat and reduce their brightness as they got hotter and hotter. They might not feel hot on the outside, but inside they are burning.
The high current was also way beyond the battery's rating and caused its voltage to quickly run down.

2) Now some LEDs are brighter than others. They are all getting the same current when in series. So the dim ones now were damaged more by the previous over-current. Maybe the ones with more damage were located above the others. Hot air rises.