# parralell leds getting dimmer?

Discussion in 'General Electronics Chat' started by solexious, Mar 12, 2008.

1. ### solexious Thread Starter Active Member

Nov 20, 2007
55
0
hello,

I'm adding some leds to a circuit, ive got a 7805 hooked up to a 9v battery and im adding leds in parralell, but every time i add one i see a slight dip in blightness. I thought adding them in paralell wouldnt do this?

How come I see this and is there a way to make it stop?

Thank you

solexious

2. ### raffter Active Member

Feb 28, 2008
113
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current IS divided everytime you add another .... add resistor PER LED ...

Ralph

3. ### antseezee Active Member

Sep 16, 2006
45
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Your 9V batter has a max output current. You have so many LEDs in parallel (which are probably drawing 10-20mA) that the max current is being drawn, and a proportional amount of current is only being provided to each one. Let's say the max output current is 100mA. You hook up 5 LEDs with the same resistance - now they're at 20mA a pop. You keep adding LEDs, and as a result, that 20mA a piece now gets lowered to 15, 10, and so forth.

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
If as raffter has mentioned, with each additional led placed in parallel with those already connected, the available current will be distributed among the leds connected in parallel.

By connecting each of the leds with its own series current limiting resistor between the output of your voltage regulator and ground, each LED will get a prescribed amount of current which will not vary as long as the output of the voltage regulator remains constant.

hgmjr

5. ### Audioguru New Member

Dec 20, 2007
9,411
896
The minimum input voltage for the 7805 regulator is 7.5V.

Measure the voltage of the battery. When its voltage drops below 7.5V from being overloaded then the 7805 regulator's output voltage will drop.

6. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
You shouldn't be using the voltage regulator anyways, depending on your configuration. It's just wasting power

You should string as many as possible together so that their forward voltages are a bit less than 9V, then use an appropriate limiting resistor. (9V - Vf X #LEDs)/(nominal current for LED) = Rlim

Steve

7. ### Audioguru New Member

Dec 20, 2007
9,411
896
Then when the battery voltage drops a little, the LEDs turn off.
You will actually see them dimming.

8. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
Yes, you're right... I looked into battery discharge curves, they're quite dramatic, which would show a lot of dimming over it's life. The only good way to fix this is to do a switching regulator. Something along the lines of the following link, except modified to accommodate more LEDs/.

http://www.maxim-ic.com/appnotes.cfm/an_pk/3532

Steve