using one resistors for number of parallel leds

[gnomeman]

resistors for leds

Hi there. I,m sure someone here can help me out.
I want to connect 20 white led's in parallel from a 5 volt power source. I found a calculator that tells me that I can do this if I put a 56k ohm resistor between each led's negative wire (20 resistors for 20 led's).
My question is: can I get away with using only one resistor for all 20 led's?

Here are the specs I have:
power source- 5 volts - 1000 ma max (would like to keep ma around 750 to be safe

led- white- 3.6v- current 25ma

Note, i do not want a series setup because the wires for the led's are burried in ground already due to a previous set up. So I want to keep them wired parallel.

Thank you in advance
Bill

 

[rkodaira]

Re: resistors for leds

IF ALL LEDS WERE EQUAL AND HAVE EXACTLY THE SAME FORWARD DROP VOLTAGE it would be possible to use only one resistor to lit all leds. The calculation would be the following:

R = (5V - 3.6V)/(0.025A x 20 leds) = 1.4V / 0.5A = 2.8 ohm (considering the max value for the current in each led - anyway, don´t do that)

But returning to real world, the forward voltage of each led is not the same of other, so doing this way, probable some will be on and other will be off (if some of them are not going to burn, or going dead in sequence, due to current exceding the max value). In the best case, some will be brighter than the others.

To solve this, you should use a small value of resistance in series with each led, to compensate the variations in the characteristics of each led.

 

[rwiggins]

resistors for leds

No. Remember: current is constant in series, voltage is constant in parallel.

from: **broken link removed**

* each 56 ohm resistor dissipates 35 mW
* the wizard thinks 1/4W resistors are fine for your application
* together, all resistors dissipate 700 mW
* together, the diodes dissipate 1800 mW
* total power dissipated by the array is 2500 mW
* the array draws current of 500 mA from the source.

 

[pranam77]

resistors for leds

Hai Bill. The 56 Ohm resistors works well for each LED. You may connect maximum 5-6 LED's in parrellel but with a 1watt resistor. But for reliability i strongly recomend resistors with each LED indidually.

 

[Kral]

Re: resistors for leds

gnomeman,
Since the light output of an LED is proportional to the current, you will get better results if you connect them in series and use a single resistor. This way all the LEDs carry the same current regardless of the voltage drops across each LED. Of course, this requires a higher voltage than the parallel arrangement. You can also compromise and use a series-parallel arrangement, depending on what power supply voltage is available.
Regards,
Kral

 

[Audioguru]

Re: resistors for leds

Notice that the series resistor for each LED is 56 ohms, not 56k ohms.

 

[gnomeman]

Re: resistors for leds

Thanks for all your info. They say that a little knoledge is a dangerous thing. Well in my case, it's costly.
I ordered a package of 100 56k ohm resistors for my project. I should have ordered 56 ohm resistors.
Can someone please tell me how many k ohms are equal to 56 ohms?

Also, what will happen to my 20 parallel leds with a 56k ohm resistor in series with each led?
Will the leds not light, will they be real dim or will they blow?
Any info would be greatly appreciated.
Bill

 

[Audioguru]

Re: resistors for leds

The 56k resistor has a value that is 1000 times too high (it is 56 thousand ohms). Then the current in each LED will be 1000 times too low. Then the brightness of the LEDs will be extremely dim.

 

[gnomeman]

Re: resistors for leds

Thanks Audioguru. I appreciate the info. I'll have to see if I can find the 56 ohm resistors online. They are not available from radio shack.
Like I said earlier, I'm new to electronics and am learning all the time. So far all my knoledge is from what I have read from the internet.
Thanks again
Bill