# 88 LEDs

Discussion in 'The Projects Forum' started by leghorn, Sep 17, 2015.

1. ### leghorn Thread Starter New Member

Sep 17, 2015
5
0
I bought 88 "Superbright 10mm Round LEDs". I need to get them all to light up. I have no idea how.
Can I use an old computer power supply? Apparently it can give either 5V or 12V. The LEDs have a forward voltage of 3.2 - 3.4. I'm not sure what that means.
Any help will be much appreciated.

2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,325
592
hi,
The forward operating voltage of ~3.2V suggests that the LED's are White.
Which means that if you connect 3 LED's in series, thats 9.6V, leaving 12V -9.6V = 2.4V to be dropped across a series current limiting resistor.
So for 88 LED's you would require 88/3 = 29.3 sets of the 3 LED's as described above.

What is the required operating current of the LED.???

E

Sep 17, 2015
5
0
4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,325
592
hi,
The link does not state an operating current, so I would assume 25mA Max and so run them at 15mA.

So, for 15mA, the series current limiting resistor will be 2.4v/0.015A = 160 ohms, use a 180 Ohm, 0.25Watt resistor for each 3 series LED's.

You appreciate that your 8 rows of 11 each will have to be built up using the 3 off LED plus series resistor networks, say 29 * 3, LED's plus resistor.

E

5. ### leghorn Thread Starter New Member

Sep 17, 2015
5
0
I think I understand.
Would a 2Watt 180 ohm resistor also work? I assume the Wattage is the max that the resistor can handle.

Does the long leg of the LED go to Positive?
And on which side do I put the resistor?

6. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,325
592
hi.
Checking the LED part no: from the link you posted, I got this data.

It shows the Longer pin on the LED is the Anode, a 2Watt resistor will work OK, buts its a big oversize.!
Do you realise that the LED's are rated as 3.6Watts!!!!.

Look at the attached circuit, you need 29 of these, when you use the 12V ex-PC power supply

What is the application.??
E

File size:
9.1 KB
Views:
7
7. ### leghorn Thread Starter New Member

Sep 17, 2015
5
0
Hi Eric,
Thank you so much for all the help. I think I can do this now.

One last question please: To make the light from the LEDs slightly brighter, can I lower the value of the resistor? At what point will I damage the LEDs?

8. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,325
592
hi,
As the LED data for that device says 3.6Watts, thats ~850mA operating current.
I do not really believe that figure for a 10mm dia LED without an heat sink.
I would initially not exceed 50mA current, that would mean a 47 Ohm resistor.

Build only one chain of the 3 , LED's and a 47R , at least 1 Watt and test it with 12volts supply, keep checking the body temperature of the LED, if exceeds 50Cdeg I would increase the value of the resistor and re-test.
If they stay cool to the touch try a 22 Ohm, if still OK, try a 10 Ohm.

I use larger 3Watt LED and I have to mount them on a heat sink to operate at 700mA and even then they run at 60Cdeg.

If you could find the full datasheet for the LED, I could post more precise information.

E

9. ### gerty AAC Fanatic!

Aug 30, 2007
1,181
319
Remember the leds are polarity sensitive, wire them up backwards and they wont work.
And you put them in series, and one is backwards, none of them will work.
If you are already aware of the polarity issue, disregard this post

10. ### Bernard AAC Fanatic!

Aug 7, 2008
4,551
477
Viewing angle 100- 120, & 20- 25. Prob. 20-25 is current in mA. 3.5 W- not likely for 30,ooo mCd.

11. ### leghorn Thread Starter New Member

Sep 17, 2015
5
0
@eric, thank you very much for all the info and help. You've enabled me to build this thing. I've just been informed that I will have to wait up to a month for the LEDs. Big Frustration!
@gerty, Thanks, I will be carefull about that.
@Bernard, thanx for the input