# LED confusion

Discussion in 'The Projects Forum' started by RodneyB, Sep 5, 2013.

1. ### RodneyB Thread Starter Active Member

Apr 28, 2012
633
13
I am at my wits end I built a Blue LED spot light using as per the attaché diagram.

I ran a 24 volt supply to a regulator which dropped the voltage from 24 to 13.9 volts It is a 6 amp supply @13.9 Volts regulator.

for 3 days solid the light was on and then for no reason that I know the LED's stopped working

PLEASE can some one lell me what I am doing wrong!

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2. ### JingleJoe Member

Jul 23, 2011
185
10
Your circuit seems sound, just did a few calculations which I'm sure you did to arrive at that resistor value

Have you tested the LEDs individually out of circuit? have you done the standard fault finding test: check the input and check the output and see if it's what you expect. The input in this case may be your 24V or 13.9V regulated voltage, the output would be the voltage accross each LED.

3. ### wayneh Expert

Sep 9, 2010
12,154
3,060
Yes, a few minutes with a voltmeter will reveal the problem. Probably blew a fuse?

4. ### ScottWang Moderator

Aug 23, 2012
4,855
767
Do you know how are the V/I of LED?

Assuming that the V_LED = 3V.
3v x 4 = 12V
13.9V - 12V = 1.9V
I = 1.9V/47Ω=40mA.

If the V_LED = 12V, V_R = 24V - 12V = 12V.
The R should be like this:
R =12V/40mA = 300 Ω
If using 80% of 40mA = 32mA
Then R=12V/32mA=375Ω, you may using 390Ω.
If V_LED isn't not 3V, then you can calculate the value as above.

Last edited: Sep 5, 2013
5. ### RodneyB Thread Starter Active Member

Apr 28, 2012
633
13
Thank you for all the help. I will check and report back my findings

6. ### wayneh Expert

Sep 9, 2010
12,154
3,060
It's unlikely the LEDs are the problem. Start at the wall socket and work towards the LEDs, not the other way around.

7. ### bance Member

Aug 11, 2012
315
34
If you haven't got a heat sink on the regulator, I would think that is where the problem is. You're dropping lots of voltage across it, and that goes in heat loss (power dissipation)

HTH Steve

8. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,771
971
just ditch the regulator..and recalculate the resistors for 24V..
more in each series string will help with resistor wattage..

bance likes this.
9. ### bance Member

Aug 11, 2012
315
34
+1 @ mcgyvr Clearly a more simple solution!

10. ### #12 Expert

Nov 30, 2010
16,345
6,829
330 ohms, 1/2 watt, for (5) 3.2V LEDs and 24 volts to produce 24.2 ma.

11. ### Ramussons Active Member

May 3, 2013
557
92
Have 2 Chains of 8 LEDs and a very small resistor. Generate less heat

Ramesh

12. ### #12 Expert

Nov 30, 2010
16,345
6,829
Two chains of 8 LEDs does not make 20 LEDs.
8 LEDs times 3.2 volts is more than the 24 volts available.