# LED Voltage

Discussion in 'General Electronics Chat' started by CrazeUK, Feb 4, 2010.

1. ### CrazeUK Thread Starter New Member

Feb 4, 2010
6
0
Hi Guys.

I am new to the forum and quite new to electronics.

I have just built my self some LED light units for a fish tank.
My problem i have is with power supplys for these units.

The LED's: 1 Watt LED, 4.6V Max.
Driver: Each driver can handle 1 x 3 Watt Led OR 3 x 1 Watt.

The setup: 3 x 1 Watt LED's on 1 driver.

So my problem is this.

I have two 12 V Adapters (Both from other devices).
a. 500mA
b. 1667mA

Now if my calculations are correct i work out:
a = 6 Watts.
b = 2 Watts

Is this correct?

If this is correct am i likely to burn out the LED's connected to a because it is delivering a higher power than a. the driver can handle and b. the LED's can handle?

Is there any way i can run both of these circuits of 1 adapter possibly a PC PSU?

Additionally, i have many 12v devices all with individual adapters on my tank.

Is it possible to have a device that i can build or purchase that will allow me to cut these off and use one single point?

p.s. apologies if these questions are plain stupid its been 11 years since i last did anything of this sort.

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
LEDs must have limiting resistors. This is not optional. If you have powered them up without then there is a good chance you have already damaged your LEDs.

LEDs, 555s, Flashers, and Light Chasers

3. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
You'll need more information than this to calculate the resistance you need than what you have. Go to your LED specifications and see what the foreward voltage is according to a certain current. For instance, you might see something like this: Vf = 3.2V IF I = 350mA. So whatever the foreward voltage drop is across your LED, you need to subtract that from your voltage that your providing through the adapter. Using the example information from above, we can conclusively find a resistance for our LED.

1. If Vf = 3.2V IF I = 350mA, and if our adapter supplies 12V, then 12V- 3.2V = 8.8V.
2. $\frac{8.8V}{0.35} ≈25Ω
$
$Austin$

Last edited: Feb 4, 2010
4. ### Externet AAC Fanatic!

Nov 29, 2005
780
58
Nope. b is 20 Watts capable

5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Google "ATX bench supply" - you will find lots of ideas for making a bench power supply from a cheap ATX form-factor PC power supply. Note that you MUST put a load on the 5v supply, or the other voltages will not regulate properly.

6. ### CrazeUK Thread Starter New Member

Feb 4, 2010
6
0
Hi guys thanks for all the responses.

Re resistor:
I thought the driver (buckpuck) was effectively the resistor (what protects the LED from irregular voltages), and all i had to do was limit the actual voltage i supplied the driver.

Essentially. the circuit looks like:

Well its howi was advised to set the led's up by the manufacturer.

I will try to get th Vf information later today.

SgtWookie: much appreciated will look it up today

7. ### cjdelphi New Member

Mar 26, 2009
272
2
They look like SSC P4 LED's... they should be capable of getting around an amp of current providing they're properly heatsinked.

1667mA @ 12 v = 12*1.667 = 20watts.

12*0.500 = 6 watts (double the capacity of which the driver can regulate the LED) now providing your driver has regulation capability that is... otherwise it will just use PWM at a high frequency and a lower frequency for a lower brightness, depending on the driver and what it's capable of.

So the question is how are you powering them all? from a transformer / car battery? because if you are then simply use a 3/5watt resistor and just pick out the right value you need.... if it's not then you'd need the driver to regulate the current / voltage... 250ma should be just inside the safe
zone without the need of heatsinking. and the driver will push 3 watts out of it which means it will need good heatsinking..

so tell me what it's for / powered by (240v, car, what)...

8. ### CrazeUK Thread Starter New Member

Feb 4, 2010
6
0
Ah whoops.

I currently am supplying them from 240v Mains, then through one of the two adpaters.

9. ### CrazeUK Thread Starter New Member

Feb 4, 2010
6
0
I have contacted the supplier.

They have said, that as long as you dont exceed the stated 4.6v per LED there is no need to regulate the amps or watts.

Which seems to be ok at the moment, i have had the LED's running at 12v (3 LED bar) for 2 days.