LED power supply

[jaharr01]

I ordered these 10 PCS 3W RGB Color High Power LED Light Emitter 4 pins with 20mm Star Base
http://www.ebay.com/itm/161057070124?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

I haven't messed with RGB LEDS before so I have a question. If I use 5 of these it says the maximum voltage is 3.6 per LED I used an LED calculator
DC Forward Voltage (VF): Red: 2.0-2.5Vdc, Green: 3.2-3.6Vdc, Blue: 3.2-3.6Vdc,
DC Forward Current (IF): Red: 400MA/ Green: 350MA/ Blue: 350MA

Solution 2: 1 x 5 array uses 5 LEDs exactly
+12V                   R = 22 ohms
                R = 22 ohms
                R = 22 ohms
                R = 22 ohms
                R = 22 ohms



The wizard says: In solution 2:
•   each 22 ohm resistor dissipates 3520 mW
•   the wizard thinks the power dissipated in your resistors is a concern 
•   together, all resistors dissipate 17600 mW
•   together, the diodes dissipate 7200 mW
•   total power dissipated by the array is 24800 mW
•   the array draws current of 2000 mA from the source.

Is this correct,should this work OK?
Jay

[PL1]

Have you considered using a 5v power supply?

12v - 3.6v = 8.4v drop across the resistor -- 70% of the total energy is wasted/dissipated as heat. (8.4v/12v)
  5v - 3.6v = 1.4v drop across the resistor -- 28% of the total energy is wasted/dissipated as heat. (1.4v/5v)

Using a 5v supply will result in about 16.66% (1.4v/8.4v) of the heat dissipation/power waste required for a 12v circuit -- assuming that you choose resistors that allow the same amount of current to pass.

Also, each channel (red, green, and blue) will need a resistor that keeps it within the voltages you mention in your OP.
3.6v applied to the red channel = very bright for a while followed by a "magic smoke" leak.   

For 5 RGB LEDs, you need to calculate for 15 channels (5 red, 5 green, and 5 blue), each channel will need a current-limiting resistor that brings the voltage down to a safe level for that color in the LED.


Scott

[jaharr01]

Yes I can,I can use a switchable power supply. It also has 5V. Will they be less bright at 5v?

[jaharr01]

They are flashers on my pinball build so they need to be bright

[PL1]

They are flashers on my pinball build so they need to be bright

Bright = good.

Overdriven = bad.

Having a larger current-limiting resistor will not make your LEDs brighter.

Picture drinking from a drinking fountain (regular 20-30mA LEDs) vs. chugging from a beer mug (your 350mA+ RGB LEDs) vs. drinking from a fully-open firehose. (your LEDs connected to 5v or 12v *without* current-limiting resistors)

You can't keep up with the firehose, it will overwhelm you unless you put a pressure/flow reducer (current-limiting resistor) inline with the hose.


Scott

[PL1]

Yes I can,I can use a switchable power supply. It also has 5V. Will they be less bright at 5v?

If you use the 12v resistor value (22 ohms) with a 5v input voltage, it will probably be noticably dimmer on your setup.

Run the calculator again to determine the correct resistor for 5v.

12v with a 22 ohm resistor and 5v with the proper resistor will be the same brightness because the calculator is figuring out what value/wattage resistor will provide the maximum desired voltage/current at the LED.


Scott

[jaharr01]

Solution 0: 1 x 5 array uses 5 LEDs exactly

+5V   
+   -|>|-   -/\/\/\-   +   R = 3.9 ohms
+   -|>|-   -/\/\/\-   +   R = 3.9 ohms
+   -|>|-   -/\/\/\-   +   R = 3.9 ohms
+   -|>|-   -/\/\/\-   +   R = 3.9 ohms
+   -|>|-   -/\/\/\-   +   R = 3.9 ohms
The wizard says: In solution 0:
each 3.9 ohm resistor dissipates 624 mW
together, all resistors dissipate 3120 mW
together, the diodes dissipate 7200 mW
total power dissipated by the array is 10320 mW
the array draws current of 2000 mA from the source.
I can do it 5v it's not a problem

[jaharr01]

And I ran the 5v for 15 channels and it was still 3.9

[PL1]

And I ran the 5v for 15 channels and it was still 3.9

If you are getting the same value resistor for red as you're getting for green/blue, you're not entering the proper values into the LED calculator.

3.9 ohm ==> 5v supply, 3.6v LED voltage (blue/green), and 400mA current draw (red) -- this will over-volt the red channels and over-amp the blue/green channels causing the reds to blow quickly and probably the blues/greens, too.

Calculate one group at a time -- channels that need the same voltage/current will use the same value/size resistor.

The values I plugged into the LED calcualtor are highlighted in red in case you want to adjust them.

    Group one -- Red channels: (5 total channels)
Power supply voltage: 5v
LED DC Forward Voltage (VF): Red: 2.0-2.5Vdc
LED DC Forward Current (IF): Red: 400mA
* You will need 6.8 ohm 2 watt resistors.
* Each 6.8 ohm resistor consumes 1088 milliwatt.
* The array draws current of 2000 mA from the source.

    Group two -- Green/Blue channels: (10 total channels)
Power supply voltage: 5v
LED DC Forward Voltage (VF): Green: 3.2-3.6Vdc, Blue: 3.2-3.6Vdc
LED DC Forward Current (IF): Green: 350mA/ Blue: 350mA
* You will need 4.7 ohm 1 watt resistors.
* Each 4.7 ohm resistor consumes 575.75 milliwatt.
* The array draws current of 3500 mA from the source.

The 5v power supply needs to be able to handle a current draw of up to 5.5A (2000 mA + 3500 mA) which corresponds to all channels at full brightness.

5v
 |   /--  Red 1 LED--- 6.8 ohm -- LED controller channel 1
 |---  Green 1 LED--- 4.7 ohm -- LED controller channel 2
 |  \--- Blue 1 LED--- 4.7 ohm -- LED controller channel 3
 |
 |   /--  Red 2 LED--- 6.8 ohm -- LED controller channel 4
 |---  Green 2 LED--- 4.7 ohm -- LED controller channel 5
 |  \--- Blue 2 LED--- 4.7 ohm -- LED controller channel 6
 |
 |   /--  Red 3 LED--- 6.8 ohm -- LED controller channel 7
 |---  Green 3 LED--- 4.7 ohm -- LED controller channel 8
 |  \--- Blue 3 LED--- 4.7 ohm -- LED controller channel 9
 |
etc.                                       


Scott

[jaharr01]

oh i understand now I ran them all the same.
Thanks

[PL1]

Glad to assist.   


Scott