Need help with simple LED grow light circuit

[dogfight14]

Hi everyone, I am currently working on an LED grow light which consists of 20 1W blue LEDs, 20 1W red LEDs, and a 10W soft white LED (shown in yellow on my diagram).

I have mocked up what I think the circuit will roughly look like, but need a bit of help with the details of it...

**broken link removed**

The LED specs are as follows:

10W WHITE LED:
* Reverse Voltage:5.0 V
* DC Forward Voltage: Typical: 10 V Max: 12V
* DC Forward Current: Typical: 900mA Max: 1050mA

1W BLUE & RED LED:
* Reverse Voltage:5.0 V
* DC Forward Voltage: Typical: 2.2 V Max: 2.4V
* DC Forward Current:300mA

If anyone can help me work out the following, there will be a donation from the project's budget

• Whether it will work; and if not, what I need to do to it
• The resistance of the resistors
• The max resistance of the variable resistor
• Any additional changes which would benefit it

Please note: I put them into the 5 parallel lines of 4 LEDs just because I thought this might be appropriate, not for any inparticular reason

Look forward to hearing what people have to say, thanks for reading,

Col

 

[betwixt]

If your specification for the white LEDs is for each LED and not for the whole chain, you cannot use this circuit. The resistor is calculated by subtracting the LED voltage from the supply voltage (12V) then dividing the result by the LED current in Amps.

In each of your chains, you have to add the voltage of the LEDS, the current will be the same for all LEDS in that chain and set approximately by the LED with lowest current demand.

So if you have 4 white LEDs in series, it would take 40V at about 0.9A to light them. Your 12V supply would not be adequate.

If your specs are correct, you should add one resistor per white LED, the value you need is (12-10)/0.9 = 2.2Ω and it's power rating is I²R = 0.9 * 0.9 * 2.2 = 1.78W so you would use a 2W rating as the next highest available.

For the blue LEDs, keep them in a chain of 4 but on each chain use a resistor of (12 - (2.2 * 4))/0.3 = 10.6Ω with a power rating of at least 0.3 * 0.3 *10.6 = 0.96W

I would not recommend using a variable resistor to control the current. They are generally not suitable for high current applications like this because the heat they produce has to be dissipated on only the part of the resistor track carrying the current. So the heat becomes localized as you rotate the control. A better way is to use PWM to pulse the complete power source on and off. It works on the principle that if the power is always on, you get most brightness, if it is always off you get nothing, so somewhere between the two you get the level you want. PWM switches the LEDs on and off too fast to see any flicker but adjusts the on to off ratio to control the apparent brightness. It is efficient and relatively simple to implement.

Brian.

 

[dogfight14]

This is a really helpful breakdown, thanks =D I'll have another go at it in a bit and see if I can make it work.

There are 40 (20 red 20 blue) LEDs at 2.2V and just 1 white LED at 10V, so this is 98V... am I correct to now think I need 100V+ power supply?! Also.. should I use the typical or max quoted value to work this out.

Thank you for your advice.

 

[betwixt]

I just spotted another mistake - you have the + and - of the supply reversed!!!

Sorry, I misunderstood your original message. You can still use 12V but wire it like this:

1. the white LED has a single 2.2Ω 2W resistor is series with it.

2. each chain of 4 red or blue LEDs has an 11Ω 1W resistor in series with it.

So you need one 2.2Ω and ten 11Ω resistors in total.

Note that you can not safely connect LEDs directly across each other, a resistor is needed. The reason is that no two LEDs have identical voltage drops so the one with the lower drop will hog the current and prevent the other one lighting properly. This can also cause reliability problems as some LEDs will pass too much current while others get very little.

Brian

 

[dogfight14]

Surley the current flows from negative to positive given the electrons negative charge?

Cheers for the help so far, I will mock up another diagram and see if I can make it better with the advice you've given. I'll post it up when I have another go.

Cheers