# LED fixture build help

#### bmplank86

Dec 17, 2013
6
Hello! I am currently building an LED fixture for my 5 gallon saltwater fish tank. I have experience in electronics, but engineering something is a little new to me.

I have 6 leds, each has a max voltage of 3.4v and I want them run at 650mah. The supply voltage is a DC power adapter of 24v at 2amps. These LEDS will be ran in series.

Total voltage of the LEDs is 20.5v. What size resistor do I need to lower the supply to spread 650mah through each LED. This is ran in series, so current is constant, correct? According to my calculations I need a 32ohm resistor. Can someone help me out?

#### mursal

Dec 13, 2013
75
24VDC - 20V5 = 3V5

3V5/650mA =

3V5/0.65A = 5R3 (5.3 Ohm) resistor

Resistor size (wattage) = 3V5 x 0.65 ≈ 3watt

Last edited:

#### duke37

Jan 9, 2011
5,364
I presume you mean 650mA (0.65A). mAh is a measure of energy such as in a battery.

The current in a series circuit doesn't need to be spread across the LEDs, it will be constant throughout the circuit.

My calculations are different from yours.
6 * 3.4 = 20.4
Voltage across resistor = 24 - 20.4 = 3.6
R = V/I = 3.6/0.65 = 5.5

This seems to be a bit low for current stabilisation and variations in PSU and LED voltages. Others may be able to advise.

#### bmplank86

Dec 17, 2013
6
Okay, I understand. You take the sum of the LEDs and subtract from supply voltage to get the left over voltage, which is what the resistor needs to limit. But what about current? Does it matter whether I use a 2amp or a 4amp power supply? If the diodes have a forward current of 650ma, and I supply it with more than that, what limits it?

#### bmplank86

Dec 17, 2013
6
Would it be safer to out the DC power supply w/ resistor and use a Meanwell driver? Constant current of 700ma and 9-48v

#### mursal

Dec 13, 2013
75
The resistor (≈6R) will limit the current flow to 650mA at that supply voltage, regardless of the available current from the supply. So it doesn't matter whether its a 2A or a 4A supply.

#### bmplank86

Dec 17, 2013
6
Right, but the resistor was calculated to make up for the extra voltage coming from the LEDs...how does that coorespond to the forward current for the LEDs? I guess I just don't understand how it lowers to the 650ma and makes up for the extra voltage.

#### duke37

Jan 9, 2011
5,364
The resistor was calculated to drop the required voltage at 650mA (not 650ma !). If you want less current, the resistor will be higher.

If the power supply can maintain its voltage, it does not matter whether it is rated at 1A or 100A. It just needs to be powerful enough for the job.

#### bmplank86

Dec 17, 2013
6
Ding. Got it. Thank you very much

#### davenn

Moderator
Sep 5, 2009
14,011
Ding. Got it. Thank you very much

You need to remember that the primary function of the resistor is to limit the current, its voltage dropping effect is a secondary effect

Dave

Last edited:

#### BobK

Jan 5, 2010
7,682
+1 on the constant current supply. For that kind of current, a simple resistor is not a good solution. They are not really all that expensive.

Bob

#### bmplank86

Dec 17, 2013
6
How much safer is a driver vs the resistor with DC power adapter?

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