# Help with CC driver for LED project

#### darky82

Joined Jan 9, 2012
21
Im after some help for a reef aquarium LED add on.
Im currently using 12v to a TC420, a 5 channel PWM/Timmer to dim on and off a array of LED's that are in a T5/LED fixture on 2 channels that are already current limited in the fixture.
It uses a constant positive and is PWM controlled through the negative side.

I want to add 4x 10w leds of 2 diffrent colours rated at 1000mA but i need them to be current limited after the PWM TC420 and run them on 500-600mA of current so i can use a heatsink without a fan.

What would the best way of doing this?
Make a circuit or buy something off say aliexpress?

#### darky82

Joined Jan 9, 2012
21
Anyone

#### crutschow

Joined Mar 14, 2008
23,141
You need to post a diagram to show what you are trying to do.

#### darky82

Joined Jan 9, 2012
21
This a quick example of how i want to set it up.
I already have white and blue on channel 1 and 2.

I need to limit the current out of the TC420 PWM controller to the 2x 10w LED's some how.

#### crutschow

Joined Mar 14, 2008
23,141
Can you not just use a properly sized resistor in series with the LEDs to limit their current?
What is the forward voltage drop of the LEDs?

#### darky82

Joined Jan 9, 2012
21
Would it over heat the resistor?
If i have 12.5v at 600mA the voltage across the LED is 9.3v

#### philba

Joined Aug 17, 2017
959
You just have to size the resistor properly.
Voltage drop is 12.5-9.3 or 3.2V
Resistor: R = V/I = 3.2/.6 = 5.33Ω
Wattage: V*I = 3.2*.6 = 1.92W, look for something at least 2.5W.

Now, you have to look at what's available in a 2.5 watt or higher resistor (it's a bit hit or miss). Mouser shows a 5.49Ω, 3 Watt resistor.

Because the resistor is a bit higher, we recalc the wattage, just to be sure (maybe we could use a smaller one).
Current: I = V/R = 3.2/5.49 = 0.583 Amps
Wattage: W = V*I = 3.2*0.583 = 1.89W
So, that resistor will work OK, you will lose .017 Amps or slightly less than 3%. I didn't see a 2.5W resistor so the 3W one will have to do. It doesn't hurt to be larger.

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#### crutschow

Joined Mar 14, 2008
23,141
Ohms law: R = V/I = 12.5/.6 = 20.833 Ohms.
You didn't include the LED voltage.

The resistor value should be (12.5V-9.3V) / .6A = 5.33Ω.
Using a 5Ω resistor, its dissipation would be (12.5-9.3)² / 5 = 2.05W.
Use a 5Ω, 5W resistor.

#### philba

Joined Aug 17, 2017
959
Yes, I updated it while you were responding. Misunderstood the 9.3V. All fixed. a 3W, 5.49Ω will do.

#### darky82

Joined Jan 9, 2012
21
You didn't include the LED voltage.

The resistor value should be (12.5V-9.3V) / .6A = 5.33Ω.
Using a 5Ω resistor, its dissipation would be (12.5-9.3)² / 5 = 2.05W.
Use a 5Ω, 5W resistor.
LED Foward voltage is 9-12v

#### crutschow

Joined Mar 14, 2008
23,141
Yes, I updated it while you were responding. Misunderstood the 9.3V. All fixed. a 3W, 5.49Ω will do.
Okay, we crossed paths in the ether(net).

Edit: But note that a 3W resistor will get quite hot dissipating near 2W.
That's why I recommend a 4 or 5W resistor.

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#### darky82

Joined Jan 9, 2012
21
Okay, we crossed paths in the ether(net).

But note that a 3W resistor will keep quite hot dissipating near 2W.
That's why I recommend a 4 or 5W resistor.
I think jaycar carry a 5W or a 10W ceramic resistor for memory

Thanks for the help

Closest I have is a 5.6 ohm 5w carbon resistor and after a minute or 2 its too hot.
The circuit is using 430mA @ 12.3v with 5.6 ohms of resistance and 660mA with a 3.3 ohm 50w aluminum body resistor.

What do i do?

Last edited by a moderator:

#### darky82

Joined Jan 9, 2012
21
Bump

#### crutschow

Joined Mar 14, 2008
23,141
Closest I have is a 5.6 ohm 5w carbon resistor and after a minute or 2 its too hot.
How hot is "too hot"?
Power resistors get hot since that's how they dissipate the power and when dissipating maximum power they will burn you if you touch them.
If they get warmer than you like then you need to use a higher power resistor or get one that mounts on a heatsink.
The circuit is using 430mA @ 12.3v with 5.6 ohms of resistance and 660mA with a 3.3 ohm 50w aluminum body resistor.
What do i do?
You use the resistance that gives you the current you want.

#### darky82

Joined Jan 9, 2012
21
Ok so its ok for a 5w resistor to get hot enough to burn if touched.

But when i put values into a ohms law calculator like 12.5v and 5 ohms it tells me 2.5A and 31.2w?

#### darky82

Joined Jan 9, 2012
21
Sorry for the dumb questions

#### crutschow

Joined Mar 14, 2008
23,141
But when i put values into a ohms law calculator like 12.5v and 5 ohms it tells me 2.5A and 31.2w?
That's correct if there's 12.5V across the resistor, but you don't have that.

You need to subtract the LED voltage drop from 12.5V to get the drop across the resistor.