19736 Views 5 Replies Latest reply: May 15, 2014 4:12 PM by davecuthbert
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# Parallel leds driver problems

Hi all

I have done extensive previous research into this but before I go ahead with my project I want to clear a few things up. I have attached a picture of my proposed design.

My questions relate to the driver itself, I am going to be running 28 5w red leds. They are ledengin and the specs as follows;

2.5v forward voltage

1A current

So based on them running in 7 strings of 4 leds. I have calculated that I will need 2.2ohm resistors and a 12vdc 7amp 140watt driver

But I'm unsure. Does the amps of the driver need to be increased by a factor of one for each parallel line added to the circuit? And also how do I know what wattage driver to use? If so this would mean a constant current driver for 7amps? But I can't find any that output that amount.

My final cicuit is going to include other leds too, but I was thinking to run these from a separate driver as their forward voyage and amps are different? So at the end I will be running 4 separate drivers for four lots of parallel led circuits. What would I need to power all 4 drivers or would each need to be power from 240vac independently. Or is there a driver out there that can do everything at once?

Any help is very much appreciated. Other than physics goes I know nothing! And that was long time ago so please if possible keep it to lamens!

Thanks guys!

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• 65 posts since
Feb 18, 2014
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May 14, 2014 5:04 PM (in response to Edgierprawn)
Parallel leds driver problems

Can you provide a link to the LED datasheet?

• 65 posts since
Feb 18, 2014
Currently Being Moderated
May 15, 2014 3:47 AM (in response to Edgierprawn)
Re: Parallel leds driver problems

The circuit you propose needs some adjustment. A switching power supply would be the most efficient solution but let's look at something simpler to implement.

The datasheet specifies an LED forward voltage of 2.5 volts at 1 amp. But the specified range (see table 3) is 2.24 to 2.9 volts. The circuit should supply 1 amp over this voltage range. To drive four series connected LEDs the compliance voltage is 8.98 to 11.6 volts. A 12 volt supply with a series resistor cannot do this. There is not enough headroom for a simple active current source (like the one shown below) so let's place three LEDs in series. Note that a current-mirror or an Op Amp based series-pass current source can drive the four LEDs in series using a 12 volt source. And there is the temperature coefficient to account for. The LED forward voltage changes by -3.6 mV/deg C. For a 100 deg C change the forward voltage drops by 0.36 volts.

For now let's place three LEDs in series and the compliance voltage range is 5.7 to 8.7 volts for tolerance and temperature. Using a 4.5 ohm resistor and a 12 volt source (not counting the LED dynamic resistance and the temperature coefficient) the current is 0.73 to 1.4 amps. If this range is too large an active current source is a solution. The circuit shown here is a basic current source that will hold the current close to 1 amp over the range of LED forward voltage. The maximum transistor dissipation is 4 watts. With nine parallel strings of three LEDs running 1 amp the power from the 12 volt source is 108 watts.

If you would like to drive seven strings of four LEDs from 12 volts I can attach a schematic for a suitable constant current source.

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• 65 posts since
Feb 18, 2014
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May 15, 2014 4:12 PM (in response to Edgierprawn)
Re: Parallel leds driver problems

Figure 7 in the datasheet shows the light output is almost linearly proportional to the current. And figure 8 shows a 30% drop in light output for a change in temp from 20 to 100 deg C. These LEDs are small and dissipate 2.5 watts and they will run hot. Thermal design is important and they will need to be soldered to a printed circuit board that is designed to get the heat out. A 1 inch square of copper top and bottom with therma vias will have the LED running about 100 deg C above ambient. Blowing air across will reduce the temp, or small heatsinks can be attached on the back of the PCB.

Given the change in light output with temperature I think holding the current within +/5% over the range of LED voltage (2.2 to 2.9 volts) will be good enough. To do this the current source impedance needs to be at least

(0.4V)/(0.05A) = 8 ohms per LED.

This means a change in LED voltage of 0.4 volts (from lot-to-lot) will cause a change in current of 0.05 amps. For a string of four LEDs the source impedance needs to be 4 x 8 ohms = 32 ohms. The maximum voltage needed for four LEDs is 4 x 2.9V = 11.6 volts. Let's call it 12 volts. So, for a simple 32 ohm series resistor at 1 amp the power supply must be 12V + 32V = 44 volts. A common 48 VDC power supply will do the job. This is simple but inefficient burning 48 watts to run four LEDs. For the seven strings the power is 7 x 48 W = 336 watts.

An active current source can do the job with a 12 volt power supply and the power burned is then only 84 watts. If you want an AC-DC power supply there are many for around \$100. I can post the active current source circuit if you'd like.

To successfully use these LEDs you will need to have a PCB designed for LED mounting and thermal management, and to place the active current regulator circuits. Without heatsinks on the back of the LEDs the PCB should be mounted in a box with 10 CFM or more of forced air flow.

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