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How do I choose a constant current driver?!

Michael Houlder

 

Scratching you head about how to choose a driver for your project? Don't worry you are not alone! 

At Future Eden we often get asked about how to select the correct driver for a particular project so we thought we would outline a few steps to point you in the right direction. Happily it is not a tricky as you might think!
PLEASE NOTE : Before we begin that this guide is for deciding upon a constant current driver such as we sell here and is not relevant for use with normal DC constant voltage supplies. 
 

Step 1 - Check you Specs! 

In order to work out your driver you will need a few pieces of information first in order to calculate the correct driver choice. 

You will need the following, which can be found on the datasheet that accompanies your LEDs. 

  • the max forward voltages of all the LEDs in your array
  • the max forward currents of all the LEDs in your array
  • Thats it! 

 

Example:  

In order to make this simpler we will use an example from our popular range of Bridgelux Cool White 3W LEDs and our popular range of drivers. 

In our example circuit we are going to use 10 x LEDs. We know by looking at the datasheet for these LEDs that the max forward voltage for these components is 4 volts. Likewise we also know that the max forward current is 750mA. Great lets move on! 

 

Step 2 - Add up the Forward Voltages

Once you have this information you need to work out how many LEDs you plan to run in your series circuit. It is then simply a case of adding up all of the max forward voltages in the circuit and making a note of this number. 

 

Example:  

In our example the total forward voltages of 10 x Cool White 3W LEDs is 10 x 4v = 40 volts. 

 

Step 3 - Choose the Driver Current

To this you find the lowest max forward current of the LEDs in your circuit. You then make sure you select a constant current driver with an output current lower than the max forward current, thus avoiding overloading the LED. 

It is worth noting here that that the closer to the max forward current of the LEDs the more heat the diode will produce, hence it is never recommended to run any power LEDs at maximum power. 

Example:  

In our example the max forward current of Cool White 3W LEDs is 750mA. As such we need to use a constant current driver that is rated at 750mA or below. Given that we don't want to produce lots of extra heat for a disproportionate increase in light we will choose 600mA constant current driver. 

 

Step 4 - Find your Driver 

Having decided what current driver you are going to use you now find one with a specification that fits the voltage figure for the circuit you worked out earlier. 

In order to do this you just need to understand how constant current drivers work : 

Basically constant current drivers produce a voltage between a given range, whilst retaining a 'constant current'. The voltage that the driver puts out is dictated by what is connected to the circuit. However what is important is that the voltage will never go higher or lower than the range. For example our 28W 600mA driver has a voltage range of 30 - 50v. Hence the driver will always put out a minimum of 30 volts and if lots of LEDs are added in series to the circuit it will increase the voltage automatically until 50v is reached.

 

Example:  

Having decided that we are going to use a 600mA driver and we know that the total forward voltages of the LED in our circuit is 40v, we need to find a driver where the 40v is well within range of the voltage output of the driver. 

If we consider 28W 600mA driver mentioned above we can see that the output range of the driver is 30-50v. As such our total 40v will sit happily within this range so this driver will be a good choice for powering our array of LEDs. 

TIP - It is worth point out that its advisable to run your drivers about 15% below the max voltages. Whilst they will happily run at these voltages running them more gently will prolong their life considerably. 

 

 

So thats it, not as tricky as you thought!?? Whilst this is something of a simplification of the process as you will be getting different voltage values at different currents, this should outline the basic steps you need to start thinking about using constant current drivers and high power LEDs. 

 

We hope this has been some use. Keep watching this space for some other commonly asked questions and some step by step solutions to send you projects into the stratosphere!

 

Thanks for reading and good luck!

 

Mickey  

Owner and Founder of Future Eden 

 

 

 

 

 

 

 

 

 

 

 



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