Current Regulators
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Current Regulators


The current regulators from LED Dynamics (Vermont), originally designed for Lumiled’s Luxeon LEDs, come in two families.

Buck regulators convert a DC input voltage to a constant current output. The input voltage can be as high as 32 VDC. The output voltage, determined by the total forward voltage drop of LEDs connected in series, can be as high as the input voltage less 2 VDC. Nominal current values of 350, 700 and 1,050 mA are available. Options include internal current adjustment and external strobing. The strobe input is used to change the duty cycle of the current pulses to achieve dimming.

Boost regulators also convert a DC input voltage to a constant current of 350 mA. However, the load voltage must be at least 2 VDC higher than the input voltage. Strobing is also an option.

The supply for all regulators used here is 24 VDC. That means, for load voltages up to 22 VDC, buck regulators are applicable and boost regulators can supply load voltages from 26 VDC to 48 VDC.

These regulators come in a very small package (less than one inch square) and are very efficient (> 90%), hence the trade name of BuckPuck and BoostPuck. This makes circuit board mounting possible without any thermal considerations.

The light fixture in this project has 8 RGB LEDs, each having a red, green and blue LED chip set. It would have been nice if the voltage drops were the same for each color. That not being the case, we have to resort to a rather complex wiring of the LEDs. The table below shows a possible solution using the least number of current regulators.

 

Red Channel

Green Channel

Blue Channel

 

V

I

 

V

I

 

V

I

 

Each

4.5

350

 

6.7

350

 

7.6

350

 

Total

36.0

 

 

53.6

 

 

60.8

 

 

2 x 4 parallel

18.0

700

Buck 700

 

 

 

 

 

 

2 x 3 parallel

 

 

 

20.1

700

Buck 700

 

 

 

2 in series

 

 

 

13.4

350

Buck 350

 

 

 

2 x 3 parallel

 

 

 

 

 

 

22.8

700

Buck 700

2 in series

 

 

 

 

 

 

15.2

350

Buck 350


The 8 LEDs can be driven, using 5 buck regulators. Having two 3 or 4-LED chains in parallel  is no problem since the statistical distribution of the voltage drops works in our favor for proper current sharing. Originally, another arrangement using a mixture of buck and boost regulators presented some problems. The strobing response is different between buck and boost regulators which causes a color change as the LEDs are dimmed.

The one voltage (22.8 V) is too high for a 24 V supply voltage. However, when the circuit was assembled, the actual voltage was only 20.3 V which is within the range of achievable output voltage from the regulator.

 

Pucks


Circuit Board for Current Regulators

 

Pucks102


Assembled Current Regulators


The resistors visible on the circuit board have a value of 1 Ohm with a 1% tolerance and are in series with the LED strings. Measuring the voltage across these resistors lets us determine the current. Note that some of the current regulators are adjustable internally by a trimmer potentiometer accessible through the board with a small watchmaker’s screwdriver. The resistors will be bypassed with jumpers after the adjustment.

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