Dimmable Moonlight driver from RapidLED Mod Author: Clifford Flanders

Disclaimer: As with any do-it-yourself project, unfamiliarity with the required tools and proper safety processes can be dangerous. This document and the information it contains should be construed as theoretical advice. The author of this document will not be held responsible for any injury, property damage, or any other way you manage to hurt yourself or others (physically or emotionally) by implementing the mods contained in this document due to misuse or misunderstanding of any information contained within.

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MOD THEORY Why modify the driver? If you don’t need to know, or don’t care, how the driver works, you can skip to the MOD INSTRUCTIONS section. The moonlight driver by RapidLED is designed to power 1 – 4 LEDs at 350mA constant current. This driver is nonadjustable, meaning the current cannot be reduced to dim the LEDs. So in some cases the full 350mA output may be too much (for example, in nano aquariums). This mod explains the theory behind the functionality of the constant current driver and how to modify it to allow the output current to be adjusted. The instructions in this document modify the control path, which means that rather than just “burning off” the extra power we do not need, we are actually controlling the driver’s output current directly. Anyone with basic soldering skills can modify their driver for only a few bucks.

How does the RapidLED constant current source work? LEDs are extremely sensitive to overcurrent; currents which exceed the LEDs specified limits will greatly reduce its useable life. A typical regulator (such as a common phone charger plugged into the wall) will maintain a constant voltage since the components in a phone are more sensitive to overvoltage rather than overcurrent (opposite of how LEDs are). Since LEDs are damaged quicker from overcurrent, a different type of regulator is required: a constant current regulator. The driver by RapidLED is built off of a linear constant current regulator design; where the term “linear” means there is no logic or clocks in the device. Linear devices tend to be cheaper to build at the cost of being less efficient than their non-linear counterparts. Like any constant current driver, its sole purpose is to maintain the designed output current regardless of the load at the output. This accomplished by increasing or decreasing the output voltage until the desired current flow is achieved. Nearly all linear constant current regulators use some sort of “sense” component (most commonly it is a resistor), which is used as a reference to measure the current flow. If we add a single LED to the circuit, the driver will adjust the voltage until the current across through “sense” resistor achieves its required setting. The addition of a second LED in the circuit will cause a voltage drop, so the driver will “sense” the decrease in the sense resistor, and will increase the voltage until the current again reaches its required setting. Figure 1 below gives a simulation based on the moonlight driver from RapidLED. A single Blue LED is connected to the output. Notice that the driver output is only enough voltage (about 4V) to produce 350mA of current through the circuit. The Vsense voltage measurement is used in the control loop of the driver, if this value decreases (meaning a “heavier” load is attached), the output voltage increases until roughly 1.085V is across the sense resistor, Rsense. When this value is reached, according to Ohm’s Law, it shows that 350mA is flowing through the 3.1ohm Rsense resistor (0.350A = 1.085V / 3.1ohms).

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LED1

8.0V

LED_Blue

6.0V

Power_out

4.0V

Green Terminal Block The driver attempts to maintain 1.085V across the Rsense resistor at Vsense all times. 0.350A = 1.085V / 3.1ohms

SEL>> 2.0V V(Power_out) 600mA

400mA

200mA

Rsense 3.1

0A 0s

0

50ms

100ms

- I(Rsense) Time

Figure 1 Figure 2 below shows the same driver, except this time with a second LED in the circuit. As you can see in the simulation, the output voltage has increased to accommodate the second LED; however, the current remains the same 350mA as before. The driver has increased the voltage to maintain 1.085V across its sense resistor (Rsense).

LED1

LED2

LED_Blue

LED_Blue

12V

8V

Power_out

4V

Green Terminal Block The driver attempts to maintain 1.085V across the Rsense resistor at Vsense all times. 0.350A = 1.085V / 3.1ohms

Rsense 3.1

SEL>> 0V V(Power_out) 600mA

400mA

200mA

0A 0s

0

50ms

100ms

- I(Rsense) Time

Figure 2 Note that there are limits to what the driver can produce; the moonlight driver by RapidLED has a maximum output voltage of 12VDC, so once the load on the driver becomes too great, the output

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is limited to 12VDC: the 350mA output current can no longer be maintained. Figure 3 shows the simulation using 4 LEDs; the output current drops to about 160mA since the driver cannot increase the output voltage beyond 12VDC. This is why RapidLED claims that no more than 4 LEDs should be connected.

LED1

LED2

LED3

LED4

20V

LED_Blue

LED_Blue

LED_Blue

LED_Blue

15V

Power_out

10V

Green Terminal Block The driver attempts to maintain 1.085V across the Rsense resistor at Vsense all times.

SEL>> 5V V(Power_out) 300mA

0.350A = 1.085V / 3.1ohms

200mA

100mA

Rsense 3.1

0A 0s

0

50ms

100ms

- I(Rsense) Time

Figure 3 Now that we understand how the driver produces a constant 350mA current, we can modify the circuit to be adjustable. Since the driver relies on the value of Rsense as its basis for controlling the current flow; if we modify the value of Rsense, then we can control the current flowing. The driver’s main goal is to maintain 1.085V across its sense resistor, so if we add resistance to the Rsense value, then the current flowing is more restricted (meaning less current flows through the LED loop as well). Figure 4 shows a two LED light with a control resistor (Rcontrol) added in series with the sense resistor (Rsense). The output current is now about 10.5mA since the voltage across both Rcontrol and Rsense together is still 1.085V. Using Ohm’s law we can verify the output current to be 1.085V / (Rsense + Rcontrol) = 10.5mA.

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LED1

LED2

LED_Blue

LED_Blue

10V

Power_out

5V

Green Terminal Block The driver attempts to maintain 1.085V across the Rsense resistor at Vsense all times.

SEL>> 0V V(Power_out) 16mA

12mA

Rcontrol 100

8mA

Rsense 3.1

4mA 0s

0

50ms

100ms

- I(Rsense) Time

Figure 4

Simply adding a resistor allows us to reduce the output current of the driver. If a different current is desired, you can manipulate Ohm’s law to give you the formula: Rcontrol = (1.085V / desired_current) - Rsense Rcontrol = (1.085V / desired_current) - 3.1 Example: (10.5mA output) Rcontrol = (1.085V / 10.5mA) – 3.1 Rcontrol = 100.2 ohms

To make the current adjustable with a turn-knob, we must add a potentiometer. Figure 5 shows the addition of a potentiometer in parallel with the Rcontrol resistor. A 1k potentiometer was selected as a starting point; too large of a potentiometer resistance value will make the adjustments more and more sensitive. As you can see in the simulation, turning the potentiometer from one end to the other controls the output current. The max range is limited by the Rsense resistor and the min range is determined by the value chosen for Rcontrol. The purpose of the potentiometer is to allow current to bypass the Rcontrol resistor. As the potentiometer is turned all the way down, all of the current flows past the Rcontrol, essentially removing it from the load path; this means that the current flow is only through the Rsense. In this case the circuit is essentially the same as the original design (1.085V now flows only through 3.1ohms which produces about 350mA of output current).

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Note that the Rcontrol resistor and wires connected to the driver are connected to the center pin of the potentiometer and one of the outer pins. P2 LED1 1k Pot

LED2

C Rcontrol 100

LED_Blue LED_Blue 1.0A

Power_out

P1

Green Terminal Block The driver attempts to maintain 1.085V across the Rsense resistor at Vsense all times.

100mA

Rsense 3.1

10mA 1.0

0

10

100

1.0K

- I(Rsense) Rpot

Figure 5

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MOD INSTRUCTIONS So how do we perform this mod on the RapidLED moonlight kit? Using the knowledge from this document, we now know we must add an Rcontrol resistor (100ohm) and a potentiometer (1kohm) in series with the Rsense resistor. Tools Required: Wire cutters Soldering Iron with Solder 100ohm resistor (1/8 Watt or greater) 1k potentiometer (1/8 Watt or greater) Pair of wires to connect the potentiometer / resistor to the driver (as long as you will need) Shrink tubing Small screwdriver or similar tool to pop off the driver cover (if necessary) Procedure: 1. First, carefully remove the cover of the driver. It may come off just by pulling on it with your hands (a screwdriver may be required to press the little tabs in on the sides). The cover should not come off forcefully. 2. You will see a resistor right next to the green terminal block where you connect your LEDs to the driver (you can see it before you even remove the cover). See Figure 6.

Figure 6

3. We want to add our resistor / potentiometer in series with this resistor. So we must first carefully cut the lead off the resistor (leave enough space at the top of the resistor to provide a sufficient solder joint). Try not to bend the resistor wires too much so they do not become fatigued and break!

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Figure 7

4. Pre-tin two wires (Pre-tin means to put some solder on the ends of the wires before trying to solder anything to them, this makes soldering parts together easier!); then carefully solder the two wires to the end of the resistor and the lead going to the circuit board (be careful not to bend these too much so you do not fatigue the joint at the board). Place a piece of shrink tubing over the lead going to the board to prevent any inadvertent shorting. (Optional) You may use hot-glue to help secure the resistor and wires so they do not break

Figure 8

5. Run the wires outside the cover of the driver and put the cover back on the driver (be careful not to tug on the wires too much and stress the internal components). Use a piece of tape to secure the wires down.

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Figure 9

6. Attach the potentiometer to the other end of the wire. Connect one of the wires from the driver to the center tab and the other wire to one of the ends of the potentiometer (it does not matter which wire from the driver goes to which tab on the potentiometer as long as one is connected to the center pin). Also solder the 100 ohm resistor between the two tabs you selected to solder the wires to; this will allow the potentiometer to bypass the 100ohm resistor and adjust the output current of the driver.

Figure 10

7. You can now test the driver, connect your LEDs according to the instructions by RapidLED, plug it in and verify you can adjust the driver via the potentiometer. You will now have an adjustable moonlight driver via turn-knob!

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Questions / Comments: For more information or to check out other mods and projects visit my website:

https://sites.google.com/site/projectsbycliff/ Feel free to contact me via email with questions or comments about the design or implementation at [email protected]

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8/7/2012