1.The theory behind the LED Resistor Selector Dial
The Theory Behind this make
LEDs are diodes with very specific characteristics. The most important values that must be followed are the (maximum) forward voltage [Vf] and the (maximum) forward current [If]. Typically, an LED powered with a voltage equal to Vf, will allow a current equal to If to pass through. The forward voltage of LEDs depends on the material they are made of (thus the color that they emit) and the number of diodes in series they have. For example, a red LED typically operates at 20mA when a voltage of 2.2 volts is applied across its leads.
So, what happens if we have 5 volts to power this 2.2V LED? There are many different LED drivers, but the simplest one is to use a resistor in series. Here is the typical circuit:
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The idea of the resistor is to generate a voltage drop across its leads. Byselecting this resistor properly, the voltage across the LED will be as much as required. To select the proper resistor value, we need first to know how much voltage we need to drop on this resistor. Suppose that the power supply has a voltage of 5 volts (Vdd) and the red LED needs 2.2 volts to operate at 20mA. The voltage across the resistor must then be:
VRES = VDD - VLED => VRES = 5 - 2.2 => VRES = 2.8 Volts
Now we can simply use the Ohm's law to calculate the resistor value. The idea is that we want a specific voltage drop (2.8 volts) at a specific current (20mA) (remember to convert the current to Amperes):
R = VRES / If => R = 2.8 / 0.02 => R = 140 Ohms
So, the resistor will be 140 Ohms! When power is provided to the above circuit, the current will climb up to 20mA. At that point, the voltage across the resistor will be 2.8 volts, so the remaining voltage (2.2 Volts) will be applied to the LED.
One more important value that has to be calculated is the power dissipation on the resistor. As you know, resistors come n different rated power values, for example 1/4 watts (250 mWatt), 1/2 watt (500 mWatt), 1 Watt etc. This value indicates how much power can be dissipated on the resistor in the form of heat. We therefore need to know this value for our circuit in order to select a properly rated resistor. The formula to calculate the power is this:
PRES = If2 x R => PRES = 0.022 x 140 => PRES = 0.056 Watts => PRES = 56 mWatts
Therefore, a typical 1/4 or even an 1/8 watts resistor is sufficient to dissipate the 56 mWatts of power.
2.Make your own LED Resistor Selector Dial.
Download fileLED Resistor Selector Dial
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| Download the PDF file from the above link | Print the PDF file. Make sure that you print it with the actual size | This A4 is what you want to print | Cut the paper in half. DO NOT cut the dashed line |
Now you should have two pieces of paper. The left side with the two big wheels is the rotating wheel, the right side with the two smaller wheels is the fixed wheel. Let's make first the rotating wheel...
Making the rotating wheel
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| Put a ruler across the dashed line | Bend the paper accros this dashed line | Now bend it back to back | If the disks are properly aligned, then the pin will go through the center marks! |
You need to make sure that the two disks are properly aligned before you go on to with the next step. Make sure that the pin goes through the center marks of the two disks.
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| Apply glue to one side and glue the two disks together | Let the glue dry for a while, remove the pin and trim the dashed line away | Then apply glue to the rest area and glue the two disks together | This is what you want to make at the end |
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| Let the glue cure for a while and then trim the paper around the disks | If the disks were properly aligned, then the disks are properly trimmed around its perimeter | Make a wider hole at the center ad you're done with the rotating disk |
Let's make the fixed wheel
Now let's make the fixed wheel.
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| First, trim the wheels. Notice that the wheels are NOT separated! | Then remove the windows from the wheels | Now bend the wheels across the dashed line. Use the pin to align them perfectly as before | Use a thicker nail to make the hole wider. |
Putting everything together
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| Get the 2 disks and place them side by side. Make sure that the wheels show the same face (resistor power or value). | Now put the rotating wheel between the fixed disk like a sandwich. |
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| Now you wanna use Y-shaped nails to fix the dial selector together | Put one of these nails through the holes at the center | Push it all the way through | And then bend the two legs of the nail. |
How to use the LED Resistor Selector Dial
You can hold the selector with one hand from the rectangular piece that extends on the left side of the fixed circle. With your other hand you can hold and rotate the rotating wheel:
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The fixed wheel (the smaller one) has the LED voltage. The rotating wheel (the larger one) has the supply voltage. You wanna match now the supply voltage with the LED voltage by rotating the bigger wheel. For example, if the supply voltage is 5 volts and the LED voltage is 2.2 volts (typical for red LED), this is how you should align the wheels:
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You are now ready to get the resistor value. From the "Resistor Value" window you can read the value for 3 different current rates: 10, 20 and 30mA:
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As you can see, for 20mA current you should select a resistor around 140 Ohms...
Finally, flip to the other side to read the power that needs to be dissipated on the resistor:
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The power is between 55 and 60 mWatts..
More LEDs in series???
What if you want to use one resistor for 5 LEDs in series? How can you use this tool to calculate the resistor value? Simply, add all the forward voltages of the LEDs and use this value instead. If for example you want to connect 5 cool white LEDs in series and each LED has forward voltage 3.3 volts, then the total forward voltage will be:
Vf_total = 5 x 3.3 = 16.5 Volts
Now, if the supply voltage is for example 22 volts and you want to allow 30 mA of current ...
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| 22 power supply aligned with the 16.5 volts required for the 5 LEDs in series | For 30mA, the proper resistor is 180 Ohms | 170 mWatts is approximately the power that needs to be dissipated on the resistor |
1.LED driving and controlling methods
there are many people who would like to know more about driving and controlling LED lights, and second because i was provided an excellent LED driver chip from Farnell for test, and i wanted to put it under the microscope. So i will place this chip against some other LED drivers to see how good it is.
The chip that I'm talking about is the A6210 from Allegro Microsystems. It is a Buck-Regulating LED Driver able to drive up to 3A load with constant current, with switching frequencies up to 2 MHz and supply voltage from 9 to 46 volts. It has an optional PWM input to control the brightness of the LED. The sense voltage is down to 0.18 volts for higher efficiency.
The previous description may sound Greek to you, but after reading this tutorial you will be able to design your own LED driver. Special thanks to Farnell and Element14 for offering three of these chips for test.
You can now start reading the theory pages:
The simplest method to drive an LED
Driving more than one LEDs with constant current
Driving more than one LEDs with constant current regardless of the supply voltage
A more efficient system to drive LEDs with constant current regardless of the supply voltage
After having explained the basic methods to drive LEDs with constant current, i explain how can someone modify these circuits to control their brightness
