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Step 6Circuit operation
To help understand the component selection in the next step, lets first review the circuit and how it works. we'll also review the maximum limits of the controller.
the main component of the circuit is the voltage regulator (either LD1585CV or LM1084-ADJ, they are nearly identical in function). the voltage regulator is a 3-pin device that allows setting a fixed output voltage using two resistors. you can review the standard circuit on page 3 of the technical datasheet for the regulator.
but we want a variable output, controlled by a dial! so all we do is to replace one of the two resistors that sets the output voltage with a variable resistor, or potentiometer. actually though, we use a resistor and potentiometer in series, because we only want to adjust about 1/2 the voltage range. so in the circuit, R2, R4, and R1 are the resistors that set the regulator's output. the regulator output is approximately proportional to: (R2+R4) / R1. so with a fixed R1, we increase R4 to increase the output voltage, and decrease it to decrease the output voltage. in the next step we'll go into the details of selecting these values.
what else? we need a current limiter. this is an important feature for driving LED's. the current through an LED is very sensitive to the voltage - if you increase voltage across the LED, current can increase a lot. this makes it very easy to accidentally put too much current through the LED, which can either burn out the LED or the power supply.
another reason it is easy to put too much current through an LED is that the voltage drop of an LED goes down when the LED heats up. reducing the voltage drop means more current at the same voltage. and putting more current through it causes it to heat up more! this phenomenon is called: "thermal runaway", and it affects both LED's and transistors.
R3 and Q1 form the current limiter part of the circuit. how does it work? R3 is the "sense resistor". all the current through the LED's goes through the R3 also. the transistor Q1 will turn on when the voltage across R3 (which is proportional to the current through it) reaches about 0.55 volts. when Q1 starts to turn on, it will act like a new, small-valued resistor R2, and thus the output voltage of the regulator will be reduced. reduced output voltage will reduce the current through the LED's and R3, and the transistor Q1 will start to turn off. this is a "feedback loop", and it happens so rapidly as to create a smooth balance point right at the desired current limit. how do we choose our maximum LED current? by choosing the value of R3 that turns on Q1 when our maximum desired current is reached.
- what are the limits of the controller?
- the voltage regulator can handle up to 28V input, and 5 amps current (although you'll need a big heatsink on it to do that much current).
- the regulator will function below 5V input, so that's low enough that you could power just 1 LED if you wanted.
- this is a simple analog controller, so if you need a scientifically reproduceable color combination, this light isn't going to do it. in fact, using this controller the colors are going to shift a teeny bit as the lamp heats up, although once it is warm things should be pretty consistent.
- C1 and Q1 are not very particular. for C1, you need at least a 35V rated capacitor, with between 10uF and 100uF. for Q1, pretty much any NPN transistor will work, although the one i specified has more consistent thermal performance than most (which will keep your current limit setting closer to what you expected if the inside of the controller box heats up).
- there are a lot of other "linear voltage regulators" that could be used here, the ones i listed have the best overall combination of low cost, flexibility, efficiency and power handling.
the main component of the circuit is the voltage regulator (either LD1585CV or LM1084-ADJ, they are nearly identical in function). the voltage regulator is a 3-pin device that allows setting a fixed output voltage using two resistors. you can review the standard circuit on page 3 of the technical datasheet for the regulator.
but we want a variable output, controlled by a dial! so all we do is to replace one of the two resistors that sets the output voltage with a variable resistor, or potentiometer. actually though, we use a resistor and potentiometer in series, because we only want to adjust about 1/2 the voltage range. so in the circuit, R2, R4, and R1 are the resistors that set the regulator's output. the regulator output is approximately proportional to: (R2+R4) / R1. so with a fixed R1, we increase R4 to increase the output voltage, and decrease it to decrease the output voltage. in the next step we'll go into the details of selecting these values.
what else? we need a current limiter. this is an important feature for driving LED's. the current through an LED is very sensitive to the voltage - if you increase voltage across the LED, current can increase a lot. this makes it very easy to accidentally put too much current through the LED, which can either burn out the LED or the power supply.
another reason it is easy to put too much current through an LED is that the voltage drop of an LED goes down when the LED heats up. reducing the voltage drop means more current at the same voltage. and putting more current through it causes it to heat up more! this phenomenon is called: "thermal runaway", and it affects both LED's and transistors.
R3 and Q1 form the current limiter part of the circuit. how does it work? R3 is the "sense resistor". all the current through the LED's goes through the R3 also. the transistor Q1 will turn on when the voltage across R3 (which is proportional to the current through it) reaches about 0.55 volts. when Q1 starts to turn on, it will act like a new, small-valued resistor R2, and thus the output voltage of the regulator will be reduced. reduced output voltage will reduce the current through the LED's and R3, and the transistor Q1 will start to turn off. this is a "feedback loop", and it happens so rapidly as to create a smooth balance point right at the desired current limit. how do we choose our maximum LED current? by choosing the value of R3 that turns on Q1 when our maximum desired current is reached.
- what are the limits of the controller?
- the voltage regulator can handle up to 28V input, and 5 amps current (although you'll need a big heatsink on it to do that much current).
- the regulator will function below 5V input, so that's low enough that you could power just 1 LED if you wanted.
- this is a simple analog controller, so if you need a scientifically reproduceable color combination, this light isn't going to do it. in fact, using this controller the colors are going to shift a teeny bit as the lamp heats up, although once it is warm things should be pretty consistent.
- C1 and Q1 are not very particular. for C1, you need at least a 35V rated capacitor, with between 10uF and 100uF. for Q1, pretty much any NPN transistor will work, although the one i specified has more consistent thermal performance than most (which will keep your current limit setting closer to what you expected if the inside of the controller box heats up).
- there are a lot of other "linear voltage regulators" that could be used here, the ones i listed have the best overall combination of low cost, flexibility, efficiency and power handling.
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First of all, this is a really cool lighting project you have here. I am fairly new to all of this so I have a few question. I want to take your analog approach to control just one high power RGB LED, this one in particular
http://www.lumex.com/pdf/SML-LX1610RGBW+A.pdf
Would i need a voltage regulator? Because im just want to get it to work hooked up to a dc power supply. The other question i had was, how will i have a feedback loop with this LED since accroding to the data sheet the RGB leds share a common node at the anode, the are in parallel. So what is best way to control the current going through each LED since RGB require different currents and voltages?
Pretty much can i implement your design for the just one of the LED i linked?
If yes, how so?
Thank you in advance.