Print PDF
Favorite
Email
Step 7Selecting resistors and dials
the exact resistor values you use depend on the number and type of LED's and the power supply voltage.
Selecting R3 sense resistor
R3 is the "sense resistor" for the current-limiting circuit. select the value of R3 as follows: R3 = 0.55 / (LED string current). so for my circuit I have R3 = 0.55 / 0.8 = 0.69 ohms. resistors don't come in every possible value, the closest match was 0.68 ohms. you also need a resistor that can handle a bit of power, use this guideline:
LED string current:
less than 200mA: standard 1/4 watt resistor, search digikey for: "yageo CFR-25JB"
less than 1200mA: 2 watt resistor, search digikey for: "panasonic ERX-2SJR", they go down to 0.47 ohms.
1200mA to 5000mA: use a couple of 2-watt resistors in parallel, or else try: "huntington ALSR-5". (the limit for the voltage regulator we are using is 5 amps).
note that the current-limit is not especially accurate, using just the calculation it might be 15% off. if you need it more accurate, you will need to build your circuit and then test a couple of resistor values. note that the current limit will decrease as the temperature of Q1 goes up - that's a feature!
Selecting R4 control slider/dial
First choose if you want a slider or dial! this is your user interface for your controls. It's less work to make a control panel using dials, because you just drill a hole for them, but you might prefer the look & feel of the sliders. I went with the sliders, but you'll see how much of a hassle they were later. any dial or slider between 2k-ohm and 10k-ohm will work. a 2k-ohm is the best because it allows precise calculation of the output voltage without testing. for sliders, i was only able to find 10k-ohm sliders which required a bit of trial-and-error to get the output voltages correct.
Slider: search digikey for: "panasonic EWA-Q12C15B14", a 10k-ohm linear taper slider
Dial: search digikey for: "CTS 270x232a", a 2k-ohm to 10k-ohm linear taper dial w/o switch
Setting R1 and R2 control range resistors
The combination of R1, R2 and R4 set the output voltage (= LED brightness) of the regulator. the output voltage is approximately equal to: 1.25 * (1 + ((R2+R4) / R1)). This becomes less accurate with larger R values. With a 2.5k dial for R4, the formula will be very close, with a 10k dial, you'll need to hand-test your R1, R2 values. below i work through two examples: one with 10k slider and one with 2.5k dial.
We need high-precision resistors for R1 & R2 (that means 1% accuracy rather than the common 5% accuracy). Search digikey for: "yageo MFR-25FBF" which will show all the available values.
So how do we use that output voltage formula? we want to choose our values of R1 & R2 so that when the dial is all the way to the left the LED's are just turned off, and so that when the dial is all the way to the right the LED's are at maximum power. To do that we need to know the voltage across the LED's where they are just turned off, and the voltage across them at our maximum set current. You can refer to the LED spec sheets from the manufacturer, or to my Power LED Data Handy Reference Chart where i've listed some typical working values.
Example 1: R1 & R2
here's the calculation for my circuit using values from the 'handy chart' above.
for 5 x red LED, our "turn on" is at 5 x 1.8 = 9V, and maximum power is at 5 x 3.8 = 19V.
for 5 x green or blue, "turn on" is at 5 x 2.4 = 12V, maximum power at 5 x 3.8 = 19V
using the regulator's output voltage formula to calculate R1 and R4 for the red led's using my 10k-ohm slider (which is 0 ohms at turn-off and 10k-ohms at maximum power):
Vout = 1.25 * (1 + ((R4+R2) / R1))
9V = 1.25 * (1 + ((0 + R2) / R1))
19V = 1.25 * (1 + ((10,000 + R2) / R1))
solving the two equations gives:
R1 = 1250 ohm
R2 = 7750 ohm
using the 10k-ohm slider there is a bit of error in the calculation compared to practice, so experimentation led to using the following values: R1 = 1.2k-ohm, R2 = 6.8k-ohm with a tested output of 9V to 20V
similarly with the green and blue (using 12V and 19V in the equations):
R1 = 1790 ohm
R2 = 15360 ohm
and experimentation led to: R1 = 1.8k-ohm, R2 = 15k-ohm with tested output: 12V to 19.5V
my power supply is 19.5V, so this is a good setup.
- the current-limit circuit will kick in at about 800mA, or at about 3.8V per LED, so at around 19V per LED string.
- the controller + current-limit resistor will drop a minimum of about 1.5V, so maximum LED voltage is actually limited to about 18V (3.6V each), the last bit of the dial's range won't do anything, and with this setup i'm not likely to hit my current limit under normal conditions (but if something overheats, it will save the day)
- this circuit will work just as well for a power supply with more than 19.5V, you just burn more power in the controller.
- if you use different-length strings of LED's, or you have a different dial, you will need to recalculate the resistor values.
Example 2: R1 & R2
Here's a second example using a 2.5k dial and strings of 3 Luxeon-3 LED's. here, the R1 & R2 calculations should be "right on". For this example we'll target "full-power" to be the spec-sheet maximum for the Luxeon-3's, which is 1000mA for the green and blue and 1400mA for the red.
first we check my handy-chart or the led spec sheet to see the turn-on voltage for the different led's, and the voltage needed at the maximum current. for the red led's: 1.8V - 3.4V per LED is the target range (no current to max current), and for the green and blue we target 2.4V - 3.9V per LED
calculating R1 & R2 for the Red channel with 2500 ohm R2:
(1.8 * 3)V = 1.25 * (1 + ((0 + R2) / R1))
(3.4 * 3)V = 1.25 * (1 + ((2500 + R2) / R1))
R1 = 650 ohm
R2 = 2160 ohm
for the Green & Blue channel:
(2.4 * 3)V = 1.25 * (1 + ((0 + R2) / R1))
(3.9 * 3)V = 1.25 * (1 + ((2500 + R2) / R1))
R1 = 694 ohm
R2 = 3303 ohm
lastly, we should point out that with a regulated 12V supply, we won't actually get the green and blue led's all the way to full power because the controller will drop 1.5V minimum leaving 3.5V at most for each LED. what are our options?
- leave it, who cares.
- switch to strings of 2 LED's rather than strings of 3. the led's will work great although the controller will need a decent heat sink since it will burn a lot more power.
- most 12V 4-amp supplies are going to be the regulated type (any computer supply for example), but on the off chance you have an un-regulated supply (it will have a large heavy transformer), you are probably fine. most un-regulated supplies put out well over their rated voltage.
- if you are handy, many computer supplies have a "trim" knob inside them that lets the factory set the output at exactly 12V. usually you can adjust the trim knob to bump the output up to 12.5 or 13V. if the case of the supply opens up with screws, it probably has a trim knob.
- you could bypass the current-limiter in our circuit (replace R3 with a wire). this will reduce the controller loss to 1.0V, and get your blue and green to about 3.7V each. is it safe? if you are sure you have a regulated 12V supply - yes. at 3.7V you're still enough below the rated maximum for the LED that random variations should not be large enough to exceed the true maximum. just don't do this for the red channel!
Selecting R3 sense resistor
R3 is the "sense resistor" for the current-limiting circuit. select the value of R3 as follows: R3 = 0.55 / (LED string current). so for my circuit I have R3 = 0.55 / 0.8 = 0.69 ohms. resistors don't come in every possible value, the closest match was 0.68 ohms. you also need a resistor that can handle a bit of power, use this guideline:
LED string current:
less than 200mA: standard 1/4 watt resistor, search digikey for: "yageo CFR-25JB"
less than 1200mA: 2 watt resistor, search digikey for: "panasonic ERX-2SJR", they go down to 0.47 ohms.
1200mA to 5000mA: use a couple of 2-watt resistors in parallel, or else try: "huntington ALSR-5". (the limit for the voltage regulator we are using is 5 amps).
note that the current-limit is not especially accurate, using just the calculation it might be 15% off. if you need it more accurate, you will need to build your circuit and then test a couple of resistor values. note that the current limit will decrease as the temperature of Q1 goes up - that's a feature!
Selecting R4 control slider/dial
First choose if you want a slider or dial! this is your user interface for your controls. It's less work to make a control panel using dials, because you just drill a hole for them, but you might prefer the look & feel of the sliders. I went with the sliders, but you'll see how much of a hassle they were later. any dial or slider between 2k-ohm and 10k-ohm will work. a 2k-ohm is the best because it allows precise calculation of the output voltage without testing. for sliders, i was only able to find 10k-ohm sliders which required a bit of trial-and-error to get the output voltages correct.
Slider: search digikey for: "panasonic EWA-Q12C15B14", a 10k-ohm linear taper slider
Dial: search digikey for: "CTS 270x232a", a 2k-ohm to 10k-ohm linear taper dial w/o switch
Setting R1 and R2 control range resistors
The combination of R1, R2 and R4 set the output voltage (= LED brightness) of the regulator. the output voltage is approximately equal to: 1.25 * (1 + ((R2+R4) / R1)). This becomes less accurate with larger R values. With a 2.5k dial for R4, the formula will be very close, with a 10k dial, you'll need to hand-test your R1, R2 values. below i work through two examples: one with 10k slider and one with 2.5k dial.
We need high-precision resistors for R1 & R2 (that means 1% accuracy rather than the common 5% accuracy). Search digikey for: "yageo MFR-25FBF" which will show all the available values.
So how do we use that output voltage formula? we want to choose our values of R1 & R2 so that when the dial is all the way to the left the LED's are just turned off, and so that when the dial is all the way to the right the LED's are at maximum power. To do that we need to know the voltage across the LED's where they are just turned off, and the voltage across them at our maximum set current. You can refer to the LED spec sheets from the manufacturer, or to my Power LED Data Handy Reference Chart where i've listed some typical working values.
Example 1: R1 & R2
here's the calculation for my circuit using values from the 'handy chart' above.
for 5 x red LED, our "turn on" is at 5 x 1.8 = 9V, and maximum power is at 5 x 3.8 = 19V.
for 5 x green or blue, "turn on" is at 5 x 2.4 = 12V, maximum power at 5 x 3.8 = 19V
using the regulator's output voltage formula to calculate R1 and R4 for the red led's using my 10k-ohm slider (which is 0 ohms at turn-off and 10k-ohms at maximum power):
Vout = 1.25 * (1 + ((R4+R2) / R1))
9V = 1.25 * (1 + ((0 + R2) / R1))
19V = 1.25 * (1 + ((10,000 + R2) / R1))
solving the two equations gives:
R1 = 1250 ohm
R2 = 7750 ohm
using the 10k-ohm slider there is a bit of error in the calculation compared to practice, so experimentation led to using the following values: R1 = 1.2k-ohm, R2 = 6.8k-ohm with a tested output of 9V to 20V
similarly with the green and blue (using 12V and 19V in the equations):
R1 = 1790 ohm
R2 = 15360 ohm
and experimentation led to: R1 = 1.8k-ohm, R2 = 15k-ohm with tested output: 12V to 19.5V
my power supply is 19.5V, so this is a good setup.
- the current-limit circuit will kick in at about 800mA, or at about 3.8V per LED, so at around 19V per LED string.
- the controller + current-limit resistor will drop a minimum of about 1.5V, so maximum LED voltage is actually limited to about 18V (3.6V each), the last bit of the dial's range won't do anything, and with this setup i'm not likely to hit my current limit under normal conditions (but if something overheats, it will save the day)
- this circuit will work just as well for a power supply with more than 19.5V, you just burn more power in the controller.
- if you use different-length strings of LED's, or you have a different dial, you will need to recalculate the resistor values.
Example 2: R1 & R2
Here's a second example using a 2.5k dial and strings of 3 Luxeon-3 LED's. here, the R1 & R2 calculations should be "right on". For this example we'll target "full-power" to be the spec-sheet maximum for the Luxeon-3's, which is 1000mA for the green and blue and 1400mA for the red.
first we check my handy-chart or the led spec sheet to see the turn-on voltage for the different led's, and the voltage needed at the maximum current. for the red led's: 1.8V - 3.4V per LED is the target range (no current to max current), and for the green and blue we target 2.4V - 3.9V per LED
calculating R1 & R2 for the Red channel with 2500 ohm R2:
(1.8 * 3)V = 1.25 * (1 + ((0 + R2) / R1))
(3.4 * 3)V = 1.25 * (1 + ((2500 + R2) / R1))
R1 = 650 ohm
R2 = 2160 ohm
for the Green & Blue channel:
(2.4 * 3)V = 1.25 * (1 + ((0 + R2) / R1))
(3.9 * 3)V = 1.25 * (1 + ((2500 + R2) / R1))
R1 = 694 ohm
R2 = 3303 ohm
lastly, we should point out that with a regulated 12V supply, we won't actually get the green and blue led's all the way to full power because the controller will drop 1.5V minimum leaving 3.5V at most for each LED. what are our options?
- leave it, who cares.
- switch to strings of 2 LED's rather than strings of 3. the led's will work great although the controller will need a decent heat sink since it will burn a lot more power.
- most 12V 4-amp supplies are going to be the regulated type (any computer supply for example), but on the off chance you have an un-regulated supply (it will have a large heavy transformer), you are probably fine. most un-regulated supplies put out well over their rated voltage.
- if you are handy, many computer supplies have a "trim" knob inside them that lets the factory set the output at exactly 12V. usually you can adjust the trim knob to bump the output up to 12.5 or 13V. if the case of the supply opens up with screws, it probably has a trim knob.
- you could bypass the current-limiter in our circuit (replace R3 with a wire). this will reduce the controller loss to 1.0V, and get your blue and green to about 3.7V each. is it safe? if you are sure you have a regulated 12V supply - yes. at 3.7V you're still enough below the rated maximum for the LED that random variations should not be large enough to exceed the true maximum. just don't do this for the red channel!
| « Previous Step | Download PDFView All Steps | Next Step » |
Thanks again for all the helpful instructables.
2.9V @ 700mA
3.35V @ 3A
I want to control it up to the max current. Seems you define R3 for a single current. Do I calculate that with the max (3A) current, giving a 0.2-Ohm resistor (closest value)? Thanks!
I'd like to construct the controllable driver with voltage regulator for use with (2) 10 watt LEDs; V= 23 volts, current = 1000ma. Cut off voltage =18 volts, max voltage= 23 volts. I'll use the 2.5K pot for R4. I need to figure values for R1&R2, but I am math/calculation deficient ,Any help will be appreciated.
your application sounds interesting, can you explain more? you want to have the light show increasing blue when the TV shows more blue, etc?