Figure 6 shows how splitting up the resistors works with different LEDs. These drivers offer much convenience that a resistor driven circuit does not. It is important to note, however, that these drivers are still using linear technology. It is crucial to understand how much power the driver will be dissipating and to make sure it falls in a safe range.
While these drivers add a lot of conveniences, they are no more efficient than using a standard LED-resistor circuit. Figure 3 shows an example of a linear eight strand LED driver. The driver is controlling three strands of the same LEDs from the previous circuit. Temperature limits the maximum power the driver can dissipate. With a maximum temperature of c, it can dissipate around 1. The chip is also limited to a maximum of 70mA per strand.
If the power was too high, an option is to place a resistor on each strand. As long as the resistor is sized correctly, it will dissipate some of the power, while the chip dissipates the rest. That trick shown in figure 4 is quite useful when there are unbalanced strand lengths. The chip controls 8 LED strands, with one strand being much shorter than the rest.
Two R resistors balance this strand by dissipating some of the excess heat. A switch-mode constant current LED driver operates similarly to a linear driver, except it uses a switching topography. A significant disadvantage of switching drivers is that they tend to be expensive. Having any switching supply onboard also introduces unwanted switch-mode noise.
Under ideal conditions, you can drive a strand of LEDs at 1A, while dissipating less than a tenth of a watt from the chip! There are also buck-boost drivers that take 5V for example as an input and can drive LED strands up to 20V. They tend to not be as efficient as a drop-down regulator, but it is still an option to consider. We have used an unusual method of driving LEDs with great success. It combines the linear drive and switch-mode drive to offer the benefits of both. It is especially useful when there are a lot of different LED colors.
For example, say we have LEDs to drive at 1 amp each, with 5 different colors. The input power supply is 24VDC, with separately controllable colors.
Yes, this is an extreme example — both in the power required, and in the number of LEDs — but we recently designed a board similar to this! An issue that arises with this approach is that at this high of power, switching drivers will only be able to drive a single strand each.
That means we would require a lot of drivers on this board. Let us first calculate the suitable value of series resistance. Now the power rating of this resistor needs to be calculated, as it determines the amount of power it can dissipate. I hope you have gained some ideas about designing the driver circuit.
I would love to see any new creative ideas on the same in the comment section below. Also, give your feedback on the article design above, so as we can keep on improving on the quality for you. Can you make a circuit on a 5w led driver circuit which can charge a battery on parallel and glows the light when there is no power supply? John you can try it with a capacitor, as it stores energy and supplies the same when supply is OFF. Can this driver be use to drive 80 LED for example?
Well, I appreciate your interest here. Thank you. Thank you so much for the circuit sir. After a long days I got a very reliable circuit.. I have two questions on this, please reply. If LEDs will be in parallel the current across each led will be divided. Will the glow reduce? The car have a from 12V to Led foward voltage is 6 — 7V, foward current is mA.
I need the circuit diagram for AC driver circuit for 5W led light. Can you help me with it please.. So as we learned from the video on constant current sources, a good, stable, low noise current source will maintain a constant current regardless of the load connected to its output! Voltage sources bench-top power supplies ramp voltage at turn-on, but the current is not controlled. This is not good for diodes which require a constantly controlled current.
A change in resistance on a constant voltage source results in a change in current. At a very general level, there are a few classes or "types" of laser drivers which you will hear commonly discussed. Constant current is just what it states, a constant output level over time, say 30 mA, in theory forever if needed.
Pulsed laser diode drivers are an interesting variation in that the output is a function of time, duty cycle being the best way to describe it.
Duty cycle is the time the current source is on - output current high divided by the total time of the pulse on and off time. A quick note about off times in current sources, they are never truly off meaning zero current , but often are at an output level low enough where the laser diode output is minimal - well below threshold.
The next section loosely defines low and high power versions of these types of drivers. The data sheets will usually also state output current magnitude and voltage, you just have to look for it. A low power driver is roughly defined as 1 mA to 5 Amps. Their are kW level drivers available in pulsed and QCW mode versions. Wavelength Electronics has an excellent video describing their current source designs. It is good information in block diagram form and easy to understand. The information presented in this video will apply to most all commercially available laser diode current sources, differences in function and features will dictate the performance and surely price.
Of course you can go much, much deeper in your understanding of laser current sources. A quick search of laser diode current sources on YouTube will result in a multitude of build your own current sources. It is highly detailed, contains great schematics for those who understand electrical design with mathematically backed design principles and the performance backed up by data and graphs. Here is quick review of the basic package styles and price ranges of constant current and pulsed current sources which are commercially available.
These are typically lower power and basic current sources providing 10 mA — mA. You will find these in your DVD player, bar code scanners, pointers etc. They are available in a wide range of output current ranges, 50 mA — Amps. The only connections to it are the AC input and the output to the laser diode load. These are available in pulsed and CW modes from mA to A or more.
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