Should you be working on any LED-related circuit, you may have seen advice or alerts to always employ a current limiting resistor.
From the beginner DIY guy to those designing and constructing LED lighting circuitboards, we have created this guide to assist everyone grasp fully when, why, and how to choose the suitable current limiting resistor.
Knowing the I-V curve of LEDs
Designing a circuit around passive semiconductor components like LEDs requires knowledge of the I-V (current vs voltage) curve, as with any other such component.
Of course, an LED is basically a diode with a non-linear I-V curve. Put another way, the correlation between input voltage and input current does not follow a linear path.
For instance, consider the forward current at 2.7 V-roughly 20 mA. Raising the voltage to 2.8 V by 0.1 V causes forward current to rise by about 30 mA to 50 mA. Forward current rises by 35 mA to 85 mA if we then raise it to 2.9 V by another 0.1 V.
The rate of forward current rise also rises as voltage rises. Tiny variations in forward voltage can lead to extremely large forward current variations.
So, constant current LED drivers are the best way to drive LEDs since they run at one current and change their output voltage to match it, therefore guaranteeing the stability of the forward current. A current limiting resistor is not required under constant current input.
If you are running constant voltage power sources, what should you do?
On the other hand, constant current power sources are often more costly and less adaptable. Nearly all LED strip products and other modules thus run on constant voltage input.
Fixed in their output voltage level, constant voltage power supplies can generate any degree of output current between 0 mA and their rated maximum, which may very well be above the rated maximum for the LEDs and LED system.
However, as we noted before, constant voltage power inputs require more modification to be used safely with LED systems because of the nonlinear connection between forward current and forward voltage, for the following reasons:
LED forward voltage does not always correspond to that of the power supply voltage level. For instance, depending on the same LED spec as above, if you have a 3.0 V constant voltage power supply, the forward current will also be limited to 135 mA.
Using the same power source, what if we wish to operate the LED at 20 mA? The LED will only require 2.7 V, not 3.0 V. But, given that most power supply units lack a variable voltage output choice, the power supply unit by itself cannot produce 2.7 V at the LED.
What should we do?
The solution is to put a resistor in series with the LED and let it "drop down" the voltage to the LED by 0.3 V.
How can we determine the value of the resistor? Using Ohm's Law, which says V=IR, we replace V with 0.3V (the voltage drop) and I with 0.02A (desired forward current). Solving for R gives us 15 Ohms.
Similar computations can be done regardless of the voltages involved-for instance, 12V and 24V LED strips.
Variations in LED forward voltage are unavoidable in mass production settings and lead to several voltage bins. Ideally, LEDs from every voltage bin have a different resistor value pairing computed to guarantee the same forward current draw, independent of the LED voltage bin. Otherwise, greater differences in forward current draw-and hence brightness-could happen.
Every one of the above lines denotes a distinct voltage bin. Different resistor requirements have to be used to obtain various forward voltages required to reach the same 60 mA for all the LED bins.
Current limiting resistors guard against rising voltages.
As we observed, the LEDs exhibit a nonlinear correlation between forward current and forward voltage. Consequently, a small rise in voltage could cause a large rise in forward current, which could cause overcurrent and device failure.
Unlike diodes, resistors have a linear relationship between forward voltage and forward current (as indicated by Ohm's Law).
So, irrespective of voltage level, a rise in the forward voltage will cause the same, proportional rise in forward current. When included into an LED circuit, this characteristic of resistors can help to offset the consequences of a rising voltage.
What causes voltage to rise?
A non-stable power source with notable noise or ripple is the first option. Should the constant voltage power supply generate non stable DC current, forward voltage and intermittent spike - and current limiting resistors will help control a related forward current spike.
The second, more consistent and widespread is a feature of LED devices themselves.
If we maintain forward current constant, an LED's forward voltage drops as it warms. LED datasheets often show this in the following temperature vs forward voltage change chart:
Designing a constant current circuit benefits from this knowledge as it informs us of the actual range of forward voltages we could observe in a system. But let's restate the same idea from a constant voltage viewpoint:
If we maintain forward voltage constant, an LED's forward current rises as it heats.
First and foremost, an LED's total power dissipation determines its heat production. Thus, the reality that forward current grows as its temperature rises is possibly disastrous since the greater forward current will raise an LED's temperature even more, therefore raising its forward current even more in a positive feedback loop. At best, this is called thermal runaway of an LED system, which will cause disastrous failures and maybe fire and smoke.
By means of its linear IV curve, a current limiting resistor helps to offset the impact of rising voltages. Furthermore, in relation to its temperature, resistors act opposite to that of LEDs; as temperature rises, resistance too rises.
Some people have called resistors used in this way a ballast resistor because of this straightforward yet helpful characteristic.
Final Thoughts
Being inherently current-controlled, LED devices don't react well to voltage changes.
Building an LED system using constant voltage power sources means you must be ready to use current limiting resistors to guarantee safe and stable operation of LED devices.
Shenzhen Benwei Lighting Technology Co., Ltd was established in 2010. It is a national high-tech enterprise integrating design, R&D, production and sales of indoor and outdoor lighting products and also can do OEM ,ODM .For more details about our offerings, please contact us at bwzm18@ledbenweilighting.com
