It is fair to say that light-emitting diodes, commonly known as “LEDs”, have revolutionised lighting and electronics in the UK and beyond.
Whether they are used in energy-efficient bulbs in people’s homes, or in car indicator lights, LEDs deliver a range of notable advantages, including brightness, durability, and low power consumption.
However, it is also true that LEDs are sensitive components requiring careful handling – specifically, proper current limiting – to prevent damage.
Why Do LEDs Require Current Limitation?
One of the major ways in which LEDs differ from traditional incandescent bulbs, is in their lack of internal ability to regulate current. So, unless a mechanism is put in place to restrict the flow of electricity, these delicate semiconductors will draw excessive power.
Once the supply voltage goes beyond the LED’s forward voltage, current will flow exponentially. Too much current generates excess heat. This can have the consequences of degrading the LED, shortening the device’s lifespan, or even just destroying the LED immediately. It can also, of course, be a fire hazard.
So, when it comes to ensuring both safety and performance from your next project using LEDs, it will be crucial to know how to calculate and implement the appropriate current-limiting resistor.
The Process for Calculating the Required Resistor Value
We mentioned concepts like “supply voltage” and “forward voltage” above. Depending on your present level of knowhow and experience, you might not be entirely familiar with these terms.
Below, then, we have set out the process for determining the correct resistance (R) for an LED circuit, which requires the application of Ohm’s Law (V = IR).
This necessitates the use of the following three values:
- Supply voltage (Vs), which is the voltage from the power source (for example, a 9V battery or 5V USB port).
- Forward voltage (Vf), which is the voltage required to light the LED. This varies depending on LED colour. Red LEDs, for instance, have a low Vf of approximately 1.8V, while blue and white LEDs have the highest Vf of roughly 3V to 4V, due to the different semiconductor materials they use.
- Forward current (If), which is the ideal operating current. This is typically measured in amperes, although you will need to be mindful of the relationship between bigger and smaller units. So, 20 milliamperes (mA), for example, is 0.02 amperes (A).
To figure out the necessary resistor for a basic LED circuit, you will need to use the formula: R = (Vs– Vf) / If. In this formula, R is the resistance in ohms (Ω), Vs is the supply voltage in volts (V), Vf is the LED forward voltage in volts (V), and If – as we touched on above – is the desired forward current in amperes (A), typically 0.01A to 0.02 A (10 to 20 mA).
Using The Right Digital Calculator Can Further Support LED Safety Efforts
You can, of course, perform calculations with the aforementioned formula “by hand”. However, this can be a slow and cumbersome process, susceptible to human error.
In that light, you might be able to best ensure safety with the LED resistor calculator for forward voltage and current on the website of the electrical component supplier, RS.
A tool like this can also be greatly useful for verifying the accuracy of any manual calculations you undertake. So, we suggest you bookmark this calculator, so that you can have it to hand for your future LED circuit projects.
