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This is just a showcase of using LEDs while taking into account variations between LEDs forward voltage (Vf) ratings. The different colors are JUST to differentiate the LEDs into groups with minor differences: any blue LED is the same as every other blue LED, which are all different from the other colors. The green LEDs represent the ideal LED for this example: Vf=2V, If=20mA.
LEDs of the same kind are expected to have similar properties but variations by any amount gets amplified in magnitude because LEDs are diodes. Diodes are exponential devices: once the forward voltage is met the diode really begins to kick on and conducts it’s rated forward current. With more and more small increases in forward voltage over its rated Vf, there will be more and more current flowing through it at an increasing rate. An increase in the Vf from 2V to 2.1V causes an exponentially larger increase in If than a Vf increase from 1.9V to 2V. The unreliability in the manufacturing of LEDs means that even two of the same type of LED from the same manufacturer can have slightly different forward voltages that result in a huge difference in current draw between the two lights. LEDs of the same kind behave much more similarly when they pass the same current through them regardless of differences in forward voltage.
In each of these 3 situations the resistor(s) ideally have a 1V drop across them, every LED *should* pass 20mA, and each LED *should* have a voltage drop of 2V. Look at the ratings of the different colors to see their variations in rated forward voltages.
In the two parallel examples there is an imbalance in the currents between the LEDs. This is caused because the LEDs still get roughly 2V each but their varying forward voltages mean they each behave differently at 2V. LEDs with a Vf of greater than 2V draw less current than expected and appear dim in comparison to the ideal LED ratings. Those with a lower Vf than 2V draw much more current than 20mA and appear much brighter, potentially even enough to cause damage. It’s clear that MULTIPLE LEDS SHOULD NOT BE RUN IN PARALLEL CONNECTED DIRECTLY TO ONE ANOTHER as in the top example where they all share the same current limiting resistor. Giving each LED its own resistor will allow them to balance currents much better as they no longer all share the same node and same voltage. The downside of this is requiring a resistor for every individual LED and the upside is much more relatively consistent current across all the LEDs.
A good way to ensure the same brightness across several LEDs is to power them in series as opposed to parallel. This means that the same current will pass through every LED regardless of any differences in forward voltages. Variations in forward voltages can result in an overall difference in current but with more LEDs this tends to average out and lead to only minor changes in expected current and the real current in practice. The downside of this is the requirement of a higher voltage than is needed to drive a single LED and the upside is ensuring the same current passes through all the LEDs. The higher voltage requirement can especially be a problem when it requires voltages above logic level voltages as that may necessitate dedicated boost converters to control the string of LEDs.
Brightness can be further controlled by using a constant current source to ensure that the LEDs are controlled at exactly the brightness you want them to have. Ideally, to get as close to the same brightness as possible among all the LEDs, either a string of LEDs in series should be driven with a constant current source or parallel LEDs should all be driven with their own individual constant current sources. This is why numerous ICs dedicated to driving LEDs exist and are readily available to order for personal/ professional projects. Typically though not much precision is required so a constant voltage with a resistor for each LED string is sufficient for a user’s needs.
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