Many beginners ask why LEDs can’t be wired in parallel to share a common current-limiting resistor as shown in Figure 1. Well they can – but it’s usually not a good idea to parallel LEDs directly.
Figure 2. IV curves for red, orange and green in parallel and current through each one.
In this example we’ll first consider the case of parallel connection of a red, orange and green LED. If the value of R1 chosen in Figure 1 were to result in a common voltage of 2.0 V applied across each of the three LEDs then we could calculate the expected current through each using Figure 2.
The green LED has the highest \(V_F\) of the three and at 2 V it will pass about 12 mA. It will be reasonably bright at this current.
The orange LED has a lower \(V_F\) and it will pass about 27 mA. It will be very bright at this current and would be close to maximum continuous rating for a typical 3 or 5 mm LED. See LED current rating for a typical specification.
The red LED has the lowest \(V_F\) and it will pass about 44 mA. This is above the 30 mA rating in the datasheet in the article above. The LED will be good and bright – for a time!
But if they’re all the same colour …?
Figure 3. For multiple LEDs of the same colour the situation is a little better – but still not ideal.
Even if all the LEDs are the same colour we can expect some variation in forward current from LED to LED. The variation might be reduced by using LEDs from the same production lot but even the manufacturers don’t rely on this and use “binning” to grade LEDs into matched lots for sensitive applications.
The best approach
Figure 4. Series connection, if the supply voltage is adequate, ensures the same current through each LED.
Figure 5. For low-voltage applications a single resistor per LED prevents current hogging by the LED with the lowest [latex]V_F[/latex].Finally, the correct way to solve the problem is to series connect for efficiency – so that the same current runs all the LEDs in the string – or, for low-voltage applications, to use a current-limiting resistor per LED.
A device’s IV curve – current versus voltage curve – is a graph of the current that will flow in the device as a function of the voltage across it.
Figure 1. IV curves for various resistors. The lines can be extended through 0, 0 to show the relationship at negative voltages and currents.
As suggested by Ohm’s Law, \( V = IR \), the relationship between current and voltage in a resistor is linear. Figure 1 clearly shows that the current increases linearly with increasing voltage and that the rate of change depends on the value of the resistor.
LEDs are rather different:
LEDs are diodes whose P-N junctions behave in a non-linear fashion. Very little current flows until the forward voltage is reached. Above this value the current increases exponentially with increasing voltage.
LEDs like all diodes conduct in one direction and do not (until the reverse breakdown voltage is reached) conduct in the opposite direction.
As LEDs’ colour is determined by the band gap1 of the semiconductor and the forward voltage, \(V_F\), also varies with the material.
Figure 2. Typical IV curves for various colours of LEDs.
The IV curves are useful for estimating the current that will flow at particular voltages, etc., and for calculating resistor values.
There are a few points worth noting from the curves:
The forward voltage matches the band gap which increases from red to violet.
It should be clear that trying to power LEDs in parallel – not recommended normally – is particularly bad idea when the colours are mixed. e.g. Connecting a red, green and blue LED in parallel on a 2.0 V supply would result in:
Red: 44 mA.
Green: 12 mA.
Blue: 3 mA.
Figure 3. The currents that would flow through a red, green and blue LED connected directly to a 2 V supply.
The red would hog the majority of the current.
IR (infra-red) LEDs have the lowest forward voltage.
Note that in this chart the curves have been extended up to 100 mA. Most small indicator LEDs cannot take this current. See LED current rating.
Plotting an IV curve
This video demonstrates measurement of the current through a green LED as voltage is adjusted. The results are plotted to generate the IV curve for the LED.
It is also possible to observe some shimmer on the multiplexed display while humming. Humming vibrates the eyes in their sockets which can create stroboscopic conditions within the eye. By humming at about 70 to 72 Hz (near music note C2) I was able to observe shimmer on the LED display.
Measuring LED forward voltage
The video clip above demonstrates measurement of LED forward voltage using a multi-function component tester. Results were:
