RGB LED current query

[talvor]

Hi,

I am currently playing with RGB LED's.  Last night I was testing the current the LED was using when each of the colours where turned on.  Note that I was using a common anode RGB LED for my testing.  I was expecting to see the current increase as each cathode of the LED was turned on.  However I noticed something very strange and I hope some one can explain what is happening.

I first started with a single current limiting resistor on the Anode as follows.

GND - Cathode - Anode - 330R - +9v

Now what I noticed is that the total current did not change as each Cathode was turned on.  In fact it looked like only a single Cathode was active as the colour of the LED was always either R, G or B and not the mixed colour as I would expect.

I then changed the circuit so the current limiting resistor was on the Cathode as follows.

GND - 330R - Cathode - Anode  - +9v

Now when each Cathode was turned on the current increased and the LED colour reflected the combination.

Can someone please explain this?

Thanks
Phillip

[madires]

Since you got a RGB LED with a common anode the resistor at the anode limits the current for R, G and B combined. If a resistor is located at each cathode it limits the current just for that color.

[gregallenwarner]

I would add that in your first test, where you expected a combination of colors, and you only saw one LED on at a time (I presume you had connected multiple cathodes to GND in order to try and turn on more than 1 LED), is because red, green, and blue LED's all have different voltage drops across them. Since current will take the path of least resistance, only the diode with the least forward voltage drop will conduct, creating a voltage potential at the common anode that is insufficient to overcome the forward voltage drop of the other two diodes.

madires had the solution with a separate current limiting resistor at each cathode.

[talvor]

I would add that in your first test, where you expected a combination of colors, and you only saw one LED on at a time (I presume you had connected multiple cathodes to GND in order to try and turn on more than 1 LED), is because red, green, and blue LED's all have different voltage drops across them. Since current will take the path of least resistance, only the diode with the least forward voltage drop will conduct, creating a voltage potential at the common anode that is insufficient to overcome the forward voltage drop of the other two diodes.

madires had the solution with a separate current limiting resistor at each cathode.

Greg, your explanation makes prefect sense.

So from the data sheet the red LED has a forward voltage of 2.4v whilst the other 2 are 3.4v.  Which means that by Ohms Law the Red LED would have the lowest resistance, hence why I was seeing the red LED only.  Is that correct?

[gregallenwarner]

So from the data sheet the red LED has a forward voltage of 2.4v whilst the other 2 are 3.4v.  Which means that by Ohms Law the Red LED would have the lowest resistance, hence why I was seeing the red LED only.  Is that correct?

You are correct, sir.

[Lupin III.]

Which means that by Ohms Law the Red LED would have the lowest resistance, hence why I was seeing the red LED only.  Is that correct?

It's a very misleading to talk about "resistance" in a LED, because it does not behave like a resistor at all. Check again what Ohm's Law tells you and compare that to the voltage-current plot of a LED. Very different!

Check the plot again, to understand what's happening here: above the forward voltage a very small increase in voltage lead to a very large increase in current. This is why you must never run a LED with constant voltage but with limited current. Luckily that's what the series resistor is for. If the current through the LED would rise to a high value, the same current has to go through the resistor. Increased current through resistor means increased voltage drop in the resistor (THAT is Ohm's law). This pushes the voltage "left over" for the LED to a level, where you're in the safe zone of the current-voltage plot again (if you used the right resistor).

[gregallenwarner]

Lupin III's explanation is more technically correct. LED's are diodes, not resistors.

If you place a voltage across a diode (with a current limiting resistor, of course!), the diode effectively "clamps" the voltage at its anode at a value equal to its forward voltage drop with respect to its cathode. For the red LED, it "clamps" this voltage to 2.4 volts. This means that there is 2.4 volts of potential at the anode with respect to the cathode. Since the other LED's in parallel have a forward voltage drop of 3.4, the 2.4 volts is insufficient to overcome this, and no current flows through them. All diodes behave this way, including silicon and germanium, which have approx 0.7 and 0.3 forward voltage drops respectively. Apply a smaller voltage than the FVD to a diode and you'll find that no current flows.