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I am trying to calculate the resistor value I need to achieve 35mA in my circuit. Vcc is 5V. Can someone explain the values I need to find in the datasheet as well as formula to use to find the correct Resistor value? I would like to learn this for future projects using transistors.
Transistor used is S8550
UV LED used is here
simulate this circuit – Schematic created using CircuitLab
You should not use an emitter follower to drive that LED, the voltage drop is too high in relation to the supply voltage.
If your GPIO is 5V you can still use the PNP 8550 transistor as so:
simulate this circuit – Schematic created using CircuitLab
The value of R2 shown is probably a bit high, and depends on the LED Vf at 35mA.
Incidentally, your measured voltage seems a bit high to me, you may be putting too much current through the LED for continuous operation.
If your GPIO is 3.3V, you should use an NPN transistor such as a 8050 and flip everything around and reduce R1 to about 860 ohms.
Note that LOW = ON in the PNP schematic shown, with an NPN transistor HIGH = ON.
I think you will need a higher voltage to supply the LED. Depending on the LED you are using, the forward voltage is between 4 to 6 volts at 20ma. This device has a much higher forward voltage drop than a standard LED. Look on the spec sheet for the Forward Voltage Vf for the exact LED you are using. Note on the second page that the Forward Voltage vs Forward Current varies greatly depending on the LED you are using. Also note that the maximum Forward Current is 40ma. Operating at 35ma is close to the maximum allowable. I agree with Roland that regulating the current would be better to protect the LED.
The function of R2 is to regulate the led current, which is very important. However, with 3.8V over the led and min.0.7 V over Q1 only leaves 0.5 V over R2. Not enough to really regulate the led current. When the led heats up, the led voltage will get a little lower, the voltage over R2 the same amount higher and the current quite a lot higher. Thermal runaway.
You need more than 5 V, or a current source with better feedback.
A fet instead of a normal transistor will give you 0.5V more voltage over R2, e.g. double voltage, and might also be cheaper.
Why not first forget about the transister (bipolar or fet) and just try to hook up the led with a resistor to 5 VDC and see if you can get a stable current? It may turn out that you will need a higher supply voltage, e.g. 7 or 10 volt. Fortunately, you can get very cheap dc/dc converters for voltages like 5 to 10 V, for under or around 10 buck (EUR/$). With a higher voltage, you can work out more interesting schematics that actually regulate your current more easily.
