Question: I want to turn a white LED on and off via my microcontroller GPIO. The ESP-01S board that I have uses an ESP8266 chip, which has a limit of 12 mA GPIO output current. However, I want to drive this LED with about 25 mA.

The transistor I have is a S8050C NPN transistor. I'm using resistor R1 to limit current for the LED and resistor R2 to limit current for the GPIO pin.

[![schematic](https://i.stack.imgur.com/mYnGp.png)](https://i.stack.imgur.com/mYnGp.png)

Edit here: <https://www.circuit-diagram.org/circuits/8a2aa3cfd1e04356b4099cee6f851846>

I know that I will have to scale R2 according to the value given for minimal current gain in the S8050C datasheet, so the current for the LED is high enough.

However, what I'm still unsure about, are the voltages: The supply voltage for the LED is 5 V. My microcontroller uses 3.3 V. Let's say the LED becomes defective and behaves like a wire - will the GPIO pin see the 5 V? Or will the transistor from collector to base function as a diode? So as long as 5 V is below the breakdown voltage, the pin is fine?

Answer: 
Here is the temperature behavior of a TDK [ferrite](https://www.dextermag.com/wp-content/uploads/2017/03/TDK_Dexter_Ferrites_Telecommunication_Materials_LAN_Pulse_Transformers.pdf) material used in 100base-T transformers, and comparison with other typical materials.

[![enter image description here](https://i.stack.imgur.com/63e27.png)](https://i.stack.imgur.com/63e27.png)

As you can see there's not a lot of effect on the intial permeability, but the incremental permeability suffers a significant drop at higher temperatures, especially the HP5 material.

If you enquire of the transformer manufacturer (TDK), perhaps they will share the material characteristics or at least the ferrite material name. Since TDK is a major supplier of ferrites they should not have a problem with that- it's probably a product made by TDK themselves.

Beyond the ferrite response there are also the temperature limitations of the wire insulation and perhaps adhesives and other materials used in the construction. Probably you're not close to their long-term limits though, given that they can withstand a short-term run through a reflow oven.
All magnetics reduce in permeability with rising temperature and transformers must have a high Q factor at 100 MHz at room temp to support the temperature range and degradation expected in Q at max temp. And max frequency of the spectrum which is not flat either.

Since Q=XL(f)/R you would expect that XL is much higher than 50 Ohms in order for the terminations to determine the transmission line impedance. You should expect your distributor to get you the temperature curves from TDK if you are an important customer. Otherwise you ought to test the magnetics properties with an RLC meter over temperature as these determine the performance along with your design and layout in a circuit Extended cable Margin measured eye-pattern Test.