Hello all,

I am trying to design a RGB LED String controller circuit via NodeMCU ESP8266 Wifi Module. I am unfamiliar with the transistors and their datasheets.

This is what I came up with for the relay controlling the LEDs.



The relay I chose has 12VDC coil voltage, 400ohm coil resistance. Here is the datasheet: https://cdn.ozdisan.com/ETicaret_Dosya/415763_1216803.pdf
I chose BC817-40 as my BJT. Assuming it has 0.4V Vce voltage when is conducting, Ic current would be Ic = (Vcc-Vce)/Rcoil = (12-0.4)/400 = 0.029 A == 30mA.
From the first graph on in the datasheet of the BC817-40 (https://cdn.ozdisan.com/ETicaret_Dosya/417241_3994580.pdf), for 30mA of Ic, hFE would be 250.


Lets call Vgpio15 as VBB. It is 3.3V when the pin is set to HIGH.

Rb = (VBB-VBE)/(Ib) = (VBB-VBE)/(Ic/hFE) = (3.3-0.7)/(0.03/250)=21.6k

When I simulated the circuit in LTSpice, Ic drops to 5mA.



When I reduce the Rb value to 1k, transistor fully conducts. Is this because of thermal assumptions are not included in the LTSpice or did I miss something?



Are my assumptions and calcutions are correct? I am using the BJT in saturation region as it is supposed to when I use it as a switch? What generic resistance value should I go to if 21.6k is correct?

 

Hi cool,
Welcome to AAC.
Please post your LTS asc file.
The base resistor should be approx 470R

E

 

The chart you took you beta from is labelled with:

Vce = 9V

That is not saturation. In saturation the beta falls sharply. Most datasheets recommend either 10 or 20 as the beta for the saturation region.

 

Thank you Eric.

Here is my asc file.

 

hi cool,
It runs OK for me, what problems did you see?

E
BTW: there is a BC817-40 in the LTSpice lib

 

When I calculated the Rb value as I described above, I found it as 21.6k but in the simulation, I had to decrease this resistance value to 1k to properly use it as a switch. Therefore I am couldn't be sure that my calculations are correct.

 

The chart you took you beta from is labelled with:

Vce = 9V

That is not saturation. In saturation the beta falls sharply. Most datasheets recommend either 10 or 20 as the beta for the saturation region.

Graph is for 5V actually. It looks like 9V due to pixelation.

Which graph or value from the datasheet should I take into account to find out correct gain value?

 

Hi cool,
Use a hfe of say 10 or 20 when driving a BJT into saturation.
#Note the Ic/Ib figure
E

 

I am unfamiliar with the transistors and their datasheets.

The rule of thumb for non-power transistors is to use a beta of 10 for saturation mode. Some BC transistors (e.g. BC547) specify 20, so you can safely use that value for those transistors.

The transistor you're using specifies 10:



This graph gives more information for different currents:

The manufacturer is guaranteeing that 10 will work for all conditions. Other values will work for conditions that aren't worst case.

Another consideration is that the relay you're using will pick-up at less than the rated voltage, so the transistor doesn't need to be in hard saturation to energize the coil.