mamadou wrote:why do i have to use 2k2 resistor instead of 22k ? is it dangerous not to replace it or it won't make any difference and the board will be fine
siwo278 wrote:In my opinion if you don't want to exchange the resistor of the base of the BC337 transistor don't change. It should work from 22k.
Of course that is up to you and I’m not saying won’t work.
My factors for calculating 2k2 are.
1) Use the lowest gain for BC337 which i have rounded down to 100
2) VOH out from microcontroller is VDD – 0.7
3) VBE for saturation = 0.7
Therefore min IB= IC/HFE
= (12/360)/100
= 330E-6
Min base bias resistance = (VOH microcontroller- VBEsat)/IB
= (4.3 - 0.7)/ 330E-6
= 10K9 ohms.
So any value higher than 10K9 there is no guarantee relay will operate.
Even if relay does operate then if transistor is not fully saturated then more power will be consumed by the transistor, heating it up unnecessary.
Personally I would use a lower value to absolutely guarantee saturation.
If you were worried that 2K2 will damage transistor then looking at Philips datasheet for BC337 you will see that maximum base current is 200mA.
One other thing it looks like the relay board EB038 uses 2K2 resistors, but it’s not very clear:
So if it's used in that board how can it be dangerous and damage other boards?