Level: transistor beginner (sorry for that)
Question: is a resistance R1 110 Kohms the good one to put before the b branche of a PN2222 transistor, if we want this transistor being in switching mode when the 0 to 30 volts variable voltage coming into the circuit is above 18 volts, and make the LED lighting above 18 volts ?
Image of the circuit: I put a green led in the circuit
Data: PN2222 transistor data: beta=300; Vce_sat 1=1; Vbe_sat=2; Vce_max=30 LED: Ic=Iled=0,025 A; Uled=2V (green led) R led= 600 ohms (Vcc-Vce_sat-Uled, with Vcc at 18 volts) Source of tension: from 0 to 30 volts Tension where the transistor should light the LED and let the electricity running (contact between c and e): 18 volts
calcul done As: Vcc = Vce + Urel, Vce = Vce_sat, Urel = Rrel x Ic, Vcc = Vce_sat + Rrel x Ic <=> Ic = (Vcc - Vce_sat) / Rrel Then, for Vcc = 18 volts: Ic = (18-1)/600=0,0283 A
And as: Ib_min = Ic / ß Then, for Vcc = 18 volts, and beta = 300, Ib_min = 0,0283 / 300 = 0,000094 A
Lets apply a 1.5 security coefficient to get ib_sat Ib_sat = Ib_min x 1.5 = 0,000094 x 1,5 = 0,00015 A
Then, as: Ve = Ur + Vbe, Vbe = Vbe_sat, Ve = R x Ib_sat + Vbe_sat, so R = (Ve - Vbe_sat) / Ib_sat which gives for Vcc = 18 volts: R = (18 – 2 ) / 0,00015 = 110212 Ohms
Is it the correct value for R to light the LED when the variable tension reach 18 volts ?