switch a PN2222 transistor at 18 volts, with variable 0 to 30 volts input

Question

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 ?

Answer 1

If you look at the data sheet of the PN2222 you will see that the hFE of the device is much lower than 300 for saturation mode. For instance with 10 mA flowing and a quite large 10 volts across collector and emitter, hFE has dropped to a minimum value of 75 and, I think it's reasonable to assume that hFE is less when operating the transistor in saturation.

Figure 4 gives some more detail for typical values (not guaranteed): -

For 10 mA collector current and a saturation voltage of 100 mV the base current might typically be 0.09 mA implying a hFE of 111 but, remember this is a typical value so I would probably assume hFE might be as low as 30.

You can also see that hFE varies quite a lot with temperature (figure 3) so assuming a hFE of 30 is probably not unreasonable in certain situations.

The trouble with your requirements is that relying on a fixed value of hFE is going to disappoint you. This is because you have drawn a fairly linear circuit and I would be tempted to put a zener diode in series with the base resistor that basically blocks significant amounts of base current until the supply exceeded 18 volts.

Answer 2

Then, if I include a Zener diode, into the circuit, that might give this:

With following values:

• R.led: 1500 ohms, where R.led = (V.e – V.ce-sat – U.led) / I.led
• I.led: 10mA
• U.led: 2V
• DZ:17V
• PN2222 Beta: between 30 and 100
• Vin: between 0 and 30 V
• Ve (switch voltage value targeted): 18V

Then, if I re-calculate the value of R with a Beta lower than the maximal value given by the datasheet, and more aligned with proable reality:

• with Beta = 100, then my calcul gives R = 95.000 ohms
• with Beta = 30, then my calcul gives R = 28.000 ohms

Equations and calculs (here Beta=30):

• I.c = (V.cc – V.ce-sat) / R.led = 0.0113 A
• I.b-min = I.c / Beta = 0.00038 A
• I.b-sat = I.b-min x 1.5 = 0.00057 A
• R = (V.e – V.be-sat) / I.b-sat = 28.235 Ohms