Switching phantom with NPN transistor?

[Ethan]

Thanks so much!

So if I wanted, could I add another NPN (T2) and connect it's collector to the previous NPN's (T1) base resistor, ground the emitter of T2 then control the whole circuit with +5V or 0V at the base resistor of T2?
So when 5V is applied to the base of T2, it would turn off T1 by bringing it's base to ground?

THis is so cool, I'm learning something about transistors! ;D

 

[KingKai]

You're getting there!

You also need a current limiting rewsistor in series with the 0 to 5V control signal and the base of T2. ~50k ohm would be plenty to prevent the destruction of T2 due to very high base-emitter current.

This resistor is usually selected to provide ust enough base current to turn the T2 transistor fully "on". Also known as saturation current. The required base current is the desired collector current by hfe (beta) of the transistor.

So... for your high-side switch.

1. determine the current required by the 48 volt load. add a little ( maybe 5%) to insure a well turned on T1.

2. divide this T1 collector current by the hfe of T1 to determine the required T1 base current.

3. size the T1 base pull-up resistor to provide the T1 base current safely. (assume zero emitter voltage for this calculation and .6V Vbe)

4. calculate the T2 collector current by assuming 0.0 volts at the base of T1 and calculate he current through the T1 base pull-up resistor. This is the current required to be shunted to ground by T2 to turn off T1.

5. divide this T2 collector current by hfe of T2 to determine the T2 base current required.

6. calculate the T2 base drive resistor to provide this T2 base current using 5V control signal on one end and 0.6V on the other.( Vbe=0.6V and Ve = 0 ground).


Hope this helps.

Peace - Irv