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How to make a voltage controlled switch using a BJT transistor?

Well i just want to know how to use a BJT transistor in a circuit to switch on at a particular voltage. Its for a charging circuit, i want to activate i transistor at 9 volts and the current from that to charge a 6 volt battery. I already tried putting a 9 zener diode at the base of a NPN but the turn on voltage was 16 Volts. I used a C5027R and Vbe is 7V and i am really confused with what Vbe really is.

Its a bit hard to comment without circuit diagrams. Vbe is literally the voltage between the base and the emitter of the transistor. If the base is saturated, the base is naturally about "one diode drop" above the emitter, or roughly 0.6...0.7 volts. Do you want to achieve a constant current charge ? Steve
Re-design7 years ago
It depends on the transistor. Check out this calculator and see if you can use it to answer any of you design questions.
http://www.calculatoredge.com/electronics/BJT.htm