I want to choose transistor that will do my job?
I have a microcontroller which i will connect to the base which will send 5V signal with 20mA current this will be connect to the base of the transistor.
VCC=27V that mean i use 3X9V battery
On load the batteries if they are PP3 will be about 8.4V/battery so, about 25V
Will the 23V device accept 25V.?
VC=23V the device i am using need 23V
Vb=5V from microcontroller
ib=20mA
ic?? not sure about it
The hfe of a particular transistor is depends upon the collector current.
example: let the collector current be 100mA and the hfe be 50, this would give a minimum base requirement
of 2mA... In order to fully saturate the transistor I would suggest a base current of 5mA.
R1&R2&transistor not sure about it??
For R1 the base resistor, assuming a +5v output from the PIC, (5 - 0.7Vbe)/
0.005 =860R,, say 820R
If your battery was 27V and the device required 23V, you just cant drop 4V across an emitter resistor as the transistor would not switch correctly with a 5V base drive.
IF you must lose 4volts put a resistor in series with your device.
Example, if Ic thru the device was 100mA, the 4V/0.1 40R, say 39R [use a 1Watt resistor]
I know that i need NPN transistor
hfe=??
Please tell me how can i select it, if still more information is needed tell me so i can find it out.