hi all
im planning on using a darlington pair to drive 20W/12V filement bulbs.
the transistors will be a BC548 type and the TIP41A.
Hfe of BC548 125
Hfe of TIP41A 15-30
the bulbs will draw 3.6A the TIP41 can handle 6A, both transistors are
well under their Max Voltage.
the base supply for the BC548 is 22.5mA
the multiple of the gain is 125*25(mid way on the TIP) =Hfe 3125
so 22.5mA * 3125 = 70312.5 or 70A.
the circuit is dynamic and the base supply will swing from 1uA to 22.5mA
Am i correct in my understanding that the 70A is the theoretical capacity
of the TIP if it could handle such a load and all i need to know is that as
im only drawing 3.6A and the TIP can handle 6A im ok.
All that matters is that the small current has been amplified to allow the
larger transistor to work efficiantly within its real world ratings.
so if as an extreme example the combined Hfe had been 500A
it doesnt matter so long as i stick to the 6A max. or do you have to
pick a pair that using their combined Hfe * Base current is =to or less than
the current handling capacity of the load bearing Transistor.
i have read some of the Darlington threads on here and also surfed the net for answers, i
am just trying to see if ive got a good enough understanding to implement the BC548
and the TIP41A in my circuit.
Regards
Fenris
im planning on using a darlington pair to drive 20W/12V filement bulbs.
the transistors will be a BC548 type and the TIP41A.
Hfe of BC548 125
Hfe of TIP41A 15-30
the bulbs will draw 3.6A the TIP41 can handle 6A, both transistors are
well under their Max Voltage.
the base supply for the BC548 is 22.5mA
the multiple of the gain is 125*25(mid way on the TIP) =Hfe 3125
so 22.5mA * 3125 = 70312.5 or 70A.
the circuit is dynamic and the base supply will swing from 1uA to 22.5mA
Am i correct in my understanding that the 70A is the theoretical capacity
of the TIP if it could handle such a load and all i need to know is that as
im only drawing 3.6A and the TIP can handle 6A im ok.
All that matters is that the small current has been amplified to allow the
larger transistor to work efficiantly within its real world ratings.
so if as an extreme example the combined Hfe had been 500A
it doesnt matter so long as i stick to the 6A max. or do you have to
pick a pair that using their combined Hfe * Base current is =to or less than
the current handling capacity of the load bearing Transistor.
i have read some of the Darlington threads on here and also surfed the net for answers, i
am just trying to see if ive got a good enough understanding to implement the BC548
and the TIP41A in my circuit.
Regards
Fenris