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βforced=Icsat/Ib which leads to Icsat=βforcedIb
βforced=IcsatIb which leads to Icsat=βforcedIb
Icsat=25∗25mA=0.625A
You can't calculate Icsat from a given base current as the load determines the collector current. For a saturated switch you can't force the collector current to be a certain value by setting the base current.
The steps are:
- You have to start by specifying the desired collector current, Ic, first. That determines R2.
Use Vcesat from the datasheet.
- Then compute Ib for that Ic; usually based on HFE_min.
- Then increase Ib by the overdrive factor.
- Then finally calculate the R1.
Assuming VBE = 0.65 to 0.7V, or better, extract more exact value from the datasheet.
See if you can do it.
If you get negative R1 that means to 610 ohm is limiting the base current more than you like. The solutions are reduce the 610ohm, use a higher gain transistor, decrease the overdrive factor.
BTW: putting LEDs in parallel doesn't always workout so well, one might be bright and the other dim.
Use this datasheet:
http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF