Go Down

Topic: BJT transistor base current calculation (Read 2 times) previous topic - next topic

MarkT

The problem with current gain is that its "poorly characterised" - which is gobbledygook for "varies a lot between devices".  So any circuit design that relies on it having a given value is a weak design.  Typically you design for the minimum value of beta in the datasheet (note that it varies with current too...).

Since the variation between devices can be as big a factor as 3 or 4 IIRC, this means your current limiting would also vary greatly.
[ I won't respond to messages, use the forum please ]

DVDdoug


The problem with current gain is that its "poorly characterised" - which is gobbledygook for "varies a lot between devices".
That's a good point!    It might OK if you are dimming one LED for a hobby project, or if you are just experimenting.  But if you are building a product on an assembly line, or if there are several transistor/LED dimmers in your project, every LED would have different brightness with the same "settings".

Typically, you'll design the circuit so that resistor values control everything.  For example, if you study op-amp based amplifiers, you will see that the large amounts of negative feedback are used, and the resistor values determine gain.  (Or you do it digitally, such as PWM dimming. ;) )

valekovski

A! Thank you for your exhaustive explanation DVDdoug! Great, my calculations seem to pan out.

Also great point MarkT! I was just thinking of possible applications, but you're quite correct, that would be a poor design, haven't really thought of that :)

Thanks everyone for your help. Cheers, Val

dhenry

For swotching applications, beta has no meaning: you should provide sufficient Ib to saturate the switch. Usually Ib is at least 1/10 that of Ic.

In your case, Ic is determined by that resistor (together with supply + the led's forward voltage drop).

So the calculationb is backwards: select your resistor to get the desired Ic and from there, get the required Ib and calculate the corresponding base resistor.

Go Up