# Calculating transistor resistors

Discussion in 'General Electronics Chat' started by Dritech, May 27, 2015.

1. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi all,

I need to use the circuit in the attached diagram where I will be driving the transistor base from a 3.3V GPIO. For the transistor collector I will be connecting a 5V supply instead af 3.3V.
What equation can I use to determine the resistors values (without assuming resistor values) ?

• ###### Capture.PNG
File size:
32.3 KB
Views:
15
Last edited: May 27, 2015
2. ### crutschow Expert

Mar 14, 2008
13,523
3,390
Nothing is attached.

3. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Sorry. I updated the post.

4. ### ScottWang Moderator

Aug 23, 2012
4,934
777
Why you not put the buzzer at the c of bjt?
And how is the V/I of buzzer?

5. ### ian field Distinguished Member

Oct 27, 2012
4,460
792
As its an emitter follower; you don't really need those extra resistors - the emitter will follow the base voltage, except that you lose 0.7V in the B/E Vf.

If you were running at 3.3V the Vbe drop might possibly prevent it working. Even at 5V you could be cutting it fine, in which case a common emitter stage would be better - if your load has to be ground referenced; you'd need a second (PNP) common emitter hanging from Vcc.

Then it starts getting complicated because the common emiter is inverting, but nothing that can't be sorted out with a few more posts.

A common emitter does of course require a resistor to limit the current into the base, find out the max current you micro can source and calculate the resistor taking Vout-high and subtractin 0.7 Vbe, and just perform Ohm's law on what's left.

Last edited: May 27, 2015
6. ### AnalogKid Distinguished Member

Aug 1, 2013
4,709
1,301
If you want the maximum loudness from the buzzer, change the configuration to a common-emitter circuit:
Eliminate R1 and the connection to GPIO pin 1 (+3.3V).
Tie the emitter to GPIO pin 6 (GND)
Connect one end of the buzzer to the collector.
Connect the other end of the buzzer to +5V.
Note: the buzzer might have polarity markings. If so, connect the + pin to +5V and the - pin to the collector.

ak

7. ### ian field Distinguished Member

Oct 27, 2012
4,460
792
As far as I can see; the TS is asking how to calculate the resistor values AS IF a common emitter were being used, I think they'll have voltage overhead problems with the emitter follower.