Some useful relationships

1.    IC = beta IB; Beta is the DC gain IC over IB, obviously.  2.    IE = 
IC + IB
You can use Ohm's Law to solve for the relationship between IB, RB and EB.
There is a 0.7 volt junction difference.

As a rule stay within 10 percent of the max current for continuous loading. 
Remember that inductive loads have an inrush current component.  The 2N3904 
is a poor choice for a relay driver but a 2N3904A is OK for small relays. 
It gets the junction heat out better.  Use a free wheeling diode in parallel 
with the relay coil.  Make sure that the cathode is at the positive terminal 
of the relay.  If not, the diode will short the coil current and draw 
excessive collector current and heat the junction causing thermal runaway. 
Bye Bye transistor!  I hope this helps.

Rich :-)



----- Original Message ----- 
From: "mrgizmo" <mrgizmo@vianet.ca>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Sunday, February 05, 2006 12:34 PM
Subject: Re: [ee] 3904 base resistor


>  I am using a 2n3904 to drive a 12V relay off a 16F628 pic at 5V input, 
> I'm
> not sure how to figure out the resistor for the base?
>
> -- 
> http://www.piclist.com PIC/SX FAQ & list archive
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