# how to find out base current

Discussion in 'Homework Help' started by jonisonvespa, Mar 25, 2012.

1. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0
hi,
been a way from electronics for a long long time, im trying to build a circuit to switch on relays and leds, cant remember how to work out the base current of a tranny resistor?, can someone please remind me also in the speck sheet what value is the base current? my circuit, (i cant seem to post a link of the sheet) its a sm sot-23

ok had another long look am i right in saying Ic is the collector current if so in the speck sheet it says 100 im asuming this is 100ma? so you use ohms law to work out the resistor? using a 5v to the base of tranny

am i correct in saying 50 ohm resistor

thank you

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Last edited: Mar 25, 2012
2. ### MrChips Moderator

Oct 2, 2009
19,115
6,146
The general rule of thumb is to make the base current 1/10 of the collector current.
So, if your load requires 20mA, set the base current to 2mA.

Last edited: Mar 25, 2012
3. ### debjit625 Well-Known Member

Apr 17, 2010
790
186
Yes and also in some european transistors the base current is 1/20 of collector current,for example BC547,BC548 .....

Good Luck

4. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0
hi mr chips
ok i think ive established the collector current for sot-23 is 100ma are you saying 1/10 of this ? so 10ma

so the resistor required is 5/.01 500 ohm resistor?

thank you

Oct 2, 2009
19,115
6,146
Looks good.

6. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0
thanks very much thats got me started

7. ### Shanjaq Member

Dec 6, 2010
12
0
Base Current to Collector Current would be the DC Current Gain(hFE) of that particular transistor? That's a dynamic property, which usually has a graph in the datasheet, or a few quoted values, like the PN2369A has an hFE=40 @ Ic=10mA and Vce=1.0V. That's an amplification ratio of 1:40 Base vs Collector current.