# Base Current of a Transistor

Discussion in 'General Electronics Chat' started by Vincenzo1309, Jun 7, 2009.

1. ### Vincenzo1309 Thread Starter Active Member

Dec 28, 2008
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Dear all,

I know that there is a need for a resistor to be connected between the pin out of a PIC and the base terminal of the transistor. The resistor is to limit the base current and prevent the transistor from damage.

So exactly how much base current will damage the transistor? is it indicated in the datasheet of the transistor?

I saw in the BD679 datasheet that puts Base current = 1A. Does it mean that 1A supply to the base terminal will damage this transistor?

Thanks alot!

2. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
Its 0.1A maximum base current not 1A.

Yes, base current higher than this limit will damage the transistor.

3. ### Vincenzo1309 Thread Starter Active Member

Dec 28, 2008
57
0

Then I am curious, why do we need to use this resistor to limit the current, since most of the PIC pin will not output high current like 0.1A?

I am currently using Basic Stamp 2, and its source current from a pin is only 20mA. But still, I am only limiting to the current to 10mA as the base current. this is a advice from a friend, I do not know the reason.
Why?

Thanks alot

Dec 28, 2008
57
0
5. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
You can also check the pdfs of BD679 here from Siemens, Motorola, SGS Thomson & Central. They all mentioned 0.1A max.

I know I should have checked all.

BD679 data sheets

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
The base-emitter junction of a transistor basically acts like a forward-biased diode. Once you get past a threshold (around 0.6v-0.7v), the current through the junction increases exponentially for a small increase in voltage.

You have a good friend, indeed.

The more current that you source or sink to/from your Stamp I/O pins, the more heat is dissipated in the Stamp itself. Getting the uC really hot will shorten its' lifespan.

Using a 510 Ohm resistor between your I/O pin and the transistor's base will limit the base current to under 10mA, assuming that your Stamp's Vcc is 5v.

7. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Looking at the datasheets, I would hazard a guess that the OnSemi on contains a typo.

The gain is round about 1000 and the max collector current 4A so a max base current of 1A does not make sense.

I think it is much more likely that the figure should be 0.1 amps in all datasheets.

8. ### Audioguru New Member

Dec 20, 2007
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896
Manufacturers of the darlington transistor recommend a gain of only 50 for saturation at a collector current of 1.5A (base current of 30mA) which is too high for a micro-controller.
For a base current of only 10mA the darlington saturates well with a collector current of only 500mA.

The darlington transistor has high current gain when its collector to emitter voltage is 3V and higher.

9. ### Vincenzo1309 Thread Starter Active Member

Dec 28, 2008
57
0
Thanks everyone for the reply and help!

So it seems that ideally the base current should be limited to 10mA.
I have limited mine to 15mA in my project.
So what happens if we limit it to a very low value, like 3 - 5mA?
The transistor will not conduct fully? Am I right? Then what will happen next?

10. ### gotumal Active Member

Mar 24, 2008
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ya, you are right. It depends on your application in which region in the transistor curve you have your quiscent point. For a switching application you have to have cut-off (very small current) and saturation. While for linear amplfier in the active region.
For high speed switching applicaions, you need to avoide your transistor to go in to the deep saturation region since it would take considerable time to get out of it when the transistor is switched off.

11. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Depending on the value of the current consumed by your collector load, with lower base currents you risk being unable to drive the transistor into saturation.

hgmjr