Base Current req to switch on Transistor

Discussion in 'Datasheets, Manuals and Component Identification' started by krishna42099, Oct 9, 2012.

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Oct 1, 2012
2. duke37

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771
Jan 9, 2011
It depends on the collector current you want.
If you want a collector current of 1A, then with a gain of 10, you will need a base current of 100mA. However to be sure that the transistor is fully on you could double this.

The maximum permitted base current is 1A.

FETs make better switches.

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Oct 5, 2012
If i can remember my math its something like IB = IC/Beta
Where IC - Colector Current
Beta - Amplification Factor ( H21e,Hfe)

4. krishna42099

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Oct 1, 2012
thx duke, I did some calculation based on a base current of 100 ma, now I confirmed it with your help.

5. krishna42099

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Oct 1, 2012
Hi duke pls pls, just one more help, could you tell me from this datasheet how much base current is required to switch this transistor completely on??

Current in the circuit is just below 1 Amp

http://www.fairchildsemi.com/ds/TI/TIP111.pdf, I would like to go with 2 milli amps...is it right??

6. duke37

5,364
771
Jan 9, 2011
The TIP111 is a Darlington device, it contains two transistors, one driving the other.
The gain at 1A collector current is 1000 so 1mA base current should be enough. 2mA would be good.

At 1A collector current there will be a voltage drop across the TIP 111 of 2.5V so there will be 2.5W of heat to get rid of. This is a normal characteristic for Darlingtons.

A small fet can pass 1A with negligible voltage drop when turned on. It takes no gate current but needs a significant voltage drive. I have only used moderate power fets which need about 10V drive but there are more sensitive logic level ones out there.

7. krishna42099

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Oct 1, 2012
Thx duke, say for example 2 watts power dissipitation, does it need any extra heat sink required?? In my project, it operates at around 30 degree C ambient temperature and anyway there is always some air flowing around it?? does it still need any heat sink??

8. duke37

5,364
771
Jan 9, 2011
The data sheet of the TIP111 says 2W maximum at 25 degC ambient.
I would put a heat sink of about 30*40mm and see how you go. If it is too hot to hold, it is too hot. Small finned heat sinks are often used on TO220 packages.