BC547 collector current when base voltage is 3.3V

Pefey's picture

Offline

Joined: Dec 12, 2019

Replies: 7
BC547 collector current when base voltage is 3.3V
April 1, 2020 - 8:04am

Hi,

I am controlling a 5V relay using a microcontroller (NodeMCU) operating on 3.3V. So the GPIO pins will only provide 3.3V. Hence I have used a BC547 to control the relay. But I am confused if the BC547 transistor will be fully closed when i give 3.3V and deliver enough current to trigger the relay 

I have attached the graph below that shows the collector current value for different base voltage. If my VBE is 3.3V then there is no collector current at ambient temperature. I think I am missing something here. Can some one pls help 

Capture.PNG

Sourav Gupta's picture

Offline

Joined: Feb 12, 2018

Replies: 282
It will work
April 2, 2020 - 10:23am

Use proper base resistor.

Pefey's picture

Offline

Joined: Dec 12, 2019

Replies: 7
Can you explain please
April 3, 2020 - 10:06am

thanks for answer gupta, but how can it be done? What value of resistor should i use? Also resistor is only used to control the current right? 

Sourav Gupta's picture

Offline

Joined: Feb 12, 2018

Replies: 282
Yes
April 4, 2020 - 2:06pm

Transistor is a current control device. Use proper resistor to control the base current.

Pefey's picture

Offline

Joined: Dec 12, 2019

Replies: 7
How much current will turn on BC547
April 7, 2020 - 8:22am

Thanks for the answer i am slowy getting to understand this.

So the above graph is useless? I also did not find a graph for base current vs collector current.

So how much base current should i provide to base pin of bc547 if my collector current should be 70mA. Please help me with this. I also want to know formulae for calculating resistor value if my base voltage is 3.3V

Sourav Gupta's picture

Offline

Joined: Feb 12, 2018

Replies: 282
Nothing is useless
April 7, 2020 - 2:30pm

All information given by the datasheet is useful from different aspects.
https://circuitdigest.com/article/npn-transistors
Follow the above link. I hope this will provide all answers of your quarries.