A little help on how to switch transistors

[MizziTDK]

Hi guys! I am currently making a project using an arduino. In this project, I will be using transistors to multiplex 7 segment displays. I know a little about transistors. But I am having a problem reading the datasheet. I'd like to use the transistor (specifically 2n2222a) as a switch but I don't know how to choose which Beta to use from the datasheet. I know that in order to saturate the transistor IB > ICsat/Beta, but how should I know beta from this datasheet:

Which beta should I use to base my calculations? Thanks!

 

[KrisBlueNZ]

It depends on what collector current you will be running the transistor at. But all but one of those lines is measured at V_CE = 10V which is not true when you're using the transistor as a saturated switch.

This is why you should use a base current several times the value you calculate by dividing I_C by h_FE if you want to ensure that the transistor is fully saturated.

You may find the following resources useful:
https://www.electronicspoint.com/resources/using-a-bipolar-transistor-to-turn-a-load-on-and-off.30/
https://www.electronicspoint.com/resources/saturation-in-transistors-bjts-why-and-how.28/

You may find it's simpler to use N-channel MOSFETs instead. These are voltage-driven and you need to ensure that V_GS is high enough to guarantee a nice low R_DS value, which means you'll probably want a "logic level gate" device which has its maximum R_DSon specified at V_GS = 4.5V and even lower. They're normally more efficient than transistors, though the best ones are so new they're only available in SMT packages. Have a look at http://www.digikey.com/product-sear...8007d,9780013&stock=1&quantity=1&pageSize=500

 

[MizziTDK]

It depends on what collector current you will be running the transistor at. But all but one of those lines is measured at V_CE = 10V which is not true when you're using the transistor as a saturated switch.

This is why you should use a base current several times the value you calculate by dividing I_C by h_FE if you want to ensure that the transistor is fully saturated.

You may find the following resources useful:
https://www.electronicspoint.com/resources/using-a-bipolar-transistor-to-turn-a-load-on-and-off.30/
https://www.electronicspoint.com/resources/saturation-in-transistors-bjts-why-and-how.28/

You may find it's simpler to use N-channel MOSFETs instead. These are voltage-driven and you need to ensure that V_GS is high enough to guarantee a nice low R_DS value, which means you'll probably want a "logic level gate" device which has its maximum R_DSon specified at V_GS = 4.5V and even lower. They're normally more efficient than transistors, though the best ones are so new they're only available in SMT packages. Have a look at http://www.digikey.com/product-search/en?pv959=5&pv959=7&pv959=8&pv959=9&pv959=11&pv959=13&pv959=18&pv959=15&FV=fff40015,fff8007d,9780013&stock=1&quantity=1&pageSize=500

Thanks for your reply. I'll surely read those articles. Well, I am still learning the BJT so I'll pass the MOSFETs for the mean time. But thanks though. I have another question. What if the Ic that I will be using is not indicated in data sheet like for example my Ic would be 14ma which is definitely not indicated in the datasheet. How should I know what is its corresponding beta?

 

[KrisBlueNZ]

A simple way is to find the two lines on the specification table that are above and below your I_C value and choose the lowest h_FE, to be on the safe side. In your case, 14 mA is between 10 mA and 150 mA so you choose the lowest of 75 and 100, which is 75.

You can also look at the h_FE vs. I_C graph in the typical characteristics section of the data sheet to see how h_FE varies with I_C. Change the vertical scale so h_FE is 75 at I_C = 10 mA, and 100 at I_C = 150 mA, and use the shape of the graph to estimate the minimum h_FE for 14 mA. It will be pretty close to 75 anyway.

But as I said in the first paragraph of post #2, those figures apply with V_CE = 10V, i.e. the transistor is a long way from saturation. That's why you need to use a large safety factor when calculating I_B to saturate the transistor at a given I_C.