Confused by transistors and base leg resistors!

[The-Kevster]

Hi all,

I'm trying to figure out the calculation for a base leg resistor but the more information I read the more there is to confuse me.

A rough diagram from google images...

My vales are as in the picture (nice when it works out) except for the resistor which I am unsure about. My relay will use 170mwatts at 5v which works out to 34ma and I'm going to calculate to 50ma for safety.

I've read a lot about saturating the transistor but also that it's not necessary and I should work out exactly what the base leg requires and use a minimum.

I was looking at using a BC548 which has a minimum hfe of 110 so I should only need to provide 0.5ma to get 50ma out. But the BC548C has a minimum hfe of 400 so I would only need 0.125ma to saturate it. Lastly I've then seen that using BC548's is outdated and I should be using a modern BC327?

any help and opinions are greatly appreciated!

Thanks,
Kev

 

[godfreyl]

560 Ohms is a good value.

I've read a lot about saturating the transistor but also that it's not necessary and I should work out exactly what the base leg requires and use a minimum.

Saturating the transistor is a good idea because you want the collecter-emitter voltage to be as low as possible.

I was looking at using a BC548 which has a minimum hfe of 110 so I should only need to provide 0.5ma to get 50ma out. But the BC548C has a minimum hfe of 400 so I would only need 0.125ma to saturate it.

No. The problem is that the current gain is much worse when Vce is low.
You can see this in the graph below, taken from a datasheet.



Only 200uA of base current is needed for 50mA collector current when Vce = 7V. However 400uA is needed when Vce = 1V.

So higher base current gets you lower Vce.

 

[WimRFP]

BC 546/7/8 is one of the most used general purpose transistors, so don't worry about availability. If you want that any BC548 type can be used (A, B or C), calculate the resistor for the A version. If you can have some power consumption from 3.3V, just go for 1 mA, this will sure saturate the transistor. At low temperatures HFE drops and Vbe rises, so some margin isn't bad. Rbase = (3.3-0.7)/1m = 2600 Ohm. that would be 2.2kOhm (E12), or 2.7kOhm, (E12, but with somewhat less base drive).


BC327 is PNP and you can't use that here.

 

[The-Kevster]

Thanks guys that's great. It seems like there's a great margin for error. If 1ma is plenty a 2.2k resistor would allow 1.5ma though. That's a good figure as I have 50ma on the pi to play with and I'm driving 7 transistors.

Thanks again!