Selecting a Transistor

Hi,

Looking to design a PCB with a relay circuit on-board. I used this tutorial here Basic Transistor Circuits to do the calculations.

In my case;
Vs = 12v
Vb = 3.3v
Rcoil = 320ohms

Based on this I did the following:

IL = 12 / 320 = 37.5mA
Thus hFE should be > 5 x 37.5mA / Ib
And here lies my first assumption. The current I put into the base could be anything as long as it’s < 10mA or so? So I subbed in Ib = 5mA
and thus hFE > 37.5

Where I’m struggling, and I assume I’m misunderstanding some theory somewhere, is how do I select a transistor based on this? I want one where the DC gain is > 37.5 right? Looking at the BC549 datasheet: http://cygnus.et.put.poznan.pl/~kklima/aue/BC549_550.pdf
it lists minimum, typical and maximum DC gains as follows:

hFE DC current gain VCE = 5 V; see Fig.2
IC = 10 ?A ? 270 ?
IC = 2 mA 420 520 800

So is the 549 suitable for this application? I assumed there would just be a maximum gain value that the transistor can achieve, I didn’t expect minimum and typical gains and that’s thrown me off slightly.

Is there a more suitable transistor for this purpose? How can I compare two transistors and decide which is more suitable?
Or does it really not matter so long as it’s capable of the gain I’m asking of it?

Thanks in advance,
Cosford.

You don't need all that.

When you're using a transistor as a switch you should completely saturate it. Give it as much current as possible.

An Arduino pin is happy at 20mA so that means something around 220 Ohms (assuming a 5V Arduino).

So is the 549 suitable for this application?

It doesn't have a very high maximum current (only 100mA). Maybe a BC337 is better.

I appreciate I probably don't need to do all that, but I'd like to try and build a solid understanding of the circuitry and electronic properties going on behind it (Electronics Engineering student).

Are you saying that, provided the transistor is entirely saturated, it really doesn't matter which specific NPN transistor I use, assuming that the gain is within spec? If so, why are there so many different (very similar) transistors available on the market?

Thanks in advance.

Cosford:
Are you saying that, provided the transistor is entirely saturated, it really doesn't matter which specific NPN transistor I use, assuming that the gain is within spec?

Yep.

Cosford:
If so, why are there so many different (very similar) transistors available on the market?

Mostly historical reasons...

Many transistor have been around for a looooong time. Every time somebody invents a slightly better transistor they can't simply stop making the old one because people have been designing stuff using it. It just gets added to the list.

Could they reduce things to half a dozen different types? Probably. Will they ever do it? Nope.

The datasheet usually has a single line description telling you what it's best suited for:
BC549: "Low noise stages in audio frequency equipment"
BC337: "Switching and Amplifier Applications"

You're doing "switching"...

The linear behaviour of transistors as analog amplifiers breaks down when the voltage
difference between collector and emitter falls below around 2V or so. The hFE parameter
simply isn't useful at Vce=0.1V (typical saturation conditions).

You need to look at the Vsat part of the datasheet (which are usually woefully inadequate
giving figures for a couple of load currents only). Often(*) there is a more useful graph
of Vsat against Ic for several levels of drive, typically Ib = Ic/10 and Ib = Ic/20.

Here you don't need to worry too much about full saturation as the current level isn't
high enough to cause thermal issues in the transistor even if Vsat is only down to 1V or
so, and the drop in voltage across the winding from 12V to 11V won't matter either.

So try 1k on the base, Ib = 2.6mA or so, will be fine.

Incidentally the statement "Give it as much current as possible." isn't useful. There are
other issues to consider such as turn off time will gets slower if over-saturated. Normally
you want to add enough base current to bring the conduction losses in the device down
to a level you can handle without heat-sinking (if possible, else with minimal heatsinking).
If you increase base current above Ic/10 you will typically dissipate more heat in the
Vbe junction and the circuit driving the base than the losses in the transistor, defeating
the object.

Old high power transistors typically need the full Ic/10 drive, more modern super-beta
transistors (usually surface mount) are much better and saturate well with as little as
2% Ic into the base. Check out the ZTX851, a rare example of a through-hole ultra
high-performance super-beta transistor: http://www.redrok.com/NPN_ZTX851_60V_5.0A_1.2W_Hfe75_TO-92.pdf

(*) Often, but not always, as for this datasheet.

Great, thanks for the help. I'll use a 337, with the base fed 3.3v through a 1k resistor.

MarkT:
Incidentally the statement "Give it as much current as possible." isn't useful. There are
other issues to consider such as turn off time will gets slower if over-saturated.

He's using a mechanical relay...

Even if he wasn't, you seem to have deleted the part where I said "When you're using a transistor as a switch". When you're using it purely as a switch then it's usually best to err on the safe side.