Why is there a need for a current limiting resistor when using a transistor

[guitchess]

Currently, I am studying transistors and the equations necessary to design circuits around them.  I have learned that when using them as a switch, the base current dictates the collector current.  If I calculate the base current to the requirements of the load is/why there be a need to limit the current after the fact?  Will the transistor limit it based on base current?  I ask because I see lots of circuits where a transistor is used to turn on LEDs, and most of them have current limiting resistors with them. 

Thanks for any input. 

[Zero999]

If the emitter is connected directly to 0V then there needs to be a base resistor since the base emitter junction acts like a diode, so the current needs to be limited in pretty much the same manner as an LED.

It's possible to buy so-called digital transistors which the series resistor built-in but they're normally SMT packages, as it's done to save space.

[Kremmen]

What hero999 says. Also, on the load side 2 things - firstly transistor hFE varies a lot so you cannot reliably set the collector current by base current alone.
Secondly, when driving a led, you seldom want the transistor to operate in the linear region due to the dissipation. So you want to drive the transistor to saturation by selecting a small(er) base resistor. But then you need to limit the collector current by another resistor.
Of course it is possible to operate the transistor in the linear region but then you need some form of feedback.

[guitchess]

So the real reason for being able to calculate a base current for a given load is circuit efficiency?  Otherwise, I could just set enough base current to make sure the transistor was fully on and forget it?

[Hideki]

How would you "set the base current" in the first place?

[Rigby]

the base of a transistor will draw enough current to fry the transistor, unless you limit that current.

do so with a resistor.

MOSFETs are voltage controlled devices and draw essentially 0 current.

[AG6QR]

If I calculate the base current to the requirements of the load is/why there be a need to limit the current after the fact?

I think I detect the heart of the misunderstanding in the sentence I've quoted above, but I'm not sure exactly where the misunderstanding is.

Calculating the base current is just a mathematical operation that tells you what base current will turn the transistor on to the extent your load requires.  That calculation, by itself, doesn't have any effect on the transistor's base-emitter junction.  Regardless of the calculations, the base-emitter junction will always draw as much current as the circuit can supply, until the transistor is destroyed by overcurrent.

The resistor is there to make sure that the base-emitter junction only has the proper amount of current available.  Enough to turn the transistor on sufficiently according to your calculations, but not enough to destroy the transistor.

If you're just using the transistor as a switch, it often doesn't hurt much to drive it with a little more current than your load requires, within reasonable limits (see the transistor's data sheet).  But if it is overdriven, it will typically take longer to switch off.  This may or may not be important, depending on your application.

[IanB]

Currently, I am studying transistors and the equations necessary to design circuits around them.  I have learned that when using them as a switch, the base current dictates the collector current.  If I calculate the base current to the requirements of the load is/why there be a need to limit the current after the fact?  Will the transistor limit it based on base current?  I ask because I see lots of circuits where a transistor is used to turn on LEDs, and most of them have current limiting resistors with them.

Imagine this situation (precisely calculating the base current) to be similar to balancing a ruler on your outstretched finger tip. It is a very precarious situation. If you don't constantly move your finger around to keep it balanced the ruler will fall over.

When designing robust and reliable circuits it is important to avoid precarious situations. One way to avoid precarious situations is to use feedback to continuously compensate for changes (like moving your finger around to keep the ruler balanced). Another way to avoid precarious situations is to arrange for the circuit to push hard and then have a big resistance against that push. (Imagine trying to move your hand through a some thick tar-like liquid. You will find your hand can only move smoothly and steadily even if you push very hard.)

So the answer to the question is that you really don't want to calculate the base current to the requirements of the load. That will be like balancing on a knife edge. What you want is to have enough base current to turn the transistor on hard, so it is pushing against the end stop and can go no further. That makes the situation stable (like pushing against a wall). Now you can independently limit the current through the LED using a suitable resistor (the resistor being like the thick liquid in the above analogy). The result is that you have separated an complex interacting system into two simple, easily controlled systems, and each of the simply controlled systems can be made to do exactly what you want in a very predictable way.

[AG6QR]

Oh, IanB's answer caught an aspect of the question I hadn't even considered.  Was the question about a current-limiting resistor on the load?  The collector side of the transistor?  I was assuming it was asking about the base-emitter current limiting resistor, but now I think maybe my assumption was wrong.

IanB's answer is correct, if the question was about a resistor on the load side.

Another way of addressing that side of the problem might be to point out that Hfe is a very poorly controlled parameter for transistors.  Look at the data sheet for details.  Data sheets often only guarantee a minimum value of Hfe, without placing any constraints on the maximum value.

Since you can't rely on a particular value of Hfe, you can't rely on a particular specific collector current.  The data sheet's constraint on the value of Hfe will let you calculate what minimum base-emitter current is required in order to guarantee that the collector will have the capability of sinking at least a given load current.  But run through the same equation with the maximum allowed value of Hfe from the data sheet (perhaps infinity?), and you'll see that collector-emitter current isn't precisely controlled by the transistor.

[vk6zgo]

Currently, I am studying transistors and the equations necessary to design circuits around them.  I have learned that when using them as a switch, the base current dictates the collector current.  If I calculate the base current to the requirements of the load is/why there be a need to limit the current after the fact?  Will the transistor limit it based on base current?  I ask because I see lots of circuits where a transistor is used to turn on LEDs, and most of them have current limiting resistors with them. 

Thanks for any input.

Transistors aren't inherently a switch,-------they are an amplifier,so in any application,be it a "Class A" audio amplifier,or a switch,the base current  dictates the collector current.

You could,adjust your base current on test,so that your collector current was  just right to light your LED.

In this mode,even though you only supply this forward bias when you need the LED on,your transistor is not a switch,but a DC  current amplifier.

Here,there comes a big "BUT"---Hfe is not controlled very accurately over millions of transistors of the same type,so you would have to "tweak" each & every such circuit individually.

This is a problem with any type of transistor circuit,so practical circuits use "work-arounds" which remove the reliance upon device parameters---to such an extent,that in many cases you can replace a device with another similar one having a totally different part number.

Getting back to your "switch"---if the transistor is taking the place of a resistor in series with an LED,all the power which is normally dissipated in that resistor must now be dissipated by the transistor.

If we include a resistor in series with the LED,& arrange for the transistor to be turned on "hard",it has only a very low voltage drop between collector & emitter for the same collector current & dissipates much less power.
At the same time,the current through the LED is mainly controlled by the series resistor.

[Richard Crowley]

Remember that the gain of a transistor is not a precise, fixed number.
They are typically specified within some range, sometimes a rather wide range.
Even if you measure the gain of the specific resistor transistor you are using, it can change with temperature, etc.
Which is another reason that for switching use, it is commonly recommended to drive the device into saturation.
Whether an emitter resistor would be recommended would depend on other circuit factors not in evidence here.

Wrote "resistor" where I meant to say "transistor".