Transistors: Connecting Bases

[rs20]

Hi guys, and thanks for your input

The input voltages and outputs (led's in this case) are not important. I was merely wondering about what is needed to successfully drive the transistors when the bases are tied together.  As in my circuit, they are not turning on.  I will post my circuit in the original thread I had started early, as that is the correct place for it.

If you try to drive a 12V led module with 5V, it's not going to turn on. That's why your circuit isn't working, and that's why input and output voltages are critically important.

So it seems that the consensus of everyone is saying to place a resistor at each base of each transistor.

Most of the more recent posts are agree to have no resistors at all, or to only put resistors for a bit of a laugh. No base resistors are called for at all.

[Falcon69]

Actually, the circuit I am using, the collectors are not common at all, in the schematic for this purpose, it is.

Also, I have it drawn with 5v for power for the bases.  It is actually 12 volts, the whole circuit is 12 volts.  If there is no resistor at the base of the transistors, They Blow. I've fried several already because I forgot to place them on the breadboard.

[c4757p]

Actually, the circuit I am using, the collectors are not common at all, in the schematic for this purpose, it is.

What? Then there is a huge, fundamental difference between what you're doing and what you're showing us. Please fix the schematic so we know what you are actually doing.

[Dinsdale]

It is actually 12 volts, the whole circuit is 12 volts.

In that case, try 10K up and 620 down.
I guess you are just playing with transistors for fun, because you don't need any transistors at all if everything is 12V.

[c4757p]

you don't need any transistors at all if everything is 12V.

 

How can you say that, knowing nothing about the rest of his circuit? 12V != high current...

[IanB]

Actually, the circuit I am using, the collectors are not common at all, in the schematic for this purpose, it is.

Also, I have it drawn with 5v for power for the bases.  It is actually 12 volts, the whole circuit is 12 volts.  If there is no resistor at the base of the transistors, They Blow. I've fried several already because I forgot to place them on the breadboard.

It is not possible to guess why the transistors are failing unless you post the actual circuit you have built on the breadboard, with actual voltages and component values.

The behavior of a circuit depends on what it actually is, and not at all on what it isn't...

[Falcon69]

Okay, I understand.  I thought that there was just a general standard for connecting the bases of multiple transistors to get them all to activate correctly.

I have posted the schematic in the other post about sequential LED's.

[LvW]

OK, thanks for the references, I am chewing through it (I am a software guy by trade, not EE, but physics and electronics was always my interest). However, as you have said - that's really an enormous overkill for what the OP was asking. Also most literature shows the current-based analysis, using load lines, characteristics, beta and such, so that's where I am coming from.

Yes - you are right. It is really surprising (funny?) that in some documents/papers - even in some textbooks - the simple formula Ic=beta*Ib (which certainly is correct) is used to describe the physical control mechanism of a BJT. Just because it is so simple? On the other hand, there are many application-oriented properties of transistor circuits which clearly show that the BJT is voltage controlled. For example, take two simple gain stages having the same DC operating point but two different beta values.
The voltage gain will be equal - independent on beta.     

[T3sl4co1l]

In the example exactly as shown, each individual emitter current is controlled by a separate resistor.  Therefore, no current sharing is necessary; the bases will each draw as much current as they need, and no base resistance is required -- anywhere, not even by the switch.

Possible gotchas: emitter followers can have a negative input impedance, which can cause oscillation.  A series "base stopper" resistor can be helpful to limit this, especially if bypass capacitors (on any combinations of two terminals, B-C-E), or long wires, are present.

If the emitters are common, however, you are paralleling transistors and must observe correct use:emitter resistors, since the effect is hFE times more reliable than base resistors (which however might still be required for the above reasons).  This is fundamentally because transistors are transconductance devices (voltage in, current out), the base current is only a convenient side effect of their construction (e.g., hFE varies all over the place, Gm follows a well defined function).

Tim

[David Hess]

Possible gotchas: emitter followers can have a negative input impedance, which can cause oscillation.  A series "base stopper" resistor can be helpful to limit this, especially if bypass capacitors (on any combinations of two terminals, B-C-E), or long wires, are present.

An emitter resistor is even more effective than a base resistor for preventing this problem and the circuit as drawn has one for each transistor although we admittedly do not know how close to the emitter it is.

[Falcon69]

Thank you everyone!

Placing a 10k resistor at the base of each transistor for the same trigger/input worked!  they all turn on now.

Thank You Thank You Thank You!

I still have some problems with the circuit, but at least the transistor turning on is solved.      

Jason

[WarSim]

LOL you actually posted for help with a cct that wasn't even the cct in question and got two pages of help. 
Even funnier that it wasn't until the second page somebody saw the floating problem of the original fake cct. 

This is a perfect example why I wait before consider helping. 


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[Falcon69]

Untrue, I believe one of the first posts said to place  10k resistor at the base of each transistor. That was what was needed. I will apply that logic to any other circuit i try and work on from now on.

[WarSim]

Yeah I know you have no idea what I am referring to and that is ok.    it is a moot point now that we know the cct given was wrong, as stated in response 27. 


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