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Transistors... driving me nuts!

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rage

New Member
Hey all! I'm an older guy working my way into electronics for kicks.

I'm having a heck of a time with my Transistors I bought from Radio Shack.

How do I know the "right" voltage to turn it fully "on"?

Transistor: 2n3904 (Datasheet)


It seems that voltage is running through from the Collector to the Emitter without the Base having power being applied to it.

Any reason why this would be?

Also, I want to be able to run a siren with a 12V DC supply with this same transistor - is that possible?

Sorry for the noob questions...

darrin

 

Hayato

Member
Hello.
Try another test. Connect the LED on the collector, and let the emitter on GND.

---+5----220----LED----Collector
 

Hero999

Banned
That should work, are you sure you've connected everything the right way round?

What you've made is an emitter follower, the voltage at the emitter will be the same as the base minus 0.7V.

The 10R resistor isn't required because the base impedance will be equal to the emitter resistor (220R in this case) multiplied by the gain of the transistor. If the transistor has a gain of 100 the base impedance will be 22000 Ohms.
 
Last edited:

Mike_2545

Super Moderator
Hey all! I'm an older guy working my way into electronics for kicks.

I'm having a heck of a time with my Transistors I bought from Radio Shack.

How do I know the "right" voltage to turn it fully "on"?

Transistor: 2n3904 (Datasheet)


It seems that voltage is running through from the Collector to the Emitter without the Base having power being applied to it.

Any reason why this would be?

Also, I want to be able to run a siren with a 12V DC supply with this same transistor - is that possible?

Sorry for the noob questions...

darrin

There is no ground in this circuit. :confused:
 

Mikebits

Well-Known Member

rage

New Member
Try not to use a digital ground in the simulator.
Sorry, that's all they gave me as a ground. I will modify it via the graphic software next time.

What number on the datasheet represents the voltage required to turn it on "fully"? I'm not sure if it's on all the way or just partially.

Thanks for all the help.
 

barf

New Member
Base must be 0.6V - 0.7V higher than the emiiter for an NPN. Your base resistor drops the extra voltage beyond the 0.6V needed
 

audioguru

Well-Known Member
Most Helpful Member
the datasheet for every transistor shows its On Characteristics Collector-Emitter saturation Voltage when it is fully turned on. Most little transistors are shown with the base current at 1/10th the collector current.

If you connected the pins of the transistor correctly and checked that it is printed with "2n3904" then it is defective (the usual RadioCrap poor quality) since it turns on when it should not turn on.
 

barf

New Member
Your base is floating when you open the switch. You should use a pull down to ensure the transistor shuts off when you open the switch. I would hook up like this:
+5V ----- switch ----- 10K ----- Base

Connect one end of a 2nd 10K resistor between switch and 1st 10K. Connect other end of 2nd 10K resistor to ground.

This way the base is grounded through 10K + 10K when you open the switch and transistor will turn off.
 

Hayato

Member
Your base is floating when you open the switch. You should use a pull down to ensure the transistor shuts off when you open the switch. I would hook up like this:
+5V ----- switch ----- 10K ----- Base

Connect one end of a 2nd 10K resistor between switch and 1st 10K. Connect other end of 2nd 10K resistor to ground.

This way the base is grounded through 10K + 10K when you open the switch and transistor will turn off.
That's right.

But a BJT would not be so "sensitive" to a floating base.
 

audioguru

Well-Known Member
Most Helpful Member
But a BJT would not be so "sensitive" to a floating base.
If it was a reject from a transistor manufacturer it would conduct when it shouldn't.
If the circuit is built on a breadboard with the long connecting wires picking up the powerful AM radio station that is down the street then it would conduct when it shouldn't.
 

rage

New Member
Thanks guys. I ran down to a local shop and picked up new ones and it worked with your help.

I might have damaged them too, so I can't pass all the blame..

darrin
 
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