NPN basics - why is it burning out!?

Thread Starter

outsidelimits

Joined Oct 28, 2020
12
I am struggling with teaching myself some basic electronics. I have been reading up on transistors and I am struggling to get them to work the way I think they should (which is most likely wrong!).
In the attached diagram can anyone let me know why the NPN get so hot it almost sticks to my finger!? The current is low, around 40mA but this transistor is a 2W which should be fine for this voltage and LED.
Am I doing something incredibly stupid!?

circuit.png
 

Papabravo

Joined Feb 24, 2006
16,773
Your diagram does not have a clear GROUND common point. The logical choice would be the negative terminals of the two batteries, The reason the transistor is getting hot is that there is a path which is drawing EXCESSIVE current. You don't really need the 2nd battery. Look up voltage divider bias and do that.
 

crutschow

Joined Mar 14, 2008
27,697
The current is low, around 40mA
The LED has no series resistor to limit the current, which it should have.
You also have the LED in the emitter of the transistor.
Normally you would put the LED in series with the collector along with a series resistor.
The transistor emitter would go to common, which would be the power supply negative terminals in this case.

Look at typical circuit diagrams for transistors to see how they should look.
 
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Thread Starter

outsidelimits

Joined Oct 28, 2020
12
That looks OK but since there is no current limiting resistor for the LED, and you have it marked as 1W, properly biased at the 4.2V you have on the schematic you'd only expect about 24mA. Is the LED extremely bright, does it get very hot?
No, the LED it's not too bright, nor does it get got, just the transistor
 

Yaakov

Joined Jan 27, 2019
3,464
Something is odd. I reproduced your circuit here and it works perfectly except that I have to limit current to the LED as expected. Do you have a wiring error? Also, do put a current limiting resistor on the LED.
 

BobaMosfet

Joined Jul 1, 2009
1,848
I am struggling with teaching myself some basic electronics. I have been reading up on transistors and I am struggling to get them to work the way I think they should (which is most likely wrong!).
In the attached diagram can anyone let me know why the NPN get so hot it almost sticks to my finger!? The current is low, around 40mA but this transistor is a 2W which should be fine for this voltage and LED.
Am I doing something incredibly stupid!?

View attachment 237039
This will help you more than you know:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Bear in mind that your understanding of the transistor you're using is limited- It probably will not handle the voltage or current you think it will.

Datasheet:
 

Attachments

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BobaMosfet

Joined Jul 1, 2009
1,848
I am struggling with teaching myself some basic electronics. I have been reading up on transistors and I am struggling to get them to work the way I think they should (which is most likely wrong!).
In the attached diagram can anyone let me know why the NPN get so hot it almost sticks to my finger!? The current is low, around 40mA but this transistor is a 2W which should be fine for this voltage and LED.
Am I doing something incredibly stupid!?

View attachment 237039
This schematic is just wrong. You are grounding the collector. Base is high @ 3V, and emitter is high at 4.2V. You cannot possibly have created this circuit and if you did, that's your problem.
 

Yaakov

Joined Jan 27, 2019
3,464
Oh my gosh you got many things or everything wrong!!
Ha. When I reproduced the circuit here, I made all the corrections you mention because that was the only way it was going to work. I made the (possibly foolish) assumption that the TS messed up the schematic since is wasn't really practical.

EDIT: I mean to his (apparent) original circuit, not the revision you made. I wanted to see if it would overload the transistor, it doesn't so long as I limit the LED current.
 
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Thread Starter

outsidelimits

Joined Oct 28, 2020
12
This schematic is just wrong. You are grounding the collector. Base is high @ 3V, and emitter is high at 4.2V. You cannot possibly have created this circuit and if you did, that's your problem.
Not really sure how to take this. Sorry if I've offended you. As I alluded to, I'm struggling through this and trying to learn. I'm aware I have a problem otherwise I wouldn't be here.
 
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