Maker Pro
Maker Pro

NPN transistor still working? Strange ..

Ratch

Mar 10, 2013
1,099
Joined
Mar 10, 2013
Messages
1,099
So then how and why does the LED blink (on and off) ? Whats the reason behind that? What controls the timing?
When the E-B breaks down, current exists through the diode and it turns on. The timing depends on how fast the cap energizes and de-energizes.

Ratch
 

n_tron

Oct 11, 2017
9
Joined
Oct 11, 2017
Messages
9
Does "Breakdown" means disappearance of the depletion layer so that the charge can flow across junction?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
A more interesting question is why the circuit operates differently from how it would if the transistor was replaced with a zener diode.
 

Ratch

Mar 10, 2013
1,099
Joined
Mar 10, 2013
Messages
1,099
Does "Breakdown" means disappearance of the depletion layer so that the charge can flow across junction?
It means its reverse breakdown voltage has been exceeded. You can study what that means by reading some of the rich variety of literature on semiconductors.

Ratch
 

n_tron

Oct 11, 2017
9
Joined
Oct 11, 2017
Messages
9
Does anyone knows of a great tutorial that explain the flow of current (+charge) travelling through various components?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Well... At one level, you're probably more interested in the flow or electrons rather than "conventional current".

At another level, maybe you should get at least a little familiar with the basics of Kirchhoff's laws. Those, along with the understanding that current doesn't flow in both directions simultaneously, can help you understand the basics.
 

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
A more interesting question is why the circuit operates differently from how it would if the transistor was replaced with a zener diode.
My thought is that a zener diode is designed to dissipate heat but the transistor base is not so that a variation in temperature could cause a variation in breakdown voltage.
A dynamic measurement of the voltage across the transistor would be interesting.

The emitter/base junction seems to be delicate, I have blown some Ge transistors just testing with a meter.
 

dorke

Jun 20, 2015
2,342
Joined
Jun 20, 2015
Messages
2,342
@n_tron,
This circuit isn't something to learn from.
If at all it may teach you how not to do things!

Here is a circuit that can teach you something worth learning,
it is also a LED blinker with a UJT transistor (quit an extinct creature).

ujt-led.jpg
 

Audioguru

Sep 24, 2016
3,656
Joined
Sep 24, 2016
Messages
3,656
Years ago when I used the emitter-base junction of a transistor as a low power zener diode I did not connect the collector. Maybe when the emitter-base has avalanche breakdown then the transistor conducts from emitter to collector?

Years ago I used a PUT as an oscillator.
 

Cannonball

May 6, 2017
193
Joined
May 6, 2017
Messages
193
A smaller cap will make the led blink faster and a larger cap will make it bling slower.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Those of us who are old enough...

Transistors operating in various breakdown conditions were a fad. Some significant early research (in the late 1950's) looked at it because it offered some interesting benefits.

This is one interesting example.

One feature which makes operating in this mode so interesting, is the exhibition of negative resistance. It should be noted that zener and avalanche effects in diodes, or just a single junction in a transistor do not exhibit this effect.

Jim Williams describes a practical circuit in Appendix B of Linear's AN-72 (p40-41). Whilst you can do better with tunnel diodes and step recovery diodes, both are far more exotic than a 2N3904 as used here. The latter article also has a number of very useful links. David Jones (eevblog) has also done at least one video describing this circuit.

The circuit presented in this thread uses breakdown of the base-emitter, rather than collector-base junction. It also lacks any base bias. The practical upshot of this is that the breakdown occurs at a lower voltage, and the rise time is not optimised (for brevity). Both factors are probably advantageous in this circuit.
 
Top