Does a capacitor discharge and recharge in a loop while power is going through it? or only when power is cut off?

Question

Beginner Question. I'm trying to understand how a simple LED flashing circuit works, here is a picture of the circuit:

I know that much like a Relay, a Transistor could be used as a switch by using it's base and passing a small amount of power to it.

Now something needs to Connect and Disconnect the power to the transistor to make it work as an automatic switch or the LED will not flash and remain On.

In the picture above, what is taking care of that power oscillation? is it the Capacitor? Does a capacitor discharge and recharge in a loop or only when its cut off power? if not then what does the capacitor do here?

Answer 1

This circuit is a negative resistance transistor oscillator. It's kind of a gimmicky circuit for a beginner, it is a very unusual use of a transistor, and is not actually using the transistor as a switch in the usual way at all. (You can tell that this is the case because the base of the transistor is not connected to anything, which is normally necessary to use it as a switch.)

See this video for some more discussion of what's happening: https://www.youtube.com/watch?v=rpGOKGrcpAk

"Negative resistance" (more properly "negative differential resistance") is a property that some electronic components have, including certain transistors when used in a reverse-biased way. (You can read more about it here: https://en.wikipedia.org/wiki/Negative_resistance#Negative_differential_resistance ) It describes a region of the I-V curve (the curve that describes a device's resistance, or how its current and voltage relate to each other in a circuit) where the current decreases as the voltage increases and vice versa, which is the reverse of the usual relationship for a passive component. I call the circuit 'gimmicky' in part because most of the cheap transistors that do this are not explicitly designed or specified to behave this way -- they happen to do it, but it's not guaranteed to work and might vary with things like temperature, or what batch your transistor happens to be from.

The capacitor does charge and discharge in a loop along with the flashing of the LED. The capacitor can't do that by itself -- the unusual property of the transistor is what triggers it to start and stop charging.

I would not recommend this circuit as an example to learn from, when learning about the use of transistors in basic electronics! It's a very cute gimmick but that's really all it is.

Answer 2

This trick circuit uses a failure mode of Vce reverse voltage > -5V to conduct up to 5mA at some threshold then go into Avalanche mode and discharge the capacitor to near -5V when the holding current of this negative resistance latch like an SCR, goes out of -Rs mode and into a +Rs mode and allows the cap to charge up again. -Rs means the voltage drops abruptly as the current rises. +Rs mode means it acts like resistance great than -5V when overstressed with reverse voltage.

Thus it forms an Relaxation Oscillator where the LED draws current from the 1k and thru the Vec which has some resistance like a zener great than at the same time charging up the cap until the DIAC-like condition triggers again.

The LED series R must not exceed the 5mA tigger current much as the energy discharge from the capacitor can burn out the Vec junction. So it works but with some unspecified stress and reduced life span.