Why does this LED blink? - Page 1
Forum section: electronics / beginners

author: Lucky-Luka
author_rank: Unknown
timestamp: February 16, 2020, 11:34:53 pm
content: Hi allI was looking at different solutions to let a LED blink on YT.I found a strange one that I haven't understood...In the comments several people mentioned an avalanche effect of transistors... Is it related to the case? What is it?Cheers

author: Nerull
author_rank: Frequent Contributor
timestamp: February 16, 2020, 11:46:30 pm
content: The capacitor slowly charges though the resistor until the transistor goes into reverse breakdown, then discharges through the transistor, which then turns back off and starts the cycle again.

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2020, 09:47:47 am
content: It works because reverse biased transistors work like spark gaps, but at much lower voltages. Here's a more comprehensive study.http://www.kerrywong.com/2014/03/19/bjt-in-reverse-avalanche-mode/Oh and this is the simplest way to make an LED flash.https://www.tme.eu/gb/details/l-56bsrd-b/special-effect-leds/kingbright-electronic/

author: Lucky-Luka
author_rank: Unknown
timestamp: February 17, 2020, 01:36:35 pm
content: Doesn't this circuit damage the IC?I would expect a current that would have destroyed it as in a normal diode.It looks like the internal behaviour is different.Is it possible to replace the IC with a zener diode?Cheers

author: Lucky-Luka
author_rank: Unknown
timestamp: February 17, 2020, 02:12:03 pm
content: Another question: why 12V?Am I wrong or the reverse breakdown voltage isn't mentioned in the datasheets?Cheers

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2020, 09:52:35 pm
content: Firstly, it's not an IC, but a single transistor. IC stands for integrated circuit, which contains more than one transistor or component on the same chip.To answer your questions, yes this circuit will damage the transistor, which is not designed to work in this mode, hence no mention of the breakdown voltage on the datasheets. What you'll more often see is a maximum reverse voltage rating for the base-emitter junction, but this will be significantly lower, than the breakdown voltage, to ensure the component can tolerate it, without damage.The circuit won't work with a zener diode, at least not very well, because they don't exhibit such an extreme drop in resistance, as the voltage increases and a sharp increase in resistance, as the current reduces. A zener diode tends to sit there, maintaining a constant voltage. You might be able to get some oscillation from higher voltage zener diodes, as they have more of an avalanche effect, but I doubt it'll be significant. I suggest trying it.

author: Whales
author_rank: Super Contributor
timestamp: February 17, 2020, 10:01:05 pm
content: Quote from: Lucky-Luka on February 17, 2020, 02:12:03 pmYes this particular behaviour is generally not specified. They don't expect it to be financially useful for them. EDIT: some might, but probably not the level of enter and exit thresholds. There's lots of other stuff not specced in many transistors, eg using mosfets backwards but below their reverse diode threshold (they work as amps).Individual transistors vary, when I made some of these oscillators a year or two back I needed 12V or a bit above for it to be reliable. Different makes of the same part (eg P2N2222) will probably have different performance here by nature of it not being specified or controlled.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 17, 2020, 11:34:31 pm
content: Here is shown the negative resistance effect when CE breaks down.Horizontal scale is 2V/divVertical scale is 1V/divThis is a 2N2222.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 18, 2020, 01:12:57 am
content: The key aspect of the avalanche behavior in reverse breakdown is that once it starts to conduct it will continue conducting even as the voltage drops below the breakdown voltage - this is the negative resistance region. This allows the capacitor to discharge all the way down to the diode forward voltage before the diode stops conducting and breaks the circuit, which also stops the avalanche effect and resets the circuit to the charging state.Zeners don't really do that.

author: Kerlin
author_rank: Regular Contributor
timestamp: February 18, 2020, 08:44:06 am
content: The simplest LED flasher is to drive the LED, through a resistor with a higher than specified current but not so much as to blow it. Just enough to cause it to get hot and go faulty.It will then flash on and off.It will even last for weeks. LOL !

author: Lucky-Luka
author_rank: Unknown
timestamp: February 19, 2020, 02:40:41 pm
content: Hi allI've tried to replicate the circuit mentioned here:http://cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.htmlI've obtained different traces on the scope.I have to say that I have a slightly bigger cap (470uF).I am a beginner so maybe I haven't set the scope properly however I notice a different trace regarding the voltage across the LED. I've seen several examples online and they obtain a precise shape: that's not the one I have obtained.Any suggestions?Cheers

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2020, 06:46:37 pm
content: The oscilloscope trace looks good, but you forgot to include the LED and where the probes are connected, in the schematic.

author: Lucky-Luka
author_rank: Unknown
timestamp: February 19, 2020, 06:49:31 pm
content: The yellow trace is related to the positive leg of the capacitor while the red trace is related to the positive leg of the LED.

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2020, 07:22:41 pm
content: The waveforms look reasonable to me. What are you expecting to see on the scope?The LED is not in your schematic at all, where is it connected?

author: Lucky-Luka
author_rank: Unknown
timestamp: February 19, 2020, 07:29:25 pm
content: ops you're right!I was expecting to see the blue trace in the attached image at the positive leg of the LED.

author: BravoV
author_rank: Super Contributor
timestamp: February 19, 2020, 08:29:25 pm
content: And if you clip both probes at the LED's 100 Ohm resistor, before and after, use scope's math function, you can see the LED current, ideal for adjusting the cap size.

author: mikerj
author_rank: Super Contributor
timestamp: February 20, 2020, 12:06:31 pm
content: Quote from: Lucky-Luka on February 19, 2020, 07:29:25 pmYour trace shows exactly what I would expect to see across the LED. The blue trace is the current through the resistor, this is explained on the website you linked toQuote