Try this circuit quiz. - Page 1
Forum section: electronics / projects

author: moffy
author_rank: Super Contributor
timestamp: January 21, 2015, 12:51:55 am
content: The circuit below: npn_cct.png, shows a circuit with only 3 components.SPICE will not help you analyse it, and yes the transistor is the correct way around for this circuit.I have also included my solution as a .zip file which is password protected. I'll give the password later to show I haven't cheated.Two questions:1. What does it do?2. Why does it do what it does?Hope you have fun, and don't be afraid of offering solutions, I found it tricky, but many of you are far more intelligent than I am.P.S. I have added my solution as a plain PDF without the password.

author: c4757p
author_rank: Super Contributor
timestamp: January 21, 2015, 01:19:14 am
content: Sure you don't mean this circuit?Thisone will do something quite interesting, and is also unSPICEable... (And if not, you're all welcome to answer my quiz too...)

author: wraper
author_rank: Unknown
timestamp: January 21, 2015, 01:33:02 am
content: Acts like about 6V zener?

author: c4757p
author_rank: Super Contributor
timestamp: January 21, 2015, 01:45:29 am
content: Quote from: blueskull on January 21, 2015, 01:44:17 amNopeNo comment on the top one as it's not my quiz

author: IanB
author_rank: Super Contributor
timestamp: January 21, 2015, 01:56:50 am
content: Quote from: moffy on January 21, 2015, 12:51:55 amIt oscillates. In the old days a similar circuit as this would have been built with a neon lamp.The transistor breaks down due to exceeding the maximum emitter-base voltage (the transistor has to be connected backwards for this to happen). R1 limits the breakdown current so that the transistor doesn't die.After the transistor is conducting C1 charges up until the transistor drops out of conduction and now C1 starts discharging through R1. Eventually the breakdown voltage across the transistor is exceeded again and the cycle repeats.

author: moffy
author_rank: Super Contributor
timestamp: January 21, 2015, 04:23:26 am
content: Quote from: IanB on January 21, 2015, 01:56:50 amYou guys are good! Especially IanB as you didn't even have to build it!My password for the zip file is: npnvco123It provides a slightly more detailed explanation based upon the current dependence of BJT secondary breakdown voltage.

author: Skimask
author_rank: Super Contributor
timestamp: January 21, 2015, 04:35:43 am
content: It's a retro encabulator.

author: cncjerry
author_rank: Unknown
timestamp: January 21, 2015, 06:45:13 am
content: Just goes to show ya that just about anything you build with a transitor turns into an oscillator, unless you are building an oscillator, of course.Jerry

author: Psi
author_rank: Super Contributor
timestamp: January 21, 2015, 07:36:20 am
content: Quote from: Skimask on January 21, 2015, 04:35:43 amhaha

author: bktemp
author_rank: Super Contributor
timestamp: January 21, 2015, 07:47:42 am
content: The bottom one generates a negative voltage:When the E-B diode junction starts conducting as a Z-diode it acts as a LED. This light it picked up by the C-B diode and is converted into a voltage of a few 100mV.

author: bktemp
author_rank: Super Contributor
timestamp: January 21, 2015, 08:33:04 am
content: Quote from: blueskull on January 21, 2015, 08:01:31 amThey do generate yellow light. Just cut a transistor open and try it yourself. It needs about 8-12V applied to the emitter-base diode in reverse direction.

author: hamster_nz
author_rank: Super Contributor
timestamp: January 21, 2015, 09:08:30 am
content: Quote from: bktemp on January 21, 2015, 08:33:04 amAnything will glow with enough voltage applied in the reverse direction

author: Skimask
author_rank: Super Contributor
timestamp: January 21, 2015, 01:15:54 pm
content: Quote from: hamster_nz on January 21, 2015, 09:08:30 amOr in any direction for that matter...

author: moffy
author_rank: Super Contributor
timestamp: January 21, 2015, 11:05:34 pm
content: Quote from: bktemplink=topic=41817.msg591630#msg591630 date=1421829184The bandgap energy of silicon is in the infrared, so any photons emitted would be in that band. The yellow glow looks like something is getting very hot.

author: c4757p
author_rank: Super Contributor
timestamp: January 21, 2015, 11:10:11 pm
content: Quote from: moffy on January 21, 2015, 11:05:34 pmLooks that way, but no, it's not. Try it. You don't need anywhere near that much current.And no, I don't knowwhyit works.

author: c4757p
author_rank: Super Contributor
timestamp: January 21, 2015, 11:12:40 pm
content: Quote from: bktemp on January 21, 2015, 07:47:42 amYup, it does.Not 100% sure on the LED bit, though. Check this out, if you have access to IEEE Xplore:"Opposite-polarity voltage generator by hole impact ionization in a silicon bipolar transistor"Also, I'm a bit embarrassed to say I forgot that the first circuit oscillates! My first thought was just "Zener".

author: moffy
author_rank: Super Contributor
timestamp: January 21, 2015, 11:28:04 pm
content: Quote from: c4757p on January 21, 2015, 11:12:40 pmI like that explanation a lot better than the photon one, has some research to back it up to!Still looks like something is getting hot in the transistor photo. Sorry to disagree.

author: c4757p
author_rank: Super Contributor
timestamp: January 21, 2015, 11:28:27 pm
content: About 200-300mV at a few µA, that's it. It's massively inefficient (a few µA for ten or so mA put in, about 3µW for some 60mW, so bad that I'm tempted to report that efficiency in decibels... -43dB), and it may or may not destroy the transistor in the long term (never did try running one for a long time to see if it could handle continuous operation)...Quote from: moffy on January 21, 2015, 11:28:04 pmFor what it's worth, I couldn't get a germanium BJT to emit visible light just now - only thing I have that could be opened up. I've done it before with a silicon though, 2N3055.

author: moffy
author_rank: Super Contributor
timestamp: January 21, 2015, 11:32:15 pm
content: The good old 2N3055, that brings back memories of all those HI-FI kit amplifiers. It really was a staple for many years.

author: IanB
author_rank: Super Contributor
timestamp: January 22, 2015, 12:22:49 am
content: Quote from: c4757p on January 21, 2015, 11:28:27 pmYou vandal!Germanium transistors are valuable antiquities. Destroying them is not cool...

author: c4757p
author_rank: Super Contributor
timestamp: January 22, 2015, 12:24:41 am
content: Useless old crap, as far as I can tell... Nasty old Soviet "RF" transistors that are more like noise generators.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2015, 02:31:14 am
content: You get a similar effect from old school vacuum tubes:I have here a 6AL5 with 6.9V on the heater (a bit hotter than 6.3V plus tolerance, but not unusual for "117V" equipment...), which measures 0.74V cathode to plate (both in parallel, 10Mohm meter).With 330k load, it drops to 0.46V,With 56k load it drops to 0.34V,10k 0.22V,1k 0.086V.Or ~4.8uW from ~2.2W input, for an efficiency of -56dB.http://en.wikipedia.org/wiki/Thermionic_converterTim

author: bktemp
author_rank: Super Contributor
timestamp: January 22, 2015, 07:11:10 am
content: The yellow light emitted by the transistor does not change the color but only the brightness when changing the current. And the color also stays the same with different transistors.The color does not look like anything glowing. It is not a reddish yellow, but has more a beige tint. Therefore I doubt it is glowing. I have no idea how it works, but it does.This is a 2N2369A with a very tiny die, but compared to a 2N3055, it seems that a much larger area is emitting light.

author: dannyf
author_rank: Super Contributor
timestamp: January 22, 2015, 11:38:20 am
content: QuoteAvalanching oscillator.

author: moffy
author_rank: Super Contributor
timestamp: January 22, 2015, 10:40:55 pm
content: Quote from: bktemp on January 22, 2015, 07:11:10 amThat is interesting that it doesn't change colour, only intensity. That does kinda rule out something hot.Quote from: dannyf on January 22, 2015, 11:38:20 amIt is indeed. If you read my npn.pdf it gives a little more detail.