Does break down voltage actually damage the diode? - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: September 16, 2018, 05:51:38 pm
content: I can think of applications where you might want to have a diode conduct over a certain voltage such as igniting lights, charging things, providing voltage spikes etc. But do they make diodes that have a "Safe" break down voltage where the diode could be run in reverse repeatedly and not fail or get damaged?Or is there another part used instead. Jullian illet has a video on a diode like this but it seems like its for lightning or other one shot protection things.:Does anyone know if the "UV light" he uses is just a black light or puts out dangerous UVA UVB? UVC can't be made by a diode can it? Usually UVC lamps look blue not purple but if it only puts out UVC you wouldn't see it at all and suspect that blue color is from the mercury or gasses in the lamps... maybe. If you look at a very strong/dangerous Xray source it looks purple. Not sure if it is or if that was just the guys eyes getting cooked by radiation.

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 05:54:20 pm
content: Yes.They are called Zener diodes.They are just like normal silicon diodes (there are some differences). But they are specified to safely break down at their rated voltage and maximum power (dissipation) ratings.There are also devices, for protecting against very large voltages etc.See TVS and MOV devices. Be careful, they are different.https://en.wikipedia.org/wiki/Transient-voltage-suppression_diodeMOVs only have a limited life, as regards overloads/surges.https://en.wikipedia.org/wiki/Varistor

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 06:03:46 pm
content: Quote from: Beamin on September 16, 2018, 05:51:38 pmYou can get UV Leds, these days. I probably have some.But I'm not sure about what types are available as regards UVA/UVB/UVC.Because the range is often increasing as they get invented, and I've not looked into them, for quite a while.The ones I have are like the Black light ones, and someone showed me his less safe ones, for making PCBs with. I'm not sure if UVC Leds are available yet.BigClive recently showed a newish (it seemed to be, but could be old) type of filament UV lamp (dangerous UV frequencies, e.g. makes Ozone).

author: Beamin
author_rank: Super Contributor
timestamp: September 16, 2018, 06:08:55 pm
content: Quote from: MK14 on September 16, 2018, 05:54:20 pm

author: Benta
author_rank: Super Contributor
timestamp: September 16, 2018, 06:12:53 pm
content: There are rectifiers that will tolerate over voltage. They are called "avalanche rated".

author: Beamin
author_rank: Super Contributor
timestamp: September 16, 2018, 06:25:45 pm
content: Quote from: MK14 on September 16, 2018, 05:54:20 pmFor some reason I knew zeners could do that but didn't think it was safe for them.I had a microwave oven that when you set it to cook it would work for about 1 second; fan starts and just as it gets to full power it shuts off including the control panel and display. You had to unplug it to get the it to work again where the timer and door light would work but repeat the same thing when fully on. I never got to take it apart but wouldn't that be a MOV heating up then as the tube reached the full 900 watts the MOV over heats and fails? Or are MOV one shot like fuses? After a MOV goes into failure mode are they supposed to be replaced? When you open a surge protector and the only circuitry is three blue disk cap like devices these are just MOVs? Is this all that's need for a good surge protector or is that a cheap low quality one? It would be better to make the surge protector a device that had two 3 pin outlets on the back and was the area of the outlet and 1" thick is that's all you need. Much better then a power strip.While on the subject of surge protectors I have a Furman AC215 power conditioner that was told it was about 300 bucks sold as a thing to make highend hifi "sound better".. maybe...I got it free when I bought a friends plasma TV back when they cost 3k$. It has a giant toroid inside it and was a 9"X5"X1" box. Should I be using this for my sensitive electronics?If not what's it good for? I don't think you need to worry about "dirty ac" when you already have a high end audio amp they should have filters in the power supply. Someday I'll test it on the scope when I find how to safely build a decoupling circuit and not blow the scope because I tied the neutral, hot and ground by mistake.

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 09:21:16 pm
content: Quote from: Beamin on September 16, 2018, 06:25:45 pmThat doesn't sound like a MOV (best to read up on them, they can somewhat rapidly wear out, each time they absorb a surge, some of the MOV burns away inside the device, weakening it each time it fires. So perhaps after 5..25 (very approximately, varies wildly, I'm not sure if MOVs can survive low energy surges, without wear or not, best to read up on them), the MOV needs replacing and can even be a fire risk. If suitable thermal fusing or other protective measures are not in place, especially as regards mains surge protectors.I'm not 100% sure (I know a bit, but not a great deal about them, currently), I've not kept up with all microwave developments.But it sounds something like, your microwaves safety overload system (thermal trip or whatever mechanism, your microwave was using), seems like it was doing its job, correctly. Apparently safely shutting down. Rather than letting out the magic smoke, and giving your local fire department, some extra work to do that night.

author: Cerebus
author_rank: Super Contributor
timestamp: September 16, 2018, 09:55:53 pm
content: Quote from: Beamin on September 16, 2018, 05:51:38 pmYou have to remember thatyouare not seeing it, you are seeing what the sensor in the camera saw - a sensor that will have a distinctly different spectral response to the human eye.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: September 16, 2018, 10:31:28 pm
content: When you select a diode with a voltage rating, that voltage rating is a guarantee that the zener voltage is that voltage or higher. When I was in school I was taught that this reverse breakdown voltage is called the zener voltage. The zener diodes are diodes that have a specific breakdown voltage or reverse bias voltage.With that said, an actual zener diode has been a little further doped to improve the knee transition so that the breakdown is a bit sharper to prevent the diode from being in the linear region too long and also at a specific voltage. I am not an expert, but this is what I was taught in college (back in the paleozoic).As far as finding the reverse breakdown voltage of a regular diode and utilizing it, you simply need to insure that you are not reverse biasing in the linear region, so as not to heat the diode too much. You can look on your scope to check this curve. You can then utilize the reverse breakdown voltage to your needs, and if you are clever, you can even utilize the knee curve to your needs if necessary. The problem with random diodes is that you will probably not find 2 that are exactly the same and that is why we select a "zener" diode at a specific rating. The search is done for us.Hope this helps...

author: Buriedcode
author_rank: Super Contributor
timestamp: September 16, 2018, 11:50:11 pm
content: Re the UV:Quote from: Beamin on September 16, 2018, 05:51:38 pmMost likely its just a ~395-405nm LED used to "cure" the epoxy. It's not ideal for that but it can work with certain adhesives - which is why you can easily find "UV cured" kits about these days. This would be at the extreme end (long wave) of UVA, although it seems the ranges vary depending on who you ask (UVA being ~315nm to 400nm). I believe there are "germicidal" (UVC ~220nm) LED sources but these are still very expensive and pretty crap efficiency which is why you can still get UV tubes.Often materials that fluoresce under UV will do so under a wide range of wavelengths, including 400nm. As its pretty visible anyway the effect isn't as dramatic as with a blacklight (that emits little visible light, and also deeper UV meaning the object appears much brighter). Buy some cheap 400nm UV LED's and test some stuff - its probably safer for the eyes than using a blacklight. One thing I noticed was, many alcohol spirits fluoresce slightly.The reason you see "purple" when looking at such sources is because they don't just produce one wavelength - nothing does - every light source (or EM) produces a range of wavelengths. And you are seeing the 380-400nm that the eye can pick up. These sources can be very narrow, such as with lasers (that produce other lines in the spectrum too, but often thin ones, <5nm) or LED's (wider but still fairly monochromatic) or very wide - "blacklights" use special glass that blocks emissions above about 400nm but produce a wide range of UV mostly invisible to you.This is why arcs and sparks can be so dangerous to the eye, sure, you can see bright light (often blue) but because they kick out such a wide range of wavelengths, invisible UV can easily permanently damage your eyes. In the case of welding, I believe IR is actually more harmful, but don't quote me!Seeing purple when looking at an x-ray sources is either because it produces a slight 400nm line from the source (which is often some kind of HV arc in glass) or indeed something in your eye fluorescing from the x-rays, either way, please don't do it. Whilst people quote ranges of wavelengths the eye can respond to, they forget we can "detect" shortwave UV and longwave IR if it is strong enough, its just not necessarily considered to be "vision" (burning sensation, or blindness from a 200W source is technically "detecting").

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2018, 03:15:57 pm
content: Quote from: Buriedcode on September 16, 2018, 11:50:11 pmI think the purple light emitted from strong X-ray sources is more likely the air being heavily ionised, causing it to emit a purple light, similar to that given off by high voltage corona discharges, rather than the eyes. Air absorbs the longer x-ray wavelengths quite will, so I'd expect this effect to be more pronounced with high powered, soft x-ray sources, than hard x-rays.

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2018, 04:06:21 pm
content: Quote from: MK14 on September 16, 2018, 06:03:46 pmSimilar filament/mercury discharge UV lamps have been around for decades. My grandmother had a clothes dryer made in the late 60s that used one to make the clothes smell fresh. I've seen sterilizing toothbrush holders with them too. I hadn't seen the lamps in decades though so I bought one to play with after seeing that video.

author: mzzj
author_rank: Super Contributor
timestamp: September 17, 2018, 04:44:42 pm
content: Quote from: tpowell1830 on September 16, 2018, 10:31:28 pmAFAIK some old diodes can actually fail catastrophically ( breakdown starts at the edges, goes around the die edges or is a "punch-trough" type instead of avalanche ... or something like that) but most modern ones can handle some reverse currenthttps://groups.google.com/forum/message/raw?msg=rec.radio.amateur.homebrew/XLRgaCb7XQE/tO2M_k_fleYJBTW Random 1N4148 reverse current:0,02uA at 100V,10uA at 128v100uA at 135v1mA at 139v

author: bson
author_rank: Unknown
timestamp: September 17, 2018, 07:32:49 pm
content: Quote from: Benta on September 16, 2018, 06:12:53 pmIndeed. More generally, diodes intended for safe breakdown operation are calledavalanche diodes, regardless of whether they technically avalanche or not.

author: Synthtech
author_rank: Regular Contributor
timestamp: September 18, 2018, 10:26:19 am
content: A lot of vintage gear that I work on use reversed biased base-emitter transistor junctions as a white noise source using standard transistors such as a 2SC945. They have been working fine in their avalanche region for decades.Whether this messes up the transistor for use as a transistor I don’t know.

author: Cerebus
author_rank: Super Contributor
timestamp: September 18, 2018, 05:30:36 pm
content: Quote from: Synthtech on September 18, 2018, 10:26:19 amIt's generally accepted that 'zenering' BE junctions is a Bad ThingTMthat will generally alter the characteristics of the transistor even if it doesn't destroy it.Probably worth a word of two about the damage mechanisms in play when you 'zener' a junction that wasn't explicitly designed for it.Gross damage can be caused by the simple mechanism of dumping too much power into the device. A diode that is designed to handle, say, 1W will generally produce that with a voltage drop of, say, 0.75 and so at a current of 1300 mA. Stick 1300mA through the same device wired as a zener at, say, 6V would produced a dissipation of 8W. Obviously at an 8 times higher thermal load something is quite likely to give, perhaps even melt.When you're not exceeding the power rating of the device the mechanism of damage is more subtle. Avalanche "zenering", such as in a transistor BE junction, inherently involves electrons that are moving fast enough to ionize the atoms that they hit going across the junction (impact ionization). If you get electrons that are moving fast enough they will have enough energy to not merely ionize atoms, but (simplistically) to start introducing faults in the crystal lattice. That's much more subtle and can alter the characteristics of the junction without destroying it.

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2018, 10:13:23 pm
content: I reverse engineered a commercial HeNe laser power supply brick that had two 2N3904 transistors wired with only the B-E pins as a zener. They were made like this for decades, I think the dead one I de-potted was about 30 years old and the failure wasn't related to those.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2018, 02:23:18 am
content: Yes, it can. Back in the bad old days of early silicon diodes, avalanche wasn't typical, and diodes were rated by maximum voltage without breakdown (as opposed to minimum breakdown voltage at some bias current).Processes weren't quite well enough controlled, and what happens is a microscopic defect breaks down, concentrating the entire applied current (~mA?) into a tiny area, which burns through and shorts the diode.Anywhere you see a rating that applies current, you are guaranteed to have at least that much avalanche capability.Although not necessarily more current than that. The defects might be less dramatic than the early diodes, but not so good as to avalanche the whole diode junction evenly. For that, there's actual avalanche rated parts. TVSs are made to break down over the whole die, and handle considerable energy.Some rectifier diodes do carry an energy rating, 1-20mJ being fairly common. In the case of schottky diodes, this is absorbed by the guard ring, a PN junction that surrounds the schottky junction. The same may be true of other types, I don't know. This is why the energy rating might be quite small relative to the overall die size.There are still plenty of diodes and transistors that can't handle avalanche. In the transistor case, you see V_CES ("collector-emitter sustain voltage"), rather than V_CEO (collector-emitter-open voltage, i.e., voltage applied C-E, with B open/unconnected).I don't know of any IGBTs rated for avalanche. Probably for good reason, like parasitic SCR activation from putting free charges everywhere.InGaN LEDs (i.e., blue, white, high efficiency green, etc.) don't seem to handle any avalanche, they die suddenly at 20-30V even with very low bias current. Older LED types (GaP, GaAs and alloys -- IR to green) usually handle a few mA, at random voltages (20 to >200V).Tim