Electronics > Beginners

Does break down voltage actually damage the diode?

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Zero999:

--- Quote from: Buriedcode on September 16, 2018, 11:50:11 pm ---Re the UV:


--- Quote from: Beamin on September 16, 2018, 05:51:38 pm --- 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.

--- End quote ---


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").

--- End quote ---

I 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.

james_s:

--- Quote from: MK14 on September 16, 2018, 06:03:46 pm ---
--- Quote from: Beamin on September 16, 2018, 05:51:38 pm ---UVC can't be made by a diode can it?

--- End quote ---

You 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).

--- End quote ---

Similar 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.

mzzj:

--- Quote from: tpowell1830 on September 16, 2018, 10:31:28 pm ---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...

--- End quote ---

AFAIK 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 current
https://groups.google.com/forum/message/raw?msg=rec.radio.amateur.homebrew/XLRgaCb7XQE/tO2M_k_fleYJ

BTW Random 1N4148 reverse current:
0,02uA at 100V,
10uA at 128v
100uA at 135v
1mA at 139v

bson:

--- Quote from: Benta on September 16, 2018, 06:12:53 pm ---There are rectifiers that will tolerate over voltage. They are called "avalanche rated".

--- End quote ---
Indeed.  More generally, diodes intended for safe breakdown operation are called avalanche diodes, regardless of whether they technically avalanche or not.

Synthtech:
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. 

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