Avalanche Diodes as EHT Stabilisation

[Murphyv310]

Hi.
On my Winter Project thread you may have seen that I used nine BYV95C diodes connected in series for EHT Stabilisation.
I have to say it is a total success with the EHT rock steady on my EHT meter of 5.5Kv.
Has anyone else tried this form of regulation?

[ppppenguin]

Can't see any reason why they won't work. It's not conceptually different to a string of zeners or a shunt stabiliser. The current available from the EHT generator is small so the diodes won't suffer. Not so sure about the EHT generator which may get a bit stressed by supplying a relatively high current continuously.

The BYV95C seems to be obsolete. I haven't looked for modern replacements.

[Murphyv310]

Hi Jeffrey.
The diodes have been fitted at the end of the EHT smoothing which comprises two 1nf caps and three resistors that add up to 2meg so total current is limited anyway, at maximum brightness which is well above normal viewing the EHT sags to 5kv. I believe these diodes are soft recovery so unlikely to fail in even a higher current EHT circuit.

[`FRANK.C]

Hi Trevor
I have never tried that, I didn't even know that avalanche diodes were available with 600V ratings.
Something to keep in mind for the future.

Frank

[Murphyv310]

Hi Frank.
you can get 1000v versions too, BYV95E is the 1Kv version. There are other types as well, I'll check later and see if there is some that are not Obsolete.

[ppppenguin]

I beleive that almost any rectifier will act as an avalanche diode though the voltage will not be predictable. It may well be stable so experiments with a string of 1N4007 might give promising results.

PS: Just tried it with a random 1N4007, 1M series resistor and my bench 5kV variable PSU. Looks like it stabilised at about 2kV across the diode.

[Murphyv310]

Good point Jeffrey.
Something else to spend some hours on 

[Geordie McBoyne]

TV service engineers will remember those avalanche diodes employed for over volts protection. In the event the HT supply voltage rising over a predetermined level the diode would conduct and shut down the supply either by crowbar action or by an alternative OVP system which initiated a trip action. The receiver would shut down followed by a supply reset.

Geordie McBoyne.

[Murphyv310]

Hitachi used them and they would short out when the power supply went into over volts.
Not really the intended use for the diode I would say.

[ppppenguin]

AFAIK all PN junction diodes exhibit avalanche breakdown. The unknowns are around how well controlled it is and what level of current it can stand before damaging the device. The zener effect appears similar but is a different phenomenon. Zener breakdown is typically below 5V.

https://en.wikipedia.org/wiki/Zener_effect

This article claims that avalanche breakdown has hysteresis, rather like in neon bulbs. There is no citation for the claim
https://en.wikipedia.org/wiki/Avalanche_breakdown