[SOLVED] high voltage zener operation

[Zak28]

Would a zener break if its regulating voltage from a source many times greater its nominal voltage? Supposing the zener is run within power limits this circuit boils the question down,


I just don't know if the zener would break or actually work as it should. Is there an actual limit to what source voltage a zener can regulate from? Agian the zeners power would not be surpassed.

 

[Audioguru]

The Zener diode does not care what voltage powers its current-limiting resistor. The resistor (R1) will become extremely hot so it must be huge or cooled somehow, and it must be large enough to prevent an arc across it.

 

[asdf44]

Parts in a circuit only “see” the voltage that is across their terminals. In this case the zener has no knowledge of the source voltage (the R is the one that sees that).

Though if 50KV is a real scenario there would be many practical hazards and a need for additional protection.

 

[smijesh]

“Warning” Do not try this circuit. Zener diode is designed for maximum 800V(I guess) because if you applied more than 800V there is a chance for arcing voltage outside the body(Body having 3mm length).
Imagine if you apply 50,000V across 3mm Zener. It is applicable to resistor also
Use potential transformer for this application.

 

[Easy peasy]

Also 5mA in the zener + say 5mA in your "load" is 10mA from the 50kV = 500 watts from the load, ~ 500 watts in the dropping resistor - not a very efficient or efficacious way to power anything from 50kV - find another source ...

 

[FvM]

if you applied more than 800V there is a chance for arcing voltage outside the body

A brief view on the zener diode I/V characteristic clarifies that this can't ever happen (at least not without blasting the zener diode chip before). Sounds like you are continuing the error of reasoning in the initial question.


In wonder what's the purpose of the circuit, what's the HV source's available current and power?

 

[KlausST]

Hi,

“Warning” Do not try this circuit. Zener diode is designed for maximum 800V(I guess) because if you applied more than 800V there is a chance for arcing voltage outside the body(Body having 3mm length).
Imagine if you apply 50,000V across 3mm Zener. It is applicable to resistor also
Use potential transformer for this application.

This post doesn´t apply to the circuit in question at all.
You don´t need to guess the zener voltage, it´s given as 12V in the schematic.
With a properly calculated current limiting resistor the voltage across the zener won´t be more than 13V.

Klaus

 

[Zak28]

...properly calculated current limiting resistor...

The resistor would be quite small considering the zener is to-3 & 50w rated.

 

[KlausST]

Hi

The resistor would be quite small considering the zener is to-3 & 50w rated.

No.
Neither the resistor value, nor the mechanical size will be small.

Klaus

 

[FvM]

The resistor would be quite small considering the zener is to-3 & 50w rated.

Makes even less sense. To get 50W power dissipation in the zener diode (about 4A zener current), you need to burn 200 kW in the current limiting resistor with 50 kV input voltage. This looks like a purely theoretical problem.

Can you please explain what you want to achieve and come back with meaningful circuit parameters?

 

[Audioguru]

This thread is a silly tremendous waste of power. Why would a company make a 12V 50W Zener diode in a TO-3 case? I looked on Digikey and there are none but they have stock of a stud-mounted one that costs $41.92US.
EDIT: High power resistor? Yes, Digikey has stock of huge 2500W power resistors that cost $101.62 each but not with the correct value to connect 80 of them in parallel to dissipate 200,000W.

 

[Nik_2213]

Be very, very careful as the voltage across that single resistor probably exceeds its 'envelope' rating, risking catastrophic internal breakdown. You may need a chain of HV-specified Resistors to be sure, to be sure.

There are also issues with mounting, creep etc etc which I am not qualified to discuss.

 

[chemelec]

Normal Resistors are Only suitable for LOW Voltage Applications

For 50KV, You would need Some HV Type Resistors to prevent Surface leakage.
Dale/Vishay makes some "ROX-6", 37KV Resistors.
This 37KV Resistor is 6 Inches long and you would need Two in Series to handle 50KV without Surface Leakage.

These resistors are usually Custom made for Specific Resistance values.
Resistance Values up to 100 Gig-Ohms or more are available.

I have numerous ones that I bought in the past.

 

[Zak28]

...the resistor value...

Infact no current limit resistor is obligatory if diodes temperature and power ratings are not exceeded.

 

[asdf44]

Yes but without a current limiting element both power and temperature will be exceeded.

 

[d123]

Hi,

At power-up ( no soft start cheating), would a circuit like this with one resistor + 12V zener or 1 million resistors + 12V zener not experience such a huge voltage and perhaps to a degree current spike in the nanoseconds while it settled that it would just burn up in an audibly perceptible "pfft" immediately or doesn't it work like that?

 

[chemelec]

The Resistor is the Current Limit.
At least if you Calculate it Correctly.

 

[FvM]

Infact no current limit resistor is obligatory if diodes temperature and power ratings are not exceeded.

How is this statement related to the post #1 circuit und question? You didn't yet explain what's the problem behind this thread.

What's the relevance of the 50 kV source? It seems just fictive.

 

[asdf44]

Hi,

At power-up ( no soft start cheating), would a circuit like this with one resistor + 12V zener or 1 million resistors + 12V zener not experience such a huge voltage and perhaps to a degree current spike in the nanoseconds while it settled that it would just burn up in an audibly perceptible "pfft" immediately or doesn't it work like that?

That depends on the parasitic C of the R and the start up transient of the source. Even assuming an instant turn on I doubt the C of a real life resistor would pose a real threat to a large zener even in this 50KV scenario (0.5CV^2). It could also be mitigated with C across the zener.
E

 

[chemelec]

Just because you have a 50,000 Volt supply, Is it actually capable of delivering 250 watts?