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Explaining a quirk in a high-voltage power supply?

For my A2 Physics practical experiment, I performed an experiment on the voltage/pressure and current/pressure relationships of an electrical arc in a pressurised environment.

I had an unexpected quirk with my 5kV power supply where the current jumped up from ~0.2 mA to 1.2mA after a certain point (45 kPa and 2-2.5 kV). The only explanation I can think of for this would be changing the internal resistance to ensure a constant output voltage.

Does this sound plausible? How could I properly explain this? Is there a better explanation you can think of for this?

Thanks

Reply 1

uberteknik

What was the spark gap you were using to create the arc?

Were you measuring the point at which a spark appeared between the electrodes?

Reply 2

pratstercs

Original post by uberteknik

What was the spark gap you were using to create the arc?

Were you measuring the point at which a spark appeared between the electrodes?

It was a ~3mm gap between the exposed ends of two wires in a sealed flask connected to a vacuum pump.

I was measuring the minimum voltage required to sustain the arc once it has been started

Reply 3

uberteknik

Original post by pratstercs

It was a ~3mm gap between the exposed ends of two wires in a sealed flask connected to a vacuum pump.

I was measuring the minimum voltage required to sustain the arc once it has been started

So the current already flowing was 0.2mA and jumped to 1.2mA as you reduced the voltage in a repeatable way? i.e. this always occurs?

Reply 4

pratstercs

Original post by uberteknik

So the current already flowing was 0.2mA and jumped to 1.2mA as you reduced the voltage in a repeatable way? i.e. this always occurs?

It was when I was increasing the pressure in the flask from the previous reading, once it hit about 45 kPa the current just jumped up while supplying the same voltage. I did it about 7 times, with the same thing happening at the same point.

Reply 5

uberteknik

That is strange since one would expect from Paschen's law that the air dielectric breakdown voltage increases with pressure.

It could be some type of coronal discharge occurring in the spark leads where a pressure seal or insulator is rupturing at a certain pressure and allowing a discharge at that point.

That would be my first thought as it seems to be pressure related.

Reply 6

uberteknik

You could try changing the pressure flask or if you think it's p.s.u. related (and if you have a second p.s.u.) try that and see if either changes the result?

Reply 7

pratstercs

Original post by uberteknik

You could try changing the pressure flask or if you think it's p.s.u. related (and if you have a second p.s.u.) try that and see if either changes the result?

I'm doing the write-up of the experiment I did a couple weeks ago, I'm kicking myself for not investigating sooner. I didn't have another psu available, every single other one in the school was broken.

It could be something like a seal, but I didn't change anything between runs so it couldn't be a permanent rupture and there didn't seem to be anything untowards pressure-wise

The results were that the voltage increased with pressure. By the way, big thanks for pointing me to Paschen's law!

Reply 8

uberteknik

It could many things but from what you are saying this is equipment related and not an experimental result.

Another thought is that as the pressure increases, the stress on a weak point in the flask where the electrodes enter the vessel may be causing a physical displacement of the spark gap in some way?

I had a similar problem several years ago when measuring the characteristics of microwave diodes under pressure. The semiconductor junction was being stressed causing a physical deformation which resulted in a sudden increase in current which disappeared when the pressure was released below a certain threshold.

No matter, as long as you write up your experiment and include the erroneous result and your attempts at finding an answer and what else you would do to track the problem down, you won't lose marks over it.

In fact, finding problems in experiments and engineering is one of the great learning tools because it forces you to fully understand what you are trying to achieve and question everything.

Good luck with tracking the problem down. Let us know if you find out what caused it. :smile:

Reply 9

pratstercs

Original post by uberteknik

Another thought is that as the pressure increases, the stress on a weak point in the flask where the electrodes enter the vessel may be causing a physical displacement of the spark gap in some way?

I had a similar problem several years ago when measuring the characteristics of microwave diodes under pressure. The semiconductor junction was being stressed causing a physical deformation which resulted in a sudden increase in current which disappeared when the pressure was released below a certain threshold.