Strange? behaviour in a NST

[Alberto]

Hi everyone.

I have bought this cheap NST on a Chinese website. It is supposed to give 10Kv but the arc can start at 6 cm. The other weird thing is if I connectit in when the wires are 6cm or less the arc start and keeps arcing. But if the wires are more than 6 cm they only generate a small spark at the tip and nothing else happens. Even if I get closer. I need to disconnect and reconnect to "restart"

And fortunately, I am very careful and never touch the wires in the plastic. They arc between them! Very good insulation ...

What you think? It is the first time that I have an NST and I do not do it if this is normal.

Thank you

Kind regards

[davekni]

Most NSTs now are inverter circuits feeding a high-frequency (~30kHz) transformer.  They have internal circuits to detect fault conditions and shut down on faults.  One fault is an arc not starting.  I'm surprised that it doesn't also shut down due to low arc voltage - but you did say that it was cheap.  (I expect that the 6cm air arc is lower voltage than a properly-working neon sign tube.)

[Alberto]

Most NSTs now are inverter circuits feeding a high-frequency (~30kHz) transformer.  They have internal circuits to detect fault conditions and shut down on faults.  One fault is an arc not starting.  I'm surprised that it doesn't also shut down due to low arc voltage - but you did say that it was cheap.  (I expect that the 6cm air arc is lower voltage than a properly-working neon sign tube.)

Thank you for your answer.

Ok ok now I understand, but, if is 10kv rated, how can it generate 6cm arc? 6cm must be around 60 kv, isn´t it?

[davekni]

Voltage depends on the shape of the electrodes (wire ends) besides distance.  However, 60kV may be reasonable.  If the 10kV rating is for operating, NSTs generate a higher starting voltage.  60kV sounds high for starting a 10kV neon tube, but I don't have much experience with this.

[Alberto]

Ok ok, thanks, You are very helpful!

[Alberto]

Ok, I have found that from wikipedia

All NSTs are designed to produce a high voltage starting pulse to a tube, then limit the current through the tube when it has started.