Unusual NST behavior
MRMILSTAR, Thu Jul 25 2019, 08:14PMMaybe someone can explain some unexpected NST behavior. I am using an old-fashioned NST for a project. It is a Magnetek rated at 15 KV and 60 ma. If I connect the NST to 120 VAC, I can draw an arc between either HV terminal and an earth ground. The NST is not grounded, the case is floating. How can this be? There does not appear to be a path for a complete circuit. I know that the center tap of the secondary is connected to the core but the core shouldn't be connected to anything else other than the case, which is floating.
Re: Unusual NST behavior
Sulaiman, Thu Jul 25 2019, 10:48PM
Draw a circuit diagram for an NST and you will be able to answer your own question.
Sulaiman, Thu Jul 25 2019, 10:48PM
Draw a circuit diagram for an NST and you will be able to answer your own question.
Re: Unusual NST behavior
MRMILSTAR, Fri Jul 26 2019, 03:16AM
The first thing that I did was to draw a schematic. I see no complete circuit path. Didn't you think that I did that before posting?
MRMILSTAR, Fri Jul 26 2019, 03:16AM
The first thing that I did was to draw a schematic. I see no complete circuit path. Didn't you think that I did that before posting?
Re: Unusual NST behavior
radiotech, Sat Aug 10 2019, 10:33AM
The 120 volt power line is connected to an earth ground. One side is directly connected, The other side
is connected also to the earth ground through an extremely low impedance. Across this impedance is
an AC voltage of 120.
The case of the NST has an electrostatic capacity to both line terminals.
The 7500 volts flows through this capacitance. Lets assume it is 1E-10 F. The current would be 0.3 mA.
That would pull an arc.
Also arcing the transformer like that is asking for trouble. You set up high voltages in unexpected
places, like across the output insulators to case.
An energized NST needs a load.
If you have a DVM, with all the power off, use the capacity scales to measure case to output, output to line.
These days a 6000 count DVM from China costs about $30. and can measure capacity over a wide range.
radiotech, Sat Aug 10 2019, 10:33AM
The 120 volt power line is connected to an earth ground. One side is directly connected, The other side
is connected also to the earth ground through an extremely low impedance. Across this impedance is
an AC voltage of 120.
The case of the NST has an electrostatic capacity to both line terminals.
The 7500 volts flows through this capacitance. Lets assume it is 1E-10 F. The current would be 0.3 mA.
That would pull an arc.
Also arcing the transformer like that is asking for trouble. You set up high voltages in unexpected
places, like across the output insulators to case.
An energized NST needs a load.
If you have a DVM, with all the power off, use the capacity scales to measure case to output, output to line.
These days a 6000 count DVM from China costs about $30. and can measure capacity over a wide range.
Re: Unusual NST behavior
MRMILSTAR, Mon Aug 12 2019, 03:16PM
That is the only logical explanation that I have read. I will measure the capacitance and find out.
MRMILSTAR, Mon Aug 12 2019, 03:16PM
That is the only logical explanation that I have read. I will measure the capacitance and find out.
Re: Unusual NST behavior
Patrick, Tue Aug 20 2019, 01:54AM
MRMILSTAR this may be useful :
Theres quite abit of bizarre and non ideal components to a NST 's behavior some deliberate, most not.
Patrick, Tue Aug 20 2019, 01:54AM
MRMILSTAR this may be useful :
Theres quite abit of bizarre and non ideal components to a NST 's behavior some deliberate, most not.
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