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Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
I have a NST that I was testing. It is an old-style iron core unit about 50 years old. It is an Acme Electric rated at 12 KV and 30 ma with a 120 volt input. This should translate to 17.5 KV peak with my 124 volt wall outlet voltage. I convert the output to DC using a full wave bridge rectifier followed by a 400 pF filter capacitor in parallel. There is no other load. When I measure the DC output, it registers as 21 KV. Can anyone explain this excess voltage level? Does this NST just have a wider tolerance?
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
That sounds about right to me. In my experience, the no-load voltage has always been a bit high. Keep in mind that these NST's were designed to illuminate neon filled tubes. These tubes need a very high voltage pulse to start-up. Then when the gas becomes conductive (and less resistive), the shunted output voltage drops drastically and the current goes up--NST's are shunted (current-limited) and usually can take a continuous short for a long, long, time. When the neon tube(s) are illuminated, I have measured only greater than a couple hundred volts on some! I have also removed shunts from some to increase the current. It can be a bit risky, but you can use a DMM to measure the current. Set the meter on the high-current range (10A-20A), make SURE the NST output connections are very securely clamped to the two NST leads and plug it in! The readings are amazingly accurate WRT the label rating. I de-tarred a 60mA 15kv NST 'till I got 70+ mA from it and then potted it wax. It is usually important to be sure the NST is used with the earth ground, as to use both sections to prevent over-voltage in open circuit cases--still NST's like a load to lower the full open-circuit voltage.
I always get the feeling that NST's were so much safer than MOT's. Hope this helps a bit.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
I measured the voltage in two ways. I used a commercial HV DC meter rated to 40 KV connected across the output of the full wave bridge. I also used a 25 microamp meter with a 1G ohm HV resistor to limit the current and make the microammeter display voltage. The commercial HV DC meter measured 20 KV while the microammeter with the series resistor measured 21 KV thus the two independent measurements are very close.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
Signification wrote ...
That sounds about right to me. In my experience, the no-load voltage has always been a bit high. Keep in mind that these NST's were designed to illuminate neon filled tubes. These tubes need a very high voltage pulse to start-up. Then when the gas becomes conductive (and less resistive), the shunted output voltage drops drastically and the current goes up--NST's are shunted (current-limited) and usually can take a continuous short for a long, long, time. When the neon tube(s) are illuminated, I have measured only greater than a couple hundred volts on some! I have also removed shunts from some to increase the current. It can be a bit risky, but you can use a DMM to measure the current. Set the meter on the high-current range (10A-20A), make SURE the NST output connections are very securely clamped to the two NST leads and plug it in! The readings are amazingly accurate WRT the label rating. I de-tarred a 60mA 15kv NST 'till I got 70+ mA from it and then potted it wax. It is usually important to be sure the NST is used with the earth ground, as to use both sections to prevent over-voltage in open circuit cases--still NST's like a load to lower the full open-circuit voltage.
I always get the feeling that NST's were so much safer than MOT's. Hope this helps a bit.
I don't really see a reason to connect the NST case to earth ground other than possibly for safety. The fact that the center point of the secondary is connected to the core should be enough to protect it by halving the voltage stress on the secondary.
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NST voltage output
