Author Topic: Trouble powering a NST with a DC:AC inverter  (Read 629 times)

Offline klugesmith

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Trouble powering a NST with a DC:AC inverter
« on: May 25, 2024, 04:24:57 AM »
Last weekend I bought a 12V-to-120V inverter to demonstrate things for sale at the electronics flea market. https://www.electronicsfleamarket.com/

One sale item was a big old NST, nominally 15 kV and 30 mA.   Its HV bushings were set up with stiff wires to make a spark gap, which worked fine plugged into the wall.

I chose an inverter nominally capable of 500 watts (continuous), with "modified sine wave" output, that came with cigarette lighter plug and clip leads for battery terminals.  Cost me US $55. 
It ran small lamp and motor loads just fine.

But when I turned it on with the NST attached, the inverter died after 10 seconds.  I could not get it back to green-light state, even with no load, and after temporarily disconnecting the 12V supply (the jump start terminal in engine compartment of a Prius).
Days later, I gave it one more try with no load, and direct connection to a new-ish car battery.

When switched on it made a popping noise and smelled like burned electronics.  The green LED went to blinking red with repeated beeping noise.  I guess that was the final consequence of latent damage.   Should I get my money back from Best Buy, or open the unit and see where the smoke came from?   

We would expect the inexpensive inverter to include protection against overcurrent, such as from a heater or an overloaded power tool.  I know that plugged into wall, a "1530" NST can't deliver even 250 watts into a resistive load.  I did not measure its input current or power factor, with output open or shorted.
My guess is that inverter was killed by extreme inrush current (could exceed 50 A) which transformers and variacs can do if switched on at a bad time in the AC cycle.   The voltage zero-crossing time is the worst time for momentarily saturating the core.

p.s. at flea market, a neighboring seller sold me a 150W inverter with lighter plug for $5.   It served me well to demonstrate the lesser curios.  My first sale was the big old NST, for $50, without being operated at the event.
« Last Edit: May 25, 2024, 04:28:37 AM by klugesmith »

Offline klugesmith

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Re: Trouble powering a NST with a DC:AC inverter
« Reply #1 on: May 27, 2024, 07:28:27 AM »
Opened the failed inverter but it wasn't obvious where the smoke had come from.

The fan-cooled heatsink has two identical-looking transistors, which I guess go with the transformer to generate about 150 volts DC.
The small flat heatsink has four transistors, which must be an H-bridge to generate 60 Hz "modified sine wave".
I haven't yet had time to measure the primary current of a NST like the one that killed the inverter.

Note that both sides of the AC output are equally "hot" with respect to the 150 volt DC bus.   I bet the latter is galvanically isolated from the 12 volt DC input, for safety. We can see a couple of 4-terminal optoisolators spanning a narrow slot in the circuit board.

[edit] After downloading and reading the inverter's product manual and warranty policy, I'm seriously thinking of sending it for repair or replacement under warranty. https://cdn.shopify.com/s/files/1/2203/2035/files/500W_Web_Manual.pdf?v=1597236095   https://www.cobra.com/pages/repair-information I will honestly say I tried to run a NST; if that constitutes abuse that will be words from them and not from me.
Step 1 will be a phone call during business hours. 
« Last Edit: May 27, 2024, 07:55:59 AM by klugesmith »

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Re: Trouble powering a NST with a DC:AC inverter
« Reply #1 on: May 27, 2024, 07:28:27 AM »

 


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