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A Festive High Voltage Load

Posted: Sun Dec 24, 2017 7:55 am
by Rich Feldman
Finally made & tested a 51,000 ohm 175 watt ( 3000 V x 58 mA ) resistive load concept. It started out as a string of 25 C9-size Christmas lights, bought for under $10 at a clearance sale last January.
These screw-base lamps are generally wired in parallel, an option sadly overlooked in otherwise good articles like this one: ... -parallel/

Current measurements this afternoon showed the lamp power to be 7 watts each at 120 V.
Resistance of the whole string is about 8 ohms cold and 82 ohms hot.

If the sockets were rewired in series, string resistance would be 625 times higher. It took about 15 minutes to make the conversion, by snipping wires here and there.
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(I found the uncut diagram in an excellent article here: ... christmas/)

Series I-V chart is derived from parallel by simply re-labeling the axes.
I lit the series string with a NST, and then with two 30 mA NST's in parallel on a Variac. Stopped at about 80% of nominal input voltage, when the NST knob voltages reached 1000 V on one side and 935 V on the other side. Next steps: a voltage divider to measure higher AC voltages. Then measurement of AC current in the 10-100 mA range without having to float the meter. Perhaps the current under test (in a wire at more than 1000 VAC) has multiple turns passing through an inductive sensor. Or put a bridge rectifier & LED side of an optoisolator in series. Are the current transformer toroids in GFCI's made of special stuff, to be usefully magnetized at 0.005 ampere-turns? How about high fidelity microphone transformers?

For the same or less money, one can get strings of C7 lamps, typically 5 W, with smaller screw bases. I've seen C7's in 4 W (for night lights) and 3 W, but never at seasonal-merchandise clearance prices.

Re: A Festive High Voltage Load

Posted: Sun Dec 24, 2017 10:08 pm
by Dennis P Brown
Nice christmas display but certainly shorting can be an issue with those lights so you might want to avoid placing these lights powered by those x-formers on your tree (lol.)

Re: A Festive High Voltage Load

Posted: Mon Dec 25, 2017 1:46 am
by Rex Allers

Nice seasonal touch to your avatar image!

Here's a copy of the image for those who might find this message after the holidays.

Rich Avatar.gif
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