‰  NOW 40 WPM  ‰  TEXT IS FROM NOVEMBER 2012 QST  PAGE 65‰

THE CASE FOR A 12 V NEGATIVE CONNECTION.  EVEN THOSE WITH SOME ISOLATION
ARE NOT ALWAYS DESIGNED FOR THIS VOLTAGE.  THE SAFEST WAY TO USE AN
INVERTER UNDER THE CONDITIONS DESCRIBED IN THE ARTICLE IS TO PLACE A GOOD
ISOLATION TRANSFORMER ON THE INVERTER OUTPUT, THEN A SAFE, GROUNDED NEUTRAL
CAN BE ESTABLISHED AT THE TRANSFORMER OUTPUT.  THE GFCIS WOULD HAVE A
BETTER CHANCE OF WORKING AS INTENDED AND THE NEGATIVE SIDE OF THE 12 V
SYSTEM COULD BE GROUNDED FOR ADDITIONAL 

12 V DC TO ABOUT 120 V THE EXACT VOLTAGE DEPENDS ON THE DESIGN OF THE UNIT.
THIS VOLTAGE IS OFTEN CALLED THE LINK BUS, BECAUSE IT LINKS THE BOOST STAGE
WITH THE SECOND STAGE, WHICH INVERTS THE HIGHER VOLTAGE DC TO AC.  THE
INVERTER CONSISTS OF FOUR TRANSISTORS, WHICH SWITCH THE HOT AND NEUTRAL
OUTPUT LINES BETWEEN THE POSITIVE OR NEGATIVE SIDES OF THE LINK BUS AT THE
DESIRED OUTPUT FREQUENCY.  THE SWITCHING OF THE HOT AND NEUTRAL TRANSISTORS
IS STAGGERED TO PRODUCE A BETTER OUTPUT VOLTAGE WAVEFORM.  FIGURE 3 SHOWS
THE APPROXIMATE TIMING OF THE SWITCHING.  AT THE START OF THE GRAPH, BOTH
THE HOT AND NEUTRAL LEADS ARE CONNECTED TO THE BUS AND THERE IS NO OUTPUT
VOLTAGE.  ABOUT 4 MS LATER THE HOT LEAD IS SWITCHED TO THE BUS AND 120 V
APPEARS BETWEEN THE HOT AND THE NEUTRAL LINE.  IN ANOTHER 4 MS THE NEUTRAL
LEAD IS SWITCHED TO THE BUS AND AGAIN NO VOLTAGE APPEARS BETWEEN THE HOT
AND NEUTRAL LEADS BECAUSE BOTH ARE AT 120 V.  AFTER ANOTHER 4 MS, THE HOT
LEAD IS SWITCHED TO THE BUS AND AGAIN AND 120 V APPEARS BETWEEN THE HOT AND
NEUTRAL LEADS, BUT NOW THE NEUTRAL IS POSITIVE AND THE HOT IS NEGATIVE.  IN
4 MS THE NEUTRAL IS SWITCHED TO THE BUS AND WE ARE BACK TO THE STARTING

GROUND FAULT CIRCUIT INTERRUPTER GFCI FAILURE I AM WRITING WITH REGARD TO
THE SEPTEMBER 2012 QST ARTICLE POWER CARTS FOR SCOUTS AND FIELD OPERATIONS.
THE MOST LIKELY CAUSE OF THE FAILURE OF THE GFCI TO RESET WHEN OPERATED
FROM AN INVERTER IS THE VOLTAGE PRESENT ON THE 120 V AC OUTPUT NEUTRAL
CONNECTION.  GFCIS ARE DESIGNED TO OPERATE IN A COMMERCIAL POWER SYSTEM
THAT, IN ACCORDANCE WITH THE NATIONAL ELECTRIC CODE, HAS A GROUNDED
NEUTRAL.  FIGURE 1 SHOWS A TYPICAL POWER SOURCE FOR A HOUSE.  A TRANSFORMER
LOCATED ON A POLE OR PAD IN THE NEIGHBORHOOD HAS A 240 V AC CENTER TAPED
SECONDARY WINDING, WHICH PROVIDES 120 V AC TO THE HOT LEADS ON EACH SIDE OF
THE CENTER TAP.  THE CENTER TAP IS CONNECTED TO THE NEUTRAL LEAD, WHICH IS
GROUNDED AT THE MAIN POWER PANEL INSIDE YOUR HOUSE.  THE ONLY VOLTAGE
PRESENT BETWEEN THE NEUTRAL AND GROUND IS THE RESULT OF VOLTAGE DROP IN THE
NEUTRAL WIRE, WHICH IS AT MOST A VOLT OR TWO.  THE TYPICAL 12 V DC TO 120 V
AC STATIC INVERTER USED IN AN UNINTERRUPTIBLE POWER SUPPLY UPS OR
STANDALONE INVERTER DOES NOT HAVE A TRANSFORMER FOR THE 120 V AC OUTPUT
BECAUSE THIS WOULD ADD GREATLY TO THE SIZE, WEIGHT AND COST OF THE UNIT.  A
ROUGH SCHEMATIC OF THE POWER PORTION OF AN INVERTER IS SHOWN IN FIGURE 2.
THE CONTROL, MONITORING AND INDICATION CIRCUITS AND RFI FILTERS HAVE BEEN
OMITTED.  THE INVERTER OPERATES IN TWO STAGES.  THE FIRST STAGE BOOSTS THE
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ