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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Inverter/Charger Question
Message-ID: <>
Date: Thu, 27 Jun 2002 12:53:55 -0400

On Thu, 27 Jun 2002 05:10:10 GMT, Owlman <> wrote:

>I recently posted a message requesting information re inverter/charger
>systems.  I was told my many that if I wanted to be able to power our
>microwave/convection oven that I would have to get a true sine wave
>inverter.  My RV servicer (in whom I've always had extreme trust) ended
>up installing a Heart modified sine wave unit in my RV.  Since I thought
>we'd discussed the need for a true sine wave unit, I was shocked to find
>it wasn't.  When I contacted him today, he said that he'd never heard of
>any problems with this unit powering a microwave and that he's installed
>numerous such units for exactly that use.  I felt a little relieved, but
>based on comments from this newsgroup, I'm also still a little nervous.
>I read through the instructions for my inverter and the only thing they
>said was that there could be problems with "some" battery chargers and
>digital clocks.  Of course, the microwave/convection has a digital clock
>and we haven't seen a problem yet.

If you got advice in this forum that a microwave has to have a true sine wave
inverter in order to work, the advice-giver doesn't have a clue.

Here is the issue.  Internally, the oven has a transformer that steps line
voltage up to about 2000 volts, plus or minus.  In a clever circuit that uses
the magnetron as both the source of microwaves and a diode, this voltage is
rectified and doubled to produce the necessary ~3500-4000 volts that it takes
to operate the magnetron.

As with any conventional rectifier circuit and particularly with a voltage
doubler, the DC voltage produced (and thus the microwave power developed) is
proportional to the PEAK value of the AC voltage.  The utility supplied power
is more or less a sine wave so the peak value is a known and is used in the

Making a sine wave is fairly expensive in an inverter circuit so simpler
methods have been developed.  For resistive and some inductive (motor) loads,
the inverter can simply toggle the polarity of the 120 volt output 120 times a
second.  This produces a 60 hz square wave.  the problem is that the peak and
average value is the same and so there is insufficient peak voltage to make
the microwave work.  A microwave plugged into a square wave inverter will
power up, the controls, lights and fan will work but it won't make any power.

The solution is to modify the switching.  One method involves switching
several different voltages in sequence so that the output is a stair step that
roughly resembles a sine wave.  As long as the peak voltage is there, the
microwave will be perfectly happy and could care less what the actual shape

Another method, common with most inexpensive inverters I've analyzed, is to
add some delay between the time when one polarity is turned off and the other
is turned on.  If the timing is chosen correctly, the resultant waveform will
have both the same RMS and peak voltage as the utility power, though it won't
look like a sine wave. The microwave will also operate happily on this
waveform.  The only difference will be that if the oven has a noisy
transformer and hums, the hum will have a sharper edge to it.

Last year I wrote a nice paper on this topic complete with waveform pictures
and posted it to my web site.  It's probably still there if you want to look.

>I know that some folks commented that they had no problem using a
>modified wave for a microwave, I'm still concerned of what such a unit
>might due long-term to the microwave.  I guess we'll have our first
>trial over July 4th weekend.

There is no long term effect.  Don't know where those urban legends get
started.  Either the oven heats satisfactorily on the inverter or it doesn't.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV input power measurements
Message-ID: <>
Date: Mon, 11 Aug 2003 22:42:56 -0400

On 10 Aug 2003 12:02:57 -0700, (Jim) wrote:

>Recently saw that BJ's club was selling Superex 1000w inverters for
>$70. Thought I might be able to use this for the microwave in the RV
>so I went and took some measurements. With the microwave operating I
>was measuring 1065-1090w, 10.8 amps at 105.5 volts (on the input to
>the RV). With the microwave off I measured 71 watts, 3.0 amps and
> From this I've determined that the Superex 1Kw inverter would be
>questionable to run the microwave with.. even though it's rated for
>2Kw surge, I haven't seen any charts that show how long a 'surge' is..
>anyone have any thoughts on this?.. BJ's has a 2500w Superex inverter,
>but they are asking $299.

I run a nominal 600 watt microwave just fine on a 1kw Vector inverter.  I
haven't yet measured the line draw but I'd expect it to be somewhere in that

The only way to know for sure about that combo is to try it.  It may very well
work.  These pseudo-sine (stepped square wave) inverters tend to not deliver
the peak voltage of a pure sine wave.  That reduces the power output and thus
the draw significantly.  I haven't measured my microwave yet but subjectively,
I know that it doesn't produce as much power on the inverter as it does on
line power.

I did a very detailed test on the Vector unit some months back for someone in
this group (sorry, bad on names.)  It will carry its rated load indefinitely
but will shut down on a time vs overload curve when one goes beyond the
rating.  The surge rating really is what it says - only for a few seconds.

One thing I did observe on the Vector was that the high voltage filter
capacitors get very hot at full load, much hotter than is conducive to long
life.  Since it appears that most of these cheap chinese made inverters use
the same circuitry, I'd not be surprised to see that on yours.  yet another
reason not to try to run it at full load for long.  In mine I arranged a
little fan to blow directly on the capacitors.  That and the built-in chassis
fan seems to do a pretty good job.

Rather than have to wonder I think I'd grab the 2.5kw inverter.  that's a nice

> The other question is that something running in the RV has a horrible
>power factor.. 71 watts and 3.0 amps?.. could this be the battery

Probably the converter.  You definitely don't want to be running the converter
from your inverter.  That's purely wasted power.

The way I wired my unit is thus.  My reefer and microwave sit adjacent to each
other and are powered off the same branch which runs through the back of the
refrigeration compartment.  I installed a DPDT relay in the compartment.  The
moveable contacts go to the two outlets.  The NC contacts go to the existing
shore power branch and the NO contacts go to the inverter.  The 120 VAC coil
is also connected to the inverter.

Operation is thus.  On AC the relay is de-energized and power flows from shore
power through the NC contacts to the refrigerator.  When the inverter is
energized, the relay activates and the loads are connected to the inverter via
the NO contacts.  Shore power is isolated.

This connection gives inverter priority.  That is, when both shore and
inverter power are available the relay selects inverter power.  I wanted it
that way so that if I run into bad power at a CG I can flip the inverter on to
get full voltage to the microwave and reefer.

I have a similar relay on the genny with genny priority.  Again, I can start
the genny and over-ride shore power if necessary.  This gives great

For the first time ever, I ran into some bad power in Pa during my 4th
vacation.  In the middle of the evening the voltage was down to 100 volts.  I
simply fired up the genny while cooking.  By the time I was through cooking
the sun had set and the voltage was back up to 110, satisfactory for the AC.
Whenever I wanted to nuke something, simply flip the inverter on and nuke at
full voltage.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Thanks, for all the responses.
Date: Sun, 16 Dec 2007 21:29:25 -0500
Message-ID: <>

On Sun, 16 Dec 2007 17:07:30 -0700, Dapper Dave <> wrote:

>1.  How about the other side of efficiency, i.e., do some or all
>appliances use more energy if being operated from a MSW source?

Nothing waveform-related.  Any difference will be because of the voltage difference
between line and the inverter.

>2. The one appliance that does not do well with MSW power is the
>microwave. It cooks at about half power when run off the inverter.

A conventional microwave oven that uses a transformer/rectifier/voltage doubler (NOT
the Panasonic Inverter oven and similar ones*) is heavily dependent on the peak
voltage of the incoming power.  The problem is that no inexpensive inverter that I've
ever seen (just don't know about expensive ones) regulates the output voltage.  The
output voltage is roughly proportional to the input minus losses in the inverter.  If
say, 80 amps traveling from the battery to the inverter terminals causes a half a
volt drop then that will be reflected as about a 5 volt drop in the output.  That has
a major effect on the microwave output.

There are a couple of ways to address this.  If the inverter has an internal
adjustment for output voltage then cranking that up a little will help.  Running the
engine during microwaving also helps since that keeps the battery voltage up.

The other way is a voltage booster transformer.  A 12 volt transformer will add 10%
(12/120).  A transformer is selected with a secondary rating approximately that of
the microwave input.  If the microwave draws 8 amps then a 10 amp transformer is
perfect.  A 5 amp one will probably work since the duty cycle is short.  This is a
fairly small transformer.  Stripping one out of an inexpensive 5 or 10 amp battery
charger is a good source.

The transformer is connected with its primary connected directly to the inverter
output and the secondary connected in series with the hot leg feeding the microwave.
Polarity matters - one way and the voltages subtract; the other way and they add.

This is the approach I used in my semi truck and am using in my MH.  The transformer
can be mounted next to the outlet where the microwave plugs in, assuming that the
inverter is connected to feed the RV's electrical system.  Or a short heavy duty
extension cord can be cut and the transformer wired in.  The voltage boost won't hurt
the microwave when operated on shore power so it can be left in place permanently.

>draws do much current that running it from batteries doesn't make much
>sense wanyway, though.

Au contraire!!  A 1000 watt input (6-700 watt output) microwave will consume about an
amp-hour a minute.  Thus, if you take 3 minutes to pop a bag of popcorn, you've only
used about 3 amp-hours from the battery (not including Perkeut, of course).  A 10
minute Hungry Man TV dinner uses on 10 amp-hours.  That's just a drop in the bucket
against a 3-400 amp-hour house battery.

The key is to have the inverter-to-battery wiring the proper size to drop absolutely
minimal voltage.  I use 3 100 amp-hour 12 volt batteries in parallel.  All the
cabling is "0".  The inverter is no more than a foot away from the batteries.  Other
than the inherent voltage drop of a heavily loaded battery, there is essentially no
voltage drop even at 100 amps.

One has to pay attention to connectors too.  I found that a heavy eye lug attached to
the steel stud on the battery post dropped a couple hundred millivolts at 100 amps
while a crimped SAE post connector dropped less than 50.

Same thing with a coffee-maker, for that matter.  In my semi, I had a Mr Coffee-type
coffee-maker.  I set it up at the beginning of a sleep period, loaded with coffee and
water and left it plugged into the inverter.  When the Screamin' Meanie pried my eyes
open :-), I swatted at the power switch until I managed to turn it on.  When the
Screamin' Meanie's snooze alarm finally woke me up, coffee was hot and waiting.  For
me a matter of survival!  The whole process used not more than 5 amp-hours.

I REALLY don't like to run my generator for just a few minutes at a time so most of
my nuking and coffee-making and bread toasting is done on battery power when I'm off
shore power.  I crank the generator only when the batteries need charging.  Or when I
need the AC, of course.

>3. I am currently in the slow process of ordering a newer charger for my
>Black & Decker drill. I wonder if the old charger failed because we
>frequently operate everything from the inverter. Also, our fairly new
>KitchenAid blender died last week. Maybe I'll reconsider my opinion that
>everything we have is happy with our Xantrex MSW inverter.

I doubt that it caused the failure.  My DeWalt charger isn't damaged by the MSW
power, it just doesn't do anything.  It uses an ancient but very good reference
design developed by Zilog back when the Z8 processor was hot spit.  I know the guy
who wrote the firmware so when I remember to ask, I'll find out why the thing doesn't
like MSW power.

The only other problem I've had, and this time the blue smoke actually leaked out,
was with the Schumacher 40 amp smart charger like Wallyworld sold a few years ago. It
apparently has an input power factor corrector circuit that demands low distortion
input power.  Mine instantly smoked when I plugged it in to the inverter.  The
replacement smoked when I plugged it in to one of my smaller generators.  The
generator waveform isn't EU-quality but it isn't THAT bad.  Just a bad design on
Schumacher's part.  Too bad.  I really liked that charger otherwise.


* I have a Panasonic inverter microwave.  It kicks ass!  It doesn't care about what
kind of power it's fed as long as it's close to 120 volts.  The microwave power is
regulated.  Variable power is just that, variable and not on/off like conventional
microwaves.  It's quite light too, since it doesn't contain that heavy transformer.

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Newbie microwave question
Date: Thu, 17 Apr 2008 20:13:06 -0400
Message-ID: <>

On 17 Apr 2008 18:02:01 -0500, nothermark <> wrote:

>Just warming up dinner and started thinking about power consumption.
>Is it reasonable to run a mh microwave off the batteries for, say, 10
>minutes.  I'm thinking of pulling over and warming up two cans of soup
>or whatever for lunch.  I'm assuming the mh will come with a 1500 w
>inverter (is this typical?) and wondering if the on board batteries
>should handle the load.

Sure.  I run mine that way all the time.  I have a nominal 600 watt oven that
draws about 900 watts from the line.  A 1000 or 1500 watt inverter runs mine
just fine.  In fact, I leave my oven connected to the inverter all the time,
even on shore power.  I have an 80 amp Intellipower converter that can supply
almost all the current the microwave needs and will top off the batteries
quickly after the cooking is done.

Some things to be aware of.  Microwave ovens are VERY sensitive to the peak
voltage, which with nominal 120 volt shore power is about 170 volts. So-called
"modified sine" inverters (actually modified square wave) output a peak
voltage in the range of 140 to 150, depending on the model.  This results in
the microwave producing less heat, even when the batteries are in tip-top
shape.  Implied in that is that you have to do everything you can to maintain
the voltage.

Most inexpensive inverters (every one I've ever had on my bench) do not
regulate the 120 volt output.  It varies with the incoming 12 volt variations.
That implies that losses in the 12 volt side have to be minimized.  Short,
very large cables and low impedance batteries.  Forget wire tables.  Use the
largest cable that will fit and keep it as short as possible.

I rearranged my battery system last year to a) move the batteries inside, b)
install three Group 29 size 12 volt batteries in parallel and c) mount the
inverter less than a foot away from the batteries and make the connections
with 4/0 welding cable.  This improved the microwave oven performance

Previously I had two Group 29 batteries mounted under the hood and connected
with several feet of 4/0 cable.  The combination of battery voltage sag under
heavy load and the tiny voltage drop across the cables resulted in a little
less output voltage and that resulted in a lot less microwave oven heat.  The
measured output of my oven was previously about half its rating.  I haven't
measured it yet with the new setup but I'm going to guesstimate that it
produces between 2/3rds and 3/4ths of the rated power.

I moved my batteries inside primarily to avoid the loss in capacity in cold
weather (I do a lot of winter camping) but the side benefit is much improved
microwave oven performance.

A 2500 watt inverter will probably be necessary for your oven.  Harbor Freight
is currently listing one in their mail flyer for $149.  It's a pretty good
inverter.  Two batteries in parallel will be marginal.  It'll work but the
heat output will be low.  OK if you have patience.  Three batteries should be
pretty decent.  The inverter will pull at least 100 amps and probably a bit
more.  Split three ways, about 35 amps per battery won't cause the voltage to
sag too much.

One more comment.  I've smoked several microwave electronic control boards
with less than good power in my concession stand.  This happened when I
overloaded my generator and the power quality deteriorated.  I now avoid
electronic controls whenever I can.  My RV microwave and the ones in my
concession stand all use mechanical timers, the kind where you turn the knob
to the desired minutes.  The mechanical timers are bullet-proof.

I selected the oven that is in my RV on two main parameters - power draw less
than 1000 watts (1kW inverters were still quite expensive back then) and the
mechanical timer.  I can't recall without looking but I believe that it is a
low end Panasonic that cost less than $50.  Substantially better quality timer
than what is on the really low end Wallyworld oven that sells for about $30.


From: John De Armond
Subject: Re: uWave ovens, was: 280V motor on 230V circuit
Date: Fri, 30 May 2008 03:00:23 -0400
Message-ID: <>

On Fri, 30 May 2008 01:50:29 GMT, (Fiat Sparks) wrote:

>Sam Goldwasser <> wrote:
>>Never seen an FR uwave. :)  Why would they use that when the basic
>>circuit is adequate and reliable (more or less!)?
>Mostly less! :-) My Panasonic inverter unit just released the magic
>smoke. First the magenetron died, and then after I replaced that, an
>IGBT in the switcher shorted and did a fair bit of collateral damage.
>I finally did find a service spanish (which I read
>poorly,) but it did at least tell me what all the small resistors and
>diodes were supposed to be (before they melted.)
>So, $100 in parts later, I now have a working microwave again. And, if
>I get tired of it, I can sell it on Craigslist for at least $35! :-/

Don't try to operate that oven from a cheap generator with a less than perfect
sine output.  That's another excuse for the blue smoke to leak out.  BTDT.

In my case I wasn't about to spend that kind of money to repair an oven that
barely cost that much, especially since I used it in my restaurant always on
high.  Therefore I yanked out all those fancy electronics and installed the
transformer/diode/cap assembly from another old oven.  I drilled a hole
through that nice touch pad and installed an Intermatic spring-wound timer
from Home Depot.

Voila, good as new and bullet-proof against nasty power.


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