Specifications

From April 2009 QST © ARRL
PRODUCT REVIEW
Mark J. Wilson, K1RO

Product Review Editor

[e-mail]
Bottom Line
Reviewed by Howard Robins, W1HSR
ARRL Contributing Editor
When I became active in the ARRL
Amateur Radio Emergency Service (ARES),
I realized that I needed to be able to run my
station during commercial power outages.
A friend, I noticed, lugged an Optima AGM
deep-cycle marine battery to field opera-
tions.
1
I learned that AGM (absorbed glass
matt) batteries are sealed, maintenance free
units that do not outgas dangerous levels of
hydrogen when charging and can take a beat-
ing. Unlike regular automotive batteries, they
are designed for many deep charge/discharge
cycles — just what’s needed for remote sta-
tion operation. So, I bought one and a battery
minder to keep it topped off.
I have since grown my battery farm to
four deep-cycle units that will source 220
ampere-hours (Ah) with an 8 A minder
that works selectively with different battery
chemistries, including AGM. I normally run
my entire station on batteries, excluding the
computer. In an emergency, I will run on
commercial power as long as it is available
and switch to battery power when it is not.
My next thought was to add a dc-to-ac
power inverter so that I could use my normal
desktop computer during power outages.
It’s a key part of my emergency station,
used for digital modes, logging and other
record keeping. I have a UPS (uninterrupt-
ible power supply) in my system, and with
forethought it should keep the computer
going long enough to allow me to switch to
emergency power.
As it turns out, the marketplace offers a
wide variety of power inverters with a wide
range of capabilities and prices. First I’ll
describe my initial experience, and then get
into our testing.
My First Inverter
The first question: How much power is
needed? I power all of the devices needed
during an outage, including the computer,
LCD monitor, printer, DSL modem and
router, via a surge protected power strip.
I purchased a Kill-A-Watt electric usage
monitor from P3 International and connected
it between my power strip and a commercial
power outlet.
2
I learned that all of the devices
on that power strip consume about 170 W.
When the monitor goes to sleep, power
consumption drops by 25 W.
I found all sorts of inverters on an Internet
search. That was the limit of my research at
the time. I got lucky when I order a 1250 W
modified sine wave (MSW) type that seemed
to be a bargain — much more power than I
needed for a price that fit my budget. (See the
sidebar, “Inverter Types,” for more informa-
tion.) The inverter worked quite well. I built
a transfer switch using a 3PDT relay with a
12 V coil and 16 A contacts, along with a
standard duplex outlet. My power strip is
now plugged into that duplex outlet, which
is wired to the load contacts on the relay.
The normally closed contacts on the relay
are wired to a plug that goes into my UPS.
The normally open relay contacts are wired
to a plug that goes into the inverter.
When commercial power fails, my com-
puter system is powered by the battery in the
UPS and the UPS sounds an alarm. All I have
to do is turn on the inverter and switch the
computer system to the ac from the inverter.
The computer system keeps running with no
interruption. I switch from commercial to
inverter power and back routinely and have
run on batteries for several hours. This works
and works well. If we have a long power out-
age, I have a 3500 W generator that could
be brought online to recharge batteries and
power other essentials.
Stupid things happen. One day I decided
to straighten out the wiring around my radio
DC to AC Power Inverters
Modified sine wave (MSW) type in-
verters offer the most power capability
for the money, but we found a wide
variation in RF interference generated
by the three units tested. The more
expensive pure sine wave (PSW)
units tested were both RF-quiet and
generated a nice sine wave.
2
www.p3international.com. Also see S. Ford,
WB8IMY, “Kill-A-Watt Electric Usage Moni-
tor,” Short Takes, QST, Jun 2006, p 61, and
D. Falcon, N2JOM, “Kill-A-Watt Electric Us-
age Monitor Revisited,” Technical Correspon-
dence, QST, Mar 2007, pp 67-68.
1
For more information on these batteries, see
www.optimabatteries.com/optima_products/
bluetop.php.
Inverter Types
To research emergency power
options for my station, I turned to two
publications available from ARRL
— Emergency Power for Radio Com-
munications and Independent Energy
Guide.
3,4
Each book devotes a chap-
ter to inverters. Emergency Power
for Radio Communications provides
an interesting historical perspective
along with practical insight into ap-
plications and use. The Independent
Energy Guide provides waveforms
and a more technical presentation. It
includes tutorials on inverter ratings
and specifications, features and op-
tions, and cost and selection.
Inverters have come a long way
from the early mechanical units some
of you may remember from the days
of tube type mobile radios. Modern
technology has made inverters small-
er, more efficient, less costly and
widely available. Today’s solid-state
inverters fall into two categories —
modified sine wave (MSW) and pure
sine wave (PSW). MSW inverters
produce stepped square waves that
resemble a sine wave. PSW inverters
produce outputs that are made up of
many finer steps, more closely ap-
proximating commercial ac. The dif-
ference is readily apparent in Figures
1 through 5, which show waveforms
of the inverter outputs while operating
under load in the ARRL Lab.
— Howard Robins, W1HSR
3
M. Bryce, WB8VGE, Emergency Power
for Radio Communications, available
from your ARRL dealer or the ARRL
Bookstore, ARRL order no. 9531. Tele-
phone [phone], or toll free in the
US 888-277-5289; www.arrl.org/shop;
[e-mail].
4
K. Jeffrey, Independent Energy Guide,
available from the ARRL Bookstore
(order no. 8601, see Note 3).
desk and accidentally shorted a couple of
relay contacts, sending 120 V ac into the
inverter output terminals. Inverters do not
like ac applied to their output terminals, so
I now have a big paperweight.
A Noisy Replacement
I found a great deal on a 2300 W MSW
inverter. I figured more is better, so I ordered
one and installed it in place of my first in-
verter. Before I powered up the new inverter,
I turned on my HF radio and heard the usual
levels of signals and noise. I powered up
the inverter with no load and heard signifi-