Greg wrote:

Here's a line of power inverters marketed to photographers, but possibly of
interest for use with radios. They offer "pure sine wave" inverters as well
as cheaper switching power supplies.

What's the difference, in layman's terms, between sine wave and switching
power supplies? What are the pros & cons for practical applications?

I've always wondered if the switching power supplies, like the Radio Shack
models, that convert 120VAC to 13VDC, are appropriate for radios.

Thanks,

Greg

They are "Pure sine wave" only when fed into a purely resistive
load. When you power a radio with one, the power diodes at the input
create a nonlinear load: Very heavy at the peaks and no load at lower
voltages. The inverter would have to be run at a very small percentage
of its rating along with some other loading or through a resonant line
filter to help even out the load to the inverter.


A lot of cheap "Wall warts" are built without caps across the
rectifiers which cause them to generate a lot of RF noise and carry it
right to the radio with the DC power. I've got a couple hundred Wall
warts and very few have the extra caps to stop the RF problems. All you
have to do with most of them is plug it in close to a radio and you'll
hear some noise but it's a lot higher if you power the radio from it.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

"Michael A. Terrell" wrote:

They are "Pure sine wave" only when fed into a purely resistive
load.

That's not true of a modern pure sine wave inverter. I don't know
what link he provided; I'm familiar with Exeltech, which makes the
best inverters out there, IMHO. They use pulse-width modulation to
synthesize the sine wave and are rated at producing that sine wave
into a load of any power factor.

Because they do use PWM, they can generate RF noise, but you can get
optional kits to clean up the RF noise from them. Obviously external
methods can be used as well.

Exeltech claims that they're used in US Embassy radio rooms all over
the world.

Now as for non-sine-wave inverters... most of the ones out there call
themselves "modified sine wave" inverters, which isn't really close to
reality. They put out a modified SQUARE wave, which will have instead
of a zero-crossing point it will have a time at 0V with the leads
shorted together. This is an attempt to make motors run more smoothly
with their power. Nonetheless, everything other than light bulbs will
be noisier and run hotter (or be destroyed) by one of these inverters.
Vaariable-speed drills and light dimmers won't work at all.

Now, these non-sine-wave inverters really do deliver 120 V RMS (or
whatever their rated voltage is), but their peak-to-peak voltage
varies wildly based on the input supply's voltage.

When you power a radio with one, the power diodes at the input
create a nonlinear load: Very heavy at the peaks and no load at lower
voltages. The inverter would have to be run at a very small percentage
of its rating along with some other loading or through a resonant line
filter to help even out the load to the inverter.


A lot of cheap "Wall warts" are built without caps across the
rectifiers which cause them to generate a lot of RF noise and carry it
right to the radio with the DC power. I've got a couple hundred Wall
warts and very few have the extra caps to stop the RF problems. All you
have to do with most of them is plug it in close to a radio and you'll
hear some noise but it's a lot higher if you power the radio from it.

That is true -- however, ICOM line lumps have the holes and traces in
place for the caps inside, so the mod is trivial.

--
Eric F. Richards

"Nature abhors a vacuum tube." -- Myron Glass,
often attributed to J. R. Pierce, Bell Labs, c. 1940