You ask a good question which I will attempt to answer.
Switch mode power supplies are more efficient since they
first convert the wall socket power to DC and then via
solid-state components to alternating current at several tens if
not hundreds of kilohertz.
Transformers at those frequencies can be made much
lighter and smaller and still be highly efficient. The output is
then rectified and filtered by components which can also be much
smaller and lighter because they are filtering out the high
frequency alternating current instead of 50 or 60-HZ alternating
current which, when filtered, produces either a 100 or 120-HZ
ripple or hum on the output.
The problem about radio equipment is that by nature, it
is very sensitive to radio frequency interference and the
chopper circuits in a switch-mode power supply produce just the
right kind of wave forms to cause interference well in to the
VHF range.
Some manufacturers of switch-mode power supplies spend
enough engineering time on filtering and noise-suppression
circuits to essentially filter it all out, but this makes for an
expensive power supply and it may start producing noise several
years down the road if some of the filtering components fail or
weaken such as electrolytic capacitors.
The old brute force system using a heavy transformer at
the power line frequency followed by a rectifier, filter to
remove the 60 or 120-HZ hum from the output, and a series-pass
voltage regulator is not nearly as likely to produce radio
frequency noise although it can happen.
Another thing about the old power supply design is that
if the pass transistors should short, the full unregulated
voltage at the output of the filter capacitors will be fed in to
whatever device you are powering. This is usually 6 or 8 volts
higher than it should be and it usually destroys the device in a
blaze of glory, well, maybe not that, but it usually does
damage.
You have to put what is called a crowbar circuit in such
a power supply to make it blow its own fuse if the output goes
too high.
Switch-mode supplies usually fail safely in that they
just die and stop producing output.
One common failure mode for switchers is the blowing out
of the chopper transistors on the high-voltage side of the
transformer. This usually blows the fuse and stops everything.
Brute force supplies are also easier to design so there
is less engineering time which also may have something to do
with why the one supply you bought is a switcher and the other
is not.
Martin
Dane Trethowan writes:
> Howdy!
>
> Just purchased some new power supplies for some equipment here.
>
> The first one I bought was something I've been after for as long as I can
> remember, this model is best described perhaps as a battery iliminator
> and can supply output voltages from 3-12 regulated.
>
> So what you say? Well firstly this power supply isn't the "run of the
> mill" type of terribly noisy wall plug pack you'd buy at your local
> Walmart, Woolworths or similar, we're talking about a bench top switch
> mode supply which delivers a 3 amp current output at any of the voltages
> listed above, at 3 inches square this supply is amazingly small given the
> amount of current it can handle.
>
> The supply comes with 2 screw terminals which have banana plugs at the
> end, the supplied accessaries include a lead with spade clips at one end
> for connection to the supply and a connector at the other to accomidate
> one of the supplied plugs which can be plugged into whatever gadget you
> wish to power, 7 of these plugs in all and I've found that I can power
> everything I have with this unit and the supplied plugs right down to my
> old and faithful Sony pocket MD recorder <smile>.
>
> The other supplied accessary is a spade clip to elligator clip lead, just
> the sort of thing you may need if you wish to test something or run a
> gadget that doesn't fit one of the supplied plugs.
>
> The supply also has overload protection.
>
> And to the second supply I bought which is just your standard regulated
> 13.8 volt DC output 2 amp supply for powering some CB radio equipment
> here.
>
> So now to the question, the first supply I mentioned was a switch mode
> type and the second supply uses the humble old transformer to supply the
> current and I ask why are transformers still being used in CB supplies? I
> thought the whole idea of a switch mode supply was to enable the use of
> high current efficiently and surely that quality would be a first feature
> in a supply to give power to radio equipment.
>
>
>
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