To facilitate portable operations with my Icom 718 high frequency transceiver I needed an alternative to the heavy and bulky traditional power supply used in my basement ham shack. The latest addition to my station equipment is the Astron Model SS-25M switching power supply. The rated output is 13.8 VDC @ 20 amps continuous duty.
Traditional non-switching power supplies use big transformer coils to produce very linear and "clean" power. Because of the circuitry involved some switching power supplies can generate unintentional radiation that manifests itself in the form of hash or static that can be heard on high frequency receivers. Obviously no good if you are trying to use that particular piece of radio spectrum for communications. After the initial voltage check I found that the Icom's receiver was just as quiet as when powered by my other supply which not incidentally is an Astron RS-35M.
The terminal blocks on the back of the SS-25M have pin holes with set screws. When I made pins from a couple brass machine screws I left them a bit long intentionally so a few threads remained exposed past the tops of the thumbscrews. The screw tips make a good place for my test leads to hook up.
The SS-25M does have a fan for cooling mounted on the back of the chassis. With my transceiver set at 50 watts of power the fan would start after 5 minutes or so of 20 word per minute cw (Morse code) transmission on 40 meters. It would shut off after a minute or two or quicker if I stopped transmitting. I'm sure the fan's cycle would change proportionately to the load placed upon the supply. While noticeable I did not find the fan's sound objectionable even operating without headphones using the radio's speaker.
The big advantage to this power supply is the light 4 pound weight and compact dimensions. Another feature I don't mind paying a little extra for is the separate volt and amp meters for monitoring output voltage and current. Electricity is invisible and dangerous and I like gadgets that tell me it's where it's supposed to be.
More on switching power supplies: http://en.wikipedia.org/wiki/Switched-mode_power_supply
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