Mobile Power Supply
A "Mobile" POWER SUPPLY - Mar 1996
I saw this neat idea for a mobile power supply in the RSGB RADCOM of March 1996. The article was in the "Euroteck" column which is devoted to translated articles from European sources. This power supply idea originally came from two other articles which discussed the use of hand-helds in mobile situations and which gave a number of ideas including getting more audio and dealing with spurii and desensing. The original articles were by PAOHPV and PA3GLK in Electron (NV) of February 94 and November 95.
I have always felt reluctant to connect my hand-helds direct to the car's 12v electrical system. Not only are the voltage excursions large resulting in unnecessary, excess dissipation in the hand-held, the electrical equipment in cars these days generates large transients which can damage sensitive solid state components. In a sinister way, this damage can be latent and not show up until sometime afterwards, at which time leaving you guessing as to the cause of the fault.
The article talks about how use of the raw 12v affects both receiver and transmitter performance. Ignition noise, alternator whine and switching transients may spoil weak signal reception; this can be overcome by filtering.
The article quotes a test on a hand- held which showed that a 23% increase in supply voltage (12 to 14.8 v) only yields a 7% increase in RF output power. The extra power being dissipated in the hand-held can easily be avoided by limiting the supply voltage.
The circuit below is taken from the article. It provides fusing, reverse voltage protection, voltage dropping and DC filtering. No doubt, commercial units of similar design would be available; however, the simplicity of the circuit readily allows for home construction and the more resourceful of our home brewers could probably fit the unit to a modified cigarette lighter plug.
Putting on my reformed smoker's hat, I can say that these days, the only useful thing about smokers is that car manufacturers still provide a neat 12v power socket in cars to cater for my radio needs.
The circuit is easily adapted to your particular hand-held power requirements and what is in your junk box. Any medium power NPN could be substituted for the series pass transistor and the output voltage can be further reduced by adding more diodes (approx .7v per diode) in the base circuit.