Fred McKenzie wrote:


Uniden BC-GPSK Serial GPS Receiver plugs into the serial port of my Ham
Radio. It uses a power adapter built into a 12 Volt lighter plug, that
converts 12 Volts DC into 5 Volts DC 1 Ampere. It has a one Ampere fuse.

After 6 months the GPS stopped working, fuse was blown. Replacement
fuse blew immediately. Opened up the adapter and found a shorted
transistor. There is also an unmarked 8 pin IC. Even if the transistor
is replaced, it still might not work. So I built up a tab-mounted 7805
regulator with a couple of capacitors for stability.

Added some heat sinking, expecting the GPS to draw nearly an Ampere.
Guess what? After hooking the GPS back up, the 7805 barely gets warm.
Temperature is about 10 degrees F above ambient. In other words the
original power adapter was greatly over specified for current. I
suspect a 78L05 would have been sufficient.

The designers must have found it expedient to use an off-the-shelf
adapter, even though it has reduced reliability and they did not need
the available current.


** Your story reminds me of an issue that cropped up when the first portable CD players appeared the mid 1980s.

Folk liked to use them in their cars and wanted a DC adaptor for the job - one that plugged into the lighter outlet. Sony and Pioneer offered custom adaptors to go with their machines but for a steep price. The spec needed was 4.5V at 1 amp - but with a trap.

Generic brand car DC adaptors were available too, for well under half the Sony price with switchable output voltages of 9, 6 and 4.5V, with a 1 amp rating - so why not use one of them ?

When tried with a generic adaptor, the player worked - but soon as you hit the "skip track" button stopped. An on/off cycle was then needed to get it going again.

Reason was when track skipping, the machines drew a brief surge of about 3 amps which the generic adaptor could not supply. Plus, if the slide switch was ever set at 9V = instant destruction of the player OR if the puny transistor inside got too hot and failed short = same result.


..... Phil



















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