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Repairing a 5 VDC adapter

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.

Fred

Reply to
Fred McKenzie
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The cancer stick socket to 5V adapter outputs to a USB port. There are specialized chips for the purpose, which include device detection, short circuit protection, and current limiting. In other words, they do more than supply 5VDC. While just 5V might be sufficient for your GPS receiver, it would not be acceptable for a smartphone, tablet, or other device that depends on a USB interface chip to identify the charger capabilities.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

** Your story reminds me of an issue that cropped up when the first portabl e 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 a daptors to go with their machines but for a steep price. The spec needed wa s 4.5V at 1 amp - but with a trap.

Generic brand car DC adaptors were available too, for well under half the S ony price with switchable output voltages of 9, 6 and 4.5V, with a 1 amp ra ting - 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 tra nsistor inside got too hot and failed short = same result.

.... Phil

bra

Reply to
Phil Allison

Current measured at the input to the regulator varies around 55 to 60 mA. This includes 5 or 10 mA for an LED pilot light. I changed to a

1/4 Amp fuse because that was the smallest I have.

I am not using the original lighter plug, but built it into an Altoids can. It should be easy to fit a 78L05 into the original plug, but the plug plus socket is bulkier than the Altoids can.

Fred

(I do not especially like Altoids, but the cans are worth it!)

Reply to
Fred McKenzie

Was this a switching regulator? I've ripped apart a bunch of car adapters for phones and just USB power and they're all essentially the same device. Some have a pilot LED, some have fuses, some are rated 12-24V in, but everything else is the same, and also circles a 8 pin chip.

Reply to
Cydrome Leader

Yes. Has fuse, pilot LED and 12-24V in. I would not be surprised if other adapters found at a thrift store have the identical circuit board.

As a purist I might attempt to repair the original adapter, but the simple 7805 circuit works well. Besides, fuses are not cheap!

Fred

Reply to
Fred McKenzie

If you have toss zener across the output as some sort of lazy over voltage protection. You can create a fusible link type fuse with a strand of wire from headphones or something really thin too.

I dug out an old multimeter that had a loop of really thin wire wrapped around the blown fuse. Guess I had none of the right type at the time.

Reply to
Cydrome Leader

I use those cheap 120 volt to 5 volt charger cubes or supplies, when I need 5 volts from 120 volts for a board level project. Saves the hell out of making your own onboard 120v to 5 supply when you can simply rip it out of the case of those dirt cheap cubes at the local junk store. :)

Reply to
M Philbrook

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