[time-nuts] Teardown of Chinese made eBay GPS antenna.
kb8tq at n1k.org
Thu Feb 22 16:48:42 EST 2018
As mentioned in another post, the part does claim to have thermal limit
built in. They appear to pulse test them at 150 ma and 16V so there is
indeed *something* that would suggest operation at 12V would be ok.
I’d guess that the thermal regulation spec applies up to 16V and past that
you are on your own…..
I’d take a look at the bypass caps on the regulator. Looking back at the
original test data, it may have simply gone unstable at the higher voltage.
A 0.1 uf on the input and 0.47 uf on the output seem to be the minimums.
> On Feb 22, 2018, at 2:48 PM, John Green <wpxs472 at gmail.com> wrote:
> For those who have been following the saga of the Chinese made, eBay
> purchased antenna that failed, I may have an answer as to why it failed.
> I had to destructively disassemble it. I just could not get it apart any
> other way. I used a Chinese version of a Dremel tool with a metal saw
> blade. After making a huge mess with plastic particles everywhere, it
> revealed a circular FR4 board with two patch antennas mounted, one atop the
> other. I assume the smaller one to be the L1, and the larger to be L2. This
> part looks almost identical to the Trimble Microcentered antenna I worked
> on recently. On the back side of this is a metal shield about 3 by 3
> inches soldered to the circular FR4 board. I switched to a abrasive wheel
> and took off some of the solder holding the shield to the board. Then,
> using a small screw driver, I went around the shield breaking the solder
> loose. The shield off revealed that the coax goes to some capacitors that
> couple RF out and through an inductor with some capacitors to ground and
> finally to a SOT23-5 package labelled LK33. This appears to be a Micrel
> MIC5203 3.3 volt regulator. It is shorted on the input side. I believe that
> putting anything over 7.5 volts on the input exceeded the power dissipation
> rating and caused it to fail. I plan on wiring up a more robust 3.3 volt
> regulator in its place and trying again. It looks like I will be able to re
> solder the shield back. The watertight integrity is gone for good. I think
> I can find a plastic box I can mount it in so I can at least experiment
> with it. I have sent a message to the seller detailing my findings. The
> Micrel part lists a 20 volt maximum input voltage, so in theory at least,
> this might have worked, and there might be some of these out there that
> don't fail.
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