Last time I mentioned about the computer software issue on a site. Well this time I had a hardware issue. And what an issue it was! My installations have climbed up from 10 units a month back to some 25 units now. All were running perfectly. My site engineer was ready to sign the MOM and come back. This is when a unit suddenly blanked out. Within next couple of hours another 4 such units had a similar problem.
Opening of the enclosure revealed that the inductor close to the power supply had blackened. The units are powered on 12 V DC. There are 2 supplies internally one of 5Vand other of 3.3V. The engineer measured the regulated voltages but found no 5V and 3.3V. There was not much he could do there so he sent all the problematic units by courier back to Pune.
At the lab we powered up the units and found that the inductors had burnt off. Our currents were 300mA @ 5V and 250mA @ 3.3V.
We are using LM2574-ADJ as the switch mode regulator. The switching frequency of 2574 is 52KHz.
The inductors which we were using were in a 'half watt through hole resistor' package. We ordered the ferrite core inductors for the same from our local x'mer manufacturer. Our units started working on them without any issues.
Since we wanted to be doubly sure that everything was working well we tested the units at 40V.
Old inductors invariably failed at this voltage but the new ones never did.
Another interesting thing that we noted was that when the old inductors when failed they had their lead would loosen up.
We have sent the modified PCB's to the site. I will inform if we develop any new problems.
Wednesday, July 1, 2009
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