Hiya
I’m building (or had built) a gps tracker using the telit GM862-GPS and its development board, to power it I built a 5V 5A switch mode power supply (so it wouldn’t get upset running on a 12V system in a car), once built it ran nicely on anywhere between 6V to 30V on the bench supply, however when i plugged it into the cars supply it blew up, the molex connector on the bottom of the telit chip set is deformed (though there does not appear to be any damage inside the chip set itself, then again trying to repair it with the available equipment would be almost impossible) and outside the chip on the develepoment board the largest either resistor or diode (its hard to tell the package is a tad black and missing) is rather blown up. the only idea i have been able to come to as to why it blew up is the aerials where on the same earth as the incoming supply has anyone seen anything like this and can anyone offer any help to get my project going again
thanks for your help
Andrew
Anything connected to a vehicle supply should be fused.
Leon
at 12v 3amp max fast blow
that is before the switch mode supply, i calculated that (in addition to testing) the TELIT box drew a max of 4.8A therefore i build a switch mode supply that could supply 5A, there by 25Watt, therefore on the 12V side a 3 amp fuse should blow if anything goes wrong (i could not find a 2.5 amp fuse which would have been my preference) and in addition the fuse did blow, however it would seam that a fuse between the switch mode supply and the Telit may have been a good idea i would still like to know what went wrong with the telit box as if it blows a fuse there is something wrong
Cheers
Where is the “Telit Box” from? Roundsolutions SFE?
It sounds very strange that you have a GM862-GPS that pulls over 2A you must have some serious problems somewhere.
You should test the power supply on the car prior to pluging in the telit, it sounds very much like a polarity problem as the supply work before you put it on the car.
The GM862-GPS will draw a lot of current only when it is transmitting, and only intermittently for short periods (peak current is 2A). I’d try a 1A slow-blow fuse, once you have found the cause of the problem. You should also put a transient suppressor across the supply input, as car electrical systems can generate high voltage spikes which could destroy the GM862. I’d also put a suitable AVX TransGuard on the 3.8 V supply.
Leon
Velocet: I got the complete kit for the telit GM862-GPS from Spark Fun, and the switch mode supply does not start if it does not get the expected polarity (thanks diodes!), with regards to you comment about it not drawing more then 2 amps, judging be the deadness of my toy i suppose it could have been faulty or the device I used to measure the current could have been mis informing me
Leon: could you make a suggestion where i would find an AVX TransGurad and how to go about using it. ps the switch mode supply it was on happy make 5.05V (with less the 0.001v ripple) using anything from 6V in to 30V and when i was bench testing it seamed to deal well with spikes etc
You should be able to get them from Farnell in NZ. The spikes can be very fast, you won’t be able to detect them without special equipment.
Leon
On the other hand if the unit went pop before the car was started then it more likely that something has touched down somewhere, bad luck.
As Leon says you should review you supply, switching frequency and protection before you try again.
I doubt it was touch down related (as both devices where in plastic cradles in the housing), and i have now throughly test my PSU without any fault showing. can the telit share an earth? i suspect it may not be able to having now had it apart and looking at an internal failure point (the ground pin on the molex connector failed)