Diode Test

[Per Hansson]

Re: Diode Test

Argh, I lost my long post, damn Firefox, that's what I get for not using Opera!

Gianni; Thank you very much for your help
Some clarification from my end, I think I know what went wrong
I do not have a lab PSU, but the first thing I tried doing was to isolate the circuit, I did this by removing the resistors L1 & L2, and cutting the trace next to them
The switching chip still saw some voltage tho, ca 0.5v...

After this I tried lifting the diode instead, after reconnecting the trace and resistors
This resulted in the Ethernet Chip becoming alive and doing it's diagnostics test with it's LED's
However the small FET also became very hot, 150°C or so (As is still evident by the burn mark on my index finger lol )

At this point the ethernet chip got the voltage I think it was supposed to get, 3.2VDC measured on L1 & L2
When I lifted the FET the ethernet chip became dead, it again only saw 0.5v

But I think my continued testing with the FET caused it to short internally, so the Ethernet chip received whatever the FET had on it's input side, and not the 3.2VDC it was supposed to have...

So had I just changed the FET originally I think the switch would have worked, but alas now I think there is no meaning to it, I have probably fried it with my testing

But I thank you very much for your help, it has still been a valuable learning experience!

[Gianni]

Re: Diode Test

At this point the ethernet chip got the voltage I think it was supposed to get, 3.2VDC measured on L1 & L2

This voltage seems more standard than 2.29V but from calculation it should be 2.29V unless I'm missing something.

When I lifted the FET the ethernet chip became dead, it again only saw 0.5v

This is correct as I wrote before anyway the voltage should be 0V not 0.5V.
From your pics I see L6 and L5 closed to the input connector, than I see L4: I think L4 and C118 are the input filter for the step-down converter and probably from L5 there is a tracks feeding other parts of the board.
I think L4 and C118 avoid ripple/noise propagation from the converter to the main input voltage and other circuit.

But I think my continued testing with the FET caused it to short internally, so the Ethernet chip received whatever the FET had on it's input side, and not the 3.2VDC it was supposed to have...

You can check it with your DMM between Drain and Source, it must be open. If you test in "diode mode" you should measure the body diode (Drain= anode Source= cathode) probably between 0.5~0.7V.
Ok if it is shorted than then input voltage went directly to the circuit and probably has damaged it.

I do not have a lab PSU, but the first thing I tried doing was to isolate the circuit, I did this by removing the resistors L1 & L2, and cutting the trace next to them
The switching chip still saw some voltage tho, ca 0.5v...

You started in the right way but without an oscilloscope it is hard to understand what's going on. With the converter disconnected you should have measured the output voltage (2.29V) but probably it was already damaged since the output was 0.5V.

Ciao
Gianni