Teensy 3.1 burned - help needed

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pirlouwi

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Hello,
i drew too much current from my Teensy 3.1
Two passive components are burned.
They are surrounded by a red and purple circle on this image:
http://i.imgur.com/QNabheO.jpg
Could someone tell me what are those components? I would like to try to replace them.
On this site, I was only able to find a logical schematic, but no placement schematic
Thanks
 
The white part is the PTC "fuse". The tan part is a 2.2 uF capacitor.
 
pirlouwi said:
What is a PTC fuse?
Click to expand...

It's really a resistor that drastically increases in value when it get hot, due to too much current flow. The higher resistance in series with the input tends to reduce the overall current, because the input is a fixed voltage. They're sometimes called self resetting fuses, because they return to the original low-resistance state when they cool down.

Generally they're used to protect wiring from burning in a short-circuit condition, much like a regular fuse, even though they're really resistors that change value.
 
and what is the value of this PTC fuse ? or it does not matter ? perhaps the cold resistance is so small that it is considered as short circuit ?
 
pirlouwi said:
and what is the value of this PTC fuse ? or it does not matter ? perhaps the cold resistance is so small that it is considered as short circuit ?
Click to expand...

If Paul doesn't have the time to answer, I'd measure the size of the PTC and then I'd fire up Digikey to find a replacement. IIRC, it's a model with a 500mA trip current to stay in USB spec. Assuming it's a 0805-size component (i.e. 0.08" long, 0.05" wide - use a dial caliper) then this PTC might be a suitable replacement:

This polyfuse has a 500mA trip current and a 200mA hold current. IIRC, the Teensy 3.x voltage regulator can't put out more than about 130mA of current, so a 200mA hold current seems adequate as long as you don't try to use VUSB or VIN to provide a lot of 5V power to external components.

A different 0805-sized PTC that will hold 500mA of current while tripping at 1A can be found here. But there is more risk with this sort of components to damage an attached USB port since the trip current exceeds the USB spec for maximum current.

Paul will forget more about this spec than I will ever know, so I hope he can chime in. Also keep in mind that the trip current will vary by application, i.e. hot conditions will lower the trip current, for example. Similarly, the current being sent through the device will vary the speed with which a PTC will react, bloating up and significantly increasing its resistance. Once the current is removed, the PTC should deflate and start conducting power again.

So, have you tried to determine the resistance across the PTC? The 500mA-trip 0805 I linked to above has a cold resistance averaging about 2 Ohms, with a stated range of 0.6-3.5 Ohms. Actual resistance for your model will depend on who made it, etc. so the first test (continuity/resistance) merely establishes if your PTC resistance is in the right ballpark.

Similarly, have you measured the capacitance of the 2.2uF ceramic cap you made unhappy? You may have managed to release the magic smoke from the Teensy as well - anything that can make the PTC smoke (like high current or high voltage) may have also damaged the voltage regulator and / or other circuits inside the Teensy.
 
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My god, thanks for so much information.
Great support.
I don't know actually if other components are damaged.
I'll try step by step, and idea by idea.
To be honest, I doubt to repare my Teensy3.1 board. Anyway, I have to try.
 
Not sure what expertise you have with a soldering iron, however, the trick to removing these small components and soldering the new ones back on is not necessarily the finest soldering tip, but use lots of flux, perhaps from a good flux pen.
 
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