Teensy 4.1 PTC fuse getting really hot, is it repairable?

[nn28]

Hi all,

I am mentoring a group of students that are using the teensy 4.1 , they have been having problems with the VIN source exceeding well above 5 volts because of faulty circuits which led to damaging the teensy. The teensy doesn't get recognized by laptops anymore and the PTC fuse gets really hot when power is applied to it. The intended way to power the teensy was to power it with a 5V LM7805C Voltage Regulator, with a 12V input and a 5V output going directly to the VIN of the board.

When inspecting the damage I could feel that the PTC fuse is getting really hot, the rest of the components are not. I am wondering what this means exactly, from existing posts on this forum I understood that the PTC fuse is rather a resistor that stabilizes the current and doesn't actually blow like a fuse. As the fuse is still getting hot I wonder if the fuse itself may be damaged and/or something on the board might still be damaged? Also, does the PTC fuse in a way save other components from getting damaged or is it more like an indication that something is already or still is damaged?

Does anyone know some troubleshooting steps I could follow in order to find the part(s) that need to be replaced to revive the teensy, that is if it is possible at all?

Thank you in advance!

[defragster]

As noted on T_4.1 spec Card - VIN should be 3.6 to 5.5 V Max.
Somewhere over 5.5V components will suffer damage.

Damaged components can go in various ways - sounds like something has gone to shorted and is over drawing current through the PTC and causing it to block the current and get hot.

This doesn't help ID the problem or nature of the damage - but seems to be showing something was indeed mistreated by high voltage explaining the hot PTC.

[nn28]

I am quiet new to the teensy boards, from my experience with Arduino boards when applying high voltage to the VIN you can clearly tell for example if an onboard voltage regulator has exploded either by the magic smoke, the puncture hole inside the regulator or it becoming very hot. The same goes for a damaged chip. But the only thing I could tell is faulty on the teensy is the PTC fuse, which gave me hope that maybe the components are saved by the fuse? In contrast, if something is still over drawing current then that definitely means something is still wrong. Or does the PTC Fuse itself also get damaged?

[BriComp]

Can you try powering it via USB?

[nn28]

Powering it via USB also results in the PTC fuse getting too hot.

[nn28]

An update: I have quite a few teensy's laying around with this problem, for troubleshooting purposes I desoldered the PTC fuse and shorted it to see what other component would get hot. I have only powered it on very briefly but it turned out to be the TPDS014 current limit switch (see picture) for the USB port. Would replacing this component fix the teensy (getting recognized by the PC again) or are there typically more components that will be damaged which would make replacing each of them not worth it?

[PaulStoffregen]

You could just remove the TPD3S014 chip. It's only needed for USB host power. Looks like you're not making use of USB host.

[nn28]

You could just remove the TPD3S014 chip. It's only needed for USB host power. Looks like you're not making use of USB host.

My students are using an external power supply indeed but to also use the serial monitor while using this external power supply I gave them USB cables of which the red wire is cut off. Is this still the correct way of doing things? Or is it not necessary anymore because the removal of the USB host?

[PaulStoffregen]

Cutting the red wire is a good approach if using external power.

Removing the TPD3S014 chip doesn't change anything.

Also, normally TPD3S014 doesn't overheat, and it certainly should not overheat if you haven't even soldered pins to the USB host port. But voltage regulator right next to it is the part known to overheat if something is shorting the 3.3V power to GND. That voltage regulator has a bottom side thermal pad which couples heat to the PCB's internal ground plane layers. Any chance it is really the part overheating and you're just observing the higher temperature when you touch the taller nearby part?

[nn28]

I did remove the TPD3S014 chip and could feel another chip getting hot afterward see picture

That voltage regulator has a bottom side thermal pad which couples heat to the PCB's internal ground plane layers. Any chance it is really the part overheating and you're just observing the higher temperature when you touch the taller nearby part?

I have more teensy's laying around in this state so I could soon double check it.

[PaulStoffregen]

U4 is the chip expected to get hot if the 3.3V power is shorted anywhere on the board.



Try measuring the 3.3V power. If you see 3.3V, then looking at chips other than U4 may be worthwhile. But if you get something much less than 3.3V, the heat is almost certainly coming from U4 and nearby parts probably also warm up.

If you see only millivolts, odds are strong the short is metal, like stray solder or debris that fell onto the board. Usually those problems can be fixed, if you're able to locate the stray metal and remove it. But if you see something like 0.4 to 0.9 volts, odds are the short is a semiconductor, probably failure inside one of the chips. Typically a semiconductor short isn't repairable.

[FrankV007]

If more than one board has this issue then I don't think debris is the problem.

You need to measure the voltages to get a 'feel' to what is going wrong. 3.3 Volts should be 3.3 Volts +/- about 50 millivolts, too high, you have issues, too low, you have issues... Too high, or even reading 5V, you have a shorted 3.3 Volt regulator. Too low, the problem is after the regulator (providing the source voltage is within spec). Many 3.3 Volt devices will fry if the voltage is 5 Volts, and using 6 Volts even on the 5 Volt rail is NOT recommended (many of the 'newer' voltage regulators cannot use 12V inputs). U2, the chip you're showing is one part away from the Micro-controller, so my thoughts is possibly a bad board. Now, the issue (source), you mentioned that you connected using a power supply... (VIN source exceeding well above 5 volts because of faulty circuits which led to damaging the Teensy.), possibly up to 12 Volts. Now the fuse is a current limiting device, not a voltage detect, dropout type of device, if the current was kept within the levels of the fuse not to 'pop' then it will not pop. If the fuse was of a resetting type, you will have similar effects, even heating up (which a PTC fuse is).

As Paul Mentioned above.... If you see only millivolts, odds are strong the short is metal, like stray solder or debris that fell onto the board. Usually those problems can be fixed, if you're able to locate the stray metal and remove it. But if you see something like 0.4 to 0.9 volts, odds are the short is a semiconductor, probably failure inside one of the chips. Typically a semiconductor short isn't repairable.

[donperryjm]

I did remove the TPD3S014 chip and could feel another chip getting hot afterward see picture




I have more teensy's laying around in this state so I could soon double check it.

Are these virgin T4.1 that are presenting these problems? Or where they used in a circuit prior?

[FrankV007]

I am mentoring a group of students that are using the teensy 4.1 , they have been having problems with the VIN source exceeding well above 5 volts because of faulty circuits which led to damaging the teensy. The teensy doesn't get recognized by laptops anymore and the PTC fuse gets really hot when power is applied to it. The intended way to power the teensy was to power it with a 5V LM7805C Voltage Regulator, with a 12V input and a 5V output going directly to the VIN of the board.

From nn28's first posting, so I am assuming they were powered up already with an unspecified voltage (but suspect 12 V) on Vin.