Teensy 3.2 availability and repair

A

alexsh1

Member
Dear all,

Unfortunately, I flied Teensy module accidently and LP38691 blown. Now it is getting hot and delivering 1.8V instead of 3.3V.
I cannot use Teensy 4.1 due to pins compatibility. Any idea where I can source Teensy 3.2 (I am based in the UK)? It is obsolete since October 2023.
I will try to repair the blown Teensy 3.2 but it will take some time as I do not have LP38691 available.

Thanks
Alex
 
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When I connect my Teensy 3.2 to USB, I get the following readings:
VUSB = 1.8V
3.3V pin = 0.12V

I suspect this is the LDO blown. Any suggestions?
 
Vusb should be 5V. Did you check fuse F1?

1704833408893.png


What voltage do you measure on capacitor C7 (2.2uF)?

Paul
 
alexsh1 said:
I suspect this is the LDO blown. Any suggestions?
Click to expand...

I'm guessing migrating to Teensy 4.0 doesn't count as a suggestion? Just keep in mind that is the long-term solution...

You could desolder the LP38691, if you have a hot air station or other appropriate equipment to desolder a SMT part with bottom side pad.

Then externally (and carefully) apply 3.3V power to the board as the LP38691 would do. If it works, then you can start shopping for a LP38691 chip. Or just continue using it with external 3.3V power.
 
PaulStoffregen said:
I'm guessing migrating to Teensy 4.0 doesn't count as a suggestion? Just keep in mind that is the long-term solution...

You could desolder the LP38691, if you have a hot air station or other appropriate equipment to desolder a SMT part with bottom side pad.

Then externally (and carefully) apply 3.3V power to the board as the LP38691 would do. If it works, then you can start shopping for a LP38691 chip. Or just continue using it with external 3.3V power.
Click to expand...

Yes, I have to re-design the whole PCB and it would take time. This is a project started a few years ago and I was using Teensy 3.2 due to its sign and AREF availability (I am using my own precision voltage source).

So you think its the LDO at fault here? Anything else I would check?
Yes, I can desolder the LP33691, but too many small components around.
 
Well, that means there is a voltage drop of (5V - 1V8) 3.2V over the fuse...
Did you measure Vusb on the Vusb pad on the Teensy?
By the way, which part got hot?

Paul
 
alexsh1 said:
Yes, the fuse is working. Otherwise, I would have 0V.
Click to expand...

It is a PTC resistor, not a real fuse that blows. During high current, self heating causes the PTC resistance to increase, which tends to limit the current when the power source is a regulated (low impedance) voltage. Use of the word "fuse" or "self resetting" is kinda misleading marketing, but sadly an industry norm. From a purely electrical theory point of view, it's important to understand these PTC devices aren't really fuses that burn. They're just resistors that increase their resistance dramatically during over current conditions.
 
PaulS said:
Well, that means there is a voltage drop of (5V - 1V8) 3.2V over the fuse...
Did you measure Vusb on the Vusb pad on the Teensy?
By the way, which part got hot?

Paul
Click to expand...
I measured VUSB on the VUSB pin. VUSB pin or pad, does not matter, it is the same. VIN and VUSB pads are not cut.
 
PaulS said:
Well, that means there is a voltage drop of (5V - 1V8) 3.2V over the fuse...
Did you measure Vusb on the Vusb pad on the Teensy?
By the way, which part got hot?

Paul
Click to expand...
Yes, as soon as I short the fuse the voltage goes up to 5V
 
PaulStoffregen said:
It is a PTC resistor, not a real fuse that blows. During high current, self heating causes the PTC resistance to increase, which tends to limit the current when the power source is a regulated (low impedance) voltage. Use of the word "fuse" or "self resetting" is kinda misleading marketing, but sadly an industry norm. From a purely electrical theory point of view, it's important to understand these PTC devices aren't really fuses that burn. They're just resistors that increase their resistance dramatically during over current conditions.
Click to expand...
Right, yes, my bad.
Thanks for pointing out
 
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