Powering Teensy 4.0 from step down converter

[madamAdam]

I'm making a midi instrument that uses a 12v power supply to power an array of COB LEDs.

I am using pololu 5v,2-5a step down voltage regulator (d24v22f5) to provide a 5v power supply for my teensy 4.0, as well as some led drivers and multiplexers. I have cut the little link between vin and vusb and this works. I still use the usb connection for uploading sketches.

All grounds are fed back to the same place - the chocolate block visible in photo.

The teensy in the photo died while I was running a basic pwm sketch and I have left everything as it was. The caps are 0.1uF and 10uF. The breadboard is reused and a bit scuffed. I have connected four extra analog inputs (A0, A11, A12, A13) on the underside of the board which are connected to rows 45 and 60 on the breadboard. The led was fading on and off happily for some time before the magic smoke appeared. The led should have a resistor but this would only have damaged the led, right?

A clue - while I was taking a picture for this post, the grd connection on the right hand side of the chocolate block came loose. It could have been loose already.

I'd be really greatful if anybody could help me to see where I'm going wrong. I've got a fresh teensy here and I really don't want it to meet the same fate.

Thanks in advance!

 

[jmarsh]

The led should have a resistor but this would only have damaged the led, right?

Noooo.
Teensy's pins can only sink or source a certain amount of current (roughly 4ma). Without a resistor that limit would be exceeded, causing internal failure.

 

[madamAdam]

Noooo.
Teensy's pins can only sink or source a certain amount of current (roughly 4ma). Without a resistor that limit would be exceeded, causing internal failure.

Ahhh - so I was thinking about voltage, when I should've been thinking about current. I've used leds without resisters in the past for quick tests and to save heating up a soldering iron, but I must've got lucky...lesson learned.

Thanks jmarsh for pointing this out.

With the loose ground wire - if it was loose, would current then possibly run through the usb grd and would this be risky?

Or can I safely assume that it was the missing resistor that caused this?

 

[jmarsh]

With the loose ground wire - if it was loose, would current then possibly run through the usb grd and would this be risky?

Not likely.
The only issue with using USB and external power at the same time (without separating the VIN/VUSB pads) is that the 5V supply lines will be joined, causing voltage regulators to fight each other.

 

[madamAdam]

Not likely.
The only issue with using USB and external power at the same time (without separating the VIN/VUSB pads) is that the 5V supply lines will be joined, causing voltage regulators to fight each other.

Good to know. Thanks for your help!