Fried an IC and want to know more.

Have a parallel LCD. Wanted to hook it up with a SerLCD backpack from Sparkfun. The SerLCD has a 5v/GND/TX screw terminal. Ok. I hook it up to the 5v out with jumper cables (the breadboard types), GND to GND. I didn't want to solder anything so I just laid the backpack in the parallel slots of the LCD (header pins).

It doesn't light up. Then I notice my arduino is dead. I unhook everything. Arduino comes on, it's running the basic blink program. When I hook up the 5v to the backpack only, the blink and power light fade away on the arduino. So I think I don't have enough juice powering it over usb. So I hook up the 9v arduino power in addition to the usb. I then lay the backpack in the LCD again. This promptly starts a cooking sound on the backpack and a nice plastic bubble forms on the IC. Smelly plastic smell. Doh!

So fine, I fried something. But really didn't learn a lot. If I had a short, should I not have hooked up the 9v adapter? Does USB have some short protection (I've seen my Mac warn about usb power draw sometimes but I didn't see it here)? I thought 5v out was 5v no matter what I have coming into the Arduino? Do shorts cause the voltage draw to max out and "run away"?

Thanks for any help. I know it's annoying helping a noob troubleshoot when you aren't there.

yes the usb is protected, twice on official design boards

once by the computer, ideally it will shut off the usb if an overload occurs, course you dont want to depend on that so there is a 500ma resetable fuse on the duino

USB will limit current to 500mA (at most; the actual number is negotiable up to 500mA).

This does not protect your device if you switch the + and - leads, short something to ground that shouldn't be shorted, etc.

Think of the USB protection as protecting the device on the other end of the USB cable - your computer.

Likewise, the newer arduino's fuse will protect you from a fire, or killing your power supply, but won't help you if you draw too much current through on of the pins and burn it out.


Seems like USB power is safer for me because I don't know what I'm doing right now. Max USB power is 5.25v though. The backpack I was trying to power was 5v in, I guess that's good enough. I'd rather not constantly fry things.

I'll try this LCD without the backpack.


Some LCD modules have nonstandard (reversed) connections for their power pins. Normally pin 1 is ground and pin 2 is +5 volts.

Here's an example of a 'backwards' one: Seeed Studio Bazaar, The IoT Hardware enabler. (scroll down to see the connections), and a review from a customer:


Thx all. Really appreciate it.

Thx all. Really appreciate it.

I'll try this LCD without the backpack.

By this do you mean the back light, most of these are LEDs and need a current limiting resistor. It sounds like you powered yours up without one. As these normally consist of LEDs in series you only need a small resistor 10 to 20R is typical.