How Does Reverse Voltage Damage Electronics?

How can wiring a battery backward damage electronics, such as Arduino?

Yes.
It will exceed many voltage limits and also generate lots of heat.

It depends on the circuit - with bipolar transistors the reverse voltage rating for the base-emitter junctions is quite small, this might become exceeded, the base-collector diodes will conduct heavily probably overloading the base circuits. In general excessive currents might flow through various diodes and zener diodes. With power MOSFETs the body diode will conduct (probably overloading the supply), with CMOS the protection diodes will all conduct (and then perhaps burn out) and the chip may go into latch-up. Any electrolytic capacitors may conduct, overheat and fail (burst). With CMOS those protection diodes will limit the tolerable reverse voltage to < -1.0V or so, assuming no inputs are connected to either rail (in which case its < -0.5V)

To my knowledge circuits with active devices (except for triacs and relays) are never designed to work with the supply connected backwards. Circuits may have a protection diode in the supply line to avoid this ever happening.

Tantalum caps glow a very bright white when connected the wrong way round.

AWOL:
Tantalum caps glow a very bright white when connected the wrong way round.

If you're going to tell people that you should suggest safety precautions and good voltage and component values! I can forsee a lot of tantalum's dying today!

I blew an (aluminium) electrolytic the other day (over voltage rather than reverse though) - "phutt" & lots of nasty smelling steam. Apparently 16V is greater than 6.3V :wink:

I blew an (aluminium) electrolytic the other day

Confetti time!

I blew one too! Kinda fun to see it start to expand, however the force of its explosion surprised me as well as it quickly filled my room with smoke
happy my hand wasn't in the way cuz it looked like it wouldhave hurt,
I had two similar looking caps, one 50v one 16v and I hooked them both up to a 24v supply lol

Modern aluminium can electrolytics have burst lines engraved, and nearly always blow upwards.
Old ones didn't have the engraving, and when they let go, you really knew about it.

AWOL:
Tantalum caps glow a very bright white when connected the wrong way round.

I refer to these as “LECs”. Light-Emitting Capacitors.

When an electrolytic (Tantalum or Aluminum) is reverse biased, the dielectric breakdown or “ungrows” itself. This creates a short between the anode and cathode plates, turning them into short-lived LECs.

So... it forces some components to short circuit? But the device doesn't turn on. If you connect a battery backward to an Arduino, can you detect voltage on some pins? Weird.

Why do you think perfectly normal things are weird?

When I run my car over water it sinks and the engine stops, weird?

You put a voltage in backwards and it screws things up, not weird!

Why do you think perfectly normal things are weird?

Because I am not fully understanding what the reverse current does to destroy the device or battery.

When I run my car over water it sinks and the engine stops, weird?

No, because I know how it happens.

dkl65:
Because I am not fully understanding what the reverse current does to destroy the device or battery.

It is because circuit design includes a lot of complex aspects of electrical engineering and material science. For example, diodes only conduct in one direction, due to the material science behind how a diode is constructed. When that diode is reverse biased, it may conduct enough current to destroy itself or allow current to flow into another component. That's the electrical engineering aspect.

That's just one example, of a single component.

There are a lot of details that go into a circuit design and even more complex details that go into the components themselves. There isn't any other general answer than: "Reverse current destroys devices because they weren't designed to conduct in reverse."

For semiconductors, the fundamental “problem” is that a reverse-biased junction behaves significantly differently than a forward-biased junction. Reverse-biasing a junction isn’t usually fundamentally harmful in itself, but assumptions made in the rest of the circuit can easily lead to “obviously harmful” behavior like conducting too much current.

Consider the common “input protection” network used on pretty much every input pin of a modern integrated circuit:

What this circuit normally does (with Vcc higher than GND) is limit the voltage from the external pin to the internal circuitry of the chip. If Vpin is higher than Vcc, the top diode conducts and “clamps” the voltage to about Vcc. Current is limited by R1 and/or the spec sheet for maximum current though the protection diodes. Similarly, if Vpin is lower than GND, the bottom diode conducts. The allowable current is relatively small; you’re supposed to keep the input voltages correct, and only VERY small currents are needed to drive the internal circuitry.

Now, if you connect the power to the chip backward, the two diodes are forward biased and will conduct pretty much all the current that they can. Since they are connected directly across the power supply, there is usually a lot of current available. In the old days of UV-eraseable chips with glass windows, it wasn’t uncommon to see the small wires that connect the chip to the IC pins actually GLOW like a light bulb. The currents involved are much larger than the chip was designed for; portions exceed the maximum temperature and fail, bridgewires or conductive paths on the IC melt or vaporize, portions of the packaging material get hot enough to turn to vapor and “explode” because they’re inside an essentially sealed environment…

Tantalum caps glow a very bright white when connected the wrong way round.

Cool! Can you show me a picture? How much voltage is required?
I never blew up a component before.

Well, you could take an extension cord and a disposable power strip out to the street... put a 470uF cap in one of the outlets, go back to the safe end of the extension cord, plug it in, and let 'er rip!

(Not that I actually recommend this. Someone could lose an eye.)

All electronic components are built containing a known quantity of smoke. This is essential to the correct operation of the device. If connected in reverse some of the smoke escapes and the device no longer works QED.

As others have said for direct current, the electricity flows in one direction. And components can only tolerate so much current until they let go of the magic smoke. If you think you will habitually mis-wire stuff, you might want to think about going with a ruddeduino, which from the site appears to put in a lot more fuses and such to protect people: http://ruggedcircuits.com/html/ruggeduino.html

Here is a link from the ruggeduino site that details a number of ways to destroy an Arduino: 10 Ways to Destroy an Arduino — Rugged CircuitsRugged Industrial Arduino Microcontrollers

10 Ways to Destroy an Arduino — Rugged CircuitsRugged Industrial Arduino Microcontrollers
It’s not enough to limit the current of each I/O pin -- the total current sourced from all I/O pins must not exceed 200mA

My Arduino must be strong; I powered 10 leds from the Arduino's pins (as indicator lights), and my Arduino (and LEDs) got warm. However, they did not all turn on at the same time. I also left out the resistors, because the lights were too dim when using a 330 ohm resistor (shouldn't have done that)! XD I use a 74HC595 shift registor now + 2 Arduino pins. But, that was 1 month ago; I still use my Arduino UNO SMD Edition today.