Computing stuff tied to the physical world

What if I turn the chip around?

In Hardware on May 8, 2013 at 00:01

Welcome to the weekly What-If series, also available via the Café wiki.

Ok, you’re all excited, you’ve built some electronic circuit – either by assembling a kit, or all on a breadboard, or perhaps you’ve even go so far as to design and create a custom PCB.

Any non-trivial circuit will have polarised components on it, whether capacitors, diodes, transistors, regulators, or… the most common one in oh so many varieties: a “chip”, with 6..40 pins, or sometimes even more.

Mr. Murphy loves chips. Because sooner or later, you’ll connect one the wrong way around. Even if you know what you’re doing, sometimes the orientation marker on a chip is fairly hard to see, especially on the smaller SMD types.

So what happens if we put things in the wrong way around?

Obvious answer: it depends (on the chip).

Comforting answer: more often than not, nothing will happen, the thing will get hot, and it’ll still work fine once you fix the problem, i.e. turn the chip around and reconnect it.

The good news is that it’s not so easy to really damage most chips, with a few precautions:

  • use a “weak” power supply, i.e. one which can’t put out to much current, as current leads to heat, and heat is usually the cause of component damage – a lab power supply with adjustable “current limiting” set to a low value is a very good idea
  • keep your hands near the ON/OFF switch when powering up a circuit for the first time, keep your eyes open, and … use your nose: bad stuff due to heat often shows itself as smoke (by then, it’s often too late), and as components getting far too hot, and starting to smell a bit
  • for low-voltage circuits, and this includes almost all digital circuits: place your fingers on several of the key components right after turning power on: if you sense anything getting hot, turn off the power – NOW!
  • sensing heat is an excellent way to save a project from serious damage: we can easily tell if something heats up to 50 °C or more, yet most silicon-based chips will be able to heat up way beyond that before actually getting damaged (125..175 °C) – so as long as you turn the power off quickly enough, chances are that nothing really will break down, and chips and resistors will often start to smell – a useful warning sign!

Note that analog circuits tend to get damaged much more easily. Put a transistor the wrong way around, and it’ll probably go to never-never land the moment power is applied.

One reason digital chips are so resilient, is the fact that they are full of ESD diodes. These tend to be on each of the I/O pins of a chip, as protection against Electrostatic Discharge. Here’s what a typical I/O pin circuit on a digital chip looks like:

JC's Grid, page 72

Nothing happens under normal conditions, since the diodes are all in blocking mode. When the I/O pin voltage rises above VCC or drops below GND, however, the diodes start to conduct, while trying to remove the charge, so that the voltage levels never reach values which might damage the sensitive oxide isolation (that’s the “O” in CMOS and MOSFET).

Now have a look at what happens when a chip gets powered up with bad voltages on two of the I/O pins (the light-blue parts are not conducting and can be ignored):

JC's Grid, page 72 copy

The way to look at this is that the pin(s) with the highest voltage will start feeding into the (internal) VCC connections, and the pins with the lowest voltage will start drawing current from the (internal) GND connections. Or, to put it a different way – some I/O pins will act as VCC and GND supply lines, albeit with some internal ESD diodes in between:

JC's Grid, page 72 copy 2

In this diagram, VCC and GND are fed from pins which were not intended as such!

As you can see, the diodes now start conducting as well, drawing a certain amount of current. If these currents are not higher than the diodes can handle (usually at least a few mA per diode), then the chip will act more or less like a short to the rest of the circuit. With a bit of luck, your power supply will decide to lower its output voltage and enter “current limiting” mode. The result: nothing works, but nothing truly dramatic happens either. It just gets hot and all the voltages end up being completely wrong.

Sooo… next time you power up your new project for the first time: stay alert, use your fingers, be ready to cut power, and… relax. If it doesn’t work right away (it hardly ever does!), you’ll usually have time to figure out the problems, fix them, and get going after all.

Note that there are no guarantees (things do occasionally break), but usually it’s fixable.

  1. It’s a pity thermal imaging cameras are so expensive – it’d be really good to have one looking at a circuit as it’s powered up.

    • That’d be neat! Hmm… I wonder whether a PIR sensor could be used. All it needs to detect is a sudden change in heat radiation – but you’d have to stay well out of the way to avoid false triggers.

  2. One of my teachers during my studies used to say that the active part in electronics is smoke. Once the smoke gets out of the device, it won’t work any more. :-)

  3. Like you say, other components are not quite so resilient. I once hooked up a DS1820 the wrong way round and it died without even releasing any smoke. Learned a valuable lesson that day: always double check the pinout diagrams!

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