[Noisebridge-discuss] Why is my AC adapter melting?

[Gopiballava Flaherty]

The complexity can vary *enormously*.

The simplest AC output ones can be just a transformer. But those are uncommon.

There are two basic types, linear and switched mode.

Linear ones can be regulated or unregulated. The absolute cheapest unregulated could have a single diode and capacitor. A slightly reasonable unregulated one would have four diodes in a bridge rectifier and a capacitor. The voltage is going to vary *enormously* based on the amount of power you draw. It might be 2 or 3 times the official rated power if there's little to no power. These are much less common than they used to be.

Regulated linear ones are inefficient - if the transformer is putting out 10V and you need 5V, then half the electricity is burned as heat.

In both of these cases, the transformer is fairly large and has lots of copper in it. They're heavy. I've had them die on me but they're fairly simple and I'd expect them to be reliable.

All modern devices use switched mode power supplies, which have loads of electronic components inside. They can be thought of roughly as small transformers with circuitry to drive the transformer at a very high frequency, and to adjust the signal driving the tiny transformer so that the output voltage is right. Depending on how much current the device draws, you need to change the driving signal.

High frequency (10s of kHz, up to a few megahertz) are a lot more efficient than 60 Hz transformers.

Better switched mode supplies have good physical and electrical isolation to prevent the high voltage 110V supply from reaching your 5V device. Cheaper ones may have very little isolation.

Thanks,

gopi.


On Apr 17, 2013, at 7:52 PM, Ari Lacenski <alacenski at gmail.com> wrote:

> I was intrigued, so I googled it and found pages and pages of people
> complaining about melting power bricks, but not the technical reason.
> I too would like to know. (egad! glad yer all right!)
> 
> Inside they're pretty simple: a pair of wire coils and some diodes for
> the DC current. I would imagine that one of the diodes failed and
> caused a short, leaving the transformer coils to turn into toaster
> elements. Confirm/deny?
> 
> Ari
> 
> 
> On Wed, Apr 17, 2013 at 6:00 PM, James Sundquist
> <sundquistjames at gmail.com> wrote:
>> I have a 9v 300ma power supply connected to an appropriate 9v 300ma device,
>> which has been happily working for the past year.  I hadn't used it in a few
>> weeks and plugged it into an outlet and found myself getting no power.
>> Confused, but easily sidetracked, I worked on something else for a half
>> hour.  I came back to the device and decided to unplug it; I found the AC
>> plug had actually grown quite soft/extremely hot to the touch.  I've now
>> tossed it in the electrical recycling bin and am just wondering if these
>> kinds of electrical meltdowns are common over time.
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