W
e won’t name the hapless
person who thought he’d
cooked his laptop. To avoid
embarrassment, we’ll simply refer to
him as ‘A.P.’ (aka, ‘Accident Prone’).
This is one of those projects we know
will be useful because A.P. kept ask-
ing ‘is it fi nished yet’ – he obviously
needed it!
That incident spooked him and why
wouldn’t it? He could have lost a lot
of work and spent quite a bit of money
and time on buying a new computer
and then setting it up, which could
have taken several days.
We do a lot of development work, in-
creasingly with Arduinos and similar
microcontroller modules. We also do
quite a bit of bread-boarding, often in
combination with the Arduinos.
When you’re doing this kind of work
and you have external power supplies
or voltage sources connected to your
circuit, that’s just asking for trouble.
You may not realise it, but when an
Arduino board (or similar) is plugged
into your computer’s USB port, you’re
just a slip away from potential disaster.
For example, say you’re running the
Arduino from a 12V plugpack, because
it’s driving some 12V relays or a motor
or whatever. So there’s a source of 12V
right near a bunch of other connections
on the Arduino board, just looking
for an excuse to fi nd its way onto the
USB 5V rail and into your computer.
One slip, and oops! It could blow up
the Arduino, your shield(s), and even
your computer.
Not only will this USB Port Protec-
tor vastly improve the chances of your
computer surviving such an event, it
may also prevent damage to the Ardui-
no board and whatever shields or other
circuitry are plugged into the USB port.
We can’t promise it will be 100% safe,
but it’s certainly a lot safer than if you
aren’t using any protection...
You might expect USB ports to have
some kind of built-in protection against
external voltages being fed in. After
all, all kinds of devices can be plugged
into these ports, including external
hard disks and amplifi ers and other
gear which may have its own, separate
power supply.
In fact, many USB ports do have
some kind of protection, for example
series PTC thermistors (‘polyswitch-
es’) to limit fault currents, transient
voltage suppressors and so on. But this
protection varies between computers
and is often absent in laptops and
notebook computers.USB PORT
PROTECTOR
by Nicholas Vinen
Using your PC or laptop to power a 5V project that you’re working on
is very convenient – but it’s so easy to make a small slip while plugging
something into a breadboard... and... oops! That’s exactly what happened
to one of our staff members. For a while after the incident, it looked like
his laptop was toast. But fortunately he was able to safely reset it and it
came back to life. But he was so lucky! Next time he won’t trust to luck –
he’ll defi nitely be using this simple, economic device.
Don’t let one
small ‘oops’
fry your PC!
Let’s face it, there’s a lot less space
inside portable computers – and
manufacturers also want to keep the
computer as light as possible and save
money where they can. That means
leaving out anything that isn’t abso-
lutely necessary.
Regardless of what sort of protection
your USB port may have, this USB Port
Protector is small and cheap, so why
not add in an extra layer of defence? If
you ever manage to activate its protec-
tion, it will have paid for its cost many
times over!Circuit description
The circuit of the USB Port Protector
is shown in Fig.1. USB plug CON1,
which plugs into your computer, is
shown on the left side, while the USB
socket, CON2, goes to the connected
device (eg, Arduino) and is on the right.
Just to be clear – the potential danger
of overload from excessive voltages or
currents comes via CON2.
The ground connection and the two
differential data lines, D+ and D–, are
wired straight through between plug
and socket (ie, CON1 and CON2) while
the 5V power fl ows through fuse F1
and positive temperature coeffi cient
thermistor PTC1.