Solder these where shown on the
overlay diagram; LED1 is green while
LED2 is red and the cathodes are ori-
ented towards the USB plug, as shown
by the ‘K’ markings on the PCB.
Now solder schottky diode D3 in
place. Add a little fl ux paste to the
pads fi rst as it’s quite large, but the
procedure is much the same as for the
other two-pin devices. Just make sure
you apply the iron for long enough to
form good solder fi llets between the
PCB and terminals of the device.
Then fl ip the board over and fi t the
four remaining SMDs on the bottom
side, as shown in Fig.5. D1, D2 and ZD1
are polarised; also pay particular atten-
tion to the location of the cathode stripe
on ZD1. The fuse is not polarised.
Finally, fi t the USB plug and socket
as shown. Both need to be pushed
down firmly onto the PCB before
soldering. The plug has a notch on
the underside which the edge of the
PCB fi ts into. Note that the USB plug
pins may be quite short and may not
protrude very far through the bottom
of the PCB, so it’s a good idea to solder
them on both sides. Just make sure you
don’t accidentally bridge the pins.TVS2 note
The TVS2 (SM2T3V3A) is in an unu-
sual package – its anode runs the full
length of its body. When soldering it,
ensure that its body is not on top of the
cathode pad (marked K) or else it could
short out the USB data signal lines.Testing
Inspect the board to verify that all the
solder joints are good and that you
have no unwanted bridges, then plug
it into a USB port on your PC. If you
have a USB charger, you could use
that instead. Check that the green LED
lights up but the red LED should not.Figs.4 and 5: top and bottom overlay diagrams for the USB Port Protector. Use these as a guide during construction.
Be careful with the polarity for TVS1, TVS2, Q1 and LEDs1 and 2. It’s easiest to start by fi tting Q1 and TVS1, then the
remainder of top-side SMD components, then the bottom-side components and fi nally, CON1 and CON2. The matching
photographs above are reproduced close to twice actual size, for clarity.
We fi nished
our USB Port Protector
with clear heatshrink tube... just
in case A.P. managed to drop
something into the PCB!You can then carefully measure
the voltage across D3. You should get
a reading in the range of 4.5-5.25V
(usually quite close to 5V), with the
red probe to its cathode (striped) end.
Now plug a small device like a USB
card reader or fl ash drive into the
socket and verify that it powers up
correctly. Try reading the contents of
the card/fl ash drive on your PC and
verify that it works normally without
any unexpected disconnection events.
If you want to verify that the USB
Port Protector will defi nitely protect
your computer, you will need a ~6V
supply and a resistor with a value
between 2.2Ω and 10Ω.
Unplug the USB Port Protector and
anything that’s plugged into it and use
a clip lead to connect the USB socket
shell to the ground terminal of your
6V supply. Connect one end of the test
resistor to the positive output of the
6V supply (battery pack, plugpack...)
and then touch the other end of the
resistor to the USB socket pin that’s
immediately adjacent to fuse F1, on
the underside of the board.
If you can do this while looking
at the top of the board, you should
see both LED1 and LED2 light up.
LED2 indicates that the protection is
operating. If you have a helper, they
could measure the voltage across D3.
It should be close to 5.5V. This
confi rms that the device
is working.Using it
To avoid accidentally shorting the 5V
supply or either of the signal lines dur-
ing use, we suggest you encapsulate
the entire device in a short piece of
heatshrink tubing, as shown above.
Clear tubing is convenient since you
can still see the components – but any
colour will work.
Cut the tubing so that it covers the
entire USB socket, up to the lip that’s
around the open end, and the base of
the USB plug, up to where it projects
from the PCB. Then it’s just a matter
of applying a little heat, eg, from a hot
air gun or hair drier.
Rotate the assembly until the tubing
has shrunk into place and try to avoid
burning yourself in the process. If it
gets too hot to hold, put it down and let
it cool before shrinking the remainder
of the tubing.
If you manage to blow the fuse, you
will simply have to cut the tubing off,
desolder the fuse, clean the old solder
off using fl ux paste and some solder
wick, solder a new fuse in place and ap-
ply a fresh length of heatshrink tubing.
Or if you’re really clever, you may be
able to cut a fl ap in the tubing around
the fuse, replace it and then glue the
fl ap back in place.Reproduced by arrangement with
SILICON CHIP magazine 2019.
http://www.siliconchip.com.au