the SOA curve for Q1 in red so that
you can compare them. As you can
see, F1 responds considerably faster
than PTC1, and in fact is very likely
to blow before Q1’s SOA is exceeded
for currents above 2A.
For fault currents between 300mA
and 2A, it’s possible that Q1 will over-
heat and fail before either F1 blows or
PTC1 acts to limit the current. And in
fact, PTC1 is not guaranteed to do any-
thing for fault currents below 1A. You
will need to notice red LED2 lighting
and resolve the fault yourself.
Still, as we said above, the ratings
for Q1 seem to be pretty conservative
and as long as the overload is limited
to no more than a second or two, we
would expect it to survive.
Looking at Fig.2, you may wonder
why we’ve bothered with the PTC
at all, given that its ‘trip’ current is
higher than the fuse blow current
over most of the graph. But keep in
mind that PTC1 is considered to be
‘tripped’ when it has reached a high
enough resistance value to keep the
fault current below 2.2A.
It will still have some effect in re-
ducing the fault current, even at lower
current levels and shorter time spans,
because its resistance will start to in-
crease well before it has fully tripped.
And you also have the option of
replacing F1 with a zero-ohm resistor
(or just soldering across the pads) and
relying on PTC1 to limit fault currents.
This does increase the risk of blowing
Q1 in a serious fault (although, as we
said, it’s pretty robust) but doing so
would also increase the chance that the
unit will survive a moderate overload
unscathed and you won’t have a blown
fuse to replace.
Note that while replacing Q1 is a bit
of a pain, it’s actually quite cheap (un-
der 50p) so if Q1 does ‘throw itself on
the grenade’ and fail while protecting
your computer from damage, at least
it isn’t an expensive failure.Sourcing the parts
Most of the parts are surface-mount de-
vices (SMDs) and they are all available
from Digi-Key, Mouser or element14.Construction
The USB Port Protector is built on a
double-sided PCB available from the
PE PCB Service. It measures 32.5 x
19mm and is coded 07105181. All but
four of the components are mounted
on the top side of the board, as shown
in the overlay diagrams, Figs.4 and
5 and matching photos. The only
through-hole components are the USB
plug and socket. (By the way, you may
notice a minor difference between the
overlay diagrams and the PCB photos:
we’ve changed TVS2 to a more suitable
part since building the prototype.)
Most of the parts are fairly easy to
solder, although some of them are
quite close together, to keep the unit
compact. It’s easiest to do in the fol-
lowing order.
Start with transistor Q1. This is in a
fairly small ECH8 package, with four
short leads on each side. The good
news is that most of the adjacent leads
are connected together so it doesn’t
matter if you bridge the pins when
soldering (in fact, it’s pretty much una-
voidable). Pin 4 is the base connection
and you need to make sure it doesn’t
short to pin 3, the emitter.Start by identifying pin 1. There
is a dot printed in the corner on the
top of the package, but you will need
a magnifier and good light to see it.
Orient the part so that it matches the
pin 1 markings on the PCB and smear
a thin layer of flux paste on all eight
of its pads.
Apply a tiny amount of solder to
the pad for pin 4, then heat this solder
while sliding the part into place. Check
that the other seven pins are correctly
located above their pads using a magni-
fier. If not, re-heat the solder joint and
carefully nudge the part. Repeat as
necessary until it’s lined up, then solder
the four pins on the opposite side of the
package. These are all joined together
so you can do it as one big solder joint.
Now apply solder to the three re-
maining pins and add a bit of fresh
solder to pin 4 as well. To tidy up the
solder joints, apply a little more flux
paste on top of the solder and then use
some solder wick to remove the excess.
Clean up the flux residue with some
methylated spirits, isopropyl alcohol
or other flux cleaner and then inspect
it visually to ensure all the solder
joints are good.
That’s the trickiest part out of the
way. Next, solder TVS1 in place, next
to Q1. It’s fairly small and its cathode
stripe will not be terribly obvious; so
again, use magnification to identify the
cathode and orient it correctly before
tacking it in place and soldering the
opposite pin.
Now solder the SMD passive com-
ponents in place; this includes five
resistors, one capacitor and the PTC
thermistor. None are polarised; just be
careful to fit each in the location shown
in Fig.4. The resistors will be printed
with a small code indicating their value
(eg, 1.2kΩ code is 122; or 12Ω x 10^2 )
but the capacitor will not be marked.
The resistor codes are also shown in
the parts list opposite.
The next components to mount
are reference REF1 and transistor Q2.
These are in identical SOT-23 packages
so don’t get them mixed up after taking
them out of their packaging. They are
polarised, but have three pins each,
so the orientation is obvious – see the
pinouts in Fig.1.
Next are the two LEDs. Usually, the
cathode is marked with a green dot
but sometimes the anode is marked
instead. The easiest way to check is
with a DMM set on diode test mode.
The LED will light up with the red
probe connected to the anode and black
to the cathode. You can confirm the
colour at the same time. Note that some
DMMs (eg, those powered by two AA
cells) may not apply sufficient voltage
to light up a green LED.1 double-sided PCB, coded 07105181,
32.5 x 19mm
1 PCB-mount USB Type A horizontal
plug (CON1)
1 PCB-mount USB Type A horizontal
socket (CON2)
[eg, Altronics P1300]
1 SMD fuse, 3216/1206 package,
1A super fast blow [Vishay
MFU1206FF01000P100]
1 SMD 1.1A PTC thermistor,
3216/1206 package
[Bourns MF-NSMF110-2]
1 30mm length of 20mm diameter
clear heatshrink tubingSemiconductors
1 AN431AN shunt reference IC, SOT-23
(REF1)
1 ECH8102 12A PNP transistor, ECH8
(Q1)
1 BC856 100mA PNP transistor, SOT-
23 (Q2)
1 high-brightness green LED,
3216/1206 package (LED1)
1 high-brightness red LED, 3216/1206
package (LED2)
1 CDSOD323-T05S transient voltage
suppressor, SOD-323 (TVS1)
1 SM2T3V3A transient voltage
suppressor, DO-216AA (TVS2)
2 BAT54SFILM dual 300mA schottky
diodes, SOT-23 (D1,D2)
1 15A 30V schottky diode, DO-214AB
(D3; MCC SK153)
Capacitors
1 100nF SMD X7R ceramic, 3216/1206
packageResistors (all SMD 3216/1206
package, 1%)
1 47kΩ (coded 4702 or 473)
1 10kΩ (coded 1002 or 103)
1 1.2kΩ (coded 122)
1 1kΩ (coded 102)
1 470Ω (coded 471)Parts list
USB Port Protector