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USB Port
Voltage
Checker
Above: the unit can be used to check for voltage
fluctuations when an external USB-powered unit
is connected in-line with the voltage checker.
By NICHOLAS VINEN
If you carry valuable data around on a USB flash drive, it’s not
a good idea to plug it into other people’s computers willy-nilly.
They could have dead or faulty USB ports and an incorrectly
wired USB port can destroy a flash drive. Tragedy! Test it first
with this handy USB Port Voltage Checker.
U
SB DEVICES ARE convenient for
many reasons and one of these is
that you can walk up to just about any
computer anywhere and plug a USB
peripheral in. This is most useful for
storage devices like flash drives and
hard drives but can apply to just about
anything.
But unless it’s your computer and
you know the ports are all working
OK, there is the possibility that your
treasured USB device will be damaged
by a faulty port. This could happen for
a few reasons. One is that front-panel
USB ports are normally plugged into
a USB pin header interface on the
computer’s motherboard via a multicore cable and these can be plugged
in incorrectly, causing the port supply
voltage to be reversed. That could easily damage a connected device. In fact,
in this case, damage is likely.
There is also the possibility that the
computer’s power supply has a poorly
regulated 5V rail, giving a port voltage
that is too high, too low or fluctuating.
This also applies for powered hubs
and other devices where a failed
84 Silicon Chip
plugpack could easily lead to trouble.
Port voltages that are too high could
also damage a connected device while
voltages that are too low (permanently
or only when current is being drawn)
could lead to erratic device operation.
Well, that’s enough about what
could go wrong. This checker will
show you when a USB port’s voltage
is in the correct range so you can plug
in your flash drive or other device with
confidence. You can even leave the
unit connected and plug the device in
piggy-back style, so you can continue
to monitor the supply voltage during
operation, to ensure it doesn’t fluctuate too much.
The USB Port Checker is just 67 x
17 x 10mm – not much longer than
flash drive. It’s built on a small doublesided PCB measuring 44 x 17mm and
is encapsulated in clear heatshrink
tubing for protection. It uses a mixture
of through-hole and surface-mount
devices to keep it compact.
Circuit description
Fig.1 shows the full circuit. The USB
plug (CON1) and socket (CON2) – both
Type-A – are wired straight through so
the function of any USB device connected to CON2 is not affected. The D+
and D- signalling lines are run close
together down the middle of the PCB
so that the digital data signals are not
affected, as well.
Schottky diode D1 rectifies the USB
supply voltage so that if the polarity
is reversed, nothing happens – no
LEDs light, including the green one,
so you know something is wrong with
the port. This is a dual diode with a
common anode connection but we’re
using them both in parallel.
Why do this? Simply because we
need one elsewhere and it’s easier to
use two identical parts than two different ones.
If all is OK, LED1 (green) is lit
and this simply runs off the rectified
voltage of around 4.7V with a 680Ω
current-limiting resistor. Typical LED
current is around (4.7V - 2V) ÷ 680Ω
= 4mA.
IC2 is a 2.5V reference “diode”
which is actually an integrated circuit
siliconchip.com.au
D1 BAT54A
+4.7V
K1
A
VOLTS
LOW
1 F
K2
LED2
100k
3
2.5V
IC2
LM285D
–2.5
CON1
CON2
1
2
3
4
+V
D–
D+
GND
TYPE A
PLUG
1
2
3
4
+V
D–
D+
GND
TYPE A
SOCKET
IC1a
2
D2 BAT54A
18k
22k*
680
1
IC1: LM393D
K1
A
A
VOLTS
OK
LED1
K2
RB#
1 F
5
IC1b
6
# OPTIONAL – SEE TEXT
* FOR USB 3.0 VERSION – SEE TEXT
7
4
160k
120k*
USB PORT VOLTAGE CHECKER
A
K1
LEDS
IC1, IC2
BAT54A
SC
K
RA#
K
2013
VOLTS
HIGH
LED3
680
8
8
4
A
K
110k*
160k
680
A
8
4
1
K2
K
A
Fig.1: complete circuit of the USB Port Checker. It’s based on dual comparator IC1 and voltage reference IC2. If the
voltage is OK, green LED1 is lit while yellow LED2 and red LED3 indicate under-voltage and over-voltage respectively. If
the supply polarity is reversed or voltage is very low, none of the LEDs light up.
with two pins (well, it has eight but
six are unconnected). It is effectively a
shunt regulator and it runs off the 4.7V
supply via a 100kΩ resistor. That gives
it a current of about (4.7V - 2.5V) ÷
100kΩ = 22µA, with its recommended
minimum being 10µA.
One half of dual low-power comparator IC1a compares this 2.5V reference
to a divided-down version of the USB
supply voltage. This is achieved with
a resistive voltage divider comprising two 160kΩ resistors and an 18kΩ
resistor. IC1a’s inverting input (pin
2) is connected to the 2.5V reference
while the voltage at the non-inverting
input is at V+ x 0.527 where 0.527 is
the divider ratio, calculated as (160kΩ
+ 18kΩ) ÷ (160kΩ x 2 + 18kΩ).
For USB 2.0, the minimum supply
voltage is specified as 4.75V. If we
plug this into the formula above, we
get 4.75V x 0.527 = 2.5V which is the
same as the 2.5V reference it is being
compared against. So when the USB
supply drops below 4.75V, pin 3 of
IC1a goes below 2.5V and IC1a’s output
switches low, turning on LED2, again
with a current of about 4mA.
At the same time, IC1a also turns off
green LED1 by pulling its anode low
through half of dual-Schottky diode
D2. Thus, if the USB voltage is too
siliconchip.com.au
low, the yellow LED turns on and the
green LED turns off.
A 1µF capacitor across LED1 ensures
that it is switched off for a minimum
period (a few milliseconds) even if
there is a brief drop in the USB voltage.
That won’t be noticeable in isolation
but if the USB voltage is dropping
below 4.75V often enough, it means
that LED1 will either dim or go off
entirely.
Note that the PCB has provision
for an SMD resistor labelled RA to
add hysteresis for comparator IC1a.
However, we don’t think it’s necessary.
Over-voltage checking
The circuit to detect over-voltage is
similar. In this case, comparator IC1b
is used, as is the same 2.5V reference
voltage from IC2. This time, however,
the division ratio is different as IC1b’s
inverting input is connected to the
other end of the 18kΩ resistor. That
means the formula to calculate the
comparator threshold is V+ x 0.473,
which means the upper threshold is
a little above 5.25V.
Again we can check this by doing
the calculation: 5.25V x 0.473 = 2.48V.
So red LED3 will turn on if the supply voltage goes much over 5.25V. As
with IC1a, IC1b’s output going low
also turns off LED1, via the other half
of dual-diode D2.
Pads for a hysteresis resistor (RB)
are supplied but as before, we don’t
think it’s necessary. If RB is fitted, its
value will need to be chosen carefully
– see below.
USB 3.0 support
The 4.75-5.25V (ie, 5V±5%) USB
supply range is from the USB 2.0
specification. The newer USB 3.0
specification allows for more current
to be drawn by USB devices and as
such, also allows a wider variation in
supply voltage, ie, 4.45-5.25V. While
we don’t think it will happen very
often, this means that with a USB 3.0
port, the under-voltage indication (ie,
yellow LED lit) could occur while operating within specifications.
If you want to accommodate this,
you can do so by changing the divider resistor values, ie, use the values
shown in red on the circuit diagram.
The divider ratios then become 0.563
and 0.476, giving an upper threshold
of 2.5V ÷ 0.476 = 5.25V and a lower
threshold of 4.44V.
Accuracy
The 2.5V version of the LM285 voltage reference has a tolerance of ±1.5%
July 2013 85
3
2
D2
BAT54A
CON1
4
3
2
1
1
680
160k^
18k*
1 F
CON2
680
4
D1
IC2
LM285
IC1
LM393
1 F
24107131
1
CON2
A
G
USB Checker
3
2
A
Y
C 2013
R
100k
A
4
680
CON1
160k #
BAT54A
REAR VIEW
FRONT VIEW
* 22k USB 3.0 VERSION
# 110k USB 3.0 VERSION
^ 120k USB 3.0 VERSION
Fig.2: follow these PCB overlay diagrams to assemble the USB Port Checker. The SMD parts (ICs, diodes and
capacitors) all go on the top side along with the LEDs and connectors, while the resistors are all fitted on the
bottom side. The two empty pads on the bottom are for optional hysteresis setting resistors.
These photos show the front & rear of the completed PCB. Take care soldering in the SMDs and check that the
diodes & ICs are correctly orientated. You can remove solder bridges across the IC pins using solder wick.
which translates into an error of about
±0.08V referenced to the USB supply
voltage. Taking into account resistor tolerances and variations in the
forward voltage of D1, the maximum
error could be more than that. There
is also a roughly +5-10mV error due to
the input bias current of IC1a and IC2a
flowing through the divider network.
An error of ±0.1V would be fairly
significant compared to the ±0.25V
specification for the USB 2.0 supply
but this is a worst case figure and
without taking any special care, our
prototype’s thresholds measured very
close to what we calculated above. You
would be unlucky to build one of these
and find it had more than ±0.05V error.
Our (randomly chosen) LM285
measured 2.4946V which is an error of
just -0.22%. One easy way to check the
accuracy of your voltage reference IC is
to use a DMM to measure the voltage
across its lower-left and upper-right
pins while power is applied. If your
reading is much lower than ours, try
reducing the value of the 100kΩ resistor feeding it (eg, to 10kΩ) as a higher
operating current should (slightly)
increase the reference voltage.
Construction
Fig.2 shows the assembly details.
Begin the construction by fitting the
The through-hole components can
then go in. The resistors go on the
other side to the SMDs (check each
one with a DMM before fitting it). The
three LEDs go on the same side as the
SMDs, with their anodes to the edge
of the board.
Finish up by fitting the two USB
connectors – the socket (CON2) is optional but recommended. Ensure that
their mounting tabs are fully pushed
into the corresponding holes on the
PCB before soldering them and then
finally the signal pins.
You can test it by simply plugging
it into a known-good USB port; the
green LED should light while the
others should remain off. If you have
a variable voltage DC supply, you can
wire it up across the USB pins (using
a spare plug perhaps) and then vary it
between 4.5V and 5.5V to check that
the yellow and red LEDs come on at
the correct voltages.
Once you’re satisfied, slide some
clear heatshrink tubing over the unit
and shrink it down.
SMD components to the PCB, which
is coded 24107131. Install the two ICs
first. Figure out which is which and
then locate pin 1 which is normally
indicated by a divot or dot in the
corner of the plastic package. It could
also be indicated by a stripe along the
top of the IC (between pins 1 & 8) or
by a bevelled edge which will be on
the pin 1 side.
Put a little solder on one of the IC
pads then place the chip in the correct position, with pin 1 at upper-left.
While heating that solder, slide it into
position. You can re-heat the solder
and adjust it if necessary, then solder
the rest of the pins. Finally, add a little
solder to the first pin you soldered, to
refresh it.
If any pins are bridged, you can clear
them using solder wick although often
all that’s required is to slide the soldering iron tip between the pins and then
back again (assuming it’s fine enough).
Now fit the two SMD dual diodes.
Their orientation should be obvious as
long as they are not upside-down, ie,
solder them with their leads touching
the PCB. Then mount the two ceramic
capacitors, again using a similar technique but make sure you wait a bit between soldering one pad and the other
to ensure the first joint has solidified
before making the second.
Hysteresis
As noted above, you probably don’t
need to add resistors for comparator
hysteresis. The advantage of hysteresis
is that a brief excursion beyond one of
the thresholds is more likely to cause
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
No.
2
1
1
1
1
1
3
86 Silicon Chip
Value
160kΩ
120kΩ
110kΩ
100kΩ
22kΩ
18kΩ
680Ω
4-Band Code (1%)
brown blue yellow brown
brown red yellow brown
brown brown yellow brown
brown black yellow brown
red red orange brown
brown grey orange brown
blue grey brown brown
5-Band Code (1%)
brown blue black orange brown
brown red black orange brown
brown brown black orange brown
brown black black orange brown
red red black red brown
brown grey black red brown
blue grey black black brown
siliconchip.com.au
USB Port Polarity: A Simple Approach
This project was inspired by reader
Bruce Pierson, who sent in details of a
simple design to check USB port supply polarity. As you can see, his design
consists of just a USB plug (surfacemounting type), a LED and a resistor,
all soldered to some Veroboard and
housed in the plastic case from a defunct
flash drive.
If all you want to do is check the supply polarity then this is not a bad idea
and it’s certainly much less complicated
than our approach. But obviously, it won’t
the LED to come on and stay on rather
than flicker so briefly that you may not
notice it. In practice, what happens as
the bus voltage crosses the threshold
is that one LED appears to fade in
while the other fades out, due to rapid
switching between them.
If you decide to add hysteresis,
choosing a value for RA is fairly simple. With RA = 10MΩ, once the supply
voltage drops below the lower voltage
of 4.75V, this threshold is changed to
about 4.77V by the fact that RA is effectively in parallel with the lower part
of the voltage divider. In other words,
it will give about 20mV of hysteresis.
A lower value resistor will give
proportionally more hysteresis. Much
more than 100mV of hysteresis is probably not desirable, giving a minimum
value of 2.2MΩ or so. Note that fitting
RA will also shift the lower threshold
needed to turn on LED2 as well, but
only very slightly.
Choosing a value for RB is more
tricky, because when IC1b’s output is
low, it is effectively in parallel with
IC2 and we must also consider that
some or all of IC1b’s input bias current
will flow through it. A sensible value
would be around 91kΩ. This forms a
220
USB TYPE A
PLUG
A
1
2
3
LED
K
4
Fig.3: Bruce Peirson’s
simple USB Tester.
K
A
give you much clue as to whether the
bus voltage is too low or too high unless
it is grossly so and you will have no way
of monitoring the bus voltage on that
port while another device is connected.
divider with the 100kΩ resistor supplying current to IC2 such that the reference voltage should be pulled down
to about 2.35V when IC1b’s output is
low, providing around 0.3V of hysteresis for the lower supply threshold.
We haven’t tried this though and
obviously, if you change the 100kΩ
resistor value you will need to scale
RB similarly.
The pads for RA and RB are designed
to accept metric 3216 or imperial
1206-sized SMD chip resistors.
Using it
Simply plug it into a USB port. If
it shows a green light, it’s OK. You
can then either unplug the checker
and connect your USB device or you
can simply leave it in and plug your
device into its socket. It should not
affect operation.
If no LEDs light, then either the
port is dead or its supply polarity is
reversed. Either way, we don’t recommend plugging anything else into that
port before you check it out. Similarly,
if you get a red LED, be careful – the
voltage may be just a touch high. Most
USB devices won’t be damaged but
you will need to measure it to be sure.
The completed PCB can be protected using
clear heatshrink tubing (so that the LEDs are
still visible). Make sure it’s working correctly
before shrinking this clear tubing into place.
siliconchip.com.au
Parts List
1 double-sided PCB with platedthrough holes, code 24107131,
44 x 17mm
1 PCB-mount USB type A plug
(CON1) (element14 2067044
or 1696544)
1 PCB-mount USB type A socket
(CON2) (element14 1696534,
Jaycar PS0916, Altronics
P1300) – optional, see text
1 60mm length 16mm-diameter
clear heatshrink tubing
Semiconductors
1 LM393D dual low-power
comparator (IC1) [SOIC8] (element14 4380563 or
2294229)
1 LM285D-2.5 micropower voltage reference (IC2) [SOIC-8]
(element14 8389195)
2 BAT54A dual common-anode
Schottky diodes (D1-D2) [SOT23] (element14 1081191)
1 green 3mm LED (LED1)
1 yellow 3mm LED (LED2)
1 red 3mm LED (LED3)
Capacitors
2 1µF 16V SMD ceramic
[3216/1206] (element14
1683655, Altronics R9950)
Resistors (0.25W, 1%)
2 160kΩ^
1 22kΩ*
1 120kΩ*
1 18kΩ^
1 110kΩ*
3 680Ω
1 100kΩ
* for USB 3.0-compatible version
^ for USB 2.0-compatible version
Note: a kit for this project is available from Jaycar, Cat. KC-5522.
If the yellow LED is lit, the low voltage is unlikely to damage anything but
your USB device may not get enough
power to operate properly. Note that
if the bus voltage is very low, it’s possible that the red LED could also light
(dimly).
Some chargers which use USB ports
can put out as much as 6V. This is most
common with high-current chargers in
the 2-3A range, such as those for tablet
computers. We believe that this is an
attempt to get the maximum current
through the USB cable. While most
devices will tolerate 6V, some could
overheat and in theory damage could
occur, so take care plugging anything
not designed for these chargers into
SC
them.
July 2013 87
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