This is only a preview of the February 2004 issue of Silicon Chip. You can view 35 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Simple Supply Rail Monitor For PCs":
Items relevant to "Studio 350 Power Amplifier Module; Pt.2":
Items relevant to "Using The Valve Preamp In A Hifi System":
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
PC Power
Monitor
By JIM ROWE
Does your PC crash intermittently? Maybe
the hard disk or something else within the
machine is not getting the right rail voltage
but how would you know? This unit lets you
easily monitor the main power rails – it clips
into your PC and has three LED bargraphs
and an alarm to indicate if any of the supply
rails swings too high or too low.
A
S WELL AS HAVING to provide a number of different DC
voltages, your PC’s power
supply has to deliver an appreciable amount of power – hundreds of
watts. This is the main reason why
switchmode power supplies are used,
because they’re much more efficient
than the older “linear” type of power
supply. However, they’re also more
complex and this tends to make them
slightly less reliable.
12 Silicon Chip
Also, some PC power supplies really
do have trouble supplying all that current and sometimes they fail to deliver
just the right voltage at critical times
– like when you are in middle of a big
download off the Internet. If you build
this unit, it will give you a visual and
audible warning of the problem so that
you can have it fixed.
Of course, apart from data loss, if
a PC’s power supply does happen to
develop a fault, this can have quite
disastrous (and costly) consequences.
Replacing a blown CPU chip can involve many hundreds of dollars, while
replacing blown DIMM modules can
be almost as costly.
Fortunately, many of the latest PC
power supplies incorporate special
circuitry to detect when any of the
main power rail voltages fail or go
high and shut down the supply if
such a fault occurs. However, such
protection circuitry does not always
do the job, so this monitoring circuit
can still be a worthwhile addition. It’s
good to know that if a fault develops,
you’ll be warned straight away so you
can “pull the plug” before much damage is done.
So that’s the idea of this project.
It’s a low-cost, easy-to-build circuit
which can continuously monitor the
main power rails in a PC and display
their status via columns of LEDs. At
the same time, whenever it senses that
any of the rail voltages has moved out
of the safe operating range (too high
or too low), it sounds a small piezo
www.siliconchip.com.au
buzzer to draw your attention to a
possible problem.
How many supply rails does it
monitor? The answer is “just three”
but they are the three that are now the
most important. These are the +12V
line (used for the motors on most disk
drives), the +5V line (used for most of
the logic on drives and plug-in cards)
and the +3.3V line (used to power the
memory modules, the chipset and
motherboard logic and the CPU).
By the way, as you can see from
Table 1, PC processor voltages have
varied a great deal in recent years.
In most cases, the processor supply
voltage(s) are derived from the +3.3V
line from the power supply, either
directly or via a DC-DC converter,
which has its output voltage(s) set
either manually by jumper shunts on
the motherboard or automatically via
“VID” (voltage identification) coding
pins on the processor itself. So in most
cases, it’s sufficient to monitor the
+3.3V line in order to keep an eye on
processor voltage.
The only exception to this is with
the latest generation of PCs using
very fast P4 processors, where the
chip’s DC-DC converter is run from
the motherboard’s auxiliary +12V line
(rather than the +3.3V line) in order to
be able to supply the extra power. In
these cases, monitoring the +12V line
is probably sufficient to keep an eye
on processor voltage, although you’d
still be advised to monitor the +3.3V
line as well because this is used for
the memory modules and the chipset.
Forget -5V and -12V
It isn’t really necessary to monitor
the -5V line any more, because this
was actually only used by a few of the
older ISA bus cards (like RS-232C serial port and modem cards). Similarly,
it’s no longer necessary to monitor the
-12V line, because this too is rarely
used in most PCs made in the last 10
years or so.
So by monitoring just the +12V, +5V
and +3.3V lines, we’re likely to be able
to detect just about any fault in a PC
power supply that could result in data
loss or damage to critical circuitry or
components.
It’s very easy to monitor the +12V
and +5V lines, because these are available from any disk drive cable connector – and there’s usually at least one of
these spare. The +3.3V line is a little
more awkward, though. You generally
www.siliconchip.com.au
Fig.1: the circuit is based on three LM3914 dot/bar display driver ICs
(IC1-IC3) – one to monitor the +12V rail, one for the +5V rail and one for
the +3.3V rail. Each IC drives five LEDs which indicate the status of each
supply rail at a glance.
have to run one or two wires connecting directly to the motherboard at the
main power connector. We’ll give you
the details of this later in the article.
How it works
To keep the project as simple as pos-
sible, each of the three power lines is
monitored by an expanded-scale LED
voltmeter circuit based on an LM3914
dot/bar display driver IC. As you can
see from the circuit diagram (Fig.1),
IC1 is used to monitor the +12V line
while IC2 and IC3 monitor the +5V
February 2004 13
Fig.2: install the parts on the PC board as shown here, taking care to ensure that all polarised parts
are oriented correctly. Note that trimpots VR1-VR3 are mounted on the copper side of the board.
and +3.3V lines respectively.
Although each LM3914 has 10 output lines, designed to drive 10 LEDs
in a normal dot or bar type display,
14 Silicon Chip
here we use only nine of the outputs
to drive a total of five LEDs per chip.
Output O6 in the centre of each chip’s
voltage range is used to drive the green
“OK” LED for that power line, while
the remaining eight outputs are connected as four tandem pairs to power
the “HIGH”, “TOO HIGH”, “LOW”
and “TOO LOW” LEDs for each supply line.
All three ICs are actually powered
from the PC’s +12V line and the LEDs
are all connected to this line as well.
This means, of course, that if the PC’s
+12V line fails completely, the complete monitoring circuit will go dead
as well. But as this in itself will be a
clear indication that your PC’s power
supply has a serious problem, we don’t
see it as a disadvantage.
As you can see, the inputs of IC2 and
IC3 are connected directly to the +5V
and +3.3V rails of the PC. However,
to allow IC1 to correctly monitor the
+12V rail, we use a simple 2:1 resistive
voltage divider to allow it to monitor
half the voltage – ie, a nominal +6V
rail which is directly proportional to
the +12V rail.
The reference voltage and sensing
range of each IC are tailored using the
resistors connected to pins 4, 6, 7 &
8 to give the correct “centre voltage”
and measuring range for each of the
three voltage rails. But each IC also
has a trimpot (VR1, VR2 and VR3), so
that each monitor can be calibrated
independently for correct indication
and alarm sensing.
By the way, calibration trimpot VR3
has a higher value than the other two
so that the centre of IC3’s sensing range
can be adjusted to suit whatever voltage is used in the PC for running the
CPU. So you’re not forced to monitor
just the motherboard’s +3.3V line; you
can monitor the actual CPU supply
voltage if you prefer. We recommend
that you do monitor the +3.3V line
though, because it’s easier to do this
and therefore less risky.
How do we do the alarm sensing?
Ah, that’s easier than you’d think. As
you can see, the three LEDs which are
used to indicate “OK”, “HIGH” and
“LOW” in each monitor are all connected directly to the +12V line. So
when any of these LEDs is illuminated
(because there’s no serious problem),
nothing else happens.
On the other hand, the LEDs at the
top and bottom of each monitoring
range (ie, LED1 and LED5, etc) are not
connected directly to +12V but instead
to an “alarm sense” rail which in turn
connects to the +12V rail via the baseemitter junction of transistor Q1.
This means that if any of the ICs
happens to detect a “TOO HIGH” or
“TOO LOW” condition and lights one
of these LEDs, this draws base current
through Q1 and turns the transistor
on. As a result, it conducts collector
current and turns on the piezo buzzer.
Nifty, don’t you think?
Construction
All the components for the power
monitor are mounted on a compact PC
board measuring 146 x 38mm and coded 07102041. This board is designed so
that it can be mounted directly behind
a 5.25-inch drive blanking plate, with
the status indicator LEDs protruding
via matching 3.5mm holes. An array
of even smaller holes at one end of the
panel allows the sound from the piezo
buzzer to emerge.
Fig.2 shows the parts layout. All
parts are mounted on the top side
of the PC board except for the three
calibration trimpots (VR1-VR3) and
the PC board terminal pins, which are
used for the power input connections.
www.siliconchip.com.au
The location and orientation of all of
the components can be seen clearly in
the board overlay diagram. As usual,
fit the wire links first, so that you don’t
forget them. The three short vertical
links can be made from tinned copper
wire or resistor lead offcuts, while the
two longer horizontal links (near the
bottom edge of the board) should be
made from insulated hookup wire.
Once the links are in, fit the six PC
board terminal pins that are used for
the input connections. As mentioned
earlier, these are fitted from the rear
of the board and soldered on that side
as well.
The fixed resistors can go in next,
making sure that you fit each one in
the correct position. That done, install
the three 2.2µF tantalum capacitors
– they all mount with their positive
leads towards the top of Fig.2. The last
capacitor to fit is the 100µF electrolytic but note that although it mounts
on the front of the board as usual,
it is mounted on its side to provide
clearance when the board is mounted
behind a blanking plate or box panel.
This capacitor is also mounted with
its positive lead uppermost.
The next components to fit are transistor Q1 and the three LM3914 ICs.
Note that the ICs all mount with their
notched (pin 1) ends facing downwards, as shown in Fig.2.
Fitting the LEDs
You’re now ready to fit the 15 LEDs.
These are all 3mm-diameter types and
there are three green LEDs, six orange
LEDs and six red LEDs as shown.
They should all be mounted with
10mm lead lengths (ie, the bottom of
each LED should be 10mm above the
board), so they they’ll later all protrude
evenly through the holes in the front
panel when the board is mounted
behind it. The easiest was to do this
is to cut a short strip of cardboard
10mm wide and then fit each column
of LEDs with their leads straddling
the cardboard strip. That way, they’ll
all be automatically set to the correct
height before their leads are soldered.
It’s a simple trick but it works well.
By the way, notice that each LED
is fitted with its cathode (flat side)
towards the right.
The last component to fit to the front
of the board is the small piezo buzzer.
This mounts directly to the board via
two pins. Because there are several different types of buzzers available, with
different pin spacings, we’ve provided
extra pads and holes on the board for
flexibility. Note that the buzzer’s negative pin should always go through the
bottom hole.
Installing the trimpots
The final components to fit are the
three trimpots, which mount on the
back (ie, copper side) of the PC board.
This is done so that they’re easy to
adjust from the back when the board is
mounted on a blanking plate or panel.
Make sure you use the 1kΩ trimpots
for VR1 and VR2, and the 5kΩ trimpot
for VR3.
Once the board is fully assembled,
you can place it aside for a few minutes
while you drill the holes in the blanking plate or box panel. You can use a
photocopy of the front panel artwork
(Fig.5) as a drilling guide and template.
Note that the holes for the LEDs and
the four board mounting holes (in the
corners) are all 3.5mm diameter, while
those for the buzzer “grille” are 2mm
in diameter.
Once the holes in the blanking plate
have all been drilled and deburred,
you might want to attach another
photocopy of the artwork to the front
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
No.
1
1
1
3
1
1
1
2
1
www.siliconchip.com.au
Value
10kΩ
4.7kΩ
3.9kΩ
1.5kΩ
1.2kΩ
1kΩ
470Ω
270Ω
220Ω
4-Band Code (1%)
brown black orange brown
yellow violet red brown
orange white red brown
brown green red brown
brown red red brown
brown black red brown
yellow violet brown brown
red violet brown brown
red red brown brown
Fig.3: this diagram shows how the
PC board is secured to the rear of
the blanking plate using 12mm
spacers and M3 x 6mm machine
screws. The LEDs protrude through
matching holes in the blanking
plate – see text.
Fig.4: here are the pin connections
for a 20-pin ATX motherboard
power connector and for a 6-pin
ATX auxiliary power connector
which is sometimes used on older
motherboards.
5-Band Code (1%)
brown black black red brown
yellow violet black brown brown
orange white black brown brown
brown green black brown brown
brown red black brown brown
brown black black brown brown
yellow violet black black brown
red violet black black brown
red red black black brown
February 2004 15
VR3
Fig.5: here are the full size artworks for the PC board and front panel. Check your board carefully for
defects by comparing it against the above pattern before installing any of the parts.
using double-sided tape, so it will
dress the panel up and give a professional look. Alternatively, you may be
able to buy a kit of parts that includes
a professionally made “sticker” for the
front panel.
The PC board assembly can now
be mounted behind the panel on four
12mm-long M3 tapped spacers and
secured using 6mm-long M3 machine
screws. Fig.3 shows the details. We
suggest that you also fit a star lockwasher under each of the rear mount-
ing screws, to ensure that they don’t
loosen with vibration.
Connecting it up
The easiest way to connect the +12V,
+5V and earth (ground) inputs of the
monitor board to the corresponding
power rails of the PC is by cannibalising the 4-pin plug and one set of wires
from a disk drive “Y adaptor” power
cable. These are readily available from
computer stores and electronics suppliers. The free ends of the wires are
The completed PC Power Rail Monitor simply clips in the front of the PC’s case,
in place of an existing drive blanking plate.
16 Silicon Chip
then soldered to the four main input
pins on the monitor board but make
sure you connect them correctly: the
red wire goes to the +5V input, the
yellow wire to the +12V input and the
two black wires to the centre ground
pins.
The 4-pin plug can then be mated
with one of the power connectors
in the PC, to make all these connections.
The connections to the PC’s +3.3V
rail are a little trickier but simple and
safe enough if you’re careful. To do
this, solder a pair of insulated hookup
leads about 500mm long to the two
remaining pins on the monitor board,
using wire with orange insulation for
the +3.3V lead and wire with black
insulation for the ground lead. That
done, remove the cover from your PC
so you can gain access to the underside
of the motherboard, just below the
main power connectors.
In most PCs made in recent years,
you should find that the main DC
power lead from the power supply
mates with the motherboard using a
20-pin Molex type plug and socket
(called the ATX power connector). If
that’s the case with your PC, you can
connect the +3.3V and ground wires
from the monitor to the underside of
the 20-pin motherboard connector,
to pins 1, 2 or 11 (orange wire) and 3
(black wire) respectively. Fig.4 shows
www.siliconchip.com.au
VR2
VR1
The above view show the completed PC board from the top, while the inset
shows how the three trimpots (VR1-VR3) are mounted on the copper side.
how to identify the pins on the motherboard ATX connector.
On some earlier model PCs, you may
find that this 20-pin ATX connector is
“missing”. Instead, there will be a pair
of 6-pin in-line main power connectors (P1 and P2), together with a third
6-pin in-line connector providing the
+3.3V power and an additional +5V
line. This is known as the 6-pin ATX
auxiliary power connector (see Fig.4)
If your PC has this arrangement, the
+3.3V lead from the monitor board
(orange) should be connected to either
pin 4 or pin 5 of the auxiliary connector (under the motherboard), while the
remaining ground wire (black) can be
connected to either pin 2 or pin 3.
If your PC is even older and doesn’t
even have the ATX auxiliary connector but just the P1 and P2 connectors,
this means that it doesn’t have a +3.3V
rail. In that case, you won’t need to
worry about monitoring the nonexistent +3.3V rail, so simply remove
the orange and black wires from the
monitor board pins and ignore the
third column of LEDs (which won’t
light anyway).
Calibration
Calibrating the monitor is quite
easy but you’ll need a reliable digital
voltmeter. The basic idea is that you
will be adjusting the relevant trimpot
www.siliconchip.com.au
for each of the monitor’s three LED
voltmeters so that the green LED glows
when the input voltage is at the correct nominal value for that power line.
When this is done, the other LEDs will
glow for the correct higher and lower
voltage levels.
Step one is to measure the +12V line
with your DVM. If it’s very close to the
correct reading (say within ±100mV
of +12V), all that you then need to do
is adjust trimpot VR1 until the green
LED glows steadily in the first column
of LEDs. In fact, you should set VR1
to the centre of the small adjustment
range over which the green LED glows.
What if the PC’s +12V rail actually
measures a little below 11.9V, or a little
above 12.1V? That’s no great problem
but it does mean that you should adjust VR1 so that one of the two orange
LEDs glows instead – ie, adjust VR1
so that either the lower orange LED
is just glowing if the voltage is just
below 11.9V, or the upper orange LED
is glowing if it’s just above 12.1V.
Calibration of the +5V and +3.3V
monitors is done in exactly the same
way. You simply measure the actual
voltage of these power rails first with
your DVM, then adjust each trimpot
so that either the green LED or one of
the orange LEDs for that monitor is
glowing, depending on the reading
on the DVM.
Parts List
1 PC board, code 07102041,
146 x 38mm
1 piezo buzzer, PC mount
6 1mm PC board terminal pins
4 12mm x M3 tapped spacers
8 M3 x 6mm machine screws
4 M3 star lockwashers
2 1kΩ horizontal trimpots (VR1,
VR2)
1 5kΩ horizontal trimpot (VR3)
Semiconductors
3 LM3914 display drivers (IC1IC3)
1 PN200 PNP transistor (Q1)
3 3mm green LEDs (LEDs 3, 8,
13)
6 3mm orange LEDs (LEDs 2, 4,
7, 9, 12, 14)
6 3mm red LEDs (LEDs 1, 5, 6,
10, 11, 15)
Capacitors
1 100µF 16V RB electrolytic
3 2.2µF 35V TAG tantalum
Resistors (0.25W, 1%)
1 10kΩ
1 1kΩ
1 4.7kΩ
1 470Ω
1 3.9kΩ
2 270Ω
3 1.5kΩ
1 220Ω
1 1.2kΩ
Once you’ve set all three trimpots in
this way, your PC Power Rail Monitor
SC
is calibrated and ready for use.
February 2004 17
|