&
Cooling Fan
Controller
Loudspeaker
Protector
By
John Clarke
This board controls up to three cooling fans, switching them on at
a preset temperature and ramping their speed up as it increases,
preventing overheating while minimising noise. It can also protect
loudspeakers from damage while also preventing power switch-on and
switch-off thumps. It isn’t just useful for amplifiers; this board is ideal for
any device that needs cooling fans.
M
any devices need forced-
air cooling when working hard
but do not need fans to be running (or perhaps only running slowly)
when they are idle or under light load
conditions. This includes large power
supplies, audio amplifiers, motor speed
controllers – just about anything that
gets hot under load.
Even devices for which passive convection cooling is adequate can have
their lifespans extended if they are
fitted with fans that switch on once
things start heating up. Those fans
might only need to run during summer,
when ambient temperatures are high.
Ideally, the fans stop or spin slowly
when only a bit of cooling is required,
to prevent the annoyance of constant
fan noise (and dust collection).
One simple method to provide cooling fans is to have a thermostat connected to the heatsink that switches
on the fan(s) whenever the temperature
exceeds a certain threshold. But, when
switched on, the fan(s) run at full speed
and make considerable noise. That is
especially bad for an audio amplifier
as it can ruin the listening experience.
A less obtrusive method is to adjust
the speed of the fan(s) so that there is
a gradual rise in speed as temperature
rises. Once the heatsink passes a certain temperature, the fan(s) run slowly
to start with; this usually provides
sufficient air movement to bring the
amplifier back to a lower temperature.
If the temperature continues to rise, the
fan will run at a progressively faster
rate, up to full speed.
By choosing the right fans, they will
be extremely quiet at slow speeds, and
the temperature can usually be controlled without making noise. Here,
we’re using PWM-controlled computer fans with brushless motors.
They are readily available at a range of
prices, start at just a few pounds each,
and generally are silent at low speeds.
Some can still move a lot of air at full
speed, though.
As this board is especially suitable
for power amplifiers, we’ve added
several extra features to it. Power
SPECIFICATIONS
DC offset reaction time: 75ms
Temperature setting range: 0-100°C (273-373K)
Fan PWM control frequency: 25kHz
Over-temperature hysteresis: 4°C (4K)
Amplifier DC offset detection: < –2V or > +2V
AC loss detection threshold: 9V AC
Relay power-up delay: typically 6s after fans are detected
Fan disconnect/failure audible alarm: 264ms burst of 3.875kHz at 1Hz
Trimpot voltage/temperature conversion: 10mV/K (2.73V = 273K = 0°C)
Over-temperature or DC fault audible alarm: 264ms burst of 3.875kHz at 0.5Hz
NTC thermistor range: 0-100°C (responds to highest temperature when two are used)
Trimpot adjustments: three – fan switch-on threshold, fan speed range and over-temperature alarm
24
Practical Electronics | February | 2023
amplifiers should include loudspeaker
protection to disconnect the speakers
if the amplifier fails. Power amplifier
failures can destroy the speakers and
even start a fire, especially if it’s a highpower amplifier.
That’s because one common failure
mode involves one or more of the output transistors failing short-circuit, possibly resulting in the entire supply rail
DC voltage (up to perhaps 80V) being
applied to the speaker. Given their
low DC resistance, any loudspeaker
connected will be quickly destroyed
by this.
At best, the loudspeaker coil will
burn out without any further damage. But a worse scenario is that the
speaker cone could catch fire, burning
the speaker box and anything else that’s
in the vicinity.
The built-in Loudspeaker Protector
Controller averts speaker damage by
disconnecting the loudspeaker from the
amplifier should the amplifier exhibit
this type of fault.
Since there is the ability to disconnect the loudspeaker from the amplifier, we can provide de-thumping features. At power-up, an amplifier can
generate a brief, uncontrolled voltage
excursion until its power supply stabilises. This will produce a thump sound
from the loudspeaker(s). We eliminated
it by adding a delay from power-up
before connecting the loudspeaker.
A similar thump can occur at switchoff. Therefore, we disconnect the loudspeaker as soon as the AC supply is lost,
before any voltage excursions from the
amplifier can cause a thump sound.
PWM fan control
Our Controller works with 4-pin PWM
fans. These fans have internal pulsewidth modulation (PWM) speed control, where the duty cycle of the waveform at a control pin is adjusted to
change the fan speed.
At low duty cycles, the fan runs
slowly and increases in speed as the
duty is increased. Our Controller can
drive up to three fans. PWM fans have
four connections: two for power (+12V
and 0V), one for speed adjustment and
one for speed feedback (RPM sensing).
These are labelled as the Control and
Sense terminals.
The sense terminal produces two
pulses per fan revolution when the terminal has a pull-up resistor connected
to a 5V supply. These pulses provide
information about the speed of the fan,
and in particular, whether the fan is
running. If the pull-up resistor is not
included, the fan will always run at full
speed when power is applied.
The fourth pin is the Control terminal and is for the PWM signal to set the
fan speed. The applied PWM signal
only needs to supply a small amount of
current as it does not directly drive the
fan motor. Internally, each fan includes
a motor driver circuit that operates
based on the PWM signal applied.
Scope 1 shows the 25kHz PWM signal that is applied to the fan. The top
yellow trace is a low duty cycle (16.7%)
waveform, and when this is applied,
the fan runs slowly. The lower white
trace shows the PWM waveform when
the duty cycle is increased to around
70%. With this higher duty cycle,
the fan runs faster but still not at full
speed. That requires a continuously
high signal.
You can find more details on this
style of PWM fan control in the PDF
at: https://bit.ly/pe-feb23-pwm
Features
As we wrote earlier, this board is applicable to a wide range of situations, but
as it’s ideal for audio amplifiers, the
following description will concentrate
on that usage.
The Controller can be used with
a mono or stereo amplifier with one
or two heatsinks. The loudspeaker
switching relay is selected to suit the
amplifier power rating; it will need a
high current rating for use with highpower amplifiers (100W or more). This
is discussed in a section below titled
Relay choices. Any relay that is used
must have a double-throw contact (ie,
SPDT or DPDT). We will describe why
that is necessary a bit later.
The Controller is presented as a bare
board and is designed to be housed
within the amplifier enclosure. It runs
from a 12V DC supply, with a current
draw possibly approaching 750mA
depending on the type of fan and how
many are used. While this 12V could
be derived from an existing amplifier
supply, a separate supply is probably
warranted, especially when more than
one fan is used.
Note that you can use the Controller without using all the features. You
can leave one thermistor disconnected
if you don’t need both, or both can be
disconnected if you are only using the
loudspeaker protection and dethumping features.
If you don’t want to connect the AC
detection input for dethumping, it can
be connected instead to the 12V DC
input. If you aren’t using the loudspeaker protection features or only
have a single channel to protect, connect the unused sense inputs to the
0V terminal.
Finally, if you want to use the
speaker protection/dethumping features but not the fan control, use a
jumper shunt to bridge pins 3 and 4
of one of the fan connectors. That prevents the Controller from showing a ‘fan
disconnection/failure’ error that would
otherwise prevent operation.
Circuit details
The entire circuit of the Controller
is shown in Fig.1; it is based around
microcontroller IC1. It monitors several inputs, including two NTC thermistors for temperature measurement,
two amplifier output voltages and an
AC input from a power transformer.
FEATURES
Suits mono and stereo audio amplifiers, or any other device which
needs thermal fan control
Onboard loudspeaker protector controller with de-thumping at switch
on and switch off
Loudspeakers are disconnected with over-temperature fault
One or two thermistors for temperature sensing
PWM control for one to three cooling fans
Over-temperature and fan-failure alarms
Temperature control range of 0-100°C
Fan detect and relay-on LED indicators
Practical Electronics | February | 2023
25
The AC input is used to sense when
the amplifier is switched on or off.
It also has three analogue inputs
connected to the wipers of trimpots
to set the temperature control parameters, plus three frequency-sensing
digital inputs for monitoring the fan
speeds (RPMs).
IC1 produces output signals for
driving the alarm piezo, LED indicators for each fan and a relay driver/
LED indicator. Under normal circumstances, the relay will switch on after
about six seconds from power-up.
This connects the amplifier output(s)
to the loudspeaker(s).
In more detail, the NTC thermistor
inputs are at CON5. Thermistor TH1
connects to the analogue input at
pin 7 of IC1 and pin 8 for TH2. Each
thermistor connects between ground
(the 0V rail) and the input pin with
a 10kW pull-up resistor to the +5V
supply. As the name suggests, negative temperature coefficient (NTC)
thermistors decrease in resistance
with increasing temperature.
For the thermistors used, the resistance at 25°C is 10kW, so in conjunction
with the 10kW pull-up resistor, they
give 2.5V DC at 25°C. As temperature
rises, this voltage falls. The resistance
and hence voltage-versus-temperature
is not linear; it follows an exponential
curve. The thermistor beta value is
3970, which allows us to calculate the
expected resistance and thus voltage at
various temperatures.
You can use an online calculator
to calculate the expected values at
any temperature. We have stored a
pre-calculated table of values from 0
to 100°C within the memory of microcontroller IC1 – one calculator is at:
https://bit.ly/pe-feb23-beta
IC1 converts the voltages to 8-bit digital values using its internal analogue-todigital converter (ADC) and then uses
the lookup table to convert them to temperatures. Temperatures below 0°C are
treated as 0°C and similarly, temperatures over 100°C are treated as 100°C.
When two thermistors are connected,
the highest temperature of either thermistor is used. That way, for a stereo
amplifier with two heatsinks, the fan
speed and other aspects will be determined by whichever is hotter.
If only one thermistor is used, the
unused input is left open, and the
pull-up resistor holds the input at 5V.
That ensures that the unused input
will have a lower temperature reading.
Trimpot adjustments
Trimpots VR1, VR2 and VR3 are for
setting how you want the fans to be
controlled. The voltage setting at the
wiper of each trimpot is directly related
to temperature in kelvin (K). A difference in 1K is equivalent to 1°C, but 0°C
= 273.15K. So to convert °C to K, simply add 273.15 and to convert K to °C,
you subtract that same value.
The conversion from voltage to temperature in our circuit is 10mV/K. So a
voltage setting of 2.73V sets a temperature of 273K, which is 0°C. For other
temperatures, add the °C value required
to 273, divide by 100, then adjust for
that voltage. For example, for a 50°C setting, you need to achieve 3.23V ([273 +
50] ÷ 100) at TP1, TP2 or TP3.
VR1 adjusts the threshold setting,
which is the lowest temperature where
the fans start running. Test point TP1
can be used to check this setting. The
voltage at pin 9 of IC1 is converted to a
10-bit digital value and then to a temperature value in °C.
VR2 sets the temperature range over
which the fans run from minimum
through to maximum duty cycle.
For example, if you set a threshold of
50°C and a range of 10°C (VR2 adjusted
for 2.83V at TP2), the fans will start to
run at the minimum duty cycle when
the thermistor temperature reaches
50°C. The duty cycle will increase linearly as temperature increases, up to
and above 60°C, where they will be
running at full speed.
As VR2 sets a temperature range,
you don’t need to readjust VR2 if you
change the threshold temperature setting with VR1.
VR3 sets the over-temperature alarm
threshold, and you can monitor this
setting at TP3. Whenever the measured
temperature is above this setting, it
will set off the piezo alarm and switch
off the relay(s) that connect the loudspeaker(s). The speaker disconnection
allows the amplifier to cool off as it is
no longer loaded.
When this alarm goes off, the fans are
set at maximum speed (if they aren’t
already) to cool down the amplifier,
and regular operation does not resume
until the temperature drops by 4°C.
Typically, this over-temperature setting would be set at least as high as
the threshold temperature plus the
speed range.
Scope 1: two PWM fan control waveforms, with a low duty cycle at the top
in yellow (so the fan runs slowly) and a high duty cycle below in white, for a
higher fan RPM, but short of full speed.
Amplifier connections
The Controller monitors the AC side of
the amplifier power supply as well as
amplifier output offset voltage. These
are wired to CON4; the AC supply voltage goes to IC1’s AN4 analogue input
at pin 16, while the amplifier outputs
go to AN5 (pin 15) and AN6 (pin 14).
AC detection is done by half-wave
rectifying the voltage from the transformer’s secondary. Diode D5 rectifies
the AC, and the resulting voltage is fed
through a low-pass filter comprising a
47kW resistor and 2.2μF capacitor.
Without any AC voltage, the AN4
analogue input at pin 16 of IC1 is held
at 0V via the 47kW pull-down resistor. When at least 9V AC is applied,
the voltage at pin 16 will exceed 2.5V.
This voltage is limited to 4.7V by zener
diode ZD3.
The time constant for the filtering has
been chosen to ensure sufficient ripple
voltage is removed from the rectified
AC while minimising the detection
period for loss of AC.
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Practical Electronics | February | 2023
The amplifier outputs are monitored
via pairs of 47kW resistors which limit
the current fed into the circuit. They
also act to level-shift the output signals from the amplifier to an average
DC level of 2.5V. Two 10μF capacitors,
in combination with these resistors, filter out the AC signal from the amplifier,
leaving only the DC level.
We have set the speaker output
over-voltage detection threshold to
be 2V on either side of 0V. Since the
pairs of 47kW resistors divide the signal level by two and add 2.5V, the normal range of voltages at pins 14 and
15 of IC1 is between 1.5V and 3.5V.
Anything outside this indicates a DC
fault in the amplifier.
Note that the 10μF capacitors are
only truly effective at removing the
AC for signal frequencies above about
100Hz. Below that, more and more of
the AC voltage will be present at the
micro inputs. The AC voltage level is
also dependent on the amplifier output level, so at low frequencies close
to 20Hz, it can exceed the offset detection threshold, especially with a highpower amplifier.
This is shown in Scope 2. The top
yellow trace is the output from a 500W
amplifier at 20Hz, with an RMS voltage of about 49.1V and 142V peakto-peak. The lower blue trace is the
waveform as presented to the AN5
input of IC1. The AC voltage is 2.36V
peak-to-peak, riding on a half-supply
DC level of 2.56V.
The horizontal lines represent the
1.5V and 3.5V thresholds. This shows
that at low frequencies and high amplifier output levels, the waveform can
exceed the offset threshold limits at
the waveform peaks.
Any standard offset detector circuit
using transistors to detect the offset will
switch off the relay whenever the AC
signal exceeds the limits. To circumvent this, the filtering would need to
be increased by using a capacitor larger
than 10μF.
However, increasing the filter capacitor will also increase the delay from
the initial detection of offset from the
amplifier and the relay switching off.
This is not ideal, as the speakers need to
be disconnected by the relay as quickly
as possible if there is a fault.
Instead, we use software logic to
determine whether there is a DC fault
or just a high-level AC voltage. The
waveform is sampled about 1000
times per second, and whenever the
offset voltage threshold is exceeded, a
75ms timer is started. If the detected
offset voltage drops to within the
offset voltage threshold boundaries
during this period, there is no DC offset, so the relay is not switched off.
Cooling Fan & Loudspeaker Protection Controller
Fig.1: there isn’t a great deal to the Controller circuit since most of the functions are handled by the firmware (software)
loaded into microcontroller IC1. At upper right there is signal conditioning so the amplifier output signals can be fed into
the micro’s ADC, with the relay driving circuitry below. The components at lower right are for the PWM fan interface
while the thermistor inputs, adjustment trimpots and indicator LEDs at left.
Practical Electronics | February | 2023
27
A genuine DC offset would continue
being detected as exceeding the offset
threshold. If DC offset is still seen at the
end of the timeout period, it will switch
the relay off and the alarm will sound.
Zener diodes ZD1 and ZD2 limit the
voltages across the possibly 16V-rated
capacitors. This can happen if the circuit is connected to an amplifier when
IC1 is not inserted into its socket. When
IC1 is in-circuit, the internal protection diodes will limit the voltage at the
input to 0.3V above the 5V supply and
0.3V below 0V.
ZD1 and ZD2 provide extra protection by limiting the voltages across the
capacitors to a maximum of 15V and
–0.6V. The 2.2kW series resistors further limit the current to the protection
diodes within IC1.
We are using a 15V zener rather than
4.7V despite the supply being 5V due to
the leakage current. A 15V zener diode
with up to 5V applied will only conduct about 0.05μA compared to 100μA
or more for a 4.7V zener diode at only
1V. That leakage current would drastically affect the half-supply voltage set
by the pairs of 47kW resistors that only
cause a 53μA current flow under quiescent conditions.
Note that if one of these two inputs
is not connected to an amplifier (eg,
your amplifier has a single channel),
that input must be tied to 0V or else it
will be detected as a DC fault.
IC1 (pin 11) via a 220W resistor. This
resistor is part of a low-pass filter to
reduce the harshness and volume to a
less piercing level.
The filtering utilises the capacitance of the transducer to filter out
some of the harmonics from the square
wave. The driving frequency is around
3.9kHz and is produced in bursts of
264ms every two seconds for both the
over temperature and amplifier offset
alarms. The fan fault alarm rate is 1Hz.
Relays
There is the option to connect two
relays, RLY1 and RLY2. These are
driven in parallel and via transistor
Q1. A high level from the RB7 output
of IC1 applied to the base of this transistor switches on the relay or relays.
Diode D6 prevents high-voltage backEMF excursion when the relay coil
switches off, thus preventing damage
to the transistor.
The amplifier’s positive speaker
output connects to the normally open
(NO) relay contact of the relay while
the plus side of the speaker connects
to the relay wiper or common (COM)
with the normally closed (NC) contact
connecting to the negative speaker output (usually earth) on the amplifier –
see Fig.3. When the relay switches on,
the amplifier output is connected to the
speaker’s positive terminal.
If the amplifier is working correctly,
the contacts will disconnect the speaker
without any problems when the relay is
switched off. However, it is not so easy
when there is an amplifier fault and the
speaker output from the amplifier has
a high positive or negative DC voltage.
Because of the high DC voltage, trying to break the speaker connection by
opening the contacts can cause an arc to
develop, and current continues to flow
through the speaker. This is where the
NC contact comes into play.
This contact closes to short out the
speaker, typically breaking any arc. If
the arc remains and current continues
to flow through the relay, the amplifier
DC supply fuse will blow.
Scope 2: the yellow trace shows a high-level 20Hz signal from a 500W amplifier
and the cyan trace below shows the signal at pin 14 of IC1. While this is an
extreme case, it demonstrates how the signal can go outside the 2V detection
window (dashed lines) even without a DC fault. Therefore, the software has been
designed to detect and ignore this case and only respond to genuine DC faults.
Fan control
There is considerable logic involved
in driving the fans. This is because
many PWM fans require a minimum
duty cycle to be applied before they
spin. Specifications for these fans give
a minimum figure of 20% duty cycle,
although most will run at lower duty
cycles than that. In fact, the fans we
used to test our prototype run at a slow
540rpm when the duty cycle is 0%.
We believe this is a feature to
improve the LED backlighting on the
fan blades, so they become a blended
wall of light as the blades spin. NonLED-lit fans are likely to stop at 0%
duty cycle. (We didn’t look specifically
for the LED lighting feature, it was just
‘part of the package’ for these low-cost
but otherwise good fans.)
The fan(s) connect to CON1-CON3,
and at least one fan needs to be connected for the circuit to work. However,
the circuit can be tricked into believing a fan is connected with a bridging
shunt between the Control and Sense
terminals (pins 3 and 4).
Power for each fan is supplied from
the 12V supply via a Schottky diode
(D1, D2 or D3), and their 12V rails are
bypassed with 100nF capacitors. The
diodes are for reverse-supply polarity
protection. The common PWM output
from pin 5 of IC1 is applied to each fan’s
Control input via a 10W resistor.
Pull-up resistors are provided for
the Sense pin on each fan, and these
pins connect to the RA3, RA0 and RA1
inputs on IC1 so it can check if each
fan is running.
Indicator LEDs driven via the RC4,
RA4 and RA5 digital outputs of IC1
via 1kW resistors show which fan is
connected and they flash if no fans
are connected.
The micro determines the minimum
duty cycle for the PWM signal that will
cause all connected fans to run the first
time the circuit is powered up. Once
found, this minimum duty and the
number and positions of connected
fans are stored in Fash memory, so the
Controller starts up faster subsequently.
The stored settings are used, provided the fans run at the stored
28
Practical Electronics | February | 2023
Piezo alarm
The external piezo transducer for the
alarm is driven via the RB6 output of
minimum duty cycle on each power-up. A check to find the minimum
duty where all the fans will run is only
done again if the number of fans connected changes, the connection position for the fans changes or if one of
the fans does not run when the stored
minimum duty cycle is applied.
The setup procedure first applies
PWM signals at about 80% duty cycle
to the fans for 10 seconds, then checks
which fans register as spinning. At this
stage, all fan LEDs will flash at 1Hz. If
no fans are detected, an error is indicated by all fan LEDs flashing and the
piezo alarm sounds. The relay(s) stay
off until a working fan is connected.
If fans are found, it determines the
minimum duty cycle that will cause all
fans to spin. After that, the LEDs associated with any connected fans are lit.
The number of fans, their positions and
the minimum duty cycle are stored in
memory, and this is indicated by all the
lit fan LEDs briefly blinking off.
The program then continues with
the usual six-second delay before
switching the relay(s) on, but only if
the checks for temperature, amplifier
offset and AC power all pass.
Subsequently, when the circuit is
powered up, it will start the six-second
delay almost immediately, provided
the fan connections have not changed.
The connected fan or fans are usually
detected within one second.
Power supply
The circuit requires a 12V DC supply,
which is applied to the fans via reverse
polarity protection diodes D1-D3. The
supply also goes to 5V for IC1 by regulator REG1 via diode D4, also for
reverse polarity protection. The 5V supply also functions as a 5V reference for
the trimpots.
Construction
The Controller is built on a double-
sided, plated-through PCB coded
01102221 that measures 95 x 74mm
and which is available from the PE
PCB Service. Fig.2 shows the assembly details.
Begin by fitting the resistors. By all
means use resistor colour codes, but
you should always check each lot using
a digital multimeter (DMM) before
installation, as the colour bands can
be misleading.
With these parts in place, mount
the diodes, taking care to orient these
as shown in Fig.2. D1, D2 and D3 are
1N5819 schottky types, while D4, D5
and D6 are standard 1N4004 diodes.
Zener diodes ZD1-ZD2 are 15V 1W
types, while ZD3 is 4.7V, 1W.
You can fit the optional socket for
IC1 now; be sure it is oriented correctly
Practical Electronics | February | 2023
Parts List – Fan and Loudspeaker Protector
1 double-sided plated-through PCB coded 01102221, 95 x 74mm from the
PE PCB Service
1-3 4-pin PWM fans to suit heatsink dissipation requirements●
1-2 lug-mount NTC thermistors, 10kW at 25°C, beta 3970 (TH1, TH2)
[Altronics R4112] OR
1-2 dipped NTC thermistors with separate securing clamps (TH1, TH2)
[Jaycar RN3440]
1-2 high-current 12V SPDT or DPDT relays (see text)
1 piezo transducer (PIEZO1) [Jaycar AB3442, Altronics S6109]
3 4-way polarised PWM fan headers, 2.54mm pitch (CON1-CON3)
[SC6071, Digi-Key WM4330-ND, Mouser 538-47053-1000] OR
3 4-way polarised headers, 2.54mm pitch, modified (CON1-CON3; see text)
[Jaycar HM3414, Altronics P5494]
4 3-way screw terminals, 5.08mm pitch (CON4)
2 2-way screw terminals, 5.08mm pitch (CON5)
4 6mm-long M3-tapped spacers
5 M3 x 6mm panhead machine screws
1 M3 hex nut
4 PCB stakes/pins (optional)
1 20-pin DIL IC socket (optional; for IC1)
● We used EZDIY 120mm PWM fans purchased from Amazon for our
prototype (search for B07X25CJT5). These are inexpensive (we paid £15
for three) and quiet, although they are not the most powerful we’ve tested.
Try Corsair ‘maglev’, Noctua or BeQuiet 4-pin PWM fans for applications
that require faster air movement or higher pressure. All computer stores
should sell suitable fans.
Semiconductors
1 PIC16F1459-I/P programmed with 0110222A.HEX, DIP-20 (IC1)
1 7805 5V 1A linear regulator, TO-220 (REG1)
1 BC337 500mA NPN transistor, TO-92 (Q1)
4 3mm high brightness red LEDs (LED1-LED4)
3 1N5819 40V 1A schottky diodes (D1-D3)
3 1N4004 400V 1A diodes (D4-D6)
2 15V 1W zener diodes (ZD1,ZD2)
1 4.7V 1W zener diode (ZD3)
Capacitors
2 100μF 16V PC electrolytic
2 10uF 16V PC electrolytic
1 2.2μF 16V PC electrolytic
6 100nF MKT polyester
Resistors (all 1% 0.5W axial metal film)
6 47kW
5 10kW
3 2.2kW
3 1kW
1 470W
1 220W
3 10W
3 10kW top adjust multi-turn trimpots (VR1-VR3)
before soldering. Next, insert the capacitors, taking care with the electrolytic
types that must be positioned with the
longer leads towards the + symbols.
Follow assembly with the trimpots.
These are all multi-turn types and
should be oriented with the screw
adjuster positioned as shown. Then
install transistor Q1.
The four 3-way screw terminal
blocks making up CON4 need to be
joined first by fitting each side-byside by sliding the dovetail mouldings
together. Make sure the wire entry side
is toward the nearest edge of the PCB
before soldering. Similarly, the two
2-way screw terminals for CON5 must
be connected and mounted with the
wire entry to the edge.
If you are using standard 4-way
polarised headers to connect the fans,
rather than the special Molex parts
listed, they need to be modified so
that you can insert the fan plugs. This
involves cutting the polarising backing
tab to remove the section behind pins
3 and 4. We used side cutters to snip
the plastic out.
When mounting CON1-CON3, be
sure to orient these headers correctly,
with the polarising tab piece away from
the PCB edge.
The LEDs can now be fitted, with the
longer leads inserted into the anode (A)
holes. Mount them such that the tops
are about the same level as the adjacent
header for LED1-LED3, and the screw
terminal for LED4.
You can now install PCB stakes/pins
at test points TP1-TP3 and TP GND,
or simply leave them off and use the
multimeter probes directly to the PCB
pads. We used a PCB pin at the GND
test point but left them off TP1-TP3.
Regulator REG1 is mounted horizontally on the board. First, bend its
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output pin is close to 5V. Typically,
these regulators are well within 100mV
of 5V. If the voltage is incorrect, check
that the input voltage at the left lead of
REG1 is at least 6V.
You now need to program a blank
PIC. First, download the HEX file
(0110222A.HEX) from the PE website
at: https://bit.ly/pe-downloads and then
load it into the chip using a PIC programmer. Now switch off power and
mount or plug in IC1, after checking
its orientation.
Fig.2: assembly of the Controller is straightforward; fit the components as
shown here, starting with the lower-profile axial parts and working your
way up to the taller devices. Watch the orientations of IC1, the diodes
(including LEDs), trimpots and electrolytic capacitors.
Fig.3: here’s a guide on how to connect one of the speaker protection relays.
If you have two amplifier channels, you can use a DPDT relay, in which case
the wiring is similar but you duplicate the speaker and amp wiring for the
second set of relay contacts, and connect the second SPEAKER + terminal to
the other AMP1/AMP2 terminal. For two separate SPST relays, do the same
but connect the second relay coil back to the other pair of relay terminals on
the controller board.
leads to pass through their mounting
holes, then secure its tab to the PCB
using the M3 x 6mm machine screw
and nut, after which the leads can
be soldered.
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Before installing IC1, check the regulator output voltage by applying 12V
across CON4’s +12V and 0V terminals.
Check that the voltage between the
regulator metal tab and the right-hand
Setting up
With power applied, adjust VR1, VR2
and VR3 for suitable temperature settings while monitoring the voltages
TP1, TP2 and TP3 respectively. We recommend starting by adjusting VR1 to
get 3.03V at TP1, giving a 30°C (303K)
fan starting temperature. Then set VR2
(Range) for 2.83V at TP2, providing a
10°C ramp range. That way, the fans
will be at full speed by 40°C.
You can initially set the over-temperature setting for VR3 to 50°C. That’s
323K, so adjust VR3 for 3.23V at TP3.
These settings may need adjusting to
optimise the way the fan speed varies
with temperature. Consider that with
a starting temperature of 30°C, the fans
will start to run as soon as you power
the device up on a hot day if the device
is not in an air-conditioned room. On a
sweltering day where it reaches 40°C,
the fans will run at full speed all the
time (which might be necessary!).
It depends on the device you are
cooling and how sensitive it is to temperature. Keep in mind that, as it’s an
external device, the thermistor will be
measuring a lower temperature than
the semiconductor junctions that are
presumably generating the heat.
You could raise the switch-on threshold temperature considerably if the
device adequately cools via convection when it isn’t running at maximum
power; the fans would then only need
to run at higher loads and temperatures.
When adjusting the range, we don’t
suggest you go too much lower than
10°C as the fans will appear to operate
in an on/off manner, particularly with
a range setting below 2°C.
If the temperature cannot be controlled using these settings, or if the fans
run at full speed most of the time, you
might need more fans (up to three maximum for this Controller), larger fans or
fans that run at a higher speed at 100%
duty cycle. Keep in mind that there are
flow-optimised fans and pressure-optimised fans (with different blade shapes).
Accuracy
Note that temperature setting accuracy is dependent on the 5V supply
Practical Electronics | February | 2023
The most common size for PWM
fans is 120 x 120mm, although they are
also available in smaller sizes like 80 x
80mm or 92 x 92mm, as well as larger
sizes like 140 x 140mm.
If your device requires lots of cooling, use the largest fans that will fit into
its case and check their air movement
specification in litres per minute (L/
min) or CFM (cubic feet per minute).
Make sure there are ventilation holes in
the case so that the air movement is not
restricted going past the heatsink fins.
Note that if you are not using the fan
control section of the Controller, pins 3
and 4 of either CON1, CON2 or CON3
must be bridged with a shorting block.
Only one such shunt is required.
A single Protector board can control up to three fans.
rail being close to 5.00V. If it is only a
few tens of millivolts different, the setting accuracy will not be affected too
much. If you need precise temperature
settings, you can multiply the required
temperature voltage (ie, the 10mV/K
value) by the actual supply voltage,
then divide by 5. Then adjust the trimpot to get that calculated voltage.
For example, if the supply is 4.95V,
multiply the required temperature voltage by 4.95 and divide by 5 (or multiply by 0.99 [4.95 ÷ 5]). For example, if
you want to set the threshold to 330K
(57°C) but the supply voltage is 4.95V,
set it to 3.267V (330 × 0.99) instead to
get it spot-on.
Relay choices
The choice of relay depends on the
amplifier power and whether you are
using the circuit with a mono or stereo
amplifier. In all cases, the relay must be
a double-throw type. That means having a normally open and a normally
closed contact for each pole.
For stereo amplifiers up to 200W,
you could use the Altronics S4310 12V
coil, 10A DPDT contacts cradle relay
with their S4318A base, or the Jaycar
SY4065 12V coil 10A DPDT contacts
cradle relay and SY4064 base.
For a mono amplifier up to 200W,
you could still use the DPDT relay but
parallel the contacts or just use one set.
For higher power amplifiers, up to about
600W, you can use the Altronics S4211
12V 30A SPDT relay for a mono amplifier, or use two for a stereo amplifier (you
can also use the Altronics S4335A).
Power supply choices
If your amplifier supply already has a
12V DC rail, you could consider powering this board from it. You need to
test how much current it draws with
the fan(s) at maximum speed and verify that the amplifier supply can safely
deliver that much current.
Practical Electronics | February | 2023
A good alternative is to use a separate enclosed switchmode supply such
as the Jaycar MP3296 (or Altronics
M8728), rated at 12V and 1.3A (shown
above). This is mains-powered, and it
should be switched on and off with
the same power switch as the amplifier
itself. Keep it away from sensitive analogue electronics like amplifier input
stages and preamps, as it may radiate
some EMI (although it shouldn’t be too
bad as it is shielded).
Fan choices
There are many 4-pin PWM fans available (mainly designed for cooling computers), and you can choose to use
up to three with our Controller, even
mixing different types if desired. Typically, larger diameter fans move more
air with less noise, as do multiple fans
when compared to a single fan. See the
parts list for some suggestions. These
fans are often available in multi-packs.
Finishing up
Mount the board in a suitable spot in
your amplifier case using threaded
standoffs and machine screws (we’ve
specified 6mm spacers to keep it compact, but you could use other lengths).
Wire up the power supply, including
the AC sense line from the transformer
secondary, or short the AC input to
+12V if you are not using that feature.
Next, wire up the thermistor(s) to
CON5 (they are not polarised so can
be wired either way around) and the
relay(s), piezo transducer and amplifier outputs (if present) to CON4. Plug
the fans in, power up the board and
check that it behaves as expected. You
can heat a thermistor with a hot air
gun and verify that the fans start, spin
faster, then slow down and stop sometime after you stop heating it.
Reproduced by arrangement with
SILICON CHIP magazine 2023.
www.siliconchip.com.au
Fig.4: if you only need the fan speed control, you can leave off some
components as shown. The insulated red wire link is needed so that the AC
detection circuitry will allow normal operation whenever power is applied.
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