The original
article comments on the fact that the cooling fan can be heard running,
and knowing that this is an annoyance to some operators I decided to
try and reduce the noise. Believing the 33 cfm rated airflow of the
fan to be significantly more than what is needed to cool the amplifier
I decided to reduce the fan speed by reducing the voltage applied to
the fan motor. This technique works well with brushless DC fans but
there is a minimum voltage limit below which the fan motor won't start
running.
To find out
just how much the voltage could be reduced I connected the fan to a
variablevoltage DC power supply and gradually increased the supply
voltage until the fan began running. The applied voltage was approximately
3.5 volts for a clean start every time, but the airflow at this voltage
was not deemed sufficient to provide the cooling required. I gradually
increased the voltage and found that about 7 volts seemed to give a
more reasonable cooling effect but with a significant reduction in fan
noise.
With a shack
supply voltage of 13.6 volts, a fan voltage of 7 volts meant that 6.6
volts would have to be dropped across a series resistor. I measured
the current drain of the fan at 7 volts and found it to be about 60
mA, so applying Ohm's Law:
6.6 volts/0.060 amps = 110 ohms
This
is the value of series resistance required to drop the necessary 6.6
volts at the observed value of current flow with 7 volts applied to
the fan motor itself. The power rating of this resistor can be found
from the Power Law, P=I²R, as:
0.060 amps x 0.060 amps x 110 ohms = 0.396 watts
At
this point I decided to try an experiment, so I connected two 100 ohm,
1watt resistors from the junk box in parallel, connected them in series
with the fan, and cranked the power supply up to 13.6 volts output.
The cooling improved noticeably but the noise increased only slightly,
so I decided to use this combination. The amount of voltage dropped
across the two resistors was measured at 4.83 volts, which meant that
8.77 volts was applied across the fan. The parallel combination of the
two resistors measured 49.7 ohms on the DVM so that meant that the current
was about 97 mA (use Ohm's Law to verify!). Using the Power Law as before,
the required wattage rating calculated to 0.47 watts. Since the two1watt
resistors in parallel can safely dissipate 2 watts there is no likelihood
of the resistors overheating. If a single 47 or 51ohm resistor is used
I recommend that it have at least a 1watt rating to provide a safety
factor.
One
note about tinkering with small DC fans: The amount of current drawn
by the fan is not linear with applied voltage, so assuming that if you
halve the applied voltage that the current will be halved is a useful
starting point but it isn't really accurate. Therefore, unless you have
a variablevoltage power supply handy you will have to determine the
exact resistance required for a specific voltage drop by the empirical
method, i.e., cutandtry. Once determined, measure the actual current
flow to determine the wattage rating you'll need (HINT: Use highwattage
resistors while you're experimenting, otherwise they may turn into firecrackers
if you're not careful).
I inserted
the two parallel resistors in the positive lead to the fan and covered
the work with a piece of heatshrink tubing. The resultant airflow provides
adequate cooling for the amplifier. While it does run a bit warmer than
before it is still just comfortably warm to the touch after 45 minutes
of 30 seconds on, 30 seconds off operation running FSK441A. Within 5
minutes of cessation of transmitting the chassis & heat sink are cool
to the touch. The fan cannot be heard running unless you put your ear
close to it and the fan in my IC706MkIIG normally masks the sound.
