[time-nuts] HP5370 power supply measurements

Charles Steinmetz csteinmetz at yandex.com
Fri Jul 15 10:44:08 EDT 2016

David wrote:

>> An external fan blowing on the heatsink keeps my
>> 5370s nice and cool, for about $10 each.
> So more noise.

You seem to have missed the next sentence, which said they are 
"extremely quiet."  So quiet that I could reduce the noise of the 
internal fan by 20dB and still not be able to tell whether the external 
fan was running without looking at it. [1]  So no, as a practical matter 
there is no additional noise.

> Surely one of the main  advantages of the SMPS route is the higher
> efficiently means less cooling air is required.  That means less noise.

Two different things.  The internal fan primarily cools the inside of 
the counter, while the power supply heat is developed on the heatsink, 
which is mainly cooled by convection outside the enclosure.  (Yes, the 
heatsink has a couple of little holes in it where air from inside the 
counter blows out -- but that is completely ineffectual at reducing the 
heatsink temperature, and I doubt that's what the designers intended. 
More likely, they were trying to minimize the amount of heat from the 
heatsink that gets inside the box, by providing a path for air to flow 
from the inside, past the heatsink, to the outside -- and at that, the 
holes seem effective.)  So, if you reduce the internal fan airflow, the 
internal circuitry will run hotter even though the power supply pass 
devices won't (much).  Further, if you replace the heatsink-mounted 
regulators with switchers that are located inside the box, you may very 
well put MORE heat load on the internal fan even though the switchers 
are much more efficient (because what loss there is would be dissipated 
inside the box, rather than outside).

Best regards,


[1]  I actually tried an internal fan that reduced the noise by 22dB, 
but it didn't have sufficient airflow to keep the interior properly cool.

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