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question on 12v dc fan noise

 
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question on 12v dc fan noise
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woodro
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Joined: 31 Oct 2004
Posts: 39

Post question on 12v dc fan noise Reply with quote
hello, i guess its quite common to pick up noise from a dc fan installed in a transmitter case. I'm wondering if the noise typically comes from the 12v fan being on the same power feed as the other gear or is it usually picking the noise up elsewhere. The power supply i will be using has both 12v and 5v dc outputs and was wondering if there would be any difference if i were to use a 5vdc fan on the 5v tap off the power supply. That way would not be using the same power feed as the transmitter. Would it make any difference to connect the 12v fan(s) on another seperate power supply all together with?

ideas?
thanks, Woody
Wed Jan 12, 2005 2:37 am View user's profile Send private message
pcs
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Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
The noise comes from the power rails partly, from
EM radiation of the fans power leads and the fan itself
and from the air flow itself.

Power leads need to be "RF blocked" with the help of capacitors
and RFC. It is good to use ceramic capacitors with low ESR and
some large 100uF or so electrolytic ones to smooth and power
spikes. This is preferably done at the fan so the power leads
radiate as little as possible.

Place the fan away from the sensitive circuitry.

Enclose the VCO in a metal shield. Strong air flow and vibration
from the fan can "modulate" the VCO coil and create FM noise.

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Wed Jan 12, 2005 4:48 pm View user's profile Send private message Visit poster's website
woodro
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Joined: 31 Oct 2004
Posts: 39

Post Reply with quote
thanks Marko
Wed Jan 12, 2005 7:57 pm View user's profile Send private message
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