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Author Topic: Slowing noisy fans down, mod to make sure the fan starts with the lower voltage  (Read 4425 times)

Posts: 538

« on: January 09, 2015, 01:58:39 PM »

Some DC powered vent and cooling fans can be annoyingly noisy, and slowing them down sometimes helps.  Assuming that the fan still does enough cooling, you can run the fan on lower voltage.  But beyond a certain point, the fan won't get started below some voltage.  But if you gently finger the blades it will stay spinning.  One way to "kick start" the fan is to have it see a higher DC supply voltage upon system power-up, and have it drop to a quieter operating voltage.  My example is a small fan that is rated for 12V and half an amp.  And I used a 200 ohm resistor in series of a 12V power source to drop the voltage and thus operating speed to a reasonably quiet and still effective cooling level.  But it won't start like this.  So I added a 1000uF cap across the resistor.  This cap is initially discharged at power-up, and the fan will see for a short while a full 12V supply, then the cap charges up, and then the fan sees a lower supply voltage.  This short period of higher DC voltage is enough to get the fan spinning, and once it is spinning, it keeps spinning at a lower rate at the lower voltage.  And be less noisy. 

Posts: 112

« Reply #1 on: January 10, 2015, 05:34:22 AM »

good idea and it will work.  Have you looked at the spec for the fan?  It should show the voltage range the fan is designed to work across, as I recall some fans that will start down around 8 volts, but generally its around 10 volts.  The spec generally also shows the air flow from the fan at different operating voltages, depending on the application, less air flow is not what you want.  If the equipment was mine, I would look for a different fan that gives the same air flow at the normal operating voltage, but less noise or find some way to use sound reducing material to drop the noise down.  would hate to see the equipment fail due to insufficient cooling.


Posts: 86

« Reply #2 on: January 11, 2015, 11:04:27 AM »

My experience has been with AC fan motors, so the following may or may not apply to your DC situation.  Series resistance or electronic(?) ceiling fan speed controls slowed the fan, but it would not reliably start.  However if I ran a 120 volt AC fan on something like 95 volts it always started very reliably, even if I tried to inhibit its rotation with a stick. This 25% decrease in voltage resulted in a very significant decrease in estimate is at least a 10 dB reduction in noise, yet in my specific situation the air flow still seemed to still be very adequate, and possibly only slightly decreased.  In my situation I simply wired a 24 volt transformer so it would buck the 120 volts.  With a 12 volt DC fan, I suspect, but cannot guarantee, that it would start just fine with something like 10 or 11 volts, and possibly less, IF the supply does in fact supply something like a stiff 10 volts.  One way to do this would be to wire several silicon diodes in series with the fan.  Each diode will have very roughly a 0.6 volt drop, so 4 diodes would result in roughly a 2.4 volt drop. (12.4volts - 2.4 volts = 10 volts).  For even a stiffer DC source you could use a voltage regulator.  Also nothing wrong with the capacitor/resistor solution unless somehow with power hits the fan stops before the capacitor discharges, however I think this is unlikely since the discharge will happen very quickly with the low resistance.
    Rick  KL7CW

Posts: 29

« Reply #3 on: January 14, 2015, 03:12:15 PM »

Another sometimes easy trick to quiet a fan is put some space between its blades and the grill.  The blades make noise as they pass by objects that are too close.  Putting even 1/4" between the fan blades and any grill or support will make a noticible difference, especially on the input side of the fan.

Posts: 4764

« Reply #4 on: January 15, 2015, 09:03:57 AM »

Fan noise is caused by the amount of air flow......which is fan speed needed for the diameter and turbulence as ILT pointed out.

The manufacturers almost always install a fan that will sufficiently cool that particular piece of gear.  Sometimes they install fans that are noisy because this is all that is available to provide the needed cooling.....Little piece of gear, lot of heat equals a small and noisy fan. 

Be careful when slowing them down.  This is why it's a good thing to have an IR temp meter available.  They're inexpensive and a fantastic tool for detecting things like temperature changes.

Take the temp at normal operating conditions and then monitor the temp as you slow the fan down.

Posts: 173

« Reply #5 on: January 15, 2015, 09:34:01 AM »

I run a 12V Radio Shack fan on 4.5V to cool my R75. It'll start on 3V fine as well but 4.5V is the point where it still runs quiet but pushes more than adequate air.
BTW I have an old "HI FI Boxer" fan by IMC Magnetics that runs on house current to cool the R71a, that is so quiet I sometimes need to doublecheck that it's running.
« Last Edit: January 15, 2015, 09:37:15 AM by HFCRUSR » Logged
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