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  #1  
Old 11-14-2018, 09:28 AM
lr172 lr172 is offline
 
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Default Determining resistor size

I am installing a fan in the dash of the 10. It is a 120mm fan and want it to run slowly most times and full blast when needed via switch. Plan to run the full 12v through one side of the DT switch and 4.5v through the other side. I have tested this on the bench and like the flow at 4.5 volts drawing 100 mA. This was tested using an LM317 in constant voltage mode.

My question is what size resistor do I need to get my 14 volt buss voltage down to 4.5 volts? Also, for computing the power for sizing, can I use the current observed at 4.5v on the LM317 based power supply?

I could use an LM317 for this, but would like to avoid throwing a little board in the harness if I can.

Larry
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  #2  
Old 11-14-2018, 09:45 AM
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Mike S Mike S is offline
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I would stick a 0-10k pot in there, adjust the pot to give the voltage or fan speed you want.

Measure the value on the pot.

Buy a resistor of that value.

Or, just keep the pot in place of a switch and resistor??
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Last edited by Mike S : 11-14-2018 at 10:31 AM.
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  #3  
Old 11-14-2018, 10:05 AM
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Carl Froehlich Carl Froehlich is offline
 
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Two speed fan(s) are really not needed.

One fan (RV-8) or two fans (side by side RVs) under the glareshield with a nice hole pattern instead of a fan screen, then just an on/off switch, Summer leave the fan on to move the avionics heat from behind the panel, winter just for those few minutes you need defrost.

No need for huge fan(s), just something to move a little air. In the RV-8 I use a single 2 fan. On the RV-10 I used two 3 fans.

Carl
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  #4  
Old 11-14-2018, 12:35 PM
Mich48041 Mich48041 is offline
 
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14.5 volts minus 4.5V fan = 10 volts dropped by resistor.
10 volts / 0.100 amps = 100 ohm resistor
The resistor will get hot if it is low wattage.
How about 5 watt rating or more?
Or search Amazon or eBay for DC Fan Speed Controller.
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  #5  
Old 11-14-2018, 12:41 PM
Mich48041 Mich48041 is offline
 
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Make your own PWM fan speed controller using a TC648
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  #6  
Old 11-14-2018, 03:43 PM
lr172 lr172 is offline
 
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Thanks for the input. I just remembered that I have a 3.3V power supply that I am installing to power my headset. I will run the fan directly off that supply. 3.3 still seems to push enough air for avionics ventilation.

Larry
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  #7  
Old 11-14-2018, 03:52 PM
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FasGlas FasGlas is offline
 
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LM7805 regulator is a buck and would work fine.
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  #8  
Old 11-14-2018, 04:03 PM
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fl-mike fl-mike is offline
 
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run it full speed all the time. KISS.
Once that big fan up front is turning, any noise is insignificant. Power consumption will be constant unless you use a switcher supply. i.e. you're just dropping voltage via heat in the dropping circuit.
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  #9  
Old 11-15-2018, 04:58 AM
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maniago maniago is offline
 
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Quote:
Originally Posted by fl-mike View Post
run it full speed all the time. KISS.
Once that big fan up front is turning, any noise is insignificant. Power consumption will be constant unless you use a switcher supply. i.e. you're just dropping voltage via heat in the dropping circuit.
....and you have a headset on.....how will you determine that you should be at low speed or high speed on your fan anyway? Not pointing fingers, just curious as to why you might want to do this.....I have 4 very small fans under the glare shield and one on each of my 3 buss solenoids in the engine compartment - they all run full bore all the time, tho probably dont need to..... .25-.5amp total.
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Last edited by maniago : 11-15-2018 at 05:04 AM.
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  #10  
Old 11-15-2018, 01:23 PM
lr172 lr172 is offline
 
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Thanks for the input. This fans moves a lot of air that just isn't necessary for cooling, but is for defrost purposes. I suppose it is no big deal to run full bore at all times. Certainly wasn't concerned about the noise; More about blowing on my face and sucking power.

Larry
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