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Determining resistor size

lr172

Well Known Member
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
 
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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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
 
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.
 
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
 
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.
 
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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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
 
I personally believe in convection.
I have two holes and grills but no fan. My assumption is that the heat behind the panel will build to the top and create it's own flow if it gets warm enough.
I assumed it would help with defrost too but nothing will help until the stuff behind the panel starts to warm up.
I have had a couple of times in very cold weather that we fogged up as soon as we shut the plane up. I keep a micro fiber for that reason.
I guess if I lived further North than Florida I would put a fan for defrost.
For those with a fan, does it help with defrost?
 
I personally believe in convection.
I have two holes and grills but no fan. My assumption is that the heat behind the panel will build to the top and create it's own flow if it gets warm enough.
I assumed it would help with defrost too but nothing will help until the stuff behind the panel starts to warm up.
I have had a couple of times in very cold weather that we fogged up as soon as we shut the plane up. I keep a micro fiber for that reason.
I guess if I lived further North than Florida I would put a fan for defrost.
For those with a fan, does it help with defrost?

Living in the great lakes area, frost is a real issue for a good part of the year. In the 6, I open the canopy to get dry air in to resolve. I believe that the air under the panel will be drier than the air up high, where I am exhausting moist air and causing condensation. However, I expect the relief will be limited and will still have to open the door. However, the fan will help to circulate that drier air across the windshield and speed the process.

Larry
 
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

Mine is connected to the NAV light switch and runs full speed when the lights are on. I also have a dimmer for interior and panel lights and am considering hooking to it to have speed control. One fan (from an HP server) already quit and I wonder if it is due to running full speed all the time (they don't in a server).

Didn't see it mentioned above but here are two equation for calculating your needs.

Voltage (V) = Current (I) * Resistance (R)
Power/Watts (P) = Voltage (V) * Current (I)
 
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