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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am searching for *simple* ideas for a fan cooling design. I have a voltage that can vary from 0 to 500v. As the voltage rises a shunt kicks into play, and I want a fan to cool the FETS. I am thinking about getting a 48v DC fan and connecting it parallel to the 25ohm dump load. I can use some series resistor to allow the proper voltage for the fan once the voltage exceeds a certain value. Does anyone have any other ideas?
Registered Member #53
Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638
You could get a thermistor and stick it to the FET and use that to control the fan. Keeps the fan power separate from working power. I have seen a few PC case fans that use a thermistor to control the fan speed.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Thanks Nik, but part of the problem is I don't have a constant voltage source. There is no battery or Mains power; there is only a variable power source that can go from 0 to 500v at any time.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
So what is happening is you have this 0-500V source, and when it gets too high you're using FETs to dump it into a 25 ohm load, correct?
One somewhat complicated way would be to put a buck/boost converter on this variable voltage source to run the fan at the appropriate voltage, and you could have the output to the fan track the current value of the voltage source, or have it track a thermistor. You could also incorporate energy storage: a rechargeable battery charged by the buck/boost converter, then another FET to PWM the fan from this, that way it can continue working when the supply goes to zero.
But you say simple... if you connect it in parallel to the 25 ohm load then you'll need roughly 2A flowing for it to power the fan at full speed, 2A*25ohm=50V, but actually more depending on how close to a current source it is. Is that the kind of current you roughly expect?
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
25 ohm dump load there can be 8 amps flowing through
I am thinking of putting a 3k/10watt resistor in series with a 48v fan. This will be parallel to the 25 ohm load. I could connect a 56v (maximum voltage for the fan) across the fan to limit the voltage. This might work.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Just find out what the current draw of the fan is and use Ohm's law to size the resistor appropriately. For instance, if it was 48V, 0.1A, then you need a resistor that passes 0.1A at (500-48) = about 450V. Hence 4500 ohms. So I guess your 3k figure was ballpark anyway.
If this doesn't give enough cooling at low voltages (and my dump load tended to run hot at low power and actually get cooler at full power) then maybe you need a constant current source/sink instead of a resistor. You can probably cobble one together out of a LM317 and a high voltage FET, or whatever.
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Fan cooling mechanism
