Quick question, i need quiet down an old cheapo fan. Would it be possible to stick a resistor inline with the 12v supply just to drop it a couple of volts so its not so noisy. I know there a better ways, the lm317(?) circuit that i use in my main box for example. But its not neccessary for variable speed just need to shut it up a bit. So would a resistor work or just over heat? I suppose i could just do the old 7v trick but i'm not sure if it would provide enough air flow. Cheers
It'll work fine provided that the resistor is of rated for sufficient power dissipation. If you want to drop 4 volts across it with a flow of about 150mA it's still less than a watt.
Put a couple of high-current diodes in series with it. Each diode will drop the voltage to the fan by 0.7V. Just make sure that they are rated to a high enough current. 2A rectifier diodes should be more than enough. _C
Yet another way to accurately lose a few volts is a zener diode in series with the fan, banded end to +12V. A 1.3W zener will do for most single fans. The common BZX85 series has several values between 2.7V & 5.1V so you can pick & choose the fan voltage (12-VZ). Bad news is Maplin's range starts at 4.7V. But ESR and Futurlec carry a wider selection.
Or you could just do the 7v trick put the 12V+ on positive, and +5v on negative lead, and you have 7v going across the fan.
I take the diodes do not get so hot in use as the resistor would? That looks like a better method for next time but at the mo i have a whole load of resistors and the uk site linked has a £2.50 delivery charge... bit much for a 10p diode. But next time i do a shop i'll get some. Thanks for the suggestions folks.
Diodes have a pretty much constant voltage drop, resistors require calculations to figure out how much voltage they will drop. What kind of resistors do you have? I think that you need in the range of 1-50 ohms for slowing down fans. Lower for high current fans.
Easy resistor calculation: Work out the fan resistance from the current or wattage written on the label. 12/current = Rfan (so a 200mA (0.2A) fan is 60 ohms) 12 x 12/wattage = Rfan (so a 3W fan is 48 ohms) Same value resistor will run the fan at 6V (approx). 1/2 value will run it at 8V 1/3 value will run it at 9V Resistor wattage needed = (Voltage over Resistor)2/Resistor value So a 24 ohm resistor on the 3W fan will drop 4V and need to be at least 16/24 = 0.67W. A 48R on the same fan will drop 6V and produce 36/48 = 0.75W (In both cases use a 1W or higher, a 1W will get very hot.)
