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Hi
Newbie here and struggling with the maths so your assistance is greatly appreciated.
I have rigged up a 12v DC 80w (9" car radiator) fan in my garage as an extractor.
I am driving it from a mains LED/Fan transformer outputting 12v DC and rated for 100W.
I want to slow the fan down and experiments with a graphite pencil as a resistor suggest I need about 5 Volts at the motor.
80w/12v is 6.6 Amps. So I guess this is what I am working with.
Keeping it simple, if I want a resistor to give me 6 volts then is it 6v/6.6a = 0.9 ohms or do I need to allow for the fact that the current will decrease at 6v so it it 6v/3.3a = 1.8 ohms?
And what watt rating do I need the resistor to be? Is it 6v x 6.6a or 3.3a (40 or 20 watts)?
Then how do I apply this to 5v output, I need a 7v drop so do i stick 5v or 7v into the formula?
I know there are other methods of achieving what I am after, I have tried a PWM unit rated for 100w but found there was excessive buzzing at the motor. I believe I can cure this with some sort of low pass filter but that gets even more complicated with capacitor and diode ratings to work out and to be honest, it's too advanced for me.
I have tried a 100W variable resistor but its min rating turned out to be about 4 ohms which was too slow.
The other method I thought of was an automotive heater blower resistor (on ebay for about £6) but I'm worried they won't have a high enough watt rating (this info isn't readily available).
I look forward to your advice, I'm sure this will turn out to be much more simple than I am making it.
Cheers
Newbie here and struggling with the maths so your assistance is greatly appreciated.
I have rigged up a 12v DC 80w (9" car radiator) fan in my garage as an extractor.
I am driving it from a mains LED/Fan transformer outputting 12v DC and rated for 100W.
I want to slow the fan down and experiments with a graphite pencil as a resistor suggest I need about 5 Volts at the motor.
80w/12v is 6.6 Amps. So I guess this is what I am working with.
Keeping it simple, if I want a resistor to give me 6 volts then is it 6v/6.6a = 0.9 ohms or do I need to allow for the fact that the current will decrease at 6v so it it 6v/3.3a = 1.8 ohms?
And what watt rating do I need the resistor to be? Is it 6v x 6.6a or 3.3a (40 or 20 watts)?
Then how do I apply this to 5v output, I need a 7v drop so do i stick 5v or 7v into the formula?
I know there are other methods of achieving what I am after, I have tried a PWM unit rated for 100w but found there was excessive buzzing at the motor. I believe I can cure this with some sort of low pass filter but that gets even more complicated with capacitor and diode ratings to work out and to be honest, it's too advanced for me.
I have tried a 100W variable resistor but its min rating turned out to be about 4 ohms which was too slow.
The other method I thought of was an automotive heater blower resistor (on ebay for about £6) but I'm worried they won't have a high enough watt rating (this info isn't readily available).
I look forward to your advice, I'm sure this will turn out to be much more simple than I am making it.
Cheers