Ohms Law, and resistor sizes.

Norman_E

Norman_E

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I need to run a small computer fan as part of the cooling system for my fridge compressor. I did the same thing some years back to cool a caravan fridge and the fan burned out when the battery charger upped the voltage. On the boat the voltage when on charge is 14.4v. The fan that I have bought has a built in temperature sensor to control its speed and is rated at 0.24 A / 12 volts, and I suspect that it will not tolerate too much over voltage. Ohms law gives a resistance of 50 ohms and a power of 2.88 watts (calculator here http://www.ohmslawcalculator.com/ohms_law_calculator.php )

The same calculator shows me that it needs a 10 ohm resistor in series with it if the circuit voltage is 14.4 and I want to keep it running at only 12 volts. Circuit power would then be over 3.5 watts. Does that mean that I need a 10 ohm resistor rated above 3.5 watts, like this 10 watt wire wound one? http://www.maplin.co.uk/p/wirewound-10-watt-10-ohm-resistor-h10r
 
You want to drop 2.4 volts at 0.24A?
That's 10ohms.

The power in the resistor is 2.4V x .24A = 0.576 W

However, if the current falls, the volts will rise, so a regulator might be better.
 
The simple answer to your question is no - if ohm's law applies the resistor would only consume a sixth of the power of the circuit so would need to be rated at a little over 500mW - say 1W to be safe.

The more correct answer is that a fan cannot be treated as if it were a resistor since when rotating it generates a back emf that tends to resist current flow. Putting a resistor in series makes life even more complicated.

It would be easier either to get a fan that could take up to 15V, or get a cheap 12V regulaor
 
Many fans use some magnetic trickery so that the motor current is minimal when stalled and it doesn't blow up when some urchin sticks something in it. You'll be able to tell because it will start slowly. If so it may behave a bit oddly if you use a series resistor. Just a thought.
 
The other problem is that when the batteries are not on charge (when the engine is not running) the battery voltage will be closer to 12 volts some where between 12.7 and 12.3 fully charger to half charged.

The risistor will then lower the voltage across the fan to somewhere around 10.5 volts.

A regulator would be better but most chip regulators need to drop about 2 volts anyway to work so you will still have the same problem.

You could use a bucks regulator and there are lots on E-Bay.

As stated before why not drive the fan from the compressor> this is what I did with and internal circulating fan and it has worked OK for 4 years now.
 
LadyInBed said:
Or tap the new fan off the fan output on the compressor in parallel to the original - that's what I did.
Click to expand...

OK if your compressor is 12 volts, but mine is a mains water cooled one without a fan. I need to add a fan to force air over the compressor, which is getting too hot because it is fitted in an enclosed space beneath a seat, and I will cut a new hole to mount the computer fan to draw air in, letting it escape via the existing grill. There is a 12 volt pump that delivers cooling water, but simply connecting in parallel with that would expose the fan to up to 14.4 volts. I like the 12 volt regulators, and am ordering one as that seems the best solution.
 
LadyInBed said:
"mine is a mains water cooled one without a fan. "
Then put in a mains fan!
Click to expand...

I had thought of a mains fan, like the sort of extractor fitted in bathrooms, but most are much noisier than computer fans. In addition the computer fan is small enough to get it in the best place to drive airflow over the compressor casing. The setup is that I have replaced the failed CLD inverter with a pair of mains contactors which close when the system is on, the thermostat is demanding cooling and the over pressure switch is in its normally closed state. One contactor runs the 230 volt compressor and the other switches in the 12 volt water pump. I believe the old inverter failed because it was too hot in the space with the compressor, and the same fact was probably the cause of the original compressor failure as well. Whilst I have not measured the temperature of the air around the compressor the boat is in Turkey and general air temperature inside the boat in summer is often well over 30 Celsius, its probably around 50 Celsius inside the compressor space when the fridge is running, hence the need to shift some air!

Having a switched 12 volt supply available makes the use of a 12 volt fan practical and the voltage regulator you linked looks just the job to keep the fan from being cooked.
 
Norman_E said:
... The fan that I have bought has a built in temperature sensor to control its speed and is rated at 0.24 A / 12 volts, and I suspect that it will not tolerate too much over voltage...
Click to expand...
Why do you suspect this? I think it will be fine. However, the sensor will be looking to run at full speed at temperatures much higher than a fridge.
 
Cheapest way for such low power I would suggest a led driver, plenty of 12/24 volt input to 12v output available on fleabay.
 
nigelmercier said:
Why do you suspect this? I think it will be fine. However, the sensor will be looking to run at full speed at temperatures much higher than a fridge.
Click to expand...

The fan goes to full speed at 38 Celsius, and the temperature in the space where the compressor sits is often above that. I suspect it might fail if fed 14.4 volts because a computer fan that I fitted a few years ago to cool a caravan fridge failed because of over voltage.

The LED driver idea had occurred to me but as the fridge is run from a modified sine wave inverter when motoring I was unsure whether such drivers would cope with the "dirty" supply, which is fine for the compressor, but probably not for electronics.
 
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