I have a piece of equipment that needs to be cooled using a fan. My power source is an 18 volt NiMH battery, and that voltage is what I need for everything else. I have a 12 volt fan that will fit the bill, but I am afraid of hooking it up to the 18 volt battery. Is there any way I can use resistors or diodes to get this down to 12 volts for this fan? If you have reference to a diagram on how to do it, that would be helpful also. Thank you.
A zener diode and resistor would do the trick. There's instructions for making the circuit including a calculator at this link Zener Diode Voltage Regulator - Electric Circuit
If it's a 'computer' fan a resistor is probably simplest. The fan will have either the current or watts rating marked on it - tell us that and we can spec a resistor for it.
Thank you so much. Yes, it is a computer fan. I actually have several, not sure of which one will fit the best yet. They are all 12 volt and they vary, .07 A., .08 A, .15 A, .22 A. Also, is the resistor wired in series with one of the leads, or does it go between the leads? Thank you again.
Another easy method would be to connect a series of rectifier diodes between the battery and the fan. Each diode will drop .7 volts. Nine diodes in series should drop your voltage to around 11.9 volts. The advantage is no heat as with a resistor. Want it to run a little faster or slower? Add or delete diodes. Radio Shack sells a 25 pack of rectifier diodes for a few dollars. RadioShack.com - Cables, Parts & Connectors: Component parts: Diodes & rectifiers: Rectifier Diodes (25-Pack) They have different reverse voltage values but all of them would work for your application.
In theory 85, 75, 40, 27 Ohms respectively, but go to the next highest standard values so you under run the fan a bit; say 100, 82, 47, 33 Ohms. Power wise I'd aim for a cooler resistor than theory might suggest and go for at least 1 Watt for the two smaller fans and 2 W for the two larger ones. (Someone else please check my numbers ) Wire in series with one lead and make sure it has some space to be hot in (or put it in the airflow). Sorry, but that is wrong - this method of dropping volts is perfectly valid, but the heat produced is the same as for a resistor solution. Ohm's Law, etc. still applies - W = VI (Volt-drop x Current). The difference is that the heat is spread over 9 blobs so each is only throwing 1/9th of the total. (You could also use 9 lower Ohms resistors in a string). The downside is you have to wire up 9 diodes and find space for them, but an upside is that the same string of diodes will do for any of the four fans
I've never heard of this type of installation. How is it wired? Does Radio Shack have them? How much heat is generated? Is it small enough to do the trick in a confined space. Thank you for the suggestion.
Thank you. How much heat will this generate? If placed near a plastic surface, will it melt it? Could I put it on the outside bottom of a power car and bring the wires through the floor to keep it cool (insulating from the floor with tin foil or something similar)?
Radio Shack P/N 276-1771 $1.25 http://www.radioshack.com/product/i...productId=2062600&support=support&tab=summary When dropping voltage, the power dissipation for a resistor or from a 3 terminal regulator such as the 7812 are the same P=V*I = (18-12) * I (Amps). The difference is the regulator will always put out 12 volts no matter what the current is, but a resistor will drop voltage in direct proportion to the current. I would use the lowest current fan yo can find that will do the job so as to minimize the heat. For your 70 ma fan, that would be 6*.07=.42 watts. shouldn't even get warm. Hook up instructions are on the package.
Thank you, sounds easy enough. I looked at the schematic and it doesn't seem totally out of my realm. The only thing that has me a bit confused is where does the ground go? This is in a plastic car body and everything else is isolated. Thank you again.
Negative from the battery goes to the center pin or it can go to the metal mounting/heatsink tab. That is also your negative connection to the fan.
