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There have been a lot of people who have cut holes in the bottoms of their amplifiers to install fans and this may help a bit (maybe more than a bit for class D amps), it does very little for class AB and class B amps. Just about the only time that this is useful is when an amp is being abused by running it below its rated load AND there is a fan blowing through the fins of the amp.
If you're building your amps into an amp rack, you need to make sure that the fans in the amp rack work with and not against the fans in the amp. If the fans in the amp are blowing against the ones in the rack, it can actually reduce the air flowing through the amp and make things worse.
Bearings vs Bushings:
Squirrel Cage Fans:
Most fans with bushings are relatively easy to repair (lubricate). Most have a split-ring retainer under the label. Sometimes it's covered by an oil seal (which just pops out). Below, you can see the split-ring as it normally is and after it's been part-way removed. To clean and lube the bushing, remove the ring completely and push the shaft for the rotor out of the bushing. The magnets will fight you a bit but it will pop out if you push it far enough. Keep track of all of the o-rings. Clean the shaft and the inside of the bushing with acetone. Use a cotton swab with most of the cotton removed to clean the inside of the bushing. For smaller fans, you'll have to remove the cotton and several layers of paper to allow it to fit in the bushing. When it's clean, use a moderately heavy oil (similar to 30 weight motor oil) or a very light grease (lithium based white greases are generally suitable). Smaller fans need lighter lubricant that the larger fans so that the fan can turn freely. The larger motors have enough torque to overcome the damping by the heavier grease.
The following diagram shows the relay controlled by a switched 12 volt source. It shows a fan and a neon tube (it could be virtually any 12 volt device) being supplied power from terminal 30 of the relay. The power source is the battery/charging system. To have a safe system, we have a fuse at the battery. With a Bosch relay, you can use any fuse up to a 30 amp fuse (the relay is rated for 30 amps). The required size of wire segments A, B and C is determined by the size of the fuse (and the current demand from the electrical accessories). If the total required current draw will be only about 10 amps, you could use a 16g wire and fuse 'A' would need to be a 10 amp fuse. If the total current draw was going to be 25-30 amps, you'd need at least a 12g wire and a 30 amp fuse. The rating of fuse 'B' is determined by the current draw of the fan and the wire connecting the fuse to the fan. If the fan draws no more than 5 amps (very likely), you could use a 5 amp fuse and a 16g wire (16g is the minimum size I'd recommend using because when you use anything smaller, it's difficult to get good, reliable connections). The size of fuse 'C' and wire 'E' are determined by the current draw of the neon. Fuses 'B' and 'C' should be as close to the relay as possible.
The main points to remember are...
If you want your cooling fans to operate only when your amplifiers are on, it will be necessary for them to be powered only when the remote output lead has 12 volts on it (when your head unit is powered up). I would NOT recommend powering them directly from the remote power lead. It would probably be damaged by the current draw of the fans. The diagram below is a connection diagram that may be used to supply power to the fan(s). If you have multiple amplifiers and signal processors, the output from terminal 30 can also be used to supply power to the remote terminals of those devices.
Possibly Helpful Links:
If you only want the fans to come on above a specific temperature, you can use a thermistor. Since you won't be designing a circuit for mass production, you want something that is fairly flexible and can produce the desired results without a lot of testing. The diagram below is a circuit which uses an op amp as a comparator with an output which goes high when pin 5 goes higher than pin 6. I decided to use a thermistor with an op-amp to prevent the self heating that you get when you try to drive a relay coil with the thermistor. Both methods work but unless you know which thermistor you need for a particular relay, you'll have to use trial and error to find the right part.
In the following demo, you can change the temperature and voltage on the potentiometer. To change the voltage on the potentiometer, click above or below the arrow (on the right side of the potentiometer). This controls the reference voltage on the negative input of the op amp. Setting it lower makes the fan come on at cooler temperatures. To change the relative temperature, click on the thermometer. The value above the thermometer is an approximation of the thermistor's resistance. When the conditions are right, the relay will engage and the fan will run. Click HERE to make the demo fill this window.
When initially setting up this circuit you need to adjust the voltage at pin 6 to 1/2 of the voltage on pin 8. This will cause the fan to switch on at approximately 150F. You can set it lower if you want the fan to come on earlier.
If you want to use an FET to control the fan, the diagram below shows you how to connect the FET to the op amp. Keep in mind that you must insulate the tab of the FET. If the tab of the FET touches to ground, the fan will run. As an alternate FET, you can use an IRFIZ44. It's a fully insulated part and won't require an insulator. If the fan draws more than ~1/2 amp, the FET might benefit from a small heat sink.
If you need more info on the circuits above, E-mail me and I'll try to help.
Possible supplier: Mouser electronics http://www.mouser.com Parts list: Thermistor: 334-4227-503 Potentiometer: 531-PT10V-100k (not critical) Op amp: 511-LM358N (critical) FET: 570-IRF540 or IRFIZ44 or IRFZ44 Bipolar transistor: 625-MPSA06 (or equal) Resistors: None are critical and can likely be purchased at Radio Shack.
You should remember:
1.A cooler amplifier will last longer and will be less likely to fail.