Using fan cooling to force air through the fins of an amplifier's heat sink, will significantly increase the reliability of an amplifier. It is especially important when the air flow to an amplifier is restricted (like when it is under a seat). If you only install one fan on the amplifier, make sure it forces air through the fins of the heat sink. As I said in another section, the semiconductors are mounted to the heat sink. Some people cut a hole in the bottom of their amps to blow air onto the circuit board. If that's the only fan, it will do very little good. If you have a fan blowing on the fins of the heat sink and want to mount another fan blowing inside the amp. it will serve to keep the transformer and large filter caps cool (not necessary unless you're driving your amp hard for long periods of time). If the amplifier has a built in fan, make sure that the added fan does not interfere with the built in fan. For example, do not try to force air into the exhaust of the fan cooled amplifier. Also, if an amplifier has a built in fan, do not make any new openings in the amplifier's case. Chances are very good that you will adversely affect the cooling of the amplifier.
Current Flow and Safety:
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...
The wire supplying power to the relay must be protected by a fuse and the fuse must be rated to open before the wire overheats and converts your vehicle into a raging inferno.
If there's a single wire carrying the current away from the relay to a fused distribution point (like wire 'C'), it must be the same size (or larger) as the main supply wire.
The fuses going to the individual electrical accessories must be rated to protect the wires supplying the current to the device(s).
If the wire feeding the individual accessories is the same as the main supply wire, the main fuse (near the power source) will protect all of the wires.
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.
Controlling a fan with a thermistor
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.
Now Available on Disc
FET Driven Fan Controller Plans
The information on the CD version of the above circuit uses only components available at your local Radio Shack (no need to have to order the parts). There are numerous highly detailed photos from multiple angles showing precisely how the components are to be layed out on a Radio Shack prototype circuit board. Even with virtually no knowledge of electronics, you can build this controller (all you have to be able to do is read and count to lay it out properly). Also included is a complete list of parts including the Radio Shack part numbers. Due to the inavailability of some of the parts in the circuits above (not available from RS), the values are significantly different. The new version also has a remote input lead that assures that the fan can only run when the remote lead has power (only when the amps are on). Also included is a printable schematic (although it's not necessary to build the controller). The total cost of the parts is ~$20 for the electronic components and the circuit board (excluding the fan, wire and solder which you may already have). If you have questions about the controller plans, feel free to E-mail me. For shipping and payment details, refer to the Front Page of Site.
All of the new demos, photos, and other content are for the CD ROM only and will not be added to the online content. If you buy the CD ROM, I will gladly give you updated files (via email) for a year or so on request (no more frequently than once a month).
For shipping and payment details, refer to the Front Page of Site. If you've already read the information on the front page of the site, you can click the button below to order the disc.
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