temperature turn on for fans

I understand how to properly use a relay for wiring cooling fans near an amplifier. I would like to take this one step further and wire the fans to turn on after an amplifier reaches a certain temperature. How can I wire this? I would like to mount a device on the amplifier heat sink that activates the fans at a certain temperature. I would like to use a relay to energize the circuit when the stereo is on and vice versa. I do not want the fans running when the radio is off even if amplifier temperature is above the "fan on" temperature. Thank you

Why the added expense of the "temp sending unit" as far as wiring goes just use the "sending unit" as the relay coil trigger with another relay that sends 12v+ only when the headunit and amps are on. you could go so far as to make a latch that would keep the fans on till the amp has cooled down even if you have turned it off( i know you said you did not want it though) #1 relay 85-        ground through "temp sensing device"?? 86-        12V+ from blue amp remote 87-        12V+ 30-        12V+ out to fans pin 30 is used to prevent 87a from constant 12v+The RIGHT way is the ONLY way.

Hmm maybe I didnt word my post well. I plan on using the remore turn on lead as you explained to energize and de-energize the relay. What I want to do is use 30 to another heat sensitive circuit that will power up the fans at a certain amplifier temperature. In other words I only want the fans to turn on when the amplifer heat sink is above 115 degrees farenheit. 115 degrees farenheit is only an example. What electronic devices opens and closes a circuit at a given temperature?

i think you worded it fine. if you wire it as shown it will do what you ask. the resaon 12v+ goes into 87 and "out" 30 is so that when de-energized pin 87a "sees" nothing. if you hook up the Temp device to the coil then the relay will only turn on when BOTH the radio is on AND the amp is at "115" this is a one relay setup. as far as the actual temp sensor....dunno yet but looking into it.The RIGHT way is the ONLY way.

Thanks resquchas. I am eagrly awaiting the best circuit design to build.

I would start with a thermister of some sort - a device whos resistance changes with temperature.  You can use that device as one half of a voltage divider circuit to turn the changing resistance in to changing voltage.

You can then use an op amp set up as a comparator to control the on/off point.  Ideally you would add hysteresis to give you different on and off points (ie turn on at 125F and turn off at 115F).

I would question, though, the necessesity of such a feature because most likely you will not have enough cooling capacity to actually drop the temperature.  I would think that once the temp hits the fan threshold the fan would just turn on and stay on until the car is turned off.  Which brings us back to just turning the fans on right away and trying to keep the temp as low as possible.

Kevin Pierson

Kevin has a valid point. what enviroment is your amp to live in. is it class A/B, D. If you are thinking of multiple fans then there are 12 volt computer fan controlers between 25 and 60 bucks. but most i have experience with require noth 12 volt and 5 volt. this is doable but to what end?The RIGHT way is the ONLY way.

KPierson wrote: I would start with a thermister of some sort - a device whos resistance changes with temperature.  You can use that device as one half of a voltage divider circuit to turn the changing resistance in to changing voltage. You can then use an op amp set up as a comparator to control the on/off point.  Ideally you would add hysteresis to give you different on and off points (ie turn on at 125F and turn off at 115F). I would question, though, the necessesity of such a feature because most likely you will not have enough cooling capacity to actually drop the temperature.  I would think that once the temp hits the fan threshold the fan would just turn on and stay on until the car is turned off.  Which brings us back to just turning the fans on right away and trying to keep the temp as low as possible.

Very good point. I do not have the knowledge base to design this circuit for one. I do agree that it probably would be best to have the fans on when the system is on. If there was a detailed schematic available for what I want I would build it. I just right that it would be awesome to have "thermostat" controlled fans.

resquchas wrote: Kevin has a valid point. what enviroment is your amp to live in. is it class A/B, D. If you are thinking of multiple fans then there are 12 volt computer fan controlers between 25 and 60 bucks. but most i have experience with require noth 12 volt and 5 volt. this is doable but to what end?

The amplifer is a class A/B 3500 watt mono amp. It is 33" long. It will be mounted in my trunk in the spare tire well. I would be using multiple fans in a push pull configuration. Do you think external fans will help cool the amplifer?

in your application external fans are a good idea if you are going to confine the amp in that enviroment. push/pull or any configuration is acceptable as long as you account for laminar airflow. bay far the easiest way to do this is to just wire up the fans to run with the amp on. try COOLERGUYS.com. they specialize in stuff like this. let me know what you decide and/or come up with. the wiring should be pretty straightforward. but i can help if needed.The RIGHT way is the ONLY way.

I call the temp switches Klixons, but that is a trade name. They are typically ~$5 or less. EG - http://www.jaycar.com.au/productView.asp?ID=ST3821 - use NO type.