I am a newbie here and last time I did something with a circuit was in grad school. I am trying to build a simple circuit and can use your help.

Problem: Controling voltage so that the current is passed when certain threshold (for voltage) is reached.

I am using AerolCool Touch 1000, a fan controller which controls speed of a 12V PC fan. The issue is that the fan is always on, it is either running at slow speed (RPM) or high speed when the temperature is hot.
I need to introduce some thing in the circuit (between fan controller and 12 V fan) so that the fan comes on when certain limit (read temperature) is reached .

I believe that the aerocool is simply controlling voltage and keeps on increasing or decreasing the voltage beased on temperature.

Any help is appreciated.

 

My first instinct is to use a mechanical switch. One brand is "Klixon".

Do you have any reason that 10 F degrees dead band is too much?
If not, keep it simple.

 

I'm not understanding the problem.

The fan needs to maintain a certain airflow through the enclosure even when it's relatively cool, or else the fan controller may be too slow in detecting that the enclosure is warming up. Keeping the air moving ensures that the fan controller gets airflow past it, so that the fan will respond more quickly when the temp is rising.

Some older Dell computer fans had the fan speed control incorporated directly into the fan itself; a thermistor monitored the temp of the air flowing into the fan from the CPU heat sink. The fan had to always be rotating in order to sense the temp of the CPU. The system worked quite well.

 

I agree, If the fan goes between on and off instead of fast or slow, I would think it would be even more annoying.

 

I'm not understanding the problem.

The fan needs to maintain a certain airflow through the enclosure even when it's relatively cool, or else the fan controller may be too slow in detecting that the enclosure warming up. Keeping the air moving ensures that the fan controller gets airflow past it, so that the fan will respond more quickly when the temp is rising.

Some older Dell computer fans had the fan speed control incorporated directly into the fan itself; a thermistor monitored the temp of the air flowing into the fan from the CPU heat sink. The fan had to always be rotating in order to sense the temp of the CPU. The system worked quite well.

Thanks all,
I am using this for a media cabinet, ideally it is recommended that the temperature in the cabinet should not go above 100F. The rig mentioned above works fine but the fans are running all the time even if no component is running in the media cabinet. Normal temperature inside the cabinet with everything off is around 75F (atleast it was yesterday). As the PS3/AV receiver starts grinding the temperature easily goes above 100F.
I am hoping that by introducing something in the circuit my fans will come on around 85 degrees and keep the temperature from rising above this point. I am sitting away from the cabinet and the fans I am using are behind the cabinet, moreover the fans are ultra quiet so I am not concerned aboout the fans going On / Off (read noise) as long as the temperature does'nt go above 85F.

Do you think a potentiometer might help? (I have a 10k trimming potentiometer sitting at home). If I introduce this between the fan controller and the fan - will it help (again I am total noob, as long as the thing wont blow up I will give it a try if you suggest)

Gist:
Current SetUp:
- Fan on at all times irrespective of the temperature
- As the temperature rises the fan RPM increases till things have cooled down again.


Desired:
- Fan should come On when the temperature rises above 85F (read as - voltage increases beyond a certain threshold)
- Fan goes Off when the temp. is below 85 degrees again.

Thanks again

 

I did some reserch and based on what I am reading online, it seems like an NPN Mosfet along with a 10k potentiometer is what I need.

This is what I am thinking,
- Apply the output (-ve terminal) from 'AeroCool 1000' to the 'Gate' of the MOSFET and also the potentiometer.
- Connect the output of potentiometer to the 'Source' of MOSFET
- Connect the 'Drain' from MOSFET to -ve terminal of the fan (+ is already connected to the supply from aerocool)
- Adjust the potentiometer so that it starts conducting when a certain voltage is reached

When the potentiometer starts conducting, I am hoping that the circuit will be complete and the fan will start spinning. As soon as the temperature goes down, circuit will stop conducting and the fan will be off again.

I hope this works but can somebody explain if I can use only the 'potentiometer' and not use the MOSFET at all - Will it work?? I dont know what MOSFET is doing in this kind of circuit.

Help please!!

 

I am not familiar with that 'AeroCool 1000' fan controller, but after a quick Google search, it appears to be intended for a computer system; where the 12v power would be turned off when the computer is shut down.

The unit should have come with several temperature sensors, and an instruction manual. From reading some reviews, you really have to read the manual in order to understand how to correctly use the control unit.
[eta]
Manual page 1: http://www.aerocool.us/images/peripheral/touch1000/Touch1000-manual-1.jpg
Manual page 2: http://www.aerocool.us/images/peripheral/touch1000/Touch1000-manual-2.jpg
The manual indicates that once the maximum fan RPM is set, the lowest fan RPM will be 50% of the maximum.
It also states that the maximum power per fan channel is 6 Watts, which means 0.5 Amperes at 12v. I have a number of computer fans that draw more power than that. You will have to choose fans carefully to avoid exceeding that limit.

I suggest that the lowest part of the media cabinet needs to be open to air flow from the room, and the fans need to be installed at the highest point of the cabinet in the rear. That way, natural convection will assist in promoting airflow through the unit.

Your controller may use PWM to control the fan speed, as that is more efficient than linear control. It also might have a switching power supply in it, which would simulate a linear control, but be far more power efficient.

If you attempt to connect a potentiometer/rheostat between the unit and the fan(s), you might be able to stop the fans from turning, but they won't reach their top speed afterwards. A MOSFET won't help, either.

You haven't described how you are powering the control unit or fans, and you have not shown us how you have installed the fans and the temperature sensors in the media cabinet. Uploading some photos of the media cabinet, fans, controller and temp sensor installation would be a big help. If you don't have a camera, make a drawing of what it looks like and upload that.

 

I will post some pictures later in the day today. I have not yet installed the unit but have already figured out the place in the cabinet and the wiring.

I am using a 120v to 12v (molex output) power supply adapter (I had this sitting around from an external hardisk enclosure). This works flawlessly with the Aerocool and the sample fan I have been experimenting with.

Adapter: (not exactly this but on similar lines and different manufacturer)
http://www.amazon.com/110v-4pin-molex-Power-Adapter/dp/B000MGG6SC

As you suggsted I am planning to use two fans behind the top portion of the cabinet and another fan in the bottom shelf and towards the front of the cabinet. I have already orderded 90CFM (Cubic feet per minute) fans running max at 2400 RPM, even if these fans run at half the speed they should hopefully cool down my cabinet efficiently as the portion I am trying to cool down is less than 30 CF (cubic feet).

I will post detailed pictures later today

Thanks again.

 

You should double-check the current/wattage requirements of the fans you have on order. If they exceed 6W or 0.5A/500mA, they will overload your Aerocool controller.

 

I suspect you are thinking in terms of too much air flow, and the size of the cabinet has very little effect on the temperature or air flow requirements.

3.413 times the watts used by the equipment = 1.08 CFM times temperature rise.
You can use this formula to determine how many CFM you really need.

 

Uploaded few images (each image have comments written directly on them).

The fans I ordered are all well within the wattage/amp limits of aerocool and you are correct, I have not really calculated CFM requirements but I am hoping that I will be having enough ammo to handle the components.

So back to original question.....how can I control the fans so that they run above certain voltage and are not on all the time. Meanwhile I will try to figure out PWM stuff.

Thanks all

 

I told you one way. Get a mechanical thermal switch or use an old mercury bulb thermostat. What do you want? A thermistor circuit? A microprocessor? Something that reads a voltage and flips a relay?

 

I told you one way. Get a mechanical thermal switch or use an old mercury bulb thermostat. What do you want? A thermistor circuit? A microprocessor? Something that reads a voltage and flips a relay?

Thanks,
Can you please suggest me a product in that case, I checked few out but they all seem to be expensive. Whole idea of doing this myself was to keep the cost low and get a kick out of doing it myself. Also will the mechnaical switch have multiple (and long) sensors to measure temperature of different components.
Also where will a mechanical switch fit in the circuit? I dont want anything which will go before aerocool because I will not be able to monitor temperature at all times as the switch will shutoff aercool when temperature is low.

I am no expert here, but thinking if a potentiometer coupled with n-mosfet will work here?
Please advice and keep on sharing new ideas.

 

So, you just want the fans to be ON or OFF then, right?

You realize that if the fans are off, that any temp sensor in the cabinet will only sense the temp right where the sensor is? Airflow is needed in order for the temp sensors to respond in a timely manner.

It's not going to be just a MOSFET and a pot. It will require at least a comparator, several capacitors, several resistors, a MOSFET and diode. If you want the fans to simply be ON or OFF, you'll need to discard the Aerocool 1000, and use a thermistor with the other parts mentioned above instead. Otherwise, the Aerocool will beep very annoyingly when the fans aren't responding like it thinks they should be responding.

If you want to monitor several different areas, you will need an additional thermistor, resistor and comparator channel per area to be sensed.