I am trying to build an easy circuit to power two fans to keep my AV equipment cool as it is in an enclosed space. I am trying to build the circuit shown here: http://www.radiolocman.com/shem/schematics.html?di=47949

The only difference is I'm using the IFR510 Power MOSFET but it suggested it as a good substitution.

The problem I'm having is when I turn it on I turn the pot down till the fan quits. I have to turn it just shy of all the way down to get the fan to stop. Once it does, I slowly heat up the thermistor to get the fan going. Once it is going I put the thermistor right next to the fan to cool it off but the fan never goes back off.

What could I do to get the fan to go back off?

 

Several questions:

How would you describe "enclosed"? Is there any opening or venting?
What are you using for a power source?
What is the rating of your fan, Voltage and current?
Do you have a heatsink on the MMOSFET?
Where are you placing the thermistor?

 

I'm not sure there's much you can do with such a simple circuit. You're suffering from hysteresis - a large band of temperature change required to change the output state of your thermostat. You can find a more elaborate, though still simple, approach here.

I'm wondering if it would help, and this is just speculation, if you were to keep the MOSFET near the thermistor, so it is also cooled by the fan. Or you might try different 510s or thermistors, if you have more of them. You might get lucky and find one that works better for you.

UPDATE: Ionic's post reminded me: You should check the voltages seen by your FET when the fan is first off and then when it turns on. The 12v power supply in a computer is not regulated well if it's not loaded, so the voltage might change, throwing off your pot setting.

 

There is a little venting but not much. I will be powering two fans, one blowing air in at the bottom right corner and then a fan blowing air out at the top left of the back panel of the entertainment center.

For the power source I'm using a 12V 2A DC power supply.

The fan came off of a computer power supply and it is not marked in any way so I'm not sure about the rating on the fan. I was actually trying to find that out when you replied.

The site where I got the schematic said that a heatsink wouldn't be necessary unless I was going to power more than 3 fans off the same MOSFET.

Right now the whole circuit is on a breadboard so I haven't mounted the thermistor anywhere yet. It will be at the top of the enclosed area to be cooled. To heat the thermistor I am holding it out and waving a lighter 10-12" below the thermistor so to gradually heat it up. Once the fan is going I move the thermistor right next to the fan to get plenty of air over it to cool it off. The fan appears to slow down but it doesn't ever kick back off.

 

Huh, your fan itself could be the problem. Operated this way, the MOSFET is not really switching on and off as it is normally intended. The current to keep your fan running is a lot less than it takes to start it. So once it cuts loose, the FET would have to have very little voltage on its gate to turn the fan back off. But for it to come back on, the gate has to go higher. The thermistor is not swinging the gate voltage far enough to get the job done.

 

Would this be a more effective circuit to use?
http://www.redcircuits.com/Page109.htm

I have all the parts except for the trimmer. I have a 10K instead of a 22K

Or another option is to add the second fan. Right now I'm just running the circuit with one fan.

 

OK, new info. I put in a second fan, this one being 12V .14A. When I heat up the thermistor the fans both come on, then the new fan goes off first with the second one stopping several seconds later. After seeing that, I took the new fan off again and tried the original setup. This time it worked. Not sure why, but after a little bit the fan does stop.

I like the idea that it stops, but am afraid to solder it up thinking it might not work once on the board. However, if it worked like this right off we wouldn't be having this discussion...

 

I don't like the idea of the fan coming to a stop.

The fan should always move some volume of air through the enclosure and over the thermistor; otherwise it will take a very long time for the thermistor to respond. Once it DOES finally respond, you may have the fans wind up spinning a good bit faster than would otherwise be necessary to get rid of the 'suddenly discovered" heat.

I have a PC that does this type of thing. It's very quiet for a long period, and suddenly the fans roar for a few moments.

I'd set up another PC to have low, constant air movement when cool; the fan acceleration was so gradual it was hardly noticeable. Far more quieter than the fan running at full speed, too.

Having at least some air moving will tend to keep the thermistor closer to the temp of the equipment, which is what you want.

 

I agree with having some air constantly moving, but only when the AV components are turned on. What I'm going to do is calibrate the desired temp first thing in the morning when the equipment has been off all night. As soon as the components are turned on they will start giving off heat and the fans will kick on. Even with the fans going I do not believe they will move enough air to get the temp back down below the desired temp until the components are turned off and the fans still run for a little while.

 

Don't forget the impact of ambient temperature. Your "room temp" might swing 15F° or more thru the year, at different times of day. Depending how sensitive your system is, that alone might be the difference between "off" and "jet engine".

 

I agree with having some air constantly moving, but only when the AV components are turned on. What I'm going to do is calibrate the desired temp first thing in the morning when the equipment has been off all night. As soon as the components are turned on they will start giving off heat and the fans will kick on. Even with the fans going I do not believe they will move enough air to get the temp back down below the desired temp until the components are turned off and the fans still run for a little while.

In this case you don't need a thermistor at all as you need the fans running all the time to keep the system as cool as you want it. Now it sounds like you need more cooling (more powerful fans). Also, your thermistor may not be rated for the correct temperature if it is needing the system to get real hot before turning on, thus making it difficult for the fans to cool it down.