Automatic Fan Controller - Help (w/ relay?) Please!

[abrodt]

Hmm okay I guess I'm going to have to rethink my strategy.  But if I replaced the NPN transistors with PNP transistors it would work (w/ a little reconfiguration)?

My buzzer is only 10mA, I had 500mA max on there when I had a transistor there, but I realized I didn't need it and just forgot to change it to 50mA max.  Yes, I wanted the buzzer to come on at a higher temperature, a temp that it wouldn't ever get to as long as the fans were running.

[Hero999]

The buzzer should work even if you used the LM393, sorry I didn't read the part about you saging 10mA, I only took notice of the 500mA written on the schematic.

Yes, it needs to be a PNP transistor.

It's possible to use a NPN transistor and short the base to 0V with the output of the comparator but that reduces the effectiveness of the hysteresis resistor because the output voltage will be clamped at 0.7V by the transistor's base-emitter junction, reducing the positive feedback.

I'd recommend using a MOSFET for the fans and driving the buzzer directly from the comparator output.

If you're going to use an N-type device on the output the operation will be reverse so you'll need to swap the +/- input connections but keeping the hysteresis resistor between the output and the + input. Incidentally, if the + input is connected to both a hysteresis resistor and another comparator's input, it will affect the other comparator's input so you'll need to factor in the state of both comparators when doing hysteresis calculations.

[abrodt]

Hey its me again

Well I've taken your suggestions into account and I've put together a new circuit.  I was hoping you would tell me what you think.  And please, don't feel the need to be nice.  I'm here to learn as well as have fun building it, so feel free to tear apart every little mistake (or anything that could be improved for that matter).

I've attempted to make it a linear fan controller.  The way my other controller is working (which it is doing its job very effectively) is kind of loud, and it isn't that big of a problem, but because it switches from 0V to 12V, the change (both on and off) is very noticeable, at least to me, and happens too often (every couple minutes).  (*A tip to anybody reading this that has dealt with this same issue - I have the probe taped to the top of my receiver, and I found that insulating it by taping a thick piece of cloth over the probe helped the quick switching immensely since it is protected from the cool air moved across by the fans, so temp of the receiver has more of an affect on it.)  My hope is that this controller won't switch on and off nearly as much (yes it does even with a large hysteresis that I gave it, and if I make it too large the receiver will overheat before the fans come on, or they will never turn back off), and when it is on, the fans won't be nearly as loud, unless I'm really cranking something, in which case it won't matter .  And since I do like loud music, I need the amount of fan power that I have, otherwise one fan would have been enough and would help with quick switching and loud noise.  But I've cranked music and measured the temp inside and the fans were on all the time keeping the case at about 93f (I also have a dedicated amp powering two subs in there making lots of heat ).

But anyway, I'd really appreciate a critique and if anything about what I intended the circuit to do isn't clear, just say so.

P1 sets the speed of the fans for when they come on.
P2 sets the temperature for when the fans come on.
P3 sets the temperature for when the buzzer comes on to alert a too high temperature.

Oh yeah I almost forgot, I figured I could use any mosfet that would handle enough power for my fans, but I was wondering if there were a few choices that have been proven over the years, kind of like I see everyone using the 2n2222 transistor for low power stuff.

Thanks so much
Adam

[Hero999]

The top comparator circuit has no pull-up resistor so it won't work.

What's M2 for? You only need to switch one side of the fan circuit and that's not how it should be done anyway.

Why is there a resistor in series with the buzzer?

[abrodt]

Okay, I'll add a 10k pull-up resistor.

M2 is the mosfet that controls the speed of the fans.  I was trying to figure out a way it could work and this is what I came up with, whether it'll work or not is a different story I guess.  I've seen similar schematics using this method all by itself without the complete turn on turn off part.  The comparator switches the positive input completely on or off, but doesn't make it linear of course.  So I put another mosfet on the negative input for the fans that will limit the voltage given, making it a linear response.  P1 will set what speed the fans will spin right when they click on (this would have to be the first potentiometer to set, followed by the other two), and then they will speed up as Th1 decreases in resistance in response to hotter temperatures, giving M2 more voltage.  But you said their is a better way?  I'm open to suggestions !

Yeah, I guess I don't need the resistor there.  I thought that resistor was there not only to limit the current to a transistor or mosfet, but also to protect IC from current surge or something.  But if not needed, I will certainly take it off.

As always, thank you for your help!

Adam

[Hero999]

What's the threshold voltage of M2?

The speed of the fans will depend on the temperature, M2's threshold and the temperature.

M2 will dissipate a lot of power and will need a large heat sink.

Speed control is normally done using PWM.

[abrodt]

I haven't picked out a mosfet for M2 yet, but I believe that most mosfets' threshold voltages are somewhere between 2-4 volts?  Any suggestions on a mosfet would be great.

Yes, that is why I will set P1 while Th1 is at the temperature I want the turn on to be for the comparator, so that right when the power clicks on for the fans from the LM393, they will start at the voltage I have P1 set at (taking into account whatever the threshold is for M2) for that specific temperature, and then only increase in speed from there.

I was actually wondering about the heat dissipation.  Do you think it will dissipate too much to be put into the small enclosure (w/o holes for heat to escape)?

I did actually do some research into PWM, but I didn't fully understand how the circuits worked, and this method seemed as good and is supposed to be quieter as well.  It seemed like to use PWM, an expensive IC was required too.  But do you think PWM is the way to go?

[Hero999]

The voltage across the fans will not be the same voltage as P1. The fans will only turn on when the current through M2 is high enough to make them spin. A tiny current flows at a MOSFET's threshold voltage, much too low to get the fans to spin. The threshold is also not very will controlled so the current is unpredictable.

The maximum power dissipated by M2 depends on the fans and will be equal to half the power consumption of the fans.

I strongly recommend using PWM for speed control, which doesn't heat the transistor up much.

[abrodt]

Okay, well I have found a circuit I'm going to try out.

http://pcbheaven.com/circuitpages/PWM_Fan_controller_using_a_555/

I'm going to use a comparator with a thermistor to have it turn on and off.  My one question is if you have any good ideas to make it as variable as possible in response to temperature.  I'm going to mess around with adding a thermistor with the pot to make it speed up at least some, but I figure it could only change it a little bit since it might change from like 100k to 90k for example in my temperature range.  It wouldn't be all the way from stalling to full power for sure.  Any ideas on how to modify it to get as much change as possible from the changing temperature?

Thanks
Adam

[Hero999]

Yes that would work, although I'd recommend making a few modifications:

1) Get rid of the 470uF capacitor across the MOSFET, it doesn't do any good and just reduces the efficiency.

2) Connect the MOSFET's gate to pin 3 which is a push-pull output and is better at driving a MOSFET than pin 7 which is just an open collector, hence the 1k pull-up.

3) Use a lower resistance MOSFET than the IRL520, I'd recommend the IRL540 which has a lower R_ON.

I've made a similar circuit before and it works well.

It's also possible to make a similar circuit using a comparator, allowing you to use an LM339, rather than having to use a separate IC for the PWM. It uses a few extra resistors but the IC will cost more than resistors which are cheap.

[abrodt]

Hero999

Thanks for your advice.  I finally went ahead and ordered a small breadboard so this will making trying different things a heck of a lot easier.  I'll try out your recommendations.
Just a side note, I believe you misread the mosfet name.  The one labeled is IRF520, not IRL520, however your advice is still good because the IRF520 Rds On is slightly lower than the IRL520, however still like twice as high as your recommendation.  And I would have ordered it too, but I already placed my order with digikey .

Thanks again!
Adam

[Hero999]

I made a typo but yes, both the IRL520 and IRF520 have a high on resistance for MOSFETs.

The difference between the IRL and IRF devices is the L devices are logic level, meaning they will pass the full current with a drain voltage of just 4.5V, the F devices require between 7V and 10V before the drain will achieve the lowest resistance possible.

I should've said the IRF540 because it has a lower resistance than the IRL540 and the gate voltage will be higher than 10V in yuor circuit. It doesn't really matter, even if you've ordered one of the other parts because their current rating is still high enough for what you want and the higher resistance shouldn't be an issue, it just might mean you need to but the MOSFET on a heat sink, which wouldn't be required with a better MOSFET.