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

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

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?

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

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

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.

Okay, so I've put together a new schematic I'd like to try out and I just wanted to make sure it should work and that I've put the transistor in correctly since I've never used one before. I thought it would be a fantastic idea to put a second comparator in with a different setpoint to drive a 106dB 2.8kH buzzer (haha my parents are going to love this ;D) to go off when the temperature gets too hot (in the event the fans have failed) so nothing burns up :). The 480 ohm resistors limit the current to 25mA coming from the LM311s, which is half of its max, so I figured that would be a good value. The TIP101G is used to power the fans. I realize 4 amps is way more than I would ever use for fans, but considering it was like 50 cents more than the low power ones... :). And the buzzer I'm using is only 10mA, so the 25mA coming from the LM311 is more than enough.

Yeah I just did a little research on transistors as switches and I see what you mean, but it's all good. I'm just glad I got it working! But to be honest, I'll probably be bored in the near future and try it out :). I'm studying to be an ME, but electronics, at least as a hobby, seems to be pretty interesting! Thanks again, Adam

Finally!!! I finally have a working fan controller. And it works brilliantly. I enclosed it in a cheap box from Radio Shack and put a red LED that is always on to show power and four :) green LED's that come on when the fans click on. And there is an override switch that can turn the fans on no matter what temp the probe is at. Hero999, Thanks for sticking with me and answering all my questions. You were a huge help and I learned a ton! I attached a few pics in case you wanted to see the final product :) Thanks again, Adam

Hmm I think I see what you mean. I guess I just need the "average rectified current" to be as much as the relay coil current is, which is only like 40mA or so. And most of them seem to be at least like 50v, so that isn't even close since I'm only using 12v. I guess what worried me is the part of the project description that said "D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. For my application the 1N4148 was good enough since the tiny relay I used was only 1 amp. However, you can use a large variety of diodes here, my next choice would be a regular purpose 1N4001 or something and should be used if your relay type can handle more then 1 amp." And my relay can handle 5 amps, so I just wanted to be sure. But now that I think about it, that shouldn't really matter since it is just the relay coil in circuit with the diode, not the other contacts handling the load of whatever is hooked up. And most of the smaller diodes handle at least 150mA, way more than my relay's coil current. And you mentioned for how much time it had to handle it also factored in. I realize that it would only be for a split second in this situation, but I'm not sure if that means it can handle even more than it says for its max for such a short time? But yeah, I see your point. I'd be hard pressed to find one that couldn't handle the job.

Okay, I know I'm asking a lot of questions, but there is something else I was wondering and forgot to ask about. I couldn't find any guidelines anywhere on how to pick the protection diode that protects the transistor or IC or whatever from voltage spikes from the relay. Are there any general rules you follow? How do I know how big a diode I need for different relays? Adam

Oh okay, that makes sense with hysteresis being how much it'll swing both ways and not the total. So I'd actually need a value a bit larger than double my original value. And that's good to hear about the pot. I don't need to worry about it burning down my house :) Since I have a 15 turn type I think I'll be okay with being able to set a fairly precise setpoint, but if I had a one turn type or something similar, I can definitely see the value in making the one turn a much smaller change. It'd be like having a volume control knob, but having silent at just before halfway and blaring at just past halfway :) The rest wouldn't be usable. Well thanks again and I'll update when I get my real comparator in, probably Tues. or Wed. Adam

Nope I haven't gotten the comparator yet; I was just hoping you could double check something for me. I was reading that article you linked to more in depth and I saw the part about calculating hysteresis. This is what I got out of it and I was hoping you could confirm or tell me I'm totally wacko. From the link, hysteresis voltage = Rp/(Rh+Rp) where Rp is the resistance of the positive input and Rh is the resistance parallel to the comparator (the one added for hysteresis in the first place). I got values from the specific 50k thermistor I'm using. http://media.digikey.com/pdf/Data%20Sheets/Murata%20PDFs/NTSD1%20Spec.pdf So the resistance at 30c (86f) is 39.8k ohms. And the resistance at 25c (77f) is 50.0k ohms. Now I'm using a 100k pot on the positive input (see previous schematic) and I'll assume it'll be set in the middle (50k) since the voltage at room temp coming in the negative input should be 6v. So that should be around the threshold point. So we'll say Rp=50k ohms. The voltage coming in at each temperature into the negative input would be 12x39.8/89.8=5.3 volts for 30c 12x50/100= 6.0 volts for 25c The difference is 6-5.3=0.7 volts. So if I want the fans to turn off approximately 5c cooler than when they turn on, then Rh needs to be the following value. 0.7=50k/(50k+Rh) ----> Rh=21.4k ohms (so a resistor close to this value) How did I do? Anywhere on track? Also on a side note, right now I have the potentiometer connected from 12v to ground. Is this okay or should I add an identical value resistor on either side of it to keep it from blowing up or something of the sorts if the pot was turned to "0" ohms resistance? I realize that it would change the hysteresis.

Okay I think I am finally getting it :) Thanks for the clarification, that helped a lot! I'll have to order again... So I'll probably be able to update early next week! Yay I think I am finally going to get something to work lol. Adam

Oops, I thought it was a comparator. Is there something I'm not understanding about this project? http://www.electronics-lab.com/projects/motor_light/003/index.html He uses one, but maybe does something special with it? Also, it does output 60mA, which is enough to drive my 37.5mA (coil) relay right? LM258 - http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-1554-5-ND My relay - http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=us&keywords=PB875-ND&x=0&y=0 My circuit is activating the relay when the potentiometer is turned enough. If I'm wrong about this stuff and I end up needing the correct comparator, is everything else in my schematic good? I'll have to try replacing the transistor on the first circuit I guess. Thanks Adam