at what point should I set a fan to come on? - Page 1
Forum section: electronics / projects

author: tron9000
author_rank: Frequent Contributor
timestamp: September 23, 2015, 09:32:46 am
content: I'm designing a circuit to cool a heatsink for my electronic load with a fan: see attachedFirst I'll explain:Basically its 2 schmitt triggers. The upper one is for the fan, the lower is to shutdown. I plan to "glue" a 1k NTC thermistor (beta=3480) to the heatsink. I calculated that when the junction temp of my MOSFETs (which are bolted to said heatsink) reach Tjmax= 175oC, the temp on the heatsink will be about 51oC. I'll admit now I haven't factored in the thermal time constant from when Tjmaxget to that temp to when the thermistor = 51oC.When it reaches this temp, it sinks the setpoint to the load circuit to 0V through Q6, thus shutting down the load, at the same time running the fan until the temp has come down.But best practice is for the fan to come on first to prevent over heating.So my question is what a sensible level for the fan to come on first? V_SHDN is at 1.1V so obviuosly the V_FAN voltage must be higher, but how much higher? are there any calculations/equations factoring heatsink power/temp? or is it just arbitary?cheers

author: German_EE
author_rank: Super Contributor
timestamp: September 23, 2015, 09:47:19 am
content: Opinions will vary on this topic but I was always told that if a semiconductor device is too hot to touch then it's too hot. So, I would aim for a switch on point somewhere between 40C and 50C. I just looked at the CMOS settings for the CPU fan in my laptop and it's set to start turning at 50C, go to half speed at 60C and full speed at 70C, this seems about right.

author: tron9000
author_rank: Frequent Contributor
timestamp: September 23, 2015, 09:52:58 am
content: so its finger in the wind: ~ 40C.sounds sensible. thanks.

author: codeboy2k
author_rank: Super Contributor
timestamp: September 23, 2015, 12:02:03 pm
content: a few things....1) Are you going to enclose the power devices? if you enclose your power devices (and heatsink) inside a case then you might find the ambient temperature is actually going to be 40C, inside the case, and you want to set this as your trip point. So make sure your temperature calculations have considered this possibility (using ambient @ 25C is a common error -- it's never 25C ). If you did not consider a proper ambient temperature then you might find that your heatsink choice is inadequate, or your fan choice is inadequate, and your 40C setting on your fan controller might just end up with the fan constantly on because your heatsink never gets below 50C.that's just an example of how it might turn out. So I'm just letting you (and the readers!) know about this possible trap so you can work it out during the design stage before you build it.2) using an LT1014 for a fan controller??this is an expensive op-amp for a fan controller. You can use a cheaper LM324 here, for sure, unless you just happen to already have 2 spares available because you are using the other 2 for your power FETs

author: tron9000
author_rank: Frequent Contributor
timestamp: September 23, 2015, 05:01:55 pm
content: yes thats pretty much the way I was going: the heatsink is in an enclosure, so perhaps then I'll have to jig my caculations, things look like they might get more interesting - much appriciatedyes I know, the reason behind this is that I have them in my spares, I have one LT1014 already doing the control on the load circuit. I could just use the LM324 and save it, have one of them, good thing about both of them, they have the same pinouts!

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 24, 2015, 05:41:33 am
content: On/off control is about the worst possible way to implement it. The thermal cycling will stress the solder joints and the on/off nature will make any noise far more noticeable than just leaving it on.

author: Berni
author_rank: Super Contributor
timestamp: September 24, 2015, 07:31:25 am
content: First of all if you get to Tjmax at 50 already then you probably should parallel together more mosfets to spread the load some more. You don't want to be operating them all the way to there.On/off fan control is not a very good way to do it. Its mostly used where they don't want to bother with electronics and they simply wire a bimetal thermo switch in series with the fan to make it kick on when the switch gets hot. What you really want to do is proportionally control the fan speed to temperature. For example 30°C is 0% speed. 40°C is 50% speed and 50°C is 100% speed. That way not only does the product run at a more stable temperature but the audible noise is reduced.(Its incredibly annoying to have to listen to a fan going on and off all the time, its a lot worse than having the fan continuously on)If you have PWM controllable fans this is a good place for a little 8 pin microcontroller to look at the temp sensors and generate a PWM signal(As well as a over temperature alarm signal). On regular 2 wire fans you can use an "opamp computer" to turn the temperature in to a analog control signal that you then boost up to proper current using transistors and run the fan from that variable voltage.If you don't want to go trough the trouble of that, perhaps you can do what Rigol does on there powersuplies. Since the processor running the front panel knows the output current it watches for any output to go over a few amps. When that happens it simply switches the fan from low speed to full speed. Its a little annoying but its better than cycling on/off all the time.

author: tron9000
author_rank: Frequent Contributor
timestamp: September 24, 2015, 07:59:09 am
content: Quote from: Berni on September 24, 2015, 07:31:25 amAh its the heatsink temp not Tjmaxat 50oC - thats what I calculated the heatsink temp to be at when Tjmax= 175oC, so I'm really pushing it, perhaps I should knock this down to 150oC. I have 4x fdp6670al (now obsolete but I have loads in stock) in parallel and the max total load current I've designed for is 10A (4x 2.5A)QuoteThat sounds simple to implement, the fan current at 12V supply is about 45mA (did see peak reading of 50mA - inrush probably). So the idea is to maintain a steady airflow, therefore a close-to-ambient temp throughout operation, instead of waiting till it reaches max temp and then blast it away - sounds logical, thanks

author: fivefish
author_rank: Frequent Contributor
timestamp: September 24, 2015, 09:38:21 am
content: Fan. Always. On.Maybe it's just me... but when I'm using my Eload to test my power supplies (at 1+ hours or more), I always need the fan on. There's never been a scenario where I'm using my Eload and I don't need the fan. So I ditch the idea of PWM and temperature activated fan, and just run the fan at full blast whenever my Eload is switched on.And when I'm done, and turn off my PSU (DUT), I leave my Eload still running for a few minutes, just to cool my heatsinks down to room temp.

author: tron9000
author_rank: Frequent Contributor
timestamp: September 24, 2015, 09:48:49 am
content: wow that's pretty cool!yeh I guess I'm being a bit too critical, I was just trying to use the spare op-amp.Looking at that board though, the BNC connector has given me an idea. If i just leave the fan on, then I could just use the spare op-amp as a buffer for an external control input from a BNC connector!well I guess I have some options, thanks for the replies!

author: rs20
author_rank: Super Contributor
timestamp: September 24, 2015, 09:53:05 am
content: Quote from: fivefish on September 24, 2015, 09:38:21 amWhy? Even if your fan is off; the Tj of your power devices will monotonically decrease from the moment the device is turned off... why go through a choreographed pfaff to cool it down faster?

author: fivefish
author_rank: Frequent Contributor
timestamp: September 24, 2015, 09:56:25 am
content: Why not? I want ice-cold heatsinks when I'm done.

author: Berni
author_rank: Super Contributor
timestamp: September 24, 2015, 10:41:53 am
content: Yeah in this case leaving the fans on for a bit longer doesn't really do much, but hey if it helps you sleep at night why not.I like the look of your load by the way. looks quite neat and it does seam like it could handle quite a bit of power. Extra points for finding a use for those stock intel heat sinks that i keep accumulating over the years since almost all of there CPUs seam to come with them weather you want one or not.

author: tron9000
author_rank: Frequent Contributor
timestamp: September 24, 2015, 10:49:19 am
content: whats the FET under the heatsink? I assume its under the heatsink?

author: fivefish
author_rank: Frequent Contributor
timestamp: September 24, 2015, 11:28:10 am
content: Quote from: tron9000 on September 24, 2015, 10:49:19 amThe solid copper core on the heatsink has been tapped and threaded, and the MOSFET bolted directly to it with a 4-40 machine screw.So far, I've only tried 2 mosfets with this board:IRFP250 personally tested at 180 watts, 30VDC @ 6Amps, 60VDC @ 3Amps, 100VDC max @ 1.5 AmpsIXFH50N50P3 personally tested at 240 watts, same as above, 120VDC max @ 2 Amps* runtime 1+ hour during my testing.BTW, PCBs are available on eBay for others that are interested