The art of electronics proportional fan controller DOESN'T WORK - Page 1
Forum section: electronics / projects

author: MaxSimmonds
author_rank: Contributor
timestamp: September 23, 2021, 08:04:56 am
content: Hey guys,Apologies for the click baity title. I'm sure it does work, it's more likely my LTSpice skills.On page 38 of this pdf (which is chapter 9 of the art of electronics, page 608 in the book)https://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdfIs a circuithttps://imgur.com/a/UYidbYiI've tried modelling this in LTSpice (because I don't understand how the op-amp action works here, given that the cap blocks DC, and there's no closed loop between the NTC bridge and op-amp output) and it doesn't work. It doesn't look like the adjust pin has any voltage across it, which to me, makes sense (the negative pin of the op amp is at a greater voltage than the positive, so it's output will really be 0V. The cap won't charge up, since it's essentially grounded?)I've attached the LTspice simulation, can anyone see if I've done something wrong, and perhaps explain how the opamp works here?Thanks very much, guys, appreciate any help!Max

author: fcb
author_rank: Super Contributor
timestamp: September 23, 2021, 08:17:26 am
content: The loop is closed by the fan (speed) cooling the heatsink and thus cooling the thermistor. Difficult to model that element.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 23, 2021, 08:21:13 am
content: Quote from: MaxSimmonds on September 23, 2021, 08:04:56 amThe 10uF cap makes the OpAmp an integrator. Total feedback is closed through the fan / air flow / temperature sensor.The circuit is your commonplace I regulator.Depending on the temperature, the OpAmp "sees" a positive or negative difference on its inputs and drives its output toward the positive or negative (GND) supply. The capacitor must be charged / discharged while the OpAmp tries to do so, so it takes some time (tau = 5s as stated) for the output to reach a rail.At DC the OpAmp has its full open loop gain, while at AC (less than 1Hz here), the gain is greatly reduced by the capacitor, down to zero at a few 10 or 100Hz.The whole loop works like this: When the system temperature reaches the setpoint, the OpAmp starts do drive its output to +12V, raising the output voltage of the LM317 (will be 1.25V above its ref pin) - consider the LM317 just a "voltage follower" with its ouput 1.25V above its ref pin.Ccharging the capacitor slows that down to some 10 seconds until the OpAmp output will reach maximum Voltage. Somewhere in between, the air flow will be sufficient for the system temperature to stay at the setpoint. Eventually the OpAmp (and fan voltage) will settle at a voltage that runs the fan at a speed that provides enough airflow to keep the system at the setpoint temperature.

author: MaxSimmonds
author_rank: Contributor
timestamp: September 23, 2021, 09:23:07 am
content: Ah I think I see my issue here. My application is not suitable for this circuit, I don't think.Would the attached circuit, have the same affect? If so, I might breadboard up the one in the art of electronics and test it in real life with the thermal link to close the loop.thanks for the explanation, guys, makes a lot more sense now!

author: capt bullshot
author_rank: Super Contributor
timestamp: September 24, 2021, 06:38:14 am
content: Quote from: MaxSimmonds on September 23, 2021, 09:23:07 amNo intend to offend you, but I'd recommend to post your schematic in an instantly readable format (e.g. picture or a pdf document) - I can guess your schematic is made using LTSpice, but usually I can't be bothered to download any proprietary format document, fire up the software (or even install, LTSpice isn't installed on this computer) and load your schematic.

author: MaxSimmonds
author_rank: Contributor
timestamp: September 24, 2021, 06:44:32 am
content: I would totally agree, but to show my naivety, I can never seem to get an inline image working on EEVblog, I upload the image, I get some HTML markup, but it never shows in the preview? It's almost as if I'm missing a button somewhere.....

author: capt bullshot
author_rank: Super Contributor
timestamp: September 24, 2021, 07:02:37 am
content: Quote from: MaxSimmonds on September 24, 2021, 06:44:32 amDoesn't work anyway here ...Upload the image as an attachment, it shows up as a small image below your post, click on it to enlarge while viewing the post.Use theCode: tags to add a link to an image that gets shown inline - the link can be the link to you uploaded image (preferred here) or some external image.

author: eblc1388
author_rank: Frequent Contributor
timestamp: September 24, 2021, 07:27:45 am
content: Quote from: MaxSimmonds on September 23, 2021, 09:23:07 amOf course one can simulate that circuit in LTSPICE by another method of operating point by specifying the characteristic of the NTC resistor as an equation.The following is the two simulations I did, by changing the circuit parameters to see how the circuit performs at different temperature. Have fun.

author: eblc1388
author_rank: Frequent Contributor
timestamp: September 24, 2021, 07:47:43 am
content: Please note the Beta value of the NTC in AOE's proposed circuit is 3934.25, calculated via a web calculator using the resistance values at 25C and 60C. The link of that website is:https://www.ametherm.com/thermistor/ntc-thermistor-betaThe B=3450 used in my simulation is the value I have used in my previous simulations.