How to create a threshold for voltage at the LM317's output? - Page 1
Forum section: electronics / projects

author: rmlazzari
author_rank: Contributor
timestamp: December 12, 2020, 10:31:50 pm
content: Hello, I'm putting together the diagram below found in this very forum, for controlling the speed of a fan through a 10k NTC. For that I would like to use an LM317 but I am encountering the following problems:1) the fan motor only starts to run when there is at least 4v. Does anyone help me so that when the LM317's output voltage falls below that 4v, no current goes to the fan? I thought of something like a zener but I confess that my knowledge is not enough for that.2) If the resistance of the thermistor becomes too low, the LM317 will be adjusted so that its output is higher than the supply, which in this case will be 12v. Will this not overload this regulator? Or will the output only remain at 12v and there will be no damage?Thankful.

author: Benta
author_rank: Super Contributor
timestamp: December 12, 2020, 10:52:31 pm
content: I'm somewhat at a loss understanding this circuit. Questions:1: what is the function of the LM334? Seems a waste of money and space to me.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2020, 11:07:29 pm
content: Quote from: rmlazzari on December 12, 2020, 10:31:50 pmWhy do you want to do that?It is of course possible, for example with the TL431, but I don't see any point in implementing it.https://www.ti.com/lit/an/slva987a/slva987a.pdfQuoteThe regulator will do into drop-out mode, if its output voltage is set too near the input voltage. Assuming the load is 100 Ohms, the output a voltage will be little less than 12V, probably just over 10V.

author: rmlazzari
author_rank: Contributor
timestamp: December 12, 2020, 11:10:02 pm
content: Quote from: Benta on December 12, 2020, 10:52:31 pmPlease sorry, you are right, I forgot to say that I am not using the LM334, I am using only the LM317 and all other components except the 334 and its resistor. I was wrong when posting the drawing that I took advantage of another topic. Even in this other topic,https://www.eevblog.com/forum/projects/_active_-dc-fan-controller/,there is discussion about the usefulness of this LM334 ...The real questions I have are the ones I already said: when the output voltage of the LM317 is <4v and when this regulator is set to an output higher than the input (12v).Thanks.

author: rmlazzari
author_rank: Contributor
timestamp: December 12, 2020, 11:15:53 pm
content: Quote from: Zero999 on December 12, 2020, 11:07:29 pmI can't see any use in feeding the motor with a current that it won't use. It would be a waste of current besides that it would be carrying the motor coils, wearing them out, heating them for nothing. In other words, it would be an optimization for all components of the circuit.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2020, 11:21:56 pm
content: Quote from: rmlazzari on December 12, 2020, 11:15:53 pmHave you measured the power consumption of the motor, when the voltage is below 4V?If it's a low power application, then you probably want to change other things in your design, for example avoiding generating the heat in the first place, so no fan and heat sink are required.

author: Benta
author_rank: Super Contributor
timestamp: December 12, 2020, 11:32:27 pm
content: Lots of information missing here.I suspect the fan is a type for PCs. These are typically brushless outrunners that won't work below ~4.5 V (for 12 V types).The 100 ohm resistor for the fan motor is a red herring.I love it when only tidbits of information are given, instead of describing a comprehensive scenario and phrasing proper questions.

author: rmlazzari
author_rank: Contributor
timestamp: December 12, 2020, 11:39:40 pm
content: Quote from: Zero999 on December 12, 2020, 11:07:29 pmThank you, I took a look and it seem exactly what I need!QuoteI tried to measure the resistance of the fan motor but I couldn't. Setting the multimeter for the diode test also didn't happen. I'm using a PC fan, but I'm using only the red and black wires. There is another one, blue, that I am despising. Even so, when I start the motor, using less than 4v on the variable source, it gives me a consumption of 60mA without the fan turning ...Well, I'll try without knowing that resistence and using that TL431. Thank you again!

author: rmlazzari
author_rank: Contributor
timestamp: December 12, 2020, 11:49:07 pm
content: Quote from: Zero999 on December 12, 2020, 11:21:56 pmIt's 60mA @ 3.9v ~ 4v, when it starts to run.This cooler circuit will cool a heatsink that cools an LM4780, which is two LM3886 in a single package.In the tests I did, the temperature reached 98 ° C. Since I would like the amplifier cabinet to be small, I need to use a relatively small heatsink.I believe that the TL431 can work and it is good to know that the LM317 will not suffer damage if it is set by the thermistor for output greater than input, I feel more relaxed.Thank you all

author: rmlazzari
author_rank: Contributor
timestamp: December 12, 2020, 11:55:54 pm
content: Quote from: Benta on December 12, 2020, 11:32:27 pmI love to feel confortable when I found hyper-super-masters of universe givin some attention to ignorant newbies.That fight for being right, effects phrase etc., instead of simply asking what is misanderstood may cause some noise in the subject. What is life about but trying to establish power over each other, right?Thank you, anyway. And good bye.

author: Benta
author_rank: Super Contributor
timestamp: December 13, 2020, 12:11:43 am
content: Quote from: rmlazzari on December 12, 2020, 11:55:54 pmCheers. The contributors here do this on their own time. We're not some support line that you're entitled to. We do it for fun.But if you hold back information, you're wasting our time, resulting in a very long thread with lots of unnecessary work for us (searching for application notes, looking up data sheets etc.).First, it's holding back the info that you're not using the LM334.Then, it's presenting a schematic that presents a 100 ohm resistor as the fan motor.Then, it's not telling us that it's a PC-type fan. I had to deduce that myself.I berated you for that, and I'll do it again. It has nothing to do with me or others being hyper-super wise guys, but no one can work with half-knowledge.That's being disrespectful to our efforts.You're very welcome here, but chill down a bit. And I'll be glad to help in the future.

author: aheid
author_rank: Regular Contributor
timestamp: December 13, 2020, 04:17:13 am
content: As mentioned, this sounds very much like an XY problem.https://en.wikipedia.org/wiki/XY_problemYou'll likely get better help if you fully describe the situation and what you are trying to accomplish.

author: srb1954
author_rank: Super Contributor
timestamp: December 13, 2020, 04:33:20 am
content: Quote from: rmlazzari on December 12, 2020, 10:31:50 pmPut a resistor in parallel with the NTC. This parallel resistor will set the minimum output voltage at low temperatures when the NTC is high resistance and ensure that the output voltage doesn't drop too below the minimum 4V.Put another low-value resistor in series with the NTC. This series resistor will ensure that the output voltage doesn't go over the fan's rated voltage at high temperatures when the NTC resistance is low.Suggested starting values are 27kfor the parallel resistor and 390for the series resistor.These two additional resistors will interact and you will need to interatively adjust them to get the desired voltage vs temperature characteristic. Even if you obtain the desired result in the simulation you will probably need to further adjust the values for the real world circuit as there will be an error introduced by the variability of the current flowing out of the LM317 adjustment pin. Normally you would use much lower values, of the order of 1k, for the resistor to ground to minimise the impact of that pin current variation.In any case you will need to have a minimum 10mA load current on an LM317 regulator output for it to maintain regulation. If this can't be guaranteed by the drain of the fan motor then a fixed resistor load will be required. Normally the output voltage divider resistors would be sized to provide that minimum 10mA load but then you would need to find a lower value NTC, say 1kto make that work.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2020, 11:48:27 am
content: Quote from: rmlazzari on December 12, 2020, 11:39:40 pmThe resistance of the fan will depend on the voltage and the speed of the rotor.60mA, at 4V is only 240mW and isn't enough to damage the motor, so there's no need to cut the power to it, when the voltage drops that low.

author: rmlazzari
author_rank: Contributor
timestamp: December 13, 2020, 03:02:07 pm
content: I would like you to apologize if I caused some discomfort mainly in the imprecision of the presentation of the question. Perhaps my question would be more appropriately placed on the forum for beginners.I would also like to thank you for the presentation of the TL431, which I did not know. For now I just learned the basics of this chip but I foresee that it can facilitate in many other circuits.Well ... I studied the options but the non-linearity of the thermistors and the difficulty in finding it at values ​​less than 10k in the retail and common trade makes it difficult to use both the LM317 and the TL431. Values like 10k between the Adj and Out terminals of the LM317 imply very high values between Adj and GND, which seems to me, even though I have little knowledge, that it would jeopardize the minimum current required to adjust this LM317. Perhaps the same happens with the TL431 but my knowledge of this chip is even less.So I have the impression that the best option is LM35 + LM358 + TIP122 even though this set takes up more of the already small space in the amplifier cabinet, much more than the LM317 + a few resistors ... Only the Darlington is already the size of the LM317, without considering the LM358 and its resistors. Furthermore, with this set it will not be possible to adjust the fan motor speed. But you can switch it on at 80 ° C and switch it off at 60 ° C. If there was more space, I would use TP35 + LM358 + BC548 and a relay, to avoid the loss of 1.4v in the TIP122, rs ... but as you can't have everything and life in practice, I will use TIP122.I thank everyone for their attention and again I leave my excuses for any discomfort.P.S .: Don't worry, dear Benta. I appreciate your guidance and I promise that I will disturb you and the forum with other issues, lol ... Thank you, dude.P.S.2: Excuse if I got misunderstood, that's a Google translation, I can't verify if the tech meaning are good.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2020, 04:29:52 pm
content: Why not use another TL431, with a different thermistor, to regulate the power to the fan?See figures 29 & 34 on the data sheet, page 27 & 28.https://www.ti.com/lit/ds/symlink/tl431.pdf

author: rmlazzari
author_rank: Contributor
timestamp: December 13, 2020, 09:38:22 pm
content: Quote from: Zero999 on December 13, 2020, 04:29:52 pmThank you very much, Zero999! Studying the datasheet, this idea seems to be much more suitable for this case than that of LM35 + LM358 + TIP122!I didn't realize that the TL431 can be used in place of the LM317. As they say here, in Brazil, "if it were a snake, I would be dead", lol ... I just didn't realize ...But I took the idea in figure 34, page 28, and I'm using the formula:Vout = 1 + R1 / R2 * 2.5 (fixed reference DO tl431, if I didn't get it wrong).Thinking that it doesn't hurt if the motor receives less than 4v, and that the voltage can be limited to its maximum by the TL431 itself, it is possible to work with one of the "legs" of the 18v-0v-18v source that is already there and that are going to the amplifier. The supply for this circuit would therefore be 18v.I'm not sure I'm on the right track. I'm still trying to calculate the resistors. But I was stuck in the calculation of R2. Would you help me with that?As the TIP122 causes a voltage drop of 1.4v and the motor should start to turn at 4v, the resistive network for 5.4v would look like this:Calculation of the relationship between R1 and R2.5.40 = 1 + R1 / R2 * 2.5v2.16 = 1 + R1 / R2R1 / R2 = 1.16Now let's see for the motor to receive 12v, remembering that this is the maximum voltage it can receive:13.4 = 1 + R1 / R2 * 2.55.36 = 1 + R1 / R2R1 / R2 = 4.36The thermistor I have here is 10k. I didn't find a 1k thermistor. I found only one store in my city that has a 2k thermistor, but I couldn't find its datasheet. So I'm finding it safer to try using the 10k, really. Lets see if it works...At 60 ° C, which is when the motor should start to turn, the thermistor I have is 1600ohm. And at 80 ° C, which is where the motor must stay with 12v, the resistance of the thermistor is 400ohm. So, to ensure that the maximum voltage to the motor is 12v and following srb1954's suggestion, I put a 390ohm resistor in series with the thermistor.R1, now with these 390ohm in series, looks like this:60 ° C - 1960ohm80 ° C - 790ohmFollowing the calculation of the relationship between R1 and R2 ...60 ° C - 2274ohm80 ° C - 3444ohmI couldn't get past that point. And I must round those values ​​up to findable values. Can you help me with that?

author: sam[PS]
author_rank: Contributor
timestamp: December 20, 2020, 06:24:18 pm
content: This sound to me like you try to take a shortcut that actually take longer.LM317 is no more than an opamp, a zener and a power darlington prewired for a certain application. Instead of trying to hack your way around a LM317 for some other application, i would use an opamp, a zener (or the suggested compensated refference) and a power darlington, it's much more flexible and allows you to apply straightforward electronic methods.Just my 2cts...

author: srb1954
author_rank: Super Contributor
timestamp: December 20, 2020, 10:19:15 pm
content: Quote from: sam on December 20, 2020, 06:24:18 pmTotally disagree. Your suggested approach is a lot more difficult overall. It will take considerably more design knowledge to implement and ultimately will probably perform more poorly.Comparing the advantages of your suggested approach against using an LM317:LM317 Separate components--------- ------------------------Circuit complexity Low HighPCB space Smaller LargerDesign effort Low HighTemperature stability Good Potentially better but only with a lot of design effortLine regulation Good Potentially better but only with a lot of design effortLoad regulation Good Potentially better but only with a lot of design effortRipple rejection Good Potentially better but only with a lot of design effortDrop-out voltage Better WorseShort circuit protection Included Requires extra circuit complexity and design effortFoldback SC protection Included Requires extra circuit complexity and design effortTransistor SOAR protection Included Requires extra circuit complexity and design effortThermal SC protection Included Not feasible to implement precise and fast-acting thermal protection in a discrete designFeedback compensation Simple Difficult design problemWe can see that, by using the LM317, we generally get better performance along with more features and at a greatly reduced complexity. With all the features and advantages of the highly integrated device it is pretty much a no-brainer to use the LM317 in a non-demanding application like this.

author: rmlazzari
author_rank: Contributor
timestamp: December 20, 2020, 10:43:20 pm
content: Thanks, sam ! The idea is to set up a scheme to dissipate heat in the smallest possible space. If the use of chips instead of discrete components leads to losses in flexibility, it gains in the item "size" and "simplicity in use". As size is, in this case, fundamental ... Can you realize the size of a PCB if you are going to assemble a simple and tiny 555 with discrete components?And just for the sake of documenting (who knows, someone researching might benefit?), I was almost putting together Mark Hannessy's very simple scheme (http://www.markhennessy.co.uk/microamp/design.htm), an NTC 10k thermistor, two transistors, four resistors and a capacitor, only with a small change: instead of muting the amplifier, when the temperature reached an appropriate level, the circuit would turn on the fan. If it needed a higher amperage, I would change the BC546 for a TIP125, adjusting the resistances.But then I remembered that PC processor coolers dissipate a lot of heat with - relative to audio sinks - little effort and space. I fixed the LM4780 to an old heatsink, which was here in the junk, tied an LM35 sensor right above the amplifier, practically stuck to it, and turned it on. That day, it was about 30 ° C here. With no signal to be amplified, the temperature stabilized at 45 ° C. I started to increase the volume and the temperature started to rise rapidly. When it reached about 65 ° C, I hung up. Since the heatsink I am using has a huge amount of aluminum, it takes a long time to both heat and cool. So I wait...Then I positioned the fan as shown in the photo below, about 1cm away from the heatsink, and turned it on at 5v, without turning on the amp. When the temperature in the LM35 reached 30 ° C, I turned on the amplifier again and surprisingly, the temperature remained at 30 ° C. I kept the amp without signal and the fan on for 20 minutes and ... 30 ° C.After that I injected an audio signal and increased the volume, with the fan still running at its minimum. Even more surprising, the temperature did not rise. I continued to increase the volume and the temperature was maintained. I left it there for more than 10 minutes at full volume, harassing the family and the neighbors and ... the temperature didn't go up! This with the fan just above the minimum you need to operate, with inconspicuous noise and consumption of 6mA.Conclusion: PC processor sinks don't work very well without ventilation, they have a lot of mass. However, when ventilated, even with poor ventilation, they are much, much more efficient than the common audio heatsinks. I must leave the fan running directly with these 5v, supplied by a minimum source (Hi-Link 5v), which fit perfectly, and without taking power from the source that powers both the amp and the preamp.(P.S .: Later, after this experiment I checked and the processors generate much more heat than audio amplifiers like this one. If I'm not mistaken this heatsink cooled a Pentium that generated about 65w of heat ...)I would like to thank everyone.Photos:brilliant scheme by Mark Hannessy for a thermo-sensor based on NTC 10kthe LM4780 mounted on the Pentium heatsink and a simulation of its proximity to the fanHi-Link 5v mini power supply