Thermo-fan controller circuit issues - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: December 11, 2017, 06:49:08 pm
content: I'm a little stumped with how to fix this.I have a fan control circuit for my DC Load. It "works", however under closer inspection it has issues. The main issue is that the current surge as the fan switches in, pulls down the Vcc for a few ms. This has a knock on effect across the whole circuit, including the fan controller and the DC load itself which shows the output current rise a few %.I noticed the load current changing and so probed the circuit with the oscilloscope and I can see what looks very much like a capacitor curve when the fan kicks in and when it knocks off.The circuit isalmostexactly like this one:http://www.electroschematics.com/6123/dc-fan-controller/The difference being I am using an LM393 comparator instead of (I assume) an op amp, so I need a 4.7K pull up on the output before the NPN. (I can also reverse it and use a PNP as the comparator is sink high, 0V low (or the other way around)). I also put a 22uF cap across the fan leads to cut the huge amount of motor noise on the circuit without it.If I probe the NPN base I can see a very clear capacitor charge/discharge as the fan comes on and off. This will be the cap across the fan. However I also see the Vcc level of the whole board drop a few tens of mV and come back up over a few ms. This causes all manor of stuff to happen including a rise in the load current though the DC load.I think the start up current draw on the fan is pretty huge and pulls the 22uF AND the caps in my power supply down as it kicks in. Either that or it drains it's bypass cap and Vcc has to charge it. No CC limiting is engaging on the power supply which is set at 200mA. The fan has 130mA written on it, but I haven't been able to get it to run higher than about 86mA.I tried upping the cap across the fan to 330uF but it kinda made it worse.The other prospect I'm thinking, because there is no resistance in series with the cap when the NPN powers on the cap charge pulls a surge of current along with the fan, causing Vcc to momentarily dip.However if I put a resistor in series with the fan I get a voltage drop and the fan slows down.Would an inductor help here to restrain the current flow to the cap and fan? Can I put the cap elsewhere? Can I arrange it so that the cap is pre-charged for the fan start up?Also, when I first made this circuit the comparator wasn't set up quite right and osculated violently at the threshold voltage. Didn't look pretty on the oscilloscope, but what it did do was effectively PWM'ing the fan at threshold to kinda soft start and I didn't have these Vcc dropping issues as with the sudden start. I'm wondering if I should botch the comparator setup and see if I can get back to that osculation thing again. Sounds a bit hacky, but it worked.

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 11, 2017, 07:05:56 pm
content: You dont say what sort of fan, brushed, brushless ?Your first problem is trying to supress noise by placing a larg capacitor across the fan, it is no wonder your power glitches as when the fan is turned on this capacitor appears as a short circuit across the supply though you have not given any details of what your supply actually is. If you have noise from brushes you should only need a capacitor aof perhaps 100nF or less to take care of that.Your next problem is the circuit you have shown, I have no idea what C1 is supposed to be doing but it should not be there as it will cause the switch to be slow and potentially burn out as a result,Your use of an LM339 is fine providing your pullup provides anough current to saturate the transistor given whatever your fan current is and secondly the LM339 can sink when the fan is supposed to be off.Some people would reccomend some hyterysis in a circuit like this to ensure the comparitor switches cleanly.I hope this helps you

author: Belrmar
author_rank: Regular Contributor
timestamp: December 11, 2017, 07:12:35 pm
content: I would recommend to use an opamp connected as a voltage follower with a RC low pass filter on the + input. if you dont have an opamp capable of driving it you can always use a bjt. but this idea is not compatible with the circuit you posted

author: paulca
author_rank: Super Contributor
timestamp: December 11, 2017, 07:52:02 pm
content: Quote from: fourtytwo42 on December 11, 2017, 07:05:56 pmFan is a brushless DC 12V 130mA CPU heatsink fan (A stock OEM AMD fan). It's simply DC input, so I imagine it's a hall sensor brushless.PSU is currently my 30V/5A bench supply but when the project is finished that will become a wall wart, probably an old external hard disk 12V supply capable of something between 500mA and 1A. The circuit shouldn't need more than 200mA.Yea, I see what you mean about the cap across the fan. So how do I suppress the fan noise without placing a cap across it? Oh... if I put a resistor in series with the cap, but parallel the fan to both, that would give some resistance on charging/discharging the cap and prevent the momentary short, but still filter some of the fan noise? The noise looks like the brushless pulsing as it's frequency is proportional to the fan speed.My circuit doesn't have a capacitor on the comparator output as it has the pull up. I did put a capacitor to ground on one of the input pins of the comparator to help stop the osculations by removing noise on the input from the voltage divider which has a thermistor on a long lead sitting right beside the fan and is noisy. I looked into hysteresis but the setup required resistors I didn't have, a 10M. At the moment the comparator switching is pretty sharp and clean.The datasheet suggests a 3k pull up, I have a 4.7K, but the voltage across the fan is about 9.xV which I assumed was the NPN dropping some voltage. I'd love to get it up to 12V. Would a lower pull up help? I suppose as long as I keep the current below the sink current limit of the comparator I should be fine.On using an opamp and aiming to ramp the voltage up based on temperature I found that the fan would "cog" or "stall" and pull high current at low voltages. It needs to be hit hard to start it. It's quite old and ran in my main PC 24/7 for 4 years, so it's had a hard ole life and still works. I think if I want slow start and ramp up I will have to go PWM or make my comparator noisy and osculating again

author: paulca
author_rank: Super Contributor
timestamp: December 11, 2017, 08:09:04 pm
content: Quote from: Belrmar on December 11, 2017, 07:12:35 pmI thought you meant an opamp instead of the comparator, but you mean an opamp in voltage follower to actually drive the fan, so there is no drop through the NPN etc.Interesting idea, but I don't believe I had an opamp capable of driving 130mA. I should check though.

author: Audioguru
author_rank: Super Contributor
timestamp: December 11, 2017, 09:37:08 pm
content: Circuits from India on Electroschematics website are discussed frequently on forums because they are wrong. This one is missing important hysteresis mentioned and shown on the datasheet of every comparator IC including the LM393 used in this circuit.