Shorting collector and emitter of a laptop fan transistor - Page 1
Forum section: electronics / beginners

author: alps
author_rank: Contributor
timestamp: September 13, 2015, 05:23:10 am
content: Hi everybody!I got a small problem concerning a laptop fan. During the first 30 minutes (more or less) it didn't work properly: it only started spinning immediately after having turned my laptop on after a system shutdown caused by the excess of CPU temperature (excess of temperature obviously due to the "absence" of the CPU fan). But then once on, the fan kept spinning for about 10-20 seconds before stopping again. So, once again the CPU temperature started rising until the forced shutdown. This kind of process occurred 3-4 times in 30 minutes. In this time span, the CPU fan seemed to be in oblivion and only worked at very high temperatures.After these 30 minutes the fan started working regularly, even for hours!Explained this problem, someone told me that it could easily depend on cold joints and a reflow of some components would have been the best solution in these cases. But I don't have a hot gun and neither do people I know. So I preferred to do some tests that someone advised.Then I managed to reset the bios but nothing changed. I forgot to say that in the first 30 minutes my motherboard often blinked (caps lock and num lock).At last, I found a quicker solution: to short collector and emitter of a darlington transistor that controls the CPU fan:Now, it would be very intesting to know any possible disadvantage of this choice, such as energy consumption, transistor lifetime (as well as the ones of the other components that might be affected by the permanent spinning of the CPU fan) and other issues that would arise.Anyway, thanks to a very small piece of copper (the end of a wire) put between collector and emitter, I tested my laptop for a few hours and it seemed that the full-speed fan didn't cause any problem and CPU temperature was well below acceptable limits.Strange as it may seem, the DDR fan didn't even turn on, perhaps because CPU was always "mild" and so processor didn't give any signal to let the DDR fan start spinning. But I did want even the DDR fan running, so I did the same trick of joining collector and emitter. Now, I have 2 fans at full speed but I need to remove the temporary piece of copper and make this modification definitive.There is just 1 mm between the 2 pins, so someone suggested to add a drop of solder in-between (without copper or any other material). Is the best solution if I don't want to waste time to unsolder the transistor? Does it guarantee a good electrical conductivity (and then a constant short between collector and emitter)? It should be a micro solder. I got a WS-70DA soldering iron that is not very precise for this kind of tasks. But I am going to practice a little bit before soldering on my motherboard.Other issue concerns the noise. With 2 fans spinning at full speed I got to wear something to protect my ears... For this reason, someone suggested to put a resistor in order to decrease a bit the speed of the fan. For a DC 5V 0.30A fan, how many ohms should I need? Consider that often the CPU fan spinned at high speed (though not full speed), when my laptop worked in normal conditions. So I cannot exagerate with the ohms. But where and how putting the resistor in such a small available space? And what kind of resistor? Only SMD resistors?

author: SL4P
author_rank: Super Contributor
timestamp: September 13, 2015, 09:53:18 am
content: Quote from: alps on September 13, 2015, 05:23:10 amSadly - you are simply avoiding the problem by hiding the symptom.The board is detecting over temp somewhere and protecting your system. Eventually, this workaround will probably come back to bite you.

author: alps
author_rank: Contributor
timestamp: September 13, 2015, 08:55:43 pm
content: Quote from: SL4P on September 13, 2015, 09:53:18 amWell, according to the last test I did, after having shorted the 2 pins, my motherboard detected no problem anymore, no more blinking. I'm led to believe that the real problem is the transistor because, once modified its normal operation and forced it to activate the fan, the motherboard cannot detect any problem that was used to detect. To use your words, I think I've hidden the symptom from the "eyesight" of the motherboard that is currently unable to recognize that problem. Now CPU temperature is quite cool, so the motherboard wouldn't have any reason to start blinking.I think this trick is working and would be very quick and cheap to make it permanent (by applying a couple of drops of solder between pins).On the other hand, an alternative, that most people advised, would be a reflow of the electrical components (in particular, of the transistor, let me say). Perhaps, at very last I should try this before the permanent modification I'm willing to do. Anyway, I would run some risks by using hot gun. If I'm not enough careful, I'm going to unsolder some of the closest components (I'm worried about the electrolytic capacitor that can be damaged by the hot gun), closest to the ones directly affected by the air flow. And what about the components on the same area but of the opposite side of the motherboard? Would they run the risk to be unsoldered and fall to the ground (for the law of gravity)?

author: Chris C
author_rank: Frequent Contributor
timestamp: September 13, 2015, 09:34:01 pm
content: If you go the resistor route:Model the fan as a resistor. A 5V 0.3A fan is 5/0.3 = 16.7ohms.Let's say you try a 3.3ohm resistor to start. New series resistance is 20ohms. Current draw is 5/20=0.25A. Total power dissipated will be 5*0.25=1.25W, with the resistor's portion being 1.25*(3.3/20)=0.21W.That's more than any SMD resistor will handle. You may also find you need a larger resistor, like a 4.7ohm, to achieve the necessary sound reduction. But a fan isn't truly a resistor, it needs and draws a higher current at startup. If you limit the power too much, it may fail to start; even if once started, it would otherwise run fine at that power. So you may have to do a little experimentation to get the right resistor value.For these reasons, I'd suggest attaching some fine wires to the transistor pads, and running them to a location where you can safely and comfortably try various resistors. Tack or tape the wire down in at least one location, to avoid stress on the solder joints. Apply tape to the motherboard under the resistor and any exposed joints to avoid accidental shorts.

author: SL4P
author_rank: Super Contributor
timestamp: September 13, 2015, 09:47:41 pm
content: ok, let me be more helpful, I thought you may have 'twigged' by now -- and you are on track.The transistor itself or physical conductors are 99% likely to be the problem.If you have the means, you could check the (gate/base) of the transistor to see that it's getting the correct PWM stream to drive the fan, but from your first comments it sounds ok.So... by shorting the (S-D / C-E) pins is simply applying full voltage to the fan, all the time.If the PWM drive is missing, then you'll have to look deeper - probably a motherboard controller, or a dedicated fan control chip.Reflowing for a single transistor sounds like overkill.. perhaps tweezers and a soldering iron.

author: amyk
author_rank: Super Contributor
timestamp: September 13, 2015, 11:48:44 pm
content: If you really want to run the fans at full speed all the time that shouldn't be a problem except for perhaps shorter lifetime of the fans, the noise, and increased dust buildup. Battery life will likely be much shorter too. Instead I'd recommend figuring out the problem itself.Try to find a schematic for the model of laptop and then you can troubleshoot the circuitry better.

author: alps
author_rank: Contributor
timestamp: September 14, 2015, 05:19:49 pm
content: Quote from: amyk on September 13, 2015, 11:48:44 pmOK! Having a simple digital multimeter and without unsoldering the suspicious transistor, what kind of tests would you recommend to do while the fan is spinning (and even immediately after the turning on of the motherboard) and while the motherboard is off?I found this schematic diagram on the internet:Which components should I check out? And how?Let me ask you a simple question that you may consider banal. The positive red probe should be connected to the pin where you want to check voltage. But the negative black probe should be connected to ground. How to identify the ground? Could it be a nearby pin? In my case, according to the above diagram of the fan circuitry, when I want to measure the voltage at the base and at the other pins (of the transistor), where can I take the ground and connect the black probe?

author: SL4P
author_rank: Super Contributor
timestamp: September 14, 2015, 09:34:56 pm
content: This type of circuit is probably the worst possible example for 'testing' with a simple DMMPretty much all you can verify is the +v an 0v rails are correct.If your DMM has a fast sample rate, and a decent bar-graph display, youmaybe able to see some action on the PWM and sense lines, but no guaranteeing they are too fast.To start somewhere, first understand what the circuit issupposedto do (it may be easier to redraw it more logically)Then, be sure that you understand what the three pins of the fan are for (not clutch, brake and accelerator!)Only Then you'll have a good starting point to understand whatshouldbe happening, what isn't happening, and why not.p.s. probing around in the tiny real-estate if a motherboard with typical fat probes is almost guaranteed to ultimately end in tears. Tread very carefully...(After you've eliminated all other possibilities, begin reading about the SMBUS on motherboards - the next level of pain!)

author: Chris C
author_rank: Frequent Contributor
timestamp: September 14, 2015, 10:27:09 pm
content: Quote from: alps on September 14, 2015, 05:19:49 pmI don't do much laptop repair, so take my advice with a grain of salt. But the very first thing I'd do is establish whether there is a signal at FAN1-ON, when the CPU is hot enough to warrant the fan being on. That would me an idea which direction to search for the issue - transistor and driver (the schematic shown), or controller.PWM is probably at 25Khz. Too high for using my usual trick of hooking up a piezo tweeter as an oscilloscope substitute. A DMM on low DC volts range will typically average the voltage. So assuming Q21 isn't failed in some way that it's shorting the base, you should see between 0.1-0.6V at the base when a PWM signal is present. Higher if you measure from the other side of R367, directly at FAN1-ON.A piezo could still be used to test FAN_SEN. Connect it between FAN_SEN and ground. Spin the fan by hand and listen for a response.Quote from: alps on September 14, 2015, 05:19:49 pmQ21 emitter is connected to ground. 's warning about accidental shorting using fat probes is wise. In this situation I'd sacrifice a spare USB cable. Cut it open to expose the '-' lead. Plug it into the laptop and connect the DMM's black lead to the '-' wire, in some way you no longer have to worry about it, and can direct your attention to the other lead exclusively.

author: poot36
author_rank: Frequent Contributor
timestamp: September 15, 2015, 05:38:11 am
content: For a ground connection for the multimeter most screw heads that hold the motherboard down are going to be ground. You can also usually clip a jumper lead to the x clamp holding the cpu heatsink down or the metal shield on any of the external ports. I would use a needle or small pin attached to a jumper lead attached to the positive meter probw to probe around with way smaller then the meter probe. By jumper lead I mean one of the ones with spring loaded clips on the ends. If your meter has a frequency setting (it may look like a square wave) use it as long as the PWM pluse it a high enough voltage it will work. I think this may be a software issue because you said that once it overheats the fan does come on. 1+ for checking the PWM output of the fan. Could cause the computer to think there is no fan connected and then not try to control it. A Mac desktop on the other hand if it detects this condition will run the fan at full speed. You can try checking the fan speeds and temperatures with a program called SpeedFan (note this program will not work on all computers).

author: jitter
author_rank: Frequent Contributor
timestamp: September 15, 2015, 07:20:50 am
content: Have you considered the fact that the fan itself maybe partially worn and only willing to spin when driven hard?Judging by the fact that the fan will spin to full speed during power on and then slow down/stop, I would consider the hole drive circuitry to be OK. I commonly find that most temperature u-controlled fans spin to full speed during power on. My PC does the same for a few seconds and then lets the fan settle down to a slow speed. A speed which I configured in the BIOS. It let's you choose min. and max. speeds and temperature thresholds.My PC doesn't do this, but I have experienced that some devices also spin up the fan shortly during power down as well.Measuring a PWM signal should ideally be done with a scope, but a DMM should show the average value. So if it were 50% on and 50% off on a 5 V supply, then I'd expect the DMM to read around 2.5 VDC.I'd probably check the laptop's BIOS first to see if there's control over the fan(s) in the BIOS. If so, set it to full speed and a very low temp. threshold.If that works, then try some slower settings and see how things behave.A partially worn fan might not want to spin slow speeds, and perhaps your repair should be the replacement of the fan.

author: alps
author_rank: Contributor
timestamp: September 15, 2015, 10:39:02 pm
content: Quote from: SL4P on September 14, 2015, 09:34:56 pmAccording to the diagram and to the picture below, would you indicate these rails and how the selector of the DMM should be set?Quote from: SL4P on September 14, 2015, 09:34:56 pmNo, my DMM is very simple and rudimentary...Quote from: Chris C on September 14, 2015, 10:27:09 pmIf I'm not wrong, the signal for temperature, coming from CPU, is converted in serial form by U27. Look at this picture taken from the service manual (http://www.datasheetarchive.com/dl/Datasheets-USER/DSAUPLD00007992.pdf) on page 88 (Socket 478 & ITP):I traced U27 on the motherboard. It should be an integrated circuit. I saw 8 pins each side, so 16 pins in total. But I'm afraid that a simple DMM isn't suitable for testing that component. Or can I do something? Someone told me that the origins of the problem can be traced in a fault of U30. I tried to find it on the motherboard but I couldn't (perhaps it's under the protective aluminium sheet that I avoided to remove). However, would you recommend some measurements for this component? Have a look at this picture from the same manual (page 109, LPC H8):Quote from: Chris C on September 14, 2015, 10:27:09 pmUnfortunately, I don't have any piezo (tweezers or anything else), but I used my DMM for measuring voltage at the base and the collector of transistor. So, as you suggested, I connected the black probe to Q21 emitter (ground) and the red one to base and then collector. At the very beginning with motherboard on, at the base I measured 0 V (when even the other smaller fan wasn't active), then 0.18 V till 0.37 V (this when the other fan was at maximum speed and the main fan still didn't spin, even if moving in small jerks), finally it reached 1.24 V (while regurlarly spinning). Then when the main fan stopped I measured 0.37 V and I noticed that the voltage got back to 1.24 when the other fan began to spin faster. Perhaps, CPU sends signals to both fans but while the smaller fan increases the speed, the main one is only able to move in jerks, even if there is the same voltage as when it's spinning. With a screw driver when I heard the other smaller fan increasing its speed, I manually helped the main fan to go and I succeeded. But then, why only after the shutdown (and an excessive temperature) does it start spinning (and at full speed)? Does this mean that the fan is faulty?Let's talk about the collector. At the very beginning with Mobo on, at the collector I measured 4.41 V, then when the other fan started spinning the voltage dropped to 0.81 V and when the other fan increased its speed I measured even less volts (from 0.58 to 0.64) and at that point I noticed that when the main fan showed signs of movement, the volts increased to 0.81 (for a fraction of second, i.e. for the duration of those signs). Finally, when regularly working it reached 0.60 V.I even measured voltage immeditely after the shutdown: 0.93 V at the base and 0.24 V at the collector.What does all this mean?Now, other components to test?Quote from: Chris C on September 14, 2015, 10:27:09 pmCould I test FAN_SEN with my DMM? There are a diode and a resistor for FAN_SEN. Should I test them? If yes, what measurements should I take?Anyway, I noticed that during the time of missed spinning (about 30 minutes), the fan moved in small jerks but without starting spinning as it should have had to. Only after the forced shutdown (caused by the excessive temperature), when I turn the Mobo on a few seconds later, the fan starts spinning (at full speed but for some seconds because then it sinks into oblivion again!), otherwise I've to manually let it go. Isn't it a kind of thermal problem? Isn't it a symptom of the so-called cold joints? Or is there a faulty SMD component (transistor or something else closer to processor, unable to trasmit the signal unless the temperature gets excessive)? Or, last, do these symptoms have the characteristics for asserting the corruption of the BIOS code? Or more simply a faulty fan?Quote from: jitter on September 15, 2015, 07:20:50 amUnfortunately, there is no control over the fans via bios.If the fan is partially worn, assuming that I don't care about laptop battery (it's exhausted since many years...), noise (unless I'll apply a resistor to decrease speed but this wouldn't be a wise choice because actually my CPU often needs the maximum speedfan) and fan lifetime, I'd prefer to short the 2 pins of the transistor more than buying a new fan (product and shipping costs, time to wait for the new fan). Then when the current fan becomes completely worn, I'll buy a new one. Even because no one can guarantee that the problem depends on the fan. Should I spend money for something that I'm not sure to be the cause of the problem?