Limiting start up current of a 12V PC fan - Page 1
Forum section: electronics / projects

author: paul_g_787
author_rank: Frequent Contributor
timestamp: April 09, 2022, 05:35:04 pm
content: Hi.I have a little project I am working on which turns on a fan using a transistor.The fan is rated at 12V 160mA which actually draws about 120mA in the real world once running.My power supply is an old router power supply I found which provides max 200mA at 11.8V. My circuit uses 11mA without the fan so this is perfect!However when the fan starts up it can draw anywhere between 180mA to 305mA!Using my bench supply I have confirmed that the fan starts fine every time with anything about 28mA.So my question is what would be a simple way to limit the inrush current to the fan as to not overload the power supply? A resistor in series maybe to cap it at a max of say 180mA ish?

author: bdunham7
author_rank: Super Contributor
timestamp: April 09, 2022, 05:41:21 pm
content: My first question would be what happens when you simply let it start up with the overload? Does the power supply have any issues with that?

author: wraper
author_rank: Unknown
timestamp: April 09, 2022, 05:43:09 pm
content: Quote from: paul_g_787 on April 09, 2022, 05:35:04 pmIs it rated for 200mA or goes into protection at such current? 200 mA sounds low for a router.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: April 09, 2022, 05:57:43 pm
content: Quote from: bdunham7 on April 09, 2022, 05:41:21 pmNothing happens. It just works fine but draws this much more thand rated current for a fraction of a second.Quote from: wraper on April 09, 2022, 05:43:09 pmIt is rated for 200mA yes.I was planning to connect a F200mA fuse to the power supply so was mainly worried about it blowing the fuse possibly.

author: wraper
author_rank: Unknown
timestamp: April 09, 2022, 06:00:26 pm
content: Then do not bother. Also 300mA is not enough to blow a 200mA fuse in a short time.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: April 09, 2022, 06:06:45 pm
content: Perhaps I just go with the mains side fuse only in that case.The absolute max current I get on the primary side of the power supply is 41mA, so I would say a 50mA fuse would do then.

author: inse
author_rank: Super Contributor
timestamp: April 09, 2022, 06:18:15 pm
content: The inrush current of the wall wart is guaranteed to blow your 50mA fuse every time you plug it in

author: bdunham7
author_rank: Super Contributor
timestamp: April 09, 2022, 06:25:10 pm
content: Quote from: paul_g_787 on April 09, 2022, 06:06:45 pmThere's probably no need to add any fuse if it is a commercially provided power supply. And your mains fusing would be intended to prevent a fire, not trip on some small nuisance overload and I'd expect it to be a lot more than 50mA. The short term overload is really not an issue unless the voltage sag affects your other circuitry. If you are determined to soft-start the fan, then using the existing switch transistor to ramp up the current would be the easiest way, although you'd have to deal with more heat for a few seconds.

author: coppercone2
author_rank: Super Contributor
timestamp: April 09, 2022, 06:28:16 pm
content: Well if you wanna get technical you can study fuse documents to supply a fuse that will take it. They have slow, medium and fast blow fuses. You would need to do I^2*T math and use lookup curves to find the correct fuse.https://www.swe-check.com.au/pages/learn_fuse_speed.php

author: CaptDon
author_rank: Super Contributor
timestamp: April 09, 2022, 06:30:53 pm
content: Don't worry about it. The design of those wall warts is so simple it will ride right through the temporary overload. That 200ma. rating is for continuous duty. It could easily take a 2:1 brief overload with a bit of voltage sag as the penalty.

author: CaptDon
author_rank: Super Contributor
timestamp: April 09, 2022, 06:33:01 pm
content: I'll bet at power up the router internal capacitors charging up probably looked like a dead short to that wall wart. Talk about overload!!

author: paul_g_787
author_rank: Frequent Contributor
timestamp: April 09, 2022, 06:42:13 pm
content: Quote from: CaptDon on April 09, 2022, 06:33:01 pmI don't even know what the router was so can't say. I only know it was a Netgear. It was from my old job and it came with two power supplies. Both the same rating but one had an EU plug on it the other a UK plug. so the EU one ended up in my box o' junk.I am just going to put the PCB inside my project and solder it to an IEC C7/C8 socket via a fuse holder.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: April 09, 2022, 06:52:23 pm
content: OK so just been doing some real-world testing using my ammeter on the 240V mains input side.At absolute maximum load when switched on, in a worst-case scenario it draws 420mA momentarily at power on.Once the device is powered up, during normal operation it draws 8.2mA. On it's absolute maximum normal usage once the fan is on it draws is 21.5mA (with a momentary spike of 41mA).What is the best practice when deciding a fuse here?Should I get a T50mA or a F500mA?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2022, 07:19:52 pm
content: On the mains side? So the power supply is drawing up to 100VA? Do you know that your meter can track the peaks well enough? (It's likely quite peaky due to the power supply's input circuit.) And, is that just inrush, or what? Inrush can be quite large for a few cycles (10s A, 100s even); it's fine.The power supply already has a fuse onboard, at least it damn well better if it claims to be approved by your country's authorities.As for secondary side fusing, don't bother: the supply won't deliver enough fault current to clear a fuse anyway. Such supplies are usually "hiccup mode" limiting, i.e. the controller boots up once in a while, tries to run, then runs out of steam when it gets nowhere, then stays off for a while.Also note that fuses are very crude limiting devices, typically opening at several times their rating, within a reasonable time frame (say 5-60s at 2-3x overload). They're there to protect the wiring, and act much too slowly to protect most any kind of electronics, for example. Such a power supply might have a 1A or higher fuse, more than enough to avoid nuisance failures due to inrush current, say, while still doing a reasonable job limiting energy to the device when/if it fails.Tim

author: paul_g_787
author_rank: Frequent Contributor
timestamp: April 09, 2022, 07:44:47 pm
content: Quote from: T3sl4co1l on April 09, 2022, 07:19:52 pmMy meter has a record function with min/max readings. I repeated the test multiple times and took the maximum "max" reading for the peak inrush current.Quote from: T3sl4co1l on April 09, 2022, 07:19:52 pmNo it does not have a fuse on the mains side which is why I need to add one to my project. Looking at the PCB the Live and Neutral go direct to a bridge rectifier IC BD1.There is a wire-jumper in between the Live and BD1 which has R1 10/2W written next to it so it looks like they potentially skimped on a 10 Ω 2W fusible resistor!!!I have seen this practice of not being fused on a few wall-wart supplies I have dismantled in the past which is why I don't generally trust them unless I take them apart myself and look.In the UK nobody actually checks or inspects anything these days which allows manufacturers to do dangerous stuff like this.Quote from: T3sl4co1l on April 09, 2022, 07:19:52 pmYes makes sense. I think I was over-thinking this and assuming the rated continuous amount was the peak current.Like you say, as it is a SMPS it will have OCP and switch off.I just tested this by connecting a 250mA load to it and the voltage went to 0V after a second or so and then recovered when I removed the load.Quote from: T3sl4co1l on April 09, 2022, 07:19:52 pmYes however I still think a fuse inside the device is best (and I believe a requirement) as in the UK our ring-main circuits for power outlets have a 32A Circuit breaker.Although the power lead will have the required 3A fuse in it anyway but that is not to be trusted as someone could use a lead with a larger fuse (which happens all the time).My plan is to wire my project as such:Live -> IEC C8 panel mount socket -> Fuse holder -> DPST switch -> Power supplyNeutral -> DPST switch (other half) -> Power Supply

author: magic
author_rank: Super Contributor
timestamp: April 09, 2022, 07:51:36 pm
content: I would look for a different PSU, then.They could have cut other corners too like transformer insulation.Look up DiodeGoneWild teardowns of junk USB chargers

author: paul_g_787
author_rank: Frequent Contributor
timestamp: April 09, 2022, 08:00:59 pm
content: Quote from: magic on April 09, 2022, 07:51:36 pmOther than the missing fuse it looks OK for this purpose. Theres is 18mm of isolation between primary and secondary, and an opto-coupler for feedback.Besides the casing it will go in is fully enclosed and double insulated and has no metal parts.Even if I used another power supply (which I would have to go and buy). I would not want to blindly trust it and would still think it is a good idea to install a fuse.

author: nctnico
author_rank: Super Contributor
timestamp: April 09, 2022, 08:39:36 pm
content: Quote from: paul_g_787 on April 09, 2022, 05:35:04 pmA beefy transistor setup as a current source should do the trick.Look here for some inspiration:https://electronics.stackexchange.com/questions/230701/cheap-low-quality-constant-current-sink-over-wide-voltage-range

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2022, 10:43:51 pm
content: Not to mention EMI. If they bypassed the fusible resistor, did they bypass the filtering as well?Tim

author: Ron68
author_rank: Newbie
timestamp: April 10, 2022, 06:58:13 pm
content: If you only need a short surge of DC current, you can store that in an electrolytic capacitor.Try choosing a value which keeps you below the 200mA inrush current for charging it initially, if you wish, and then see what happens when you run the fan.If you are able to store any amount of energy so you have it available beforehand, it will reduce the inrush from mains as you connect the load.If both the board and the fan are powered on simultaneously, you may split the process to get a little time to charge the cap, effecitively splitting the load, by turning the fan on a couple of seconds after the board detects power.

author: robzy
author_rank: Regular Contributor
timestamp: April 11, 2022, 02:18:49 pm
content: What's the model number written on the power supply?

author: coppercone2
author_rank: Super Contributor
timestamp: April 11, 2022, 09:36:51 pm
content: I thought the rule of thumb when a fuse inrush is greater then ten times the operating current of the fuse you need to use a slow blow fuse. I guess it depends on duration but it seems reminiscent of what people say about torroidal transformer fusing.So I would think based on stuff I have read, you want a 30% oversized fuse for 21.5mA, so that means a 30mA slow-blow fuse would be what you want? (maybe 35mA for higher ambient temperature).How well does this part do checked against a more specific analysis?https://www.digikey.com/en/products/detail/littelfuse-inc/0218-032MXEP/777567

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 24, 2022, 11:29:17 am
content: Quote from: robzy on April 11, 2022, 02:18:49 pmJust managed to find the original plastic casing for it which amalgamated with the every growing pile on my desk.On the back it says:NETGEARL.T.E. Power SupplyP/N: 332-10682-01Model: MU06-N120050-C5Input 100-240V ~ 50/60Hz 0.2AOutput 12V 0.5AIntertekI think Intertek is the manufacturer.This power supply came with an old router, it is an EU plug Wall Wart. Of course I didn't need this because I used the also supplied UK plug version so this just ended up in my box 'O junk.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 24, 2022, 02:39:13 pm
content: OK so I am re-visiting this project now as I have time.I left it at the state the project is working but I need to pick the fuse for the input of the mains power supply.It is important to note that when the circuit is running it is unknown weather the fan will either be on or off and therefore present two scenarios of load to the power supply.To take these measurements I connected the BM786 inseries with the mains Live wireof the power supply because this is obviously where I need the fuse to go.When the circuit is running in "standby" mode (just two LEDs basically) it is using a very stable 4.7mA.When the circuit is running in "on" mode (with the fan fully powered on) it is using a stable 167mASo these readings look as I expected.So next I tested using BM786 as an ammeter in record mode I took max inrush readings for both standby and on states of the circuit.In "standby" mode I had the max inrush of between 5-7mA.In "on" mode (with the fan switching on at the same time as the power supply) I got the following readings:312mA362mA326mA357mA344mA365mAWhich is an Average of 344mA inrush current (max 365mA).Would I be correct to say that these readings differ due to the power supply switching on at different points in the 50Hz Sine wave cycle, so the load is initially presented differently until the power supply capacitors charge up?So the circuit uses 167mA (let's call it 170mA) with an inrush of 365mA.And of course the power supply is rated at a continuous 200mA.Of course these readings are only as accurate as my BM786 multimeter but I think they is good enough for this case.So I am thinking that aT200mAfuse would be most suitable here.Which would make sense as 200mA is the power supply's rating.What do you all think? Does anyone agree with my choice of fuse?This is the datasheet from one of the T200mA fuses I was looking at:https://www.farnell.com/datasheets/2869805.pdfhttps://cpc.farnell.com/multicomp/mc000875/fuse-5x20mm-glass-time-delay-200ma/dp/FF03088There are also other makes of fuses available:https://cpc.farnell.com/siba/70-007-65-0-2a/fuse-20x5-ceramic-slow-blow-200ma/dp/FF02836https://cpc.farnell.com/littelfuse/0218-200mxp/fuse-glass-antisurge-200ma/dp/FF02859

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 24, 2022, 05:46:29 pm
content: What's the sampling rate of the BM786? Seems like the figure should be a good bit larger, on the mains side. But it only lasts for a cycle so most meters will skim right over it.A better test might be a (mains frequency) current transformer (or any other AC or AC/DC reading amprobe), and oscilloscope. Assuming you have both, of course...Tim

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 24, 2022, 08:56:02 pm
content: Quote from: T3sl4co1l on June 24, 2022, 05:46:29 pmThe manual says for AC record mode;Added uncertainty to specified accuracy: +/- 300dMin Amps signal duration: 460msREC nominal update rate per second: 5Unfortunately I do not have a working oscilloscpe currently.So I thought about what you said and decided to repeat the test to get a larger number of results. I used an old digital timer to turn the power on and off every 5 seconds and left it running for 2 hours.The max inrush current reading I got on the BM786 was 623mA.After more observations I found that the circuit draws approx 320mA of current at approx 1 second after power up dropping to 200mA in about 4-5 seconds (I assume due to the fan ramping up) and then dropping to a stable 167mA /168mA at approx 6-7 seconds of power on time.I already tested a T160mA fuse and that blew after about 2 mins 10 seconds of running the circuit on max load. This seems to correlate almost exactly with the datasheet curve.I am thinking as they are only super cheap (less than 20p each) I will buy some T200mA, and T250mA. (I already have in stock some T315mA and higher values).So I can just try the lower value, then switch to a higher value if need be. Then if one does blow I can compare the blow time to the datasheet curve.I am confident either T200mA or T250mA will be correct.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 24, 2022, 11:03:09 pm
content: A simple way is to use a relatively big MOSfet (some "oversized" TO220 jobbie) and connect it between the fan and GND. Also connect one side of a resistor to the gate (something like 1M Ohm is fine). Then touch the 12V rail with a finger of one hand, and the free pin of the 1M ohm resistor with a finger of the other hand. The gate will get charged slowly, and the transistor turns on lowly.When you connect the resistor to GND, it will turn off again. You can influence the switching speed with the resistor value and/or placing a capacitor between gate and source. (Between Gate and Drain may even work better).

author: mariush
author_rank: Super Contributor
timestamp: June 25, 2022, 01:48:33 am
content: Regarding fuse and currents ... keep in mind that your AC voltage can fluctuate ... at heavy usage times you could have 210..220v AC , at 3-4 AM or when there's very little usage in your area you could have 240v ACWith low AC voltage on the input, your inrush current will be higher but not by a significant amount. I would set a fuse of at least 250mA, but honestly probably would use a 400mA or even higher fuse.A lot of switching power supplies will go in hiccup mode or shut down if you pull too much current, so you won't protect your circuit this way, the psu will be happy to keep rebooting and deliver as much as it can.If something blows up inside the psu, it will most likely blow shorted, so you'll have more than 400mA or whatever fuse value you select.As for limiting the power to the fan, there's simple solutions ... could use a NTC ... has a high resistance when cold, resistance goes lower when it gets warm.but it would probably be easier to just change the power supply.

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2022, 03:02:51 am
content: Since you are already using a transistor to control the fan, I would configure the transistor to limit the current, which typically includes adding a low value of resistor in series with the emitter and limiting the base voltage. If you do this, keep in mind that the transistor will be operating in its linear region so power dissipation must be considered.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 25, 2022, 03:07:04 am
content: What sort of transistor is it now? A FET or a BJT?Also with a BJT you can probably create an RC combination at the base to switch it slower, but you can't go as high as with MOSfets for the resistor because transistors always need some base current. Using a darlington just to lower the base current may help here.

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2022, 03:11:25 am
content: That reminds me that if you are using a power MOSFET to control the fan, then an RC circuit at the gate can be used to limit how quickly the transistor switches on limiting startup current.

author: viperidae
author_rank: Frequent Contributor
timestamp: June 25, 2022, 06:50:40 am
content: Are you aware that your power supply is rated for 500mA output? The 0.2A refers to the maximum input current.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 25, 2022, 12:14:35 pm
content: You do not need to use a fuse before or after a commercial wallwart. Fuse provides nothing useful, so don't waste your time adding it. Also as others have said, these supplies can supply over the rated current for some fractions of seconds just fine. The router it powered definitely had significant capacitance and high inrush current, possibly more than your fan.If you want to improve resilience against lightning strike induced overvoltages etc., then you need something more advanced than just fuse, e.g. varistors (MOVs) plus fuse (or polyfuse).To me, your solution (wallwart + fan & other things, average consumption way below the wallwart rating, inrush 150% for way below 1 second) is just fine, no need to worry..... and if you really misread the current rating and it's a 500mA supply, then there is even less of a problem.

author: rstofer
author_rank: Super Contributor
timestamp: June 25, 2022, 02:27:34 pm
content: I would pick a time delay fuse at about 1.5 times the 500 mA wall wart rating. 750 mA seems about right. I could even go with a time delay fuse rated 500 mA and it would probably work fine. Buy an assortment and test them by shorting the power supply. Use this as a learning experience.Any device with inrush should probably be protected with a time delay fuse. Nuisance tripping is a bother...Your use of a DMM to measure inrush is dubious at best. Five samples per second overlooks a lot of detail. There could be a 100A spike lasting just milliseconds that won't be captured at all (or just get averaged into the reading over 200 ms) but it would still pop a normal fuse.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 25, 2022, 02:38:06 pm
content: Quote from: rstofer on June 25, 2022, 02:27:34 pmSwitch-mode wall-warts most often use hickup short circuit protection strategy. You can test it by shorting the output and measuring current, for example using a a multimeter with 10A range. Hickup is evidenced by very short current peaks around double-triple the rating every second or so; peaks can be so short the multimeter won't capture all of the bursts, or shows wrong value, but you'll get the idea what's happening. Duty cycle is so low there is hardly any heating in the short, and this is also why fuses never blow, so it's pointless to add one.Adding fuse makes sense if you have something iffy downstream, like you are misusing a power supply to charge a battery. Then fuse would protect against short circuit and battery sourcing current into the short - or into the supply.