Understanding circuit for inrush current limiting - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 05:43:30 pm
content: Hey guys,Im building a couple of linear PS and they are fairly beefy, both around 300W.The capacitor bank is also pretty hefty so ill need a inrush current limiting.On the Texas Instruments website they have the attached circuit but I do not understand it.The position of that zener holding the 12V point but at the same time tied to ground doesnt make sense to me. Also the way contacts 1 and 3 of the relay are tied up together seems very weird.Im not criticizing the circuit I just dont understand it.So why is diode 103 there? Wouldnt it be simpler to have the anode of the tied to ground (with a current limiting resistor)?Can someone help me with this, please?https://e2e.ti.com/blogs_/b/powerhouse/archive/2015/03/31/powertips-how-to-limit-inrush-current-in-an-ac-dc-power-supply

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 06:22:36 pm
content: So this is my case:I have a transformer that is rated at 25A and has 21V on the secondary. It has 30,000uF capacitor bank.So the inrush current would be Vrms minus the diodo drops and that divided by my series resistance right?Then we get 30V - 2V divided by 82R/10W resulting inrush a current of around 340mA. Now, I used that value because thats what I have for 10W resistors. Will that be enough? If I use the circuit above and after a couple seconds trigger the relay and bypass the resistors wont I have a huge current dump?Or should I use a lower value like a 10R/10W (that I also have laying around) and use a 555 to wait one sec and then trigger the relay and ignore the power on the resistor given that it is just for 1 second?That would give me still only 2.8A of inrush current and Im not convinced that bypassing the 10R resistor after one second wont give me a huge current spike.

author: Benta
author_rank: Super Contributor
timestamp: December 19, 2018, 06:36:55 pm
content: As it is not apparent what "GND2" and "12V2" is, impossible to say. More info is needed to answer your question.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 06:38:53 pm
content: Quote from: Benta on December 19, 2018, 06:36:55 pmI cant, that is all it is provided on an article title"Power Tips: How to limit inrush current in an AC/DC power supply"The link is herehttps://e2e.ti.com/blogs_/b/powerhouse/archive/2015/03/31/powertips-how-to-limit-inrush-current-in-an-ac-dc-power-supply

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 06:43:30 pm
content: But that is fine, I think the main idea of the circuit is to bypass the resistor after some time has passed and some current has raised the voltage of the caps.So what am I searching for here? Is there a specific current that I should let pass or a minimum voltage for the caps to reach?What is the science here?

author: rstofer
author_rank: Super Contributor
timestamp: December 19, 2018, 06:52:42 pm
content: D103 seems to be a kickback diode to dissipate the collapsing field of the relay when it is de-energized.Inrush current of a transformer can be determined by measuring its DC resistance. You shouldn't be surprised if it is 10 times nominal full load.I wouldn't spend much time worrying about the rating of the inrush resistor. It will be switched out of the circuit soon after turn-on. The resistor value isn't magic either. Too low and the inrush will still be high, too high and the downstream circuitry won't get fully past inrush. I would have a tendency to build it the way it's shown unless I can prove it won't work - and I can't.That strange relay connection doesn't do anything, the designer figured he had to do 'something' with the unused contact and that's what he chose.There's no charging resistor shown for C109 so the relay may pick up very fast depending on the output impedance of the 12V2 source. I'm not sure where that source comes from and how long it might take to have enough voltage to pick up the relay. If 12V2 is derived from the downstream PS then it will take some time (maybe a few cycles) before it is up to voltage. That's pretty much what you want!

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 07:00:58 pm
content: Quote from: rstofer on December 19, 2018, 06:52:42 pmIm sorry if I was not clear, that is not my circuit. that is just the example shown by texas of how to limit inrush current. My situation is completely different. I described it on reply#1

author: rstofer
author_rank: Super Contributor
timestamp: December 19, 2018, 10:34:31 pm
content: No, it wasn't clear. You linked the TI paper and embedded the schematic in your first post. Your specific questions in the OP were all related to the TI diagram.Then, in your Reply #1, you talk about the resistor in terms of the secondary voltage as though that is the voltage seen by the resistor. Are you just trying to throttle the capacitor charging current? Or the transformer inrush?

author: rstofer
author_rank: Super Contributor
timestamp: December 19, 2018, 10:45:32 pm
content: As to charging the capacitors (if that's what you are limiting), Tau = R*C and it takes 5*Tau to get close to the final voltage.Tau = 82 * (30,000 *10-6) = 2.46 seconds5*Tau = 12.3 seconds.

author: floobydust
author_rank: Super Contributor
timestamp: December 19, 2018, 10:50:24 pm
content: That TI circuit is wrong, two 75k ohm resistors R104, R105 in series with a 12V 30mA relay coil???Needs over a kV to pull in!

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 11:36:13 pm
content: Quote from: rstofer on December 19, 2018, 10:34:31 pmYou are absolutely right, my post was a mess. Sorry about that. At first it seems to me that the 30mF capacitor bank is a much bigger problem than the transformer inrush current, I could be totally wrong so please point that out if it is the case.I dont see a lot of inrush current limiting resistors in simple project schematics that use a transformer. Are they always needed?if we consider that a load wont be connected to the supply as it turns on then it doesnt really matter the power rating of the transformer, right? The transformer in that case would pull the same current be it a 1W transformer or a 1000W transformer, correct?Now, if we consider a capacitor bank that size as a load then I think it might be usefull to limit the transformer inrush current, but Im kinda talking out of my ass now. I think my question is this.Given that Im building a linear PS and the capacitor bank is rather large, what is the considerations about inrush current?I did some research and it seems that limiting it with a power resistor and then bypassing the resistor is the way to go on PSs with the power rating Im working on. Other sources suggest using a NTC but it gets reaaaally specific and I dont have a large selection to choose from where I live.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 11:40:42 pm
content: Quote from: rstofer on December 19, 2018, 10:45:32 pmI did not know about that! that is awesome! thank you!So do I need to wait for 5*Tau or can I have a minimum charge to trigger a bypass relay?I guess it is like you said on reply#5, that the resistor value isnt magic. So what could be a common sense approach to that?If Im expecting them to charge up to around 30V, should I let the relay go around 25V?

author: rstofer
author_rank: Super Contributor
timestamp: December 20, 2018, 12:49:24 am
content: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.htmlThe charging current falls off pretty fast. Again, Tau = R*C or 2.4 secondsI = V/R*(e-t/Tau) so it will be reduced to 37% in 2.4 seconds. The maximum current occurs at t=0 and is equal to V/R.You know the maximum inrush, V/R, now all you need to decide is how long should you wait. I would think 1 Tau is about right.Assuming you are just limiting the capacitor current.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 21, 2018, 12:22:52 am
content: Quote from: rstofer on December 20, 2018, 12:49:24 amOne last question,If I use the resistor and relay circuit on the primary side of the transformer will I achieve inrush current limiting for both capacitor bank and transformer?

author: rstofer
author_rank: Super Contributor
timestamp: December 21, 2018, 01:00:14 am
content: Quote from: AngraMelo on December 21, 2018, 12:22:52 amYes... It's hard to say, without seeing your circuit, just how effective the current limiting will be. Your transformer is rated about 500 VA and at 120V that is about 4A full load. Starting inrush MIGHT be as high as 10x or 40A. But that would only last a couple of cycles. Still, the entire line voltage and the inrush current are working on the resistor. Think in terms of P = E2/R = 1440 Watts. Not for long but it just seems like the wrong way to do things.You could model your design in LTSpice. You'll have to search the web to find a transformer model, there isn't one in the default setup.Another approach:https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 21, 2018, 02:43:55 am
content: What kind of transformer do you have?Toroidial: high inrush current, low leakage. A double-whammy."Shell" (conventional, windings in layers on top of each other on an E core usually): modest inrush, modest leakage (still fairly low). Very common. No one really cared, back in the day; why do you?*Bank wound (bobbin is divided, windings are side by side): Low inrush, high leakage. Very easy to start even a high capacitance load... but, the voltage droop under load is higher (poorer regulation), and then you need more headroom for a given output voltage at max current.*Which is a good question, why? If just for curiosity in avoiding that turn-on "clunk" or dimming of the lights... ? (I mean, don't forget that doing it as an exercise, so you know how to do it when you finally need to, has value!) Is it to meet a standard? Is it even more practical than that, e.g. you've had inrush knocking out your household appliances or generator or something, and want to make something that'll play nicely in that limited environment?Anyway, regarding startup resistors: the power rating needs to be adequate to handle the startingenergy. That's contradictory (power is not energy), so bear with me. Generally, more powerful resistors are also bigger resistors. For example, a 100W resistor is usually good for 10-100J (5ms pulse). The dissipated energy is more-or-less equal to the stored energy in the capacitors.So, 30mF at 30V is 13.5J, so you should expect to dissipate about as much in the resistor during startup.Probably a 10 to 50W resistor would be adequate, depending. (I'd say, check the datasheet, but often, they don't say -- and then you need to read a lot of datasheets to get a feel for what resistors of a given style/type may be capable of. Yeah, it's tedious; well, in short, that's one of the things I get paid for...)Now, how to control the resistor? Preferably, it should be in circuit for:1. Only a brief moment, during startup;2. Until the output is fully charged (say to 90% of nominal); and3. Removed from circuit if condition 2 is not met in the expected time frame.Why 3? Consider what happens if 2 never occurs, or the bypass relay never closes. That poor resistor is left supplying the entire circuit in operation. It quickly burns out (hopefully without making a smoking or sparking mess -- vitreous/enameled resistors are a good choice here), leaving you with a broken supply. Or, it burns out somehow, and the relay (which would be timed in this case) closes anyway, dumping inrush current to begin with, but also slowly damaging its contacts due to the surge current.This is easily arranged with a few more transistors, resistors and capacitors.You don't have to go the extra mile in this case -- I wouldn't be afraid to use the first attached circuit, with suitable changes of course (the resistors as all have noted). That implements #1 and #2. This final #3 step is only what you want for industrial reliability.Tim

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 21, 2018, 03:32:26 am
content: Hey Tim,Great considerations! I was thinking about using a 5W because Im using a 1.2sec delay to bypass the resistor and in that time the resistor does not show any signs of getting even remotely warm. But given your message Ill stay on the safe side and use a 10W.Now, I dont think I have enough skills to make a circuit to save the PS in case the relay goes to shit.Would you point me in the right direction so I could start researching that?thank you very much

author: soldar
author_rank: Super Contributor
timestamp: December 21, 2018, 07:19:24 pm
content: Quote from: AngraMelo on December 19, 2018, 05:43:30 pmIt is quite simple. Initially R103 limits the inrush current. When the PSU is outputting 12V then relay K100 is activated and shorts the resistor.D102 and D103 are there to absorb the flyback current when the 12V is switched off. Not entirely necessary IMHO.IMHO that circuit is good but overengineered and can be simplified quite a bit. In fact, most computer PSUs just use a NTC to limit inrush and, depending on your needs, that may be enough for you too.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 21, 2018, 07:50:10 pm
content: Quote from: soldar on December 21, 2018, 07:19:24 pmSoldar,what is that zener doing there? if you hold both ends of the coil at 12V potential how can there be current flow?Also, 2 75k resistors in parallel gives the coil a current of 0.3mA, you would need 1.2kV to drive that relay!I still have no idea of what is going on.

author: soldar
author_rank: Super Contributor
timestamp: December 21, 2018, 08:15:12 pm
content: Yeah, I overlooked the two 75K resistors which make no sense and should be shorted.The diode is there to dissipate the energy stored in the relay coil when it is turned off. It is a very common and standard protection. The Zenner serves the same purpose and can be omitted and the circuit would still work correctly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 21, 2018, 08:24:52 pm
content: The zener and diode aren't necessary for a 12V supply, but they are helpful for a switched source. It's an appnote with other apparent errors, don't read into it too deeply.Tim

author: soldar
author_rank: Super Contributor
timestamp: December 21, 2018, 09:02:54 pm
content: Yeah, it seems like they extracted that from somewhere and did not adapt it correctly. I would ignore it as there are simpler solutions.For 300 Watt probably a NTC would suffice. Most computer PSU are of higher wattage and just use an NTC.If you want to go the relay way you can just power the relay from the filter caps. Suppose the input is 230Vac. Once rectified you will have in the order of 300Vdc. You could use a relay for 200~300 Vdc, maybe with an appropriate resistor in series. The relay will be open until the caps voltage reaches a certain point and will then close.A relay solution is more intellectually "better" but a relay can give you problems much more probably than a NTC.In any case you need a good filter to block noise and transients in both directions.If cost and space are not at a premium you can look into fitting a filter with some serious inductance which would, not only seriously filter the regular current but also the inrush. I have occasionally used the primary or secondary of a transformer for this purpose.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 22, 2018, 01:50:03 am
content: thank you guys for helping.What I would like to know is, if I use a NTC or the resistor/relay on the primary side of the transformer will I effectively be limiting the inrush current for the transformer as well as the capacitor bank?It is way easier to find a NTC (and relay) to the primary side currents then the secondary.

author: soldar
author_rank: Super Contributor
timestamp: December 22, 2018, 08:36:53 am
content: Quote from: AngraMelo on December 22, 2018, 01:50:03 amI am confused by your post. What do you mean?

author: spec
author_rank: Frequent Contributor
timestamp: December 22, 2018, 08:54:20 am
content: Quote from: AngraMelo on December 22, 2018, 01:50:03 amHi AngraMelo,Yes, in most cases the NTC thermistor is placed on the mains side of the PSU transformer and it limits the power to all circuit elements after the NTC thermistor.Bear in mind that the NTC thermistor will run very hot and should be mounted off the PCB and with a length of self-lead so that the NTC thermistor heat does not crystallize the solder joint or discolor the PCB. You can get stand-off ceramic beads for this purpose. Sometimes high melting point solder is used for soldering thermistors and other power devices to a PCB.Also bear in mind that the NTC thermistor, even when at a high operating temperature, will add resistance to the primary winding which will contribute to the overall resistance/impedance of the secondary winding.There are a whole range of NTC thermistors available and there will be one to suit your application just fine. But unless you are using a torodial transformer, in-rush current control may not be required.https://www.digikey.co.uk/products/en/sensors-transducers/temperature-sensors-ntc-thermistors/508?FV=fff803f7,fff80044

author: soldar
author_rank: Super Contributor
timestamp: December 22, 2018, 12:28:56 pm
content: Pretty much all computer PSUs add inrush limitation and line filtering and I would include it in any design of mine. Filtering may be legally required but, in any case, protects the device from external transients and noise and also prevents transients from the device from going out.I do not remember when I last bought any components for this because I have a crate full of old PSUs and I just get my components from there.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 22, 2018, 02:25:01 pm
content: Quote from: spec on December 22, 2018, 08:54:20 amHey Spec! Im trying to find a way to solve this problem by using readily available components. I cant use digikey/mouser, the importing process for my country is extremelly expensive, complicated and it might result in me purchasing the components and it getting stuck in customs for no reason.For you to have an idea, our local "component distributor" has 12 types of NTCs only. Thats it. I mean that is his whole selection of NTCs, 12 of the same no-name brand with different initial resistances at 25 degrees.So even though I went to the trouble of making all the calculations based on the links that were sent me here (which by the way are great and I learned a lot) it still serves me no purpose because I dont have access to an array of NTCs to choose from. So all my projects need to be "bent" so I can use what is avaliable in the market.My concern is that I might easily solve this problem with the relay/resistor system on a 30Vdc power supply, but I have a couple of 65Vdc that need the same treatment, now trying to find a relay that can take that is not easy. So if I can to things at the primary side I would be dealing with 220Vac and smaller currents which gives me a bit more freedom when choosing m components and making my projects.So if I limit the inrush current on the primary side using the relay/resistor system I would achieve the same goal with easy to get components.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 22, 2018, 02:46:52 pm
content: So for example,I have a 700W transformer, 220ac primary and 2 secondary winding, each 45Vac capable of 7.5A each.If I want to limit the inrush current for the primary side, what I have available is this:Fist my math: Max allowed peak current on primary = 2AVpeak = (Vrms + 5%)x1.414 = (220 +11)x1.414 = 326VpeakRminimum = 326/2 = ~162ohmsI would have to bite the bullet and go for the 100R NTC, which has no markings, no brand, is 5mm (I have no idea how the size comes into play, probably heat dissipation but with no datasheet no way to be precise)And there is our candidate:https://proesi.com.br/termistor-ntc-100r-5mm.html

author: soldar
author_rank: Super Contributor
timestamp: December 22, 2018, 04:21:27 pm
content: The value of the NTC is not critical and probably anyone from a junked computer PSU will do the job. The junk is a great source of free components.The solution with resistor and relay has already been mentioned.Another thing I have done is to install a sliding switch which first connects the transformer through a resistor and then directly.And as a last resort you could install no filter and no inrush current limitation. A rather long, thin power supply cord could provide some resistance while being able to dissipate some heat.There are many ways to deal with this and it comes down to what components you have access to.

author: spec
author_rank: Frequent Contributor
timestamp: December 23, 2018, 08:58:26 am
content: Quote from: AngraMelo on December 22, 2018, 02:25:01 pmAhh right- got the picture.The answer to your question is yes. Whatever system you use to limit the inrush current, the components that follow will have limited inrush.

author: spec
author_rank: Frequent Contributor
timestamp: December 23, 2018, 09:04:34 am
content: Quote from: AngraMelo on December 22, 2018, 02:46:52 pmYour calculations are probably fine, but you are being over-cautious when you say that the maximum peak current allowable is 2A. Transformers, and many other components, will take quite high peak currents, for a short period without a problem. So, if I were you, I would try your circuit with just a fuse and see how it goes.By the way, computer PSUs and the like, do not have a transformer input. Instead they have a diode rectifier feeding a large reservoir capacitor to generate 309V DC peak from 220V RMS AC input. The 309V is then fed to a switch mode PSU circuit, which switches at 50kHz to 4MHz and produces the various DC supply lines required by the equipment. In computer PSUs the SMPS is nearly always a fly-back type.The bottom line is that the reservoir capacitor has a practically zero impedance so, with that architecture, it is essential to limit the inrush current to protect both the rectifier diodes and reservoir capacitor.About your unknown thermistor, you can can determine its characteristics by measurement. The tricky bit is determining the thermistor maximum working voltage, but even that is not too much of a problem, but you would not get a definitive answer for all thermistors of that type.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 23, 2018, 06:56:35 pm
content: Here is a circuit to limit inrush current to a transformer using an NTC/relay and it explains why the relay should be included in the circuit. Good explanation of the circuit and hints on how to adapt it to other situations. Most of the previous circuit was a line filter for connecting to a transformerless or SMPS.https://www.instructables.com/id/Controlling-the-Inrush-Current-Required-by-Large-T/

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 23, 2018, 08:26:52 pm
content: For a beginner, it is hard to know what is actually critical and what isnt.Great to hear that I if I pick more or less the right NTC I could get away with it, Ill get some, make the tests and come back to you guys.I heard all sorts of things like: inrush current will slowly kill your capacitors and transformer...So I ended up considering this a situation that needed a "precise" solution, when in fact, it doesnt.I guess if Im in the right ball park of current limiting it would be way better than just having the Ac line dumping current directly.So Ill make do with what we have on hand.PS: Spec, there is no way around it, Ill have to learn Eagle. Im having a lot of problems with undesired oscillations and Im pretty sure it´s due to my horrific board layout tecniques that involve jumping everything with tiny wires. I quickly discored that some voltage regulators and Opamps are not happy with that.I already have the software and Im downloading some libraries.

author: spec
author_rank: Frequent Contributor
timestamp: December 23, 2018, 09:45:18 pm
content: Quote from: AngraMelo on December 23, 2018, 08:26:52 pmExcellent move learning EAGLE (version 9.2.2 now). It is very easy to use for doing circuits once you get to know its funny ways.Yes, a good layout and decoupling are vital for many high-gain functions like opamps, power amplifiers and PSUs. A good layout etc is also imperative for high speed circuits, both analogue and digital. And when you get to microwave frequencies, even 5mm of extra track/wire will wreck the performance. It amazes me that opamp circuits, for example, ever work when you consider that a typical opamp has a gain of 120dBs, which is a voltage gain of one million. That means that 1uV at the input would result in 1V at the output. You inject 1uV into a circuit just by looking at it.Luckily, there are some simple rules that are easy to understand and do, that will ensure that you get the best performance and minimum problems.But, apart from simple circuits, you nearly always get problems with the first build.

author: spec
author_rank: Frequent Contributor
timestamp: December 23, 2018, 10:08:06 pm
content: Quote from: soldar on December 22, 2018, 12:28:56 pmComputer PSUs do not have a transformer input- they have a rectifier/ capacitor input which has quite different inrush characteristics to the OPs circuit.Yes, it is amazing the components you can get from junk, and computer PSUs are a rich source. One of my favorite sources of components are the bases of failed EL ceiling lights: capacitors solid and electrolytic, transistors, inverter transformer.

author: Jwillis
author_rank: Super Contributor
timestamp: December 24, 2018, 01:50:36 am
content: Inrush current is more damaging to the rectifier and can shorten the life of the filter capacitors.Can also be damaging to the transformer because the coil acts like a dead short until the field is created.Generally it's best to have inrush current limiting on the primary side. In your circuit place your transformer between C104 and the bridge rectifier.This picture will generally outline the operation of your circuit.The limiting resistor may be a ceramic or NTC .Ceramic will work just a well in this circuit .NTC Thermistors are not necessary in this circuit if you don't have any.The charge time of filter capacitors can be slowed by increasing the resistance of limiting resistor .Wattage can be increased with parallel resistors . For example two 20 ohm 10W ceramic resistors in parallel will be 10 Ohm 20W.If you are limited to component supply you can parallel lower wattage resistors to get the power rating you require.

author: spec
author_rank: Frequent Contributor
timestamp: December 24, 2018, 03:10:24 am
content: Quote from: Jwillis on December 24, 2018, 01:50:36 amPerhaps you are thinking of a mains transformer's magnetizing current.AFAIK, most DIY and stock transformer/rectifier inputs, as opposed to rectifier/capacitor inputs, do not have any inrush current limiting per se. Don't forget that the input voltage is a sine wave of 50Hz or 60Hz, depending on your local. It does not have an infinitely fast dv/dt. In fact, the waveform is quite slow.The primary of a mains transformer has a significant resistance, not to mention the source resistance and inductance of your mains supply and the unit's fuse, wiring and mains switch resistance. So there can never be a dead short with an implied infinite current flowing, although a torodial transformer's primary resistance can be quite low.If no field is generated, there will be no transformer action, and thus no induced voltage in the secondary so, thus, no current through the rectifier or into the reservoir capacitor.Note that mains transformers, diodes, and reservoir capacitors normally handle huge currents every half cycle of the input sine wave, during the conduction angle. Rectifier diodes especially are designed to handle massive peak currents for this reason.As an example, take a look at the datasheet for the humble 1N500x family of rectifier diodes: only 3 amp average current rating, but able to handle a 200 amp peak:https://www.vishay.com/docs/88516/1n5400.pdf

author: Jwillis
author_rank: Super Contributor
timestamp: December 24, 2018, 08:29:20 am
content: Your right spec.In most electronics the transformer is quite small,which is to say a low VA .So the current draw will be quite small.The inrush current becomes a problem on large VA and very large VA transformers.A transformer draws inrush current that can exceed saturation current at power up.These current transients can exceed 10 times the rated current of the transformer and 50 times the rating on a toroidal. The inrush current is most damaging in the first microseconds of start up when the field is null to the weakest and the magnitude of the inrush current depends on the point on the AC wave the transformer is switched on. At the AC peak voltage theirs no inrush on start up.Its when the AC voltage is zero on start up you get the highest inrush current that can exceed the maximum saturation current of the transformer.This all starts in the first cycle of the AC wave .1/60 of a second for 60Hz and 1/50 of a second for 50Hz and go several cycles until the transients die away.On large transformers it can take several seconds for the transients to die off and the field balances out.For rectifiers the current load of the bulk capacitors may not exceed the pulse peak for any period of time but can exceed the average power rating of the rectifier for an extended period of time until the bulk capacitors are charged.This is what can shorten the life of rectifiers.Inrush current becomes more of an issue as the power rating of the transformer increases and the size of the bulk capacitors increase.Normally transformers under 300 VA won't pose to much of a problem.Over 300 VA with bulk caps exceeding 20000uF, inrush current limiting may be something to consider.When they get as high as 800 to 1200 VA with capacitors over 50000uF then it gets really exciting.But theirs really no reason not to put current limiting in smaller power systems aside from the potential cost increase.

author: soldar
author_rank: Super Contributor
timestamp: December 24, 2018, 09:42:03 am
content: Quote from: spec on December 23, 2018, 10:08:06 pmI would say most PSUs today are switchers and have no mains transformer.I have several DELL laptops powered by external brick-type PSUs and they have no inrush protection and will blow fuses and trip breakers like crazy. About a year ago, in a hotel in London, it blew the fuse of the plug/adapter and I had to repair the fuse with a strand of wire. All this with no tools to speak of.So now all my bricks are fitted with a cord which is as thin as I can get it. This adds a certain resistance and the heat is dissipated away from the PSU. This is a very good way of limiting inrush current and also even out the spikes of current which go through the diodes in normal use. And a very light cable is a plus when traveling.I also add a NTC in the cable itself. It looks like a tiny wart.So I solve the inrush problem without even touching the PSU itself, only by replacing the main power cable.Quote from: spec on December 23, 2018, 10:08:06 pmI have crates and crates of electronic junk which I use as source for simple basic components. A basic line filter and inrush limiter is something I would not even consider designing or buying as I can get it from my junk pile. Heat sinks, diodes, transistors, caps, etc.And when I feel like killing some time and relieving some stress I will spend a while removing components from boards. Sometimes slowly and carefully with a soldering iron. Sometimes I will hold a board over an open flame and remove the components much faster.I have crates with dozens of transformers and have built countless linear power supplies pretty much with recycled junk.I really need to stop collecting junk.

author: spec
author_rank: Frequent Contributor
timestamp: December 24, 2018, 08:20:50 pm
content: Quote from: soldar on December 24, 2018, 09:42:03 amDittoBut also you find that some so called junk only has a minor fault, like a corroded solder joint, or even just a blown fuse. I got hold of a load of PAT/ground drop units at one time, which were only scrapped because they no longer complied with some new standard or another. They were in a bulletproof metal case and had a very nice 10A mains transformer. I built a Rolls Royce battery charger from one. They had no surge limiter and they made a great 'boing' sound when you switched them on, but my battery charger was still going strong after 30 years when it was crushed when a building collapsed on it.