Author Topic: Understanding circuit for inrush current limiting  (Read 8975 times)

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Offline soldar

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Re: Understanding circuit for inrush current limiting
« Reply #25 on: December 22, 2018, 12:28:56 pm »
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. 
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Offline AngraMeloTopic starter

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Re: Understanding circuit for inrush current limiting
« Reply #26 on: December 22, 2018, 02:25:01 pm »
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.
Hi 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

Hey 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.

« Last Edit: December 22, 2018, 02:27:32 pm by AngraMelo »
 

Offline AngraMeloTopic starter

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Re: Understanding circuit for inrush current limiting
« Reply #27 on: December 22, 2018, 02:46:52 pm »
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 = 2A
Vpeak = (Vrms + 5%)x1.414 = (220 +11)x1.414 = 326Vpeak
Rminimum = 326/2 = ~162ohms

I 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
 

Offline soldar

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Re: Understanding circuit for inrush current limiting
« Reply #28 on: December 22, 2018, 04:21:27 pm »
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.

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Offline spec

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Re: Understanding circuit for inrush current limiting
« Reply #29 on: December 23, 2018, 08:58:26 am »
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.
Hi 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

Hey 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.
Ahh 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.
 
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Offline spec

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Re: Understanding circuit for inrush current limiting
« Reply #30 on: December 23, 2018, 09:04:34 am »
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 = 2A
Vpeak = (Vrms + 5%)x1.414 = (220 +11)x1.414 = 326Vpeak
Rminimum = 326/2 = ~162ohms

I 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

Your 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.
« Last Edit: December 23, 2018, 09:54:50 am by spec »
 
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Offline ArthurDent

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Re: Understanding circuit for inrush current limiting
« Reply #31 on: December 23, 2018, 06:56:35 pm »
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/ 
 
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Offline AngraMeloTopic starter

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Re: Understanding circuit for inrush current limiting
« Reply #32 on: December 23, 2018, 08:26:52 pm »
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.
 

Offline spec

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Re: Understanding circuit for inrush current limiting
« Reply #33 on: December 23, 2018, 09:45:18 pm »

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 techniques that involve jumping everything with tiny wires. I quickly discovered that some voltage regulators and opamps are not happy with that. I already have the software and I'm downloading some libraries.
Excellent 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.
« Last Edit: December 23, 2018, 09:58:10 pm by spec »
 

Offline spec

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Re: Understanding circuit for inrush current limiting
« Reply #34 on: December 23, 2018, 10:08:06 pm »
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.

Computer 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.
« Last Edit: December 24, 2018, 02:43:49 am by spec »
 

Offline Jwillis

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Re: Understanding circuit for inrush current limiting
« Reply #35 on: December 24, 2018, 01:50:36 am »
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.
 

Offline spec

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Re: Understanding circuit for inrush current limiting
« Reply #36 on: December 24, 2018, 03:10:24 am »
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.
Perhaps 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
« Last Edit: December 24, 2018, 03:53:22 am by spec »
 

Offline Jwillis

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Re: Understanding circuit for inrush current limiting
« Reply #37 on: December 24, 2018, 08:29:20 am »
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.
 
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Offline soldar

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Re: Understanding circuit for inrush current limiting
« Reply #38 on: December 24, 2018, 09:42:03 am »
Computer PSUs do not have a transformer input- they have a rectifier/ capacitor input which has quite different inrush characteristics to the OPs circuit.
I 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.

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.
I 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.  :)
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Offline spec

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Re: Understanding circuit for inrush current limiting
« Reply #39 on: December 24, 2018, 08:20:50 pm »
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.  :)
;D Ditto

But 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.
« Last Edit: December 24, 2018, 08:38:42 pm by spec »
 


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