Electronics > Beginners
Understanding circuit for inrush current limiting
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soldar:
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. 
AngraMelo:

--- Quote from: spec on December 22, 2018, 08:54:20 am ---
--- Quote from: AngraMelo on December 22, 2018, 01:50:03 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.

--- End quote ---
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

--- End quote ---

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.

AngraMelo:
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
soldar:
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.

spec:

--- Quote from: AngraMelo on December 22, 2018, 02:25:01 pm ---
--- Quote from: spec on December 22, 2018, 08:54:20 am ---
--- Quote from: AngraMelo on December 22, 2018, 01:50:03 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.

--- End quote ---
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

--- End quote ---

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.

--- End quote ---
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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