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For Inrush Current in Power Supply!!

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mananshah93

New Member
I am designing 7.5V , 2Amp. power supply from 230 V,50 Hz. AC using LM338,
230V to 9V ,2.5Amp. transformer...
Now, after some working out here in forum, I found my Filter capacitor value around 16667uF....
However, one other important term I come to know is Inrush Current limiter...
For, that I found two common technics 1.Resistor and 2. NTC thermistor method..

Now, the question is how to find out which value is suitable for my application???

some links for details..\
http://www.electro-tech-online.com/custompdfs/2009/08/an3001_en.pdf
http://www.electro-tech-online.com/custompdfs/2009/08/article_EP_0306_inrush.pdf
Inrush Current Limiters for 240vac | Ametherm

And datasheet of SG series surge guard..
Datasheet Archive - SG-170 datasheet - SG-170 application note - SG-170 pdf

Thanks...
 

Hero999

Banned
I wouldn't worry about it for such a low powered power supply, just use a 100mA slow blow fuse and it should be fine.
 

mananshah93

New Member
I wouldn't worry about it for such a low powered power supply, just use a 100mA slow blow fuse and it should be fine.
Can you tell me how you have calculated 100mA slow fuse for my 7.5V, 2Amp. power supply!!!
I am eager to know the calculation......

Thanks..
 

MikeMl

Well-Known Member
Most Helpful Member
Did you notice that all of these inrush current limiters are used with AC Line powered SMPSes; NOT linear transformer-powered supplies like you intend to build?

The reason is that the transformer naturally provides the current-limiting due to core saturation.

Don't worry about it; just build it...
 
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mananshah93

New Member
Did you notice that all of these inrush current limiters are used with AC Line powered SMPSes; NOT linear transformer-powered supplies like you intend to build?

The reason is that the transformer naturally provides the current-limiting due to core saturation.

Don't worry about it; just build it...

Thanks for information...but I want to know calculation for slow fuse , how 100mA slow fuse can provide protection for 2Amp. load???
 

MikeMl

Well-Known Member
Most Helpful Member
Thanks for information...but I want to know calculation for slow fuse , how 100mA slow fuse can provide protection for 2Amp. load???
Ask Hero?

Your transformer is like 10:1, so for 2A of load current, your steady-state line current will be ~0.2A. The initial surge to charge the filter cap will only last a few cycles, like 0.1sec. The peak line current will be limited by the transformer to ??? (3X to 5X the line current?). So get a 0.25A time-delay fuse that will take a 5X surge for 0.1sec.
 

mneary

New Member
Thanks for information...but I want to know calculation for slow fuse , how 100mA slow fuse can provide protection for 2Amp. load???
The 100mA slow fuse is on the 230V side of the transformer, where the current is typically 65mA plus any transformer losses.

The slow fuse should account for the inrush charging the capacitor. If your transformer isn't oversize, it'll have some internal resistance that will naturally keep the current to sensible levels.
 

mananshah93

New Member
Ask Hero?

Your transformer is like 10:1, so for 2A of load current, your steady-state line current will be ~0.2A. The initial surge to charge the filter cap will only last a few cycles, like 0.1sec. The peak line current will be limited by the transformer to ??? (3X to 5X the line current?). So get a 0.25A time-delay fuse that will take a 5X surge for 0.1sec.
I think hero99 has done calculation based on transformer 20:1 (230 to 9) so for 2 Amp. he has calculated 100mA...
 

Hero999

Banned
Yes, if the transformer is 2.5A, 9V it wil be 22.5VA

The primary current will be 230/22.5 = 97.8mA

I made a mistake, the transformer won't be perfect so the primary current will be more like 120mA so use a 200mA or 250mA slow blow fuse.
 
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