NTC Inrush Current Limiter - Replacement

[stenzer]

Hi,

I'm repairing a power supply board and I have noticed that the housing of an NTC, used as a inrush current limiter, shows cracks. Although it seems to be still functional, I want to replace it as I have to order replacement parts anyhow. As the NTC has been glued to keep it in place I'm not completely able to read the package marking. The closest I can came up is SCK-0510, which would be close to the ~5.5 Ohm I have measured wit a DMM. Measured diameter is ~19.5 mm and the thickness is ~4.5 mm.

The closest repleacement I can find is NTL10-005M, but it is barely available (at least at my preferred distributers). So my question would be how to select a suitable replacement? E.g. would the B57364S0509M000 fit? The resistance at 50°C as well as at 100°C of the B57364S0509M000 are similar of the SCK-0510 at the same temperature points. The most obvious differencee for me would be the maximum current of 10 A for the SCK-0510 compared to the 8.5 A of the B57364S0509M000 one. But here TDK is specifying the maximum current for the B57364S0509M000 up to 65°C compared to 25°C only for the SCK-0510. At this point I should mention the input circuitry is protected by a Time Lag (T)/Time Delay fuse with 5 A.

As I'm not really experienced with the selection of NTCs for inrush current limiting functions, I would appriciaty any guidance and input.

BR

 

[KlausST]

Hi,

marking shows:
*****
???SCK
???510
*****
And it´s green and about 20mm in diameter.
Even your measured resistance meets the datasheet value.

--> Thus I´d say it´s the SCK-0510 device.

********
For sure the NTC needs to meet the power supply requirement. Sadly we don´t have any information about input voltage/input current.

********
Regarding "glue".
The NTC is a therma device. It needs to heat up for normal operation. Thus any added thermal capacity and added thermal conductivity will change the NTC´s behaviour.
--> Thus I would avoid to add glue.
The glue will lead to reduced thermal resistance, which means it needs higher power dissipation to get to it´s working temperature and thus leads to higher voltage drop.

Klaus

 

[stenzer]

Hi,

For sure the NTC needs to meet the power supply requirement. Sadly we don´t have any information about input voltage/input current.

the most important information I have not mentioned of course .

Required power stated on the housing of the device is 500 W @ 230 VAC 50Hz.

BR

 

[KlausST]

Hi,

this means at least 2.2A at full load.
I wonder how the NTC works at no load condition. The NTC will see only a couple of mA.. resulting in low heating and rather high voltage drop.

Since I tend to over-engineer things, I probably would short circuit the NTC with a delay... especially for low load cases.

****
For your application .. I´d say the NTC selection is not very critical. As long as the continous current is specified to be 3A or more ... it will limit the inrush current anyways.
It´s main function is to protect against fuse trip on power up.

Klaus

 

[stenzer]

Thank you for the replay and your opinion!

I will use the B57364S0509M000 as replacement.