Author Topic: Replacing unknown NTC for inrush current  (Read 2650 times)

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Offline razvanm

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Replacing unknown NTC for inrush current
« on: May 23, 2017, 08:17:34 am »
Hi,

Two days ago my 45Amp battery charger (Waeco PerfectCharge IU4512) died while charging an empty AGM DeepCycle battery rated at 260Ah. I have recently bought two such batteries and using my DC electronic load I discharged each to find out the true capacity (100% until 10.8V).
While the charger was able to fully recharge the first battery, 10 minutes after hooking it up to the second one, it blew the fuse (6.3A @ 250V) and also tripped the main breaker for that rail. I replaced the internal fuse and it blew it again.

Long story short, I started this repair and found out the bridge rectifier (KBPC2508) was shorted (neg. to AC diode). Replaced the bridge with a new one and higher ratings (25A @ 800V vs 50A @ 1000V) and I was good to go. The unit works but each time I start it, the NTC sparks a little. The ceramic surrounding has a tiny crack, about 5mm long, so I put a shrinking tube over to avoid other strange things like arcing with other components nearby.

I have no service manual and on the NTC component there are no writings, other than "NTC" marked on the board.
How would I find the correct value for it? It measures 9,4Ohm @ 19C but I have no idea about the AMP rating. Picture attached below.


BTW, the capacitors are 2 x 1000uF / 200V / 85C

Any suggestion appreciated!

Thanks,
Raz
« Last Edit: May 23, 2017, 08:21:37 am by razvanm »
 

Offline Seekonk

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Re: Replacing unknown NTC for inrush current
« Reply #1 on: May 23, 2017, 11:48:10 am »
That is the least of you problems and if it reads 9 ohms it probably survived.  Measure the resistance of the power FET G-S.  The gate almost always shorts.
 

Offline razvanm

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Re: Replacing unknown NTC for inrush current
« Reply #2 on: May 23, 2017, 12:27:15 pm »
That is the least of you problems and if it reads 9 ohms it probably survived.  Measure the resistance of the power FET G-S.  The gate almost always shorts.
@Seekonk, please read the full text, especially the second paragraph.
 

Offline joseph nicholas

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Re: Replacing unknown NTC for inrush current
« Reply #3 on: May 23, 2017, 12:45:01 pm »
The ntc's I've seen in ATX power supplies are all black.  Are you certain the part is correct?  Just to add more uncertainty to the mix.  I came across a fuse in my oscilloscope with a specification for size and voltage but no amp rating.
 

Offline razvanm

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Re: Replacing unknown NTC for inrush current
« Reply #4 on: May 23, 2017, 12:54:09 pm »
The ntc's I've seen in ATX power supplies are all black.  Are you certain the part is correct?  Just to add more uncertainty to the mix.  I came across a fuse in my oscilloscope with a specification for size and voltage but no amp rating.
joseph nicholas, on the PCB silkscreen it is marked as "NTC" and the symbol "-/\/\/\-" but the component itself has no text on either sides. I agree, most ntc's I've seen come in black but there are also green ones. See here.
 

Offline Seekonk

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Re: Replacing unknown NTC for inrush current
« Reply #5 on: May 23, 2017, 01:02:25 pm »
Look for something like this

http://www.ebay.com/itm/SCK20206-Thermal-Resistor-6A-/122204440656?hash=item1c73f3c850:g:eF8AAOSw44BYFVuL

Search your sources for "SCK resistor"  the last digit is the current. Found a lot of 4A which may be sufficient with a 6A fuse.  Preceding 20 is the ohms. Just assuming 10 ohms. As low as 4 ohms would work.  Some even just install a 1-2 ohm resistor, only to limit capacitor inrush current .
 

Offline Kjelt

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Re: Replacing unknown NTC for inrush current
« Reply #6 on: May 23, 2017, 01:04:35 pm »
So if the fuse is 6A the nominal current will be what something like 2 - 3A ?
So why not just go for a 6A ICL-NTC thermistor, special subcategory of NTC:

http://uk.farnell.com/w/c/circuit-protection/thermistors/ntc-thermistors/in-rush-current-limiting-icl-ntc-thermistors?st=NTC

10 ohms and 6-7A you have ten choices left:
http://uk.farnell.com/w/c/circuit-protection/thermistors/ntc-thermistors/in-rush-current-limiting-icl-ntc-thermistors?zero-power-resistance-at-25degc=10ohm&maximum-steady-state-current-at-25degc=6.4a|6.5a|6a|7.5a|7a|8a&st=NTC
« Last Edit: May 23, 2017, 01:08:01 pm by Kjelt »
 

Offline razvanm

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Re: Replacing unknown NTC for inrush current
« Reply #7 on: May 23, 2017, 01:44:17 pm »
So if the fuse is 6A the nominal current will be what something like 2 - 3A ?
So why not just go for a 6A ICL-NTC thermistor, special subcategory of NTC:

http://uk.farnell.com/w/c/circuit-protection/thermistors/ntc-thermistors/in-rush-current-limiting-icl-ntc-thermistors?st=NTC

10 ohms and 6-7A you have ten choices left:
http://uk.farnell.com/w/c/circuit-protection/thermistors/ntc-thermistors/in-rush-current-limiting-icl-ntc-thermistors?zero-power-resistance-at-25degc=10ohm&maximum-steady-state-current-at-25degc=6.4a|6.5a|6a|7.5a|7a|8a&st=NTC

Kjelt, thank you. I thought about using a value equal to the fuse rating or a bit bigger. I think I'll go bigger in order to avoid damaging the NTC before the fuse blows.
 

Offline razvanm

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Re: Replacing unknown NTC for inrush current
« Reply #8 on: May 23, 2017, 01:46:11 pm »
Look for something like this

http://www.ebay.com/itm/SCK20206-Thermal-Resistor-6A-/122204440656?hash=item1c73f3c850:g:eF8AAOSw44BYFVuL

Search your sources for "SCK resistor"  the last digit is the current. Found a lot of 4A which may be sufficient with a 6A fuse.  Preceding 20 is the ohms. Just assuming 10 ohms. As low as 4 ohms would work.  Some even just install a 1-2 ohm resistor, only to limit capacitor inrush current .
Seekonk, that looks close to the actual component. Thank you!
 


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