Replacing unknown NTC for inrush current - Page 1
Forum section: electronics / repair

author: razvanm
author_rank: Contributor
timestamp: May 23, 2017, 08:17:34 am
content: 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 / 85CAny suggestion appreciated!Thanks,Raz

author: Seekonk
author_rank: Super Contributor
timestamp: May 23, 2017, 11:48:10 am
content: 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.

author: razvanm
author_rank: Contributor
timestamp: May 23, 2017, 12:27:15 pm
content: Quote from: Seekonk on May 23, 2017, 11:48:10 am@Seekonk, please read the full text, especially the second paragraph.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: May 23, 2017, 12:45:01 pm
content: 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.

author: razvanm
author_rank: Contributor
timestamp: May 23, 2017, 12:54:09 pm
content: Quote from: joseph nicholas on May 23, 2017, 12:45:01 pmjoseph 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. Seehere.

author: Seekonk
author_rank: Super Contributor
timestamp: May 23, 2017, 01:02:25 pm
content: Look for something like thishttp://www.ebay.com/itm/SCK20206-Thermal-Resistor-6A-/122204440656?hash=item1c73f3c850:g:eF8AAOSw44BYFVuLSearch 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 .

author: Kjelt
author_rank: Super Contributor
timestamp: May 23, 2017, 01:04:35 pm
content: 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=NTC10 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

author: razvanm
author_rank: Contributor
timestamp: May 23, 2017, 01:44:17 pm
content: Quote from: Kjelt on May 23, 2017, 01:04:35 pmKjelt, 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.

author: razvanm
author_rank: Contributor
timestamp: May 23, 2017, 01:46:11 pm
content: Quote from: Seekonk on May 23, 2017, 01:02:25 pmSeekonk, that looks close to the actual component. Thank you!