Help Identifying Burnt Resistor on NCM Battery Charger SM4830 - Page 1
Forum section: electronics / repair

author: JayCal
author_rank: Newbie
timestamp: July 28, 2024, 06:21:19 am
content: Hello everyone,I have an Battery Charger Model SM4830 (54.6V, 3A output). A component inside has burnt out, and I'm trying to identify it and find its value to replace it.The burnt component is in a slot marked "R1".It is connected to the third pin of a KBP310 Bridge Rectifier.The other pin of the burnt component goes to the positive (live) wire of the charging output.The component is white, flat, and looks like a ceramic capacitor but is in a resistor slot.I've attached some photos of the circuit board and the burnt component area. Any help in identifying this component and its value would be greatly appreciated!Thank you!Jay Cal

author: fzabkar
author_rank: Super Contributor
timestamp: July 28, 2024, 06:32:47 am
content: It looks like an NTC resistor for inrush current limiting.This is just one datasheet that I picked at random:https://www.topdiode.com/pdf/MF72-NTC-Thermistor.pdfI would check the semis in that area. These resistors don't usually fail on their own.

author: JayCal
author_rank: Newbie
timestamp: July 28, 2024, 06:42:16 am
content: Thanks Fzabkar...Any ideas as to its value or even links for a replacement component to purchase online?

author: fzabkar
author_rank: Super Contributor
timestamp: July 28, 2024, 06:47:23 am
content: Quote from: JayCal on July 28, 2024, 06:42:16 amCan you read anything off the remnants?The dimensions of the device give you some idea as to its power rating. The rating sticker of the charger says 180W for the input, so that should give you a ballpark figure for the steady-state current.

author: JayCal
author_rank: Newbie
timestamp: July 28, 2024, 06:48:33 am
content: All good... I asked Chat GPT...."so i found out it is a power NTC thermistor can you find the most likely value and size for a circuit which outputs 54.6v at 3A"ChatGPT:Given that the component in question is a power NTC (Negative Temperature Coefficient) thermistor, it is used to limit inrush current when the charger is first powered on. These thermistors have specific resistance values and current ratings suitable for different applications.For a circuit that outputs 54.6V at 3A, we can make some educated guesses on the appropriate NTC thermistor specifications:Current Rating: Since your charger outputs 3A, the NTC thermistor should be rated to handle at least this current, preferably higher to ensure reliability and longevity.Resistance Value: The initial resistance should be high enough to limit inrush current but should drop to a low enough value when the thermistor heats up to not affect normal operation significantly.Here are some common values for power NTC thermistors used in similar applications:Resistance (R25): Between 5Ω and 10Ω at 25°C. Lower values are used for circuits with higher inrush currents.Maximum Steady-State Current: At least 3A, but preferably around 5A or more to ensure it can handle the current without issues.Dissipation Constant: A value that allows the thermistor to quickly decrease its resistance after the initial inrush current is limited.Examples of Suitable NTC ThermistorsNTC Thermistor 5D-9Resistance: 5Ω at 25°CMaximum Current: 4ADiameter: 9mmNTC Thermistor 10D-9Resistance: 10Ω at 25°CMaximum Current: 3ADiameter: 9mmRecommendationsConsidering the requirements of your charger (54.6V, 3A), the 5D-9 or 10D-9 thermistors are likely suitable choices. Here are their detailed specifications:5D-9 ThermistorInitial Resistance (R25): 5ΩMax Steady-State Current: 4ADiameter: 9mmSuitable for circuits where the initial inrush current needs to be significantly limited.10D-9 ThermistorInitial Resistance (R25): 10ΩMax Steady-State Current: 3ADiameter: 9mmProvides more resistance initially, which can be useful for circuits with very high inrush currents.ConclusionBased on the charger’s specifications, a 5D-9 NTC thermistor would be a good starting point. It offers a balance between limiting the inrush current and handling the steady-state current of your charger. You can find these thermistors on electronics component websites such as DigiKey, Mouser, or even local electronics stores.Make sure to check the physical size and ensure it fits in your charger’s PCB layout. If you have access to the charger's schematic or can find a detailed service manual, that can provide the exact specifications required.

author: fzabkar
author_rank: Super Contributor
timestamp: July 28, 2024, 06:54:29 am
content: ChatGPT is selecting a resistor for the DC output rather than the AC input.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 28, 2024, 08:45:44 am
content: Do not ignore the final sentence of the first reply.