Newsletter-Spring 2022
Compared to the dramatic fluctuation of the PC industry, applications of industrial control, medical, automotive, smart meters, and green energy are considered more stable industries. Firstohm has been committed to improving the quality of MELF resistors for these industries for decades.
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★FIRSTOHM e-commerce
Date: July 11-13, 2023
★ Carbon or ceramic composition resistors
Alternative Parts
If you consider the in-circuit performance attainable by using carbon or ceramic composition resistors that could handle high surges, peak power or energy - example applications included power charging/ discharging, inrush current limiting, RC Snubbers, High Voltage Power Supplies, etc.
The Firstohm SSR Series is currently offered with the lead-time of only 8~10 weeks - while the unit price is significantly more cost-effective compared to carbon or ceramic composition resistors. Ex : The Ohmite OX/OY Series is available in two power ratings:
• OX Series - 1W Ceramic Composition Resistor
• OY Series - 2W Ceramic Composition Resistor
The SSR resistor is available from 1W~3W @70DegC, 400V and with a Max Permissible Surge Voltage of 25KV and can be considered as a replacement to the OHMITE OX/OY Series.
★ Application of Pre-Charge Resistor
Before the core device of the electric vehicle is powered on, the voltage at both ends is zero, which is equivalent to a short circuit at the moment of power-on, resulting in a large charging current, which can easily cause damage to the power device. Therefore, it is necessary to connect a pre-charge resistor in series in the charging circuit during the pre-charge process to limit the amount of the pre-charge current.
From market, the commonly used pre-charge resistors included Aluminum Housed Power Resistors, Cement Resistors and High-Power Through Hole Wirewound Resistors, etc.
FIRSTOHM SWM series are SMD Wirewound Resistor, it possesses advantages such as smaller size, SMD type and multi-national patents with a high reliability in anti-surge characteristics, which are very suitable for pre-charge applications.
Refer to below case study, the capacitance value is 100 mF. If input voltage is supplied 60V, pre-charge resistance is 22 ohm, the voltage changing time is 1.1 second, the maximum power reach 163.64W, therefore, when high voltage source connect to capacitor, it is requested to limit surge current, in order to avoid electric impact to the other components.
※Test Result : Passed !
a.) Test Conditions:
Ambient temperature : 25℃
Pre-Charging RES. : SWM200,22R,5% *4PCS
Input Voltage : 60VDC
Capacitor : 100 mF
Charge/Discharge : Charging time @5 Sec. / Discharging time@55 Sec.
Number of switching : 1,000
τ=RC/2= (22Ω*100mF) /2 =1.1 Sec.
E=1/2CV2 =1/2(100mF) (60V)2=180J
b.) Evaluated circuit
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