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The Importance Of Current Sense Resistor In Switch Mode Power Supply
In monitor power supply where pulse width modulation (pwm) ic UC3842are used. Pin 3 of this power ic is called I-sense which meanscurrent sense and this pin is connected to source pin of POWER FET.The purpose of this current sense circuit is for overcurrent protection.This circuit usually samples the voltage drop across a resistor in serieswith the switching fet transistor. If the current rises abnormally due tosome short circuit in the secondary side such as a shorted secondary diodeor horizontal output transistor (HOT), the voltage will exceed a referencelevel and shutdown the pulse generator which is the PWM ic. This circuitprovides protection to components in the event a problem occurs.In some cases where overload happen-the power section will blow and most of the time the components changed are fuse, power fet, pwm ic,zener diode and current sense resistor.
Do you know that a small change in this current sense resistor valuehave a great impact on the secondary output? A little change in valuesay from 0.22 ohm to 0.44 ohm can caused your ouput voltage to dropinto half! I have seen a lot of repairer can't detect this type ofproblem (low output voltage) due to that they used analog or digitalmeter that don't have the low ohms range. You can't differentiate if the low ohms resistor has its value changed by using normal meters.I personally use the Dick Smith ESR Meter which have the capabilitiesto check resistor value down to 0.01 ohm! If you don't have this special meter you may just direct replace the current sense resistorto see if the generated secondary output will back to normal voltage.
Another tip is that if the current sense resistor in the monitorB+ circuit have changed value it will caused the output in theboost circuit to become low and you will have a small width displayand of course the high voltage will become low as well. Instead of24Kv it becomes 15-18Kv.
Jestine Yong is a electronic repairer and a writer. For more information on electronic repair please visit his website atwww.noahtec.com/electronic-repair-articles.htm
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