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Current sense resistor is usually located in the switch mode power
supply section. Newer version of monitor-the b+ (boost)
The Importance Of Current Sense Resistor In Switch Mode Power Supply
Current sense resistor is usually located in the switch mode power
supply section. Newer version of monitor-the b+ (boost) circuit also
have this current sense resistor. If you repair electronic equipment
especially the switch mode power supplies, the current sensing resistor
is connected to the source pin of power FET (field effect transistor)
and the other end to HOT GROUND. The value of current sense resistor is
in the range of 0.1 ohm to about 1 ohm. The values are depends on the
design of each power supply.
In monitor power supply where pulse
width modulation (pwm) ic UC3842 are used. Pin 3 of this power ic is
called I-sense which means current 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 series with the switching fet transistor. If
the current rises abnormally due to some short circuit in the secondary
side such as a shorted secondary diode or horizontal output transistor
(HOT), the voltage will exceed a reference level and shutdown the pulse
generator which is the PWM ic. This circuit provides 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 value have a great impact on the secondary output? A little
change in value say from 0.22 ohm to 0.44 ohm can caused your ouput
voltage to drop into half! I have seen a lot of repairer can't detect
this type of problem (low output voltage) due to that they used analog
or digital meter 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
capabilities to check resistor value down to 0.01 ohm! If you don't
have this special meter you may just direct replace the current sense
resistor to see if the generated secondary output will back to normal
voltage.
Another tip is that if the current sense resistor in the
monitor B+ circuit have changed value it will caused the output in the
boost circuit to become low and you will have a small width display and
of course the high voltage will become low as well. Instead of 24Kv it
becomes 15-18Kv.
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