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
citcuit 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.