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
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.