Prtoech2u.com
 

 

The Importance Of Current Sense Resistor In Switch Mode Power Supplies

current-sense-resistor 

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.

 

testing electronic componentFind Burnt Resistor

 

 

Home
Repairing Articles
Free Repairing Tips
Download Service Manual
TFT LCD Display Equivalents
Repairing Bookstore
Other Resources
Contact Us
Free Subscribe Now
Links
Site Map

LCD TV Repair

Plasma TV Repair

Projection Tv Repair

Learn Basic Electronic

Computer Repair Course

Learn Laptop Repair

CRT Tv Repair Course

LCD Monitor Repair ebook

How to testing Electronic Components like a Pro!

Power Supply Repair Guide- all types of SMPS


Testimony

1 Feb 2007

Many thanks!  I have
downloaded the file and
will - hopefully - now be
able to fix the screen.
Like your website -
very helpful...
thanks again.
- Mike

31 Dec 2006

thank you for this
great site that i
found what i
search about.
-Moomin

15 Jan 2008

Thank you very
much for your
excellent site!
I know there would
be lots of people
around the world
who will appreciate
your very neat,
well-organised site.
-Matt Straford


DONATE Help the P2U

If you find my free stuff useful, why not leave me a little something with PayPal.

Donations are greatly appreciated, and help with the costs of providing FREE service manual & server fees.