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.
Jestine Yong is a electronic repairer and a writer. For more information on electronic repair please visit his website at
http://www.noahtec.com/electronic-repair-articles.htm
Things to Look for When Buying a Computer Microphone
15 July 2008
Computer microphones are a valuable addition to any PC and they greatly increase interactivity and communication levels. You can use a microphone for performing various tasks, both business and fun related. For example, you can hold a conference with your business partners, over the Internet, or you can chat with your friends online, instead of doing that on the phone. Of course, in order to enjoy accurate and high quality communication, you have to choose a good PC microphone. There is a huge variety of models on the market, ranging from microphones that you can buy for a few dollars and going all the way to expensive, high tech desktop microphones. So which PC microphone is right for you?
Price Vs Quality
You can pay $5 for a microphone, or you can pay $50. So where’s the difference, apart from the price? Well, cheap microphones have low prices for some good reasons: they offer average to low quality sound, they break down more easily and you can’t use them successfully in all applications that support a microphone. The materials they are manufactured from have to be cheap, which eliminates the idea of high quality, so the end product is of lower reliability. And at this price, you can be sure bulk manufacturing is present and quality tests are quick and far from rigorous. Microphones that come for a higher price have, in most cases, better sound quality and more features. Such microphones will have noise canceling filters, which help diminish background noise. Distortion filters and integrated circuits also help keep voice sounds clear and accurate. The materials used for more expensive microphones also last longer - plastic doesn’t crack so easily and if you drop it on the floor you know that the internal components are well protected.
Choosing the right computer microphone
Determining what price range you are in and what model to purchase depends heavily on the purpose you have in mind for the microphone. Are you going to use it just to chat with friends online? In that case, spending a lot of money on a professional computer microphone is not justified. Do you plan to hold business conferences over the Internet? Then maybe you should get a better microphone that will allow the discussion to be continuous and clear, without noise distortion. Giving vocal commands to your computer or dictating to word processing software with speech recognition capabilities is another use for desktop microphones. Quality should be first here - speech recognition software is very sensitive and demands a high quality microphone and sound card. Recording your voice and inserting it into music tracks is possible, but a high quality microphone is absolutely essential in this case.
Mantius Cazaubon is a successful author and publisher of http://the-microphone-guide.com, a resource for information and tips on buying computer microphones online.