Every electrical technician knows the difference between DC (Direct Current) and AC (Alternating Current). Every electrical technician also realizes the importance of taking accurate current measurements to protect conductors from exceeding their insulators' ability to withstand heat or assuring devices under power work properly. However, does every electrical technician realize that electrical current measurements aren't always what they appear to be?

Direct Current (DC) is straightforward. When we use a multimeter to measure direct current, it is what it is. However, the plot thickens when we are dealing with Alternating Current (AC). AC current travels back and forth down a conductor and can best be described in graphical format. The most common graphical description of AC current is a sine wave. Because the amplitude of the sine wave continuously changes over the wave period (one complete cycle), at any given point in time, a current measurement would not be the same. Therefore, how do we accurately measure AC Current?

One method to measure AC current would be take current measurements at increments across one complete cycle and average them together. This would give us an average value of the current. If the current is a perfect sine wave, mathematically, the average value is always 0.636 times the value of the peak amplitude.

Another method to measure current is based on the current's ability to perform work when applied to a resistive load. The laws of physics tell us that when current passes through a resistive load, it dissipates energy in the form of heat, mechanical motion, radiation or other forms of energy. If the resistive load is a heating element and the resistive load stays constant, then the laws of physics tell us that the heat produced is directly proportionate to the current passing through the load. Therefore, if we measure the heat, we will know the current.

Mathematically, the relationship between heat and current is such that the heat produced is proportional to the square of the current applied to a resistance.

(Power or Heat) = (Current) ^2 * (Resistance)

If the current is continuously changing, as in AC current, the heat produced is proportional to the average (or mean) of the square of the current applied to a resistance:

(Power or Heat) = Average [ (Current) ^2 * (Resistance) ]

Using algebra, the above formula can be rewritten to read:

Current = Square Root [ (Power or Heat) / (Resistance) ]

AND this is called the Root Mean Square Current or RMS Current.

For AC currents that are graphically represented by a sine wave, the RMS current will always be 0.707 times the peak current. With that said, we can calculate current by multiplying peak measurements by 0.707 if the current is a perfect sine wave. However, perfect sine waves are rare in most commercial and industrial applications. This is because resistive loads in commercial applications are not linear which results in unpredictable or variable current requirements.

In order to get a True RMS measurement, we can measure the heat dissipated by a constant resistive load and perform the above calculations. The result is a True RMS measurement.

Now that we got all the technical discussion out of the way, which is the best method to calculate current? Should we 1.) measure a current average 2.) multiply current peaks by 0.707 to get an RMS current, or 3.) measure the heat from a resistor and calculate a True RMS current value?

Although Global Test Supply sells multimeters that can calculate current using any of the above methods, the most accurate way to calculate current in my opinion is a True RMS method. Average current values often are 40% less than True RMS values and that could mean the difference between blown circuit breakers, malfunctioning motors, or worst case, potential fire hazards. True RMS multimeters only cost about 20-30% more than the alternative. How much is an accurate current reading worth to you?

Robert Preville is the President and CEO of Global Test Supply, LLC, a distributor of test and measurement equipment, and welcomes question from those looking for a http://www.globaltestsupply.comVoip Home Service Tv Phone Internet
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Heres a list of tips and tricks to consult at two a.m. when youre trying to put your site to bed and that d*mn script just wont work.

1. Make a note of the error message, and type it in Google. Leave out your unique paths and file names. Someone else has had the same problem, and some nice person will have posted a solution.

2. If it's a server error, repeat what caused the error. Go quickly to the error log on your web hosting control panel. Any clues there?

3. Read the README file. Have it open on-screen when you're setting up the script.

4. Use a text editor like Wordpad or Notepad to edit scripts. Simply put, your editor should not change anything beyond the bare text that you type on screen. Microsoft Word will alter your formatting, with unhappy consequences.

5. Avoid wrapping lines. Check that your editor is not set to do this.

6. Avoid putting characters like " , ' ; in if you don't know what you're doing.

The server may read these as programming code. Apostrophes and other non-alphabet characters can be 'escaped' out of a script by putting a backward slash in front of them.

7. Get the paths to required files correct in the script itself and in the html files involved. Telnet to your site, and use the pwd command: pwd somename (without the apostrophes) to find where directories and programs are on your server.

8. Windows (MS-DOS) may insert carriage returns which cause errors in Unix. In Perl, You need to have a line break after #!/usr/bin/perl. It appears to be there, but you may have to insert it manually via Telnet. Otherwise you may get an error.

9. Permissions: Set permissions of scripts to 705 (e.g. 'CHMOD 705 yourscript.cgi' in a telnet session). CHMOD 606 for text files the script may need to read or write to. CHMOD 707 any file or sub-directory the script author asks you to. The rest can usually be left at the default 644.

(The use of 0 here in permissions is to stop people in the same group as you peeking at your files).

10. Upload scripts in ASCII mode only, or you'll get errors in them (the text will be shifted around if you upload in Binary).

FTP files with the extensions .tar.gz, .zip, and image files, in BINARY mode only.

11. Check whether you've put a trailing forwardslash / where you're not supposed to, or not inserted one when you're supposed to!

12. Email addresses in a script may need a backwardslash before the @ to work.

13. Don't change too many things at once. Do the absolute minimum to get the script working, then change one thing at a time.

14. If you are changing the HTML code of a form page that posts to the script: Clear your browser's cache. You may still be loading an old version of your page, with tags that your script now won't recognise.

Your ISP may also be using its cache of your web page. Try connecting via another ISP, or use a proxy service like http://www.Anonymouse.ws to view your page.

15. The error 'document contains no data' in your browser usually means a path or a file name is wrong. It can also mean the file you're calling is 0 bytes in size.

16. Don't try setting scripts up when you're tired. If you've been working for hours, take a break, or go to bed!

17. It's a fact that, after all this, your script might have a bug. Check any discussion boards at the vendor's website, or general webmaster forums, to see if this is a common problem. Report exactly what problems you're having, and your server set-up, to the programmers. It's unlikely you're the first person to have it.

T. O' Donnell ( http://www.tigertom.com/personal-loans-uk.shtml ) is a credit broker and cynic living in London, UK.Hdmi Inputs Do Not Work On Samsung Tv
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