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Old 4th January 2004, 02:04 AM   #1
newbie1 is offline newbie1  United States
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Default DC Power Supply Has AC Signal?

Hi I am new to the forum and have a question about DC current in general.

I just picked up a DC power supply from Radio Shack (13.8V 3A). It seems to register the correct DC voltage when measured. However, I noticed that there is an AC voltage of 29V as well. I tried testing a 9V battery for AC voltage and it registered 19.4V. This made me suspect there is something wrong with my meter?

Just thought I would ask the experts first, and if I am wrong and this is a normal phenomenon would 29V of AC from the Radio Shack PS harm a DC circuit?

Thanks
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Old 4th January 2004, 03:25 AM   #2
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Your radioshack power supply is almost certainly a SMPS (switch mode power supply) and will have some high frequency ripple (AC), given the high frequency, it's a little unpredictable how your meter will measure it(it would almost certainly give you a bogus figure). The 9 volt battert basically shouldn't register any AC so I'd suspect that your meter is no good. Try putting a fresh battery in the meter, you'd be amazed at how somethings behave with a flat battery.
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Old 4th January 2004, 06:11 PM   #3
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As the webmaster mentioned the ripple is standard. The most accurate way I know of measuring it would be with an oscilloscope, but not many people have one of those just lying around. The meter should read the AC wave "riding" on the DC signal..... but 29V seems way too high for a 14 V powersupply.
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Old 4th January 2004, 08:07 PM   #4
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This is clearly a result of the way your meter works (or doesn't) on the AC range and has nothing to do with ripple, the AC battery reading proves this.

You do not have 19.4V AC at the terminals of your battery, end of story.
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