Why does this rectifier have a large AC output voltage?

AI Thread Summary
The discussion revolves around unexpected AC voltage readings from a bridge rectifier designed for a simple LED circuit. Users noted that there is still an AC voltage present at points expected to output DC, with measurements showing nearly double the transformer voltage. The readings were taken using a multimeter, which can yield peak voltage readings that may confuse users. Suggestions were made to incorporate a capacitor to stabilize the power supply and reduce noise. The conversation emphasizes the importance of understanding multimeter settings and proper measurement techniques in rectifier circuits.
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I made a (bridge) rectifier for a simple circuit that would make several LEDs light up. When it was finished, I used a multimeter to measure the voltages between several points and got the readings you can see in http://users.pandora.be/k-a-d/rectifier.jpg". Why is there still an AC voltage between the two points that were supposed to be DC-only? And why is it almost twice the original transformer voltage? The resistor code is pretty blurry in the image; it's 2.2k if it helps.
 

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How are you getting that 20V AC number? I'm guessing it's with a multi-meter of some sort. A cheap way to make AC voltage measurements is to use half a bridge rectifier, measure peak voltage, and multiply by 2.

You could test this by measuring the AC voltage of a battery using the same tool.

P.S. You might want to stick a capacitor in there to clean up the power supply. Otherwise it will be noisy.
 
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Indeed, the reading comes from a multimeter. I simply switched between "V DC" and "V AC".
 
Take a look at http://www.allaboutcircuits.com/vol_3/chpt_3/4.html" to see where you're getting the AC.

Regards
 
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