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raif 1st June 2005 08:31 PM

ac voltage after rectification
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Hello everyone,

My multimeter reads 37 v dc after rectification and smoothing with a cap across output terminals. I use a 24 v 330 va transformer and a single secondary connected to four mur 860 diode bridge.
However when I check ac voltage it shows 70 v ac! after rectification. What is wrong?


Bill Fitzpatrick 1st June 2005 08:56 PM

Re: ac voltage after rectification

Originally posted by raif
What is wrong?

You're not supposed to measure DC with the meter set to AC so it's your measurement technique that's wrong.

raif 1st June 2005 09:14 PM

not too fast
I measure dc voltage with multimeter set to dc. The problem is when I check ac voltage across output terminals( obviously, multimeter set to ac) it shows 70 volts ac!


Netlist 1st June 2005 09:20 PM

Do you have another meter to try? Or a scope to watch?

/Hugo :)

homer09 1st June 2005 11:09 PM

Bill is right. measure AC with AC setting and DC with DC setting.

You can't measure AC on a DC signal.

BUT, you can measure how much DC on an AC signal with your meter set to DC.

the reason for this is simple. DC is direct current, AC is DC but alternating at 50/60 Hz. when you set your meter to AC, it assumes its measuring AC, but it will give you about double your DC voltage because there is no switching (which it is assuming there is).

you can measure DC on an AC signal because the AC is switching too fast to be picked up by DC meter.

raif 2nd June 2005 08:52 AM

meter is set to ac to measure ac
thanks for replies, but I am trying to measure if there is any stray ac voltages after rectification(ripple voltage) and smoothing and I get 70 volt ac reading(meter set to ac). Is this because dc currents are too fast and the meter sees it as ac ?

pinkmouse 2nd June 2005 09:07 AM

You need a scope, for the reasons Homer ellucidates above.

Magura 2nd June 2005 10:14 AM


Originally posted by pinkmouse
You need a scope, for the reasons Homer ellucidates above.

Hmm, is that stricktly so? I was under the impression that a true RMS meter would be able to measure ac on a dc supply???


Jennice 2nd June 2005 11:05 AM

I agree... Testing in "AC" mode should give some ripple voltage only (after the bridge rectifier), but not a seriously high voltage.
The DC mode should tell the intended working voltage after the rectifier.

A scope can tell more, but should not be needed to find the error.

Check for correct connection to the bridge. Otherwise disconnect it and test every one of the 4 diodes to see if one is shorted.
If you do not have a diode tester, a battery and a resistor should work. Current should flow in one direction, leaving about 0.7 V drop across a diode. If you change polarity, no current should flow, and no voltage drop should occur across the resistor.


Magura 2nd June 2005 11:19 AM

I just checked up on it.

When measuring with a true RMS meter you only measure the ripple when set to measure AC on a DC supply. I have no means of testing if it is so with a non true RMS meter, as I don't have such :(

Magura :)

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