# ac voltage after rectification

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#### raif

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?

Thanks.

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#### Bill Fitzpatrick

raif said:
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

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!

thanks

#### Netlist

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

/Hugo

#### homer09

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

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

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

#### Magura

pinkmouse said:
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???

Magura

#### Jennice

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.

Jennice

#### Magura

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

#### Kermit

I happen to have two different multimeters, one really cheap one and a more expensive one.
When measuring a 6V DC supply the cheap multimeter measures 13V when set to measure AC and the more expensive one measures 0,01V when set to AC.
None of the multimeters are true RMS as far as i know....
So I guess the answer lies in the price tag on your multimeter

#### jan.didden

Paid Member
Re: not too fast

raif said:
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!

thanks

This is normal for some cheap multimeters. An AC sine voltage of 70V would have an average values of something in the area of your 37V. Since such a cheap meter is average responding, it is calibrated to show 70V whenever it gets 37V average on AC scale. And of course those 37V DC ARE 37V average to that meter.

Use a series cap to the meter, that will allow you to read the AC ripple.

Jan Didden

#### cowanrg

I have a true RMS meter, and i have on several occassions had it mistakenly set to AC instead of DC, and sure enough, it measures the slight ac ripple. its very low, so it has confused me at first until i realize its on ac instead of dc. my other cheap meter just measures some wrong value, not the ripple.

#### raif

caltek cm1200t

Thanks everyone the culprit is the cheap multimeter. Any suggestions for a decent one which will also measure inductance.

Thanks

#### pinkmouse

Well, you learn something new every day! 'Cos I've got a scope it's something I never bothered to check, now I'll have to go through all my DMMs and see if any work that way.

#### Magura

Re: caltek cm1200t

raif said:
Thanks everyone the culprit is the cheap multimeter. Any suggestions for a decent one which will also measure inductance.

Thanks

AFAIK there is only one meter that is worth buying that fits your needs. Fluke 110.

Magura

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