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Ac+dc

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dr.power

Member
Hi guys,

how to measure just the DC level of a signal consisting of an AC+DC level without any scope?

Thanks beforehand
 
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BrownOut

Banned
Well, I guess it depends on your meter. I was thinking about our lab meters, which average the readings. A small hand-held meter might not have the capability. You might need to take several readings and average them. Or, you can use an old-fashoned "dial" type meter. If the AC is above a few tens of herts, the intetia of the dial will average the reading.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

My bench meter does AC+DC and it's a little over 2 years old. It provides for AC+DC RMS measurements.
Some of the cheaper digital meters will average, some will go nuts with AC on DC.
The old analog DMMs do average quite well as Brownout pointed out.

You can make your own averaging circuit which will provide a true average but not RMS. There's a catch though, in that the signal when it goes to its lower voltage it has insignificant impedance right down to the lower voltage level, which excludes diode bridge circuits unless there is some significant resistive load already there. With a 10 meg ohm input impedance DC volt meter, you can connect a 10k resistor in series with a cap like 10uf (of the right voltage rating of course) and that will average the AC part of the signal quite well (adjust cap higher for lower frequencies than about 50Hz). The reading is then taken across the cap. Very little is lost as long as the meter has 10 meg input impedance.
If the low level of the signal doesnt have significant impedance then you have to buffer it or connect a load resistor and do some more math to calculate the true average after making the measurement across the cap.
 
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kiriakos-gr

Member
What is the point having an non RMS AC+DC volts measurement, especially today ( 2011), that bandwidth is also important !!
The U1272A its hand held and has 100KHz bandwidth.
And if you are a lucky American, currently it is on a serious discount offer by Agilent it self,
at about 230$. ( 100$ less than the original street price).
 
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BrownOut

Banned
The question was measuring only DC in the presense of AC. So, he wants an average AC meaurement, not RMS. For example, a sine wave on a DC level would average to zero, leaving only the DC level.
 
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ronsimpson

Well-Known Member
Most Helpful Member
If you add a RC low pass filter on the input of a low cost meter it will read the DC level.
Where the RC time constant is long compared to the AC frequency.
 

MrAl

Well-Known Member
Most Helpful Member
If you add a RC low pass filter on the input of a low cost meter it will read the DC level.
Where the RC time constant is long compared to the AC frequency.

Hi there ron,

I mentioned this back in post #6 of this thread, but note the exception i also warned about.
 

ronsimpson

Well-Known Member
Most Helpful Member
I mentioned this back in post #6 of this thread, but note the exception i also warned about.
Every week I hear this fight. "I need to measure........".
"For $0.03 I can get you within 3%."
"Three percent...no good."
"You are at 100% now."
"How much for 0.01%?"
"$12.50"
"How about 2%"
 

MrAl

Well-Known Member
Most Helpful Member
Every week I hear this fight. "I need to measure........".
"For $0.03 I can get you within 3%."
"Three percent...no good."
"You are at 100% now."
"How much for 0.01%?"
"$12.50"
"How about 2%"

Hi there ron,

What are you trying to say...care to rephrase that? :)
 

kiriakos-gr

Member
I think that he speaks about accuracy in contrast of the multimeters pricing.

But the trap of doing such comparisons, is that AC is harder target for high accuracy, only pricey DMM can offer accuracy and bandwidth.
And you need those both, if you are dealing with modern inverters and sinusoidal waves that those inverters had born with the use of high tech electronics and the use of square waves.
 

dr.power

Member
The question was measuring only DC in the presense of AC. So, he wants an average AC meaurement, not RMS. For example, a sine wave on a DC level would average to zero, leaving only the DC level.

Yea, the average level is equal to DC level for a signal containing AC+DC.
We know that a DC voltmeter measures the average levels, so connecting a DC voltmeter to a signal having AC+DC levels will lead to a DC reading because the average level of the AC signal would be set to zero. Am I right?
 
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MrAl

Well-Known Member
Most Helpful Member
Hi,

Not all DC meters can do this. For example, try to measure the average DC value of a full wave rectified waveform without a smoothing capacitor with some meters and you'll see a display that bounces all over the place. I know this because i've tried it several times. Add a smoothing capacitor and you read peak volts not average. Add a resistor and capacitor and you still read peak volts because the circuit has nothing to discharge it.

An analog meter is great for this kind of thing.
 

kiriakos-gr

Member
try to measure the average DC value of a full wave rectified waveform without a smoothing capacitor with some meters and you'll see a display that bounces all over the place. I know this because i've tried it several times.

Please make famous the brand and model of this DMM :)
 
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MrAl

Well-Known Member
Most Helpful Member
Hi again,

Any meter that uses digital averaging will see this same problem because it cant sync to the waveform. Could be the cheaper ones because that's what i was referring to, one Radio Shack and one Velleman.
 

The Electrician

Active Member
Hi again,

Any meter that uses digital averaging will see this same problem because it cant sync to the waveform. Could be the cheaper ones because that's what i was referring to, one Radio Shack and one Velleman.

I don't think that just any meter that uses digital averaging will see this problem. The common technique is to use a crystal oscillator to derive a reference frequency close to the grid frequency. Maybe the cheapest ones don't do that, although it would seem that nowadays a crystal controlled oscillator isn't particularly expensive.

The cheapest DVM I own is also a Radio Shack. I created a test waveform by connecting a 20 volt DC supply in series with the secondary of a small transformer providing 25.2 VAC. I supplied the primary of the transformer with a variac. This allowed me to create a 20 volt DC voltage with various amounts of AC riding on the DC.

I've attached 3 images showing 3 of the 5 waveforms I measured. The reference level of the scope was offset 2 cm down so that all the waveforms would be on screen.

I started out with 20 VDC and added various AC voltages of zero, 20 V P-P, 40 V P-P, 60 V P-P, and 80 V P-P. Notice that for the last two, the instantaneous voltage goes negative.

I measured the AC+DC waveforms and got the following results with the Radio Shack on its DC range:

Waveform
1.) 19.92 VDC
2.) 19.91 VDC
3.) 19.88 VDC
4.) 19.78 VDC
5.) 19.45 VDC

There was no bouncing of the readings.

I also made the measurements with a Fluke 187 meter on its DC range.

1.) 20.014 VDC
2.) 20.013 VDC
3.) 20.013 VDC
4.) 20.014 VDC
5.) 20.014 VDC

The problem the Radio Shack is having is due to the limited dynamic range of its ADC. This is the same reason that for RMS measurements DVMs have a maximum crest factor for rated accuracy.
 

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