Ac to dc voltmeter question

[Crambone]

I have a 150Vdc analog meter and want to read 0-120 ac voltage with it. Can I simply use a bridge rectifier to convert the ac to dc?

[Michael Craft]

Is there a specific reason you're trying to read AC voltage using this meter? Because generic, handheld DMMs are very cheap nowadays, and they will read AC voltage.

But to answer your question... yea, you can use a FWBR and capacitor. With no load, the voltage on the cap will equal the peak AC voltage minus 1.4. If, for example, you measure 168.0 VDC across the capacitor, the peak voltage of the AC would be 169.4 V. If we assume the AC voltage is a perfect sine wave, the RMS of the AC voltage would be 169.4 divided by the square root of 2, or 119.8 V RMS.

[radiolistener]

I have a 150Vdc analog meter and want to read 0-120 ac voltage with it. Can I simply use a bridge rectifier to convert the ac to dc?

No, but you can use it to measure peak Voltage (Vpk) of AC signal.

In order to measure effective Voltage of AC signal, you're needs a true RMS multimeter or to use RMS to DC converter

[Gyro]

As it's an analogue voltmeter it won't read Vpk (it would need the addition of a 'reservoir' capacitor to do that). It will mechanically integrate the pulsing DC waveform from the rectifier to read 'average' voltage.

This is the way that Analogue multimeters have read AC voltage for generations. Without an RMS to DC converter they are described as 'Average responding, calibrated for sine wave". Such meters can only be accurate for sine waves, not waveforms of other crest factors, eg, squarewaves. You would almost certainly need to calibrate the meter scale to known AC voltages [Edit: you might just get away with adjusting the resistance in series with the meter movement - at least on a 150V meter, the forward voltage drops of the diodes become insignificant.


As this is the Beginners section, I should also add that if the 120V is mains then you should take all suitable safety precautions for insulation and fault conditions - at the very least, a low current fuse to protect against shorts or bridge rectifier failure.

[Vovk_Z]

No, but you can use it to measure peak Voltage (Vpk) of AC signal.

Why no? In general - yes. Rectifying is a general method to measure an AC sine voltage. 

[radiolistener]

Why no? In general - yes. Rectifying is a general method to measure an AC sine voltage. 

No, because rectified output will depends on Voltage of signal waveform peaks.
But effective Voltage measurement needs integration of Voltage for entire waveform.

If you know that the signal waveform is ideal sine, you can calculate effective Voltage by multiplying peak Voltage with factor 1/sqrt(2) = 0.7071. But since in real world waveform will have some distortions, that method give you error, and that error will be as high as waveform is different from ideal sine.

For example, the mains have distorted sine because it is loaded with a lot of switched mode power supplies. So, the method to measure peak Voltage and then multiply it with 0.7071 leads to a high error.

In addition diode rectifier has some Voltage drop, which needs to be taken into account in order to measure proper peak Voltage. And since analog Voltmeter has relative low input impedance, it also needs to be taken into account, because it will discharge capacitor on the recifier output between waveform peaks. So, even measurement of peak Voltage is not just simple.

[Zenith]

Yes and the old style analogue multimeters used such an arrangement, but you can't just do it and assume it will accurately convert the DC meter into an AC meter. Those meters had a scale which was calibrated to agree with the RMS value of a sine wave - "average responding". That was fair assumption as the AC they'd be used with was line, or line through a transformer, or audio up to a KHz or so. They'd give an inaccurate result for a square wave or noise, or anything but a reasonably pure sine wave. There are analogue true RMS meters, but they are much more complicated and rely on measuring the heating effect of the current using techniques such as matched thermistors.

AC theory and Root Mean Square (RMS) values are well worth looking into.

However, assuming this is mains (line) voltage with its obvious dangers you want to measure, you'd do better to buy an average responding meter made for the purpose. Panel meters are  available cheaply on ebay and other places. A decent quality average responding DMM doesn't cost that much. True RMS reading DMMs are more, but still affordable.

[Vovk_Z]

Why no? In general - yes. Rectifying is a general method to measure an AC sine voltage. 

No, because rectified output will depends on Voltage of signal waveform peaks.

1. Have you seen 'sine voltage' in my reply?
2. 0-120 VAC voltage range means TS possibly wants to measure a mains voltage. It is close enough to the sine waveform.
3. 99% of all old AC voltmeters used rectifiers.

[radiolistener]

3. 99% of all old AC voltmeters used rectifiers.

yes, they have some kind of diode rectifier and Voltage correction, but they all shows incorrect Voltage for mains. They was usable 40-50 years ago, when all home electronics was analog with linear loading and no SMPS.

For example, sometimes I can see difference up to 30-40 Volts on the mains when measure it with DT830 and with TRMS BM867S.

[2N2222A]

Based on what others have said, simply put, if you connect the meter to a bridge rectifier (without a capacitor) you'll have a meter that measures average AC Voltage, minus a little bit for diode Voltage drop. The meter can probably be adjusted to set the zero point at 1.4V for that.

RMS Voltage of a pure sine wave is 0.707 of peak.
Average Voltage of a pure sine wave is 0.637 of peak.
Average is .90 as much as RMS of a sine wave.

So your meter will measure .90 as much as your typical AC Volt meter, which measures RMS Voltage but ONLY when measuring a true sine wave. They are average responding but set to true RMS but only on a true sine wave. Adjust your typical cheap meter down to .9 and it'll become an average Voltage meter just like what you're making.

Some meters are really cheap and are not even average responding. They measure peak Voltage with only one diode and then show you .707 of that. They typically only have 750VAC and 200VAC on the dial.

[TimFox]

In this context, "average responding" means that it measures the time average of the absolute value of the voltage (displaying the rms value for a sine wave with that average), while "rms" means that it measures the time average of the square of the voltage (displaying the square root of that mean).  As pointed out above, it is easy to compare "average" to "rms" to "peak-to-peak" or "peak" for a well-defined waveform, such as sinusoidal, square wave, or DC.  "True rms" should give the correct answer independent of waveform, with practical limits such as "crest factor" (ratio of peak amplitude to rms).

[EPAIII]

"In order to measure effective Voltage of AC signal, you're needs a true RMS multimeter or to use RMS to DC converter".

This, while technically true, is misleading. Most, dare I say over 99%, of the VOMs available today or in the past do NOT USE A TRUE RMS measurement but they do routinely read sinusoidal AC Voltages with a fair amount of accuracy. To do so they employ two techniques. First, they simply assume that the AC Voltage is a sine wave. All sine waves have the same conversion factor (the reciprocal of the square root of two or 0.707). So a simple resistor divider can be used to incorporate that factor. The second thing they do is to use a separate AC Volt scale on the meter (analog meters) or some form of compensating circuitry (analog and digital meters) to account for or to eliminate entirely the 1.4V or less loss due to the Voltage drops across a full wave, diode rectifier.

The point is, even inexpensive VOMs, which do not have true RMS circuitry, can and do provide accurate AC Voltage readings as long as the AC Voltage is a sine wave.

For other AC waveforms, like a triangular wave, you do need either a true RMS style meter or a different conversion factor for your particular waveform.

How well does this assumption of the AC being sinusoidal in form work in the real world? I worked for over 45 years in electronics and I can count the number of times when I really needed a true RMS meter on the fingers of one of my hands. When dealing with other waveforms I usually just used a scope.

I have a 150Vdc analog meter and want to read 0-120 ac voltage with it. Can I simply use a bridge rectifier to convert the ac to dc?

No, but you can use it to measure peak Voltage (Vpk) of AC signal.

In order to measure effective Voltage of AC signal, you're needs a true RMS multimeter or to use RMS to DC converter

[2N2222A]

Most, dare I say over 99%, of the VOMs available today or in the past do NOT USE A TRUE RMS measurement but they do routinely read sinusoidal AC Voltages with a fair amount of accuracy.

In the rest of your quote it looks like you're referring to the really cheap 200V / 750V AC scale meters that just use a rectifier and just measure the peak. Most of the better meters divide the AC Voltage down something low like 200mV and then use an operational amplifier circuit to convert that in to average DC Voltage. That is then adjusted up by 1.11x to display the correct RMS Voltage assuming the input is a true sine wave.

[EPAIII]

I am sure that many "better" meters do use more advanced circuitry to read AC Voltages. But I was talking about what "most" meters do. And "most" meters are not "better" meters. They do not have this type of advanced circuitry.

The OP is talking about measuring AC Voltages with a "150Vdc analog meter". Does that really sound like he has a "better" meter? It seems to me that he/she has this one meter and wants to extend it's use. Perhaps it is a better analog meter. But it is a simple, single scale, analog, DC meter.

So, YES, I am talking about the majority of meters. The ones that sell new for under $50 USD. I have worked on many of them. They do not have any amplifiers. They do not have any true RMS circuitry. Many of them do not have any way of moving the needle if a one Volt, P-P AC signal is being measured because of the diode drop. And meters like this do have non-linear scales for low AC Voltage ranges because of the diode drop.

This person does not need to be told a true RMS meter is what is needed for AC measurements because that is simply not true 99.99% of the time. What he/she does need to know is what can be done with the meter at hand and what a simple VOM can also do. Electronically he/she needs to learn to walk before learning about hopping, skipping, and jumping while running as fast as possible.

Of course, I could be wrong. Perhaps the $15 USD meters that I see at the home supply and hardware stores have advanced to include true RMS capability. I would love to have such a meter at that price and will look at them at the next opportunity.

Well, I just quickly searched Amazon for a true RMS meter and found one for $9.99 USD. Anybody want to buy one of these? It is hardly to be described as a "better" meter and I certainly am not going to spend even that amount on such a meter. I wonder just how they arrive at a "true RMS" measurement. Want to bet they don't specify "sine wave only" in the fine print? But perhaps the prices have come down quite a bit.

https://www.amazon.com/AstroAI-Multimeter-Resistance-Transistors-Temperature/dp/B071JL6LLL/ref=sr_1_1_sspa?keywords=true+RMS+voltmeter&qid=1667026682&qu=eyJxc2MiOiIyLjQ3IiwicXNhIjoiMS41OSIsInFzcCI6IjAuMDAifQ%3D%3D&sr=8-1-spons&psc=1

Most, dare I say over 99%, of the VOMs available today or in the past do NOT USE A TRUE RMS measurement but they do routinely read sinusoidal AC Voltages with a fair amount of accuracy.

In the rest of your quote it looks like you're referring to the really cheap 200V / 750V AC scale meters that just use a rectifier and just measure the peak. Most of the better meters divide the AC Voltage down something low like 200mV and then use an operational amplifier circuit to convert that in to average DC Voltage. That is then adjusted up by 1.11x to display the correct RMS Voltage assuming the input is a true sine wave.

[radiolistener]

So, YES, I am talking about the majority of meters. The ones that sell new for under $50 USD. I have worked on many of them. They do not have any amplifiers. They do not have any true RMS circuitry.

Many of modern multimeters below 50 USD have TrueRMS.

For example, I bought UT210E on aliexpress for 44 USD, it has TrueRMS AC Voltmeter.

Cheap Chinese Richmeters RM111 also has True RMS, it cost about 15-20 USD:
https://www.aliexpress.com/item/32835638876.html

[Vovk_Z]

Many of modern multimeters below 50 USD have TrueRMS.
For example, I bought UT210E on aliexpress for 44 USD, it has TrueRMS AC Voltmeter.

I must add that if you think that its 'TrueRMS' is a kind of 'real True RMS' and is much better than just a simple rectifier? Yes, it is better (especially for small voltages), but the real difference is not that strong. Typical 'TrueRMS' works only within a small range of signal cross-factor. I would rather call it 'partially-true-RMS' or 'limited-TrueRMS'.
(I have the same UT210E DMM among others btw).
Sometimes manufacturers write the false 'TrueRMS' labels even on meters with rectifiers or active rectifiers.

[MrAl]

Let me see if i can help with this.

First, there are true RMS meters and there are non true RMS meters.
The true RMS meters have to one way or another perform the calculation:
Vrms=squareroot(mean(V^2))

which means they have to square the voltage:
y1=V^2

then take the mean (Tp is the period over which the integration is performed):
y2=integrate(y1,t,0,Tp)/Tp

then take the square root of that:
Vrms=sqrt(y2)

Alternately, they use a resistor as load and measure the heat the resistor develops over a reasonable time period.  This is a rather old technique though i doubt any do it today.

That's how to get a true, true RMS reading.

But since RMS is RMS, you can claim the reading is RMS if you assume that the waveform is a true sine wave and if you measure either of two things:
1.  Average voltage
2.  Peak voltage

and then do a calculation to convert to pseudo RMS.  The reading that appears on the meter will be the same as a true RMS meter, but only if the line voltage waveform remains to be a true sine wave with no distortion, and the amount of distortion amounts to a corresponding inaccuracy.  Typically sine wave flat top under some appliance loads so this would not be too uncommon.

For the Average measurement, to convert to a calculated RMS value you would multiply by 1.1111.
For the peak measurement, to convert to a calculated RMS value you would multiply by 0.7071 which also means you can divide by 1.4142 .
Since dividing a voltage down is simpler than multiplying a voltage up, measuring the peak is a simple way to get to the RMS calculation because then you can either use a resistor voltage divider (two resistors) or a single resistor in series with your existing DC meter.
You first have to rectify the mains AC voltage with a diode bridge and a decent size filter capacitor value.  The capacitor value should be large enough to filter the ripple so you get a decent kind of DC voltage. This places the cap voltage at around 1000uf but we can calculate a better value knowing the resistance of the meter when reading 120vdc.  The voltage rating should be much higher than the line voltage, which will be 120v, so a rating of 240vdc should be good.

To use a voltage divider you would calculate two resistor values that provide a division by 1.4 which means if you have R2 on the bottom of the divider and R1 on the top, the values can be calculated as either:
R1=sqrt(2)*R2-R2

where you pick a value for R2 then calculate R1, or from:
R2=R1/(sqrt(2)-1)

where you pick a value for R1 then calculate R2.

Since R1 is the top resistor and the meter will be across R2, to ensure the proper power rating if the meter shorts out the min value for R1 if R1 is made a 1/2 watt resistor and at 120vrms would be:
R1=57600 Ohms

The max mains is a bit higher around 135vrms and that puts the value at 72900 Ohms.

You can then calculate R2.  For the value of 72900 Ohms for R1 the value of R2 comes out to:
R2=175996 Ohms.

There is a catch though.  The internal meter resistance will be in parallel with that R2 so you have to calculate the parallel combination to find the right value for R2.  The total would be 175996 in this case, and so the parallel combination would really be:
Rp=(R2*Rm)/(R2+Rm)

and then use Rp instead of R2 as the bottom resistor.

A better way is to use a single series resistor.  In this case you have to know the resistance of the meter when it is reading 120vdc because that is what it will be reading when this is done.  Then R2 becomes the meter resistance alone.  This is the way many volt meters are done.

As a final note, the calculations and the voltage drop of the diodes and ripple factor dont always come out exact.  If you want to get closer, you have to "calibrate" the meter.  This would mean using another meter of known accuracy or simply adjusting a resistor value to get the meter to read 120vdc when the full mains voltage is applied.  This assumes the mains voltage is 120vac RMS at some point.  If it is not, then you will still get a reading that is comparative to what the line voltage is at the time of calibration.  This is often good enough as when time goes on you will still know if the line went higher or lower, and by what percentage it went up or down.  For example, if you read 120vdc now and 108vdc later, you can assume the line voltage went down by 10 percent.  The calibration phase can make up for a lot of different types of inaccuracies even if the wave isnt pure sine, as long as the waveform shape does not change too much over time.
Where i live we have this problem in the summer months where the line voltage can drop to as low as 100vac or even lower sometimes on the very hot days.

If you need any more information im sure someone else can help with this too or just post a message what it is you need to know and when i check the forum again later i'll reply again.

As someone else said, you do have to be very careful with this kind of voltage.  All though most people just get a shock that doesnt do long term damage, there are plenty of circumstances where you can actually get killed and there's no coming back to fix the circuit after that.

Good luck to you.

[radiolistener]

I must add that if you think that its 'TrueRMS' is a kind of 'real True RMS' and is much better than just a simple rectifier? Yes, it is better (especially for small voltages), but the real difference is not that strong.

Yes, True RMS are better because they have less error for mains measurement.
For example, at the moment diode rectifier Voltmeter shows 232 V on my mains, while TRMS BM867S DMM shows 237 Vrms. But sometimes I see error up to 20-30 V for the mains measured with diode rectifier Voltmeter.

Typical 'TrueRMS' works only within a small range of signal cross-factor. I would rather call it 'partially-true-RMS' or 'limited-TrueRMS'.
(I have the same UT210E DMM among others btw).

this is because any device has a limited working bandwidth. Regarding to UT210E, I tested it with signal generator - it has bandwidth 3350 Hz and perfectly flat frequency response up to 1400 Hz.

In many cases 2-3 kHz bandwidth is enough for mains measurements. Just because higher frequencies are filtered with a long mains wires and Y capactiors in the devices. BM867S has about 20 kHz bandwidth, BM869S has 100 kHz bandwidth. If you want more wide bandwidth then it's better to use oscilloscope with RMS measurement.

[Vovk_Z]

For example, at the moment diode rectifier Voltmeter shows 232 V on my mains, while TRMS BM867S DMM shows 237 Vrms. But sometimes I see error up to 20-30 V for the mains measured with diode rectifier Voltmeter.

Ok, that's Brymen, Brymen is a reputable manufacturer, so you have beaten me. .

[Vovk_Z]

BM867S has about 20 kHz bandwidth, BM869S has 100 kHz bandwidth. If you want more wide bandwidth then it's better to use oscilloscope with RMS measurement.

Yes, an oscilloscope can be ok, but we have to consider its bit rate (8-10-12 bit ADC?) so it depends on a voltage level.  It is ok for a mains voltage, but it is not sufficient for ultra-low-voltage measurements.

I have a B3-57 AC microvoltmeter with a 5 MHz bandwidth, '30 uV' in the smallest range, and a converter of a heat type.  IDK how to name the heat type RMS converter of B3-57 - 'UltraMegaRealTrueRMS'? Or 'MoreThanRealTrueRMS'? 

[TimFox]

Usual name in US is "thermal converter" for such rms meters.  The converter itself could tolerate a huge crest factor, but there is usually a finite-amplitude amplifier ahead of it in the voltmeter.