Mains 120 Voltage through full bridge rectifier could you explain the following?

[JJ_023]

A full bridge rectifier is an unregulated linear power supply. It's invalid to measure unregulated power supplies without a load. As in, connecting straight to the multimeter is invalid. I have a 15W 9VAC power supply that merely contains a transformer. Under no load, it measures 13-15VAC. My 15W 9VDC unregulated linear power supply measures 12-15VDC under no load. Measuring across a high wattage / low ohm resistor that draws ample current, the AC and DC power supplies read correctly. At least within 10% but the voltages on power supplies are nominal in any case.

Been said but rms voltage of AC with DC offset is sqrt(AC² + DC²). 114VDC and 35VAC yield 120Vrms, which is close enough to 124Vrms. AC measurement jumping up to 50VAC on dual display doesn't mean anything when you aren't measuring under load. As in, an unregulated 1A power won't measure correctly outputting under 1mA.

Note that switching mode power supplies are inherently regulated and can measure correctly under no load, i.e. directly connected to multimeter.

Phase shift between voltage and current affects the power readings, not the voltage readings.

Also been said but you should check the input voltage. I live in the US and my nominal 120Vrms supply out of the outlet is between 121-125Vrms when I measure it.

I appreciate your explanation.  I think it was my fault for not posting with more detail.  I am not looking for an explanation of the numbers or how the circuit works.  What I was trying to figure out was the discrepancy having to do with the actual meter.  Which in my case turned out to be faulty autoranging.  I do appreciate you taking the time to respond.  Thank you.

[MrAl]

Hello there,

You can't really measure just the output of the bridge you first have to measure the input of the bridge.  That's because the input line could be something other than 120vac. The normal range is 108vac to 132vac so you could possibly have anything between those two.  Normally you would get something close to 120vac, but you should use your meter for that anyway so you will know how your meter interprets that voltage.  It could also vary significantly over the course of one day.

The input at the bridge rectifier is about 122 V.

Hello again,

Ok then you would multiply the theoretical values by 122/120.

I have a question though.  What is going to be the purpose for measuring this quantity?  Normally you don't really measure that until you have some load or some filter capacitance or something else like that.  Is this a study of some type?

I was just honestly playing around there is no particular purpose. 

I have  a bridge rectifier that is set up to go into mains directly.  So all I really have to do is just plug it into the wall and everything is insulated and there are jacks on the output for the multimeter.  Because of me playing around  I now have this question I am trying to figure out with this particular meter.

Hi again,

Oh ok no problem, so this more or less falls into the category of self study, which is always a good thing.  You can learn a lot like that.

The bottom line to your original question seems to me to go like this.
If you have meter A and you read 80v, and you have meter B and you read 90v, then the two meters are using a different method to measure the voltage, or there is something wrong with one of the meters.  You could try a third meter C to see if it matches with either one of the first two.

The same would apply to different settings on a single meter.  If setting A produces 80v and setting B produces a reading of 90v, then the two settings are using a different technique to measure the voltage, or else the meter is not made quite right.  Again another meter might help to decide which reading is right, or at least close to right.

You should be aware that a true RMS reading is done differently than a regular AC measurement on a run of the mill meter.  A true RMS method is done in a way that samples the wave in different places and builds up a reading.  A regular AC measurement just measures the peak or the average and uses a correction factor (sometimes called the 'crest factor') to display the reading.  The two would read vastly different values on a full wave rectified waveform.

I am not sure what measurements would be useful here either, except the true RMS value or the average DC value.  Once you add a capacitor though, the average DC value will change.  The capacitor can store energy during the low voltage time periods and thus the average DC voltage becomes higher in value, effectively giving you more power output.

The theoretical values of the full wave output is of interest though.  You can also do a spectrum analysis to see the levels of the different harmonics.

[JJ_023]

Hi again,

Oh ok no problem, so this more or less falls into the category of self study, which is always a good thing.  You can learn a lot like that. 

Thank you for response.

I love learning and do so every chance I get.  To me self-study has been fairly easy especially with the wealth of information out there nowadays.  I also know that things in the real world are different than textbooks and simplified equations.  So I also try to learn from others that already went through the growing pains on whatever particular subject.

The bottom line to your original question seems to me to go like this.
If you have meter A and you read 80v, and you have meter B and you read 90v, then the two meters are using a different method to measure the voltage, or there is something wrong with one of the meters.  You could try a third meter C to see if it matches with either one of the first two.

This is just common sense and this was my oversight.  I should have pulled out my other meters to double check.

The same would apply to different settings on a single meter.  If setting A produces 80v and setting B produces a reading of 90v, then the two settings are using a different technique to measure the voltage, or else the meter is not made quite right.  Again another meter might help to decide which reading is right, or at least close to right.

The conclusion that I came to based on someone else’s explanation and actually testing the meter is that the auto range function was not doing its job properly.  Because when I went up to a higher range it showed the correct voltage (I assume it is correct but I did verify it with 2 other meters).

You should be aware that a true RMS reading is done differently than a regular AC measurement on a run of the mill meter.  A true RMS method is done in a way that samples the wave in different places and builds up a reading.  A regular AC measurement just measures the peak or the average and uses a correction factor (sometimes called the 'crest factor') to display the reading.  The two would read vastly different values on a full wave rectified waveform.

I appreciate the detail.  I understand this. I try to buy only “true RMS” meters.

I am not sure what measurements would be useful here either, except the true RMS value or the average DC value.  Once you add a capacitor though, the average DC value will change.  The capacitor can store energy during the low voltage time periods and thus the average DC voltage becomes higher in value, effectively giving you more power output.

This part I already know from my studies.  It was suggested that I try a high value resistor by somebody else. 

The theoretical values of the full wave output is of interest though.  You can also do a spectrum analysis to see the levels of the different harmonics.

I would love to.  However I don’t really have access to a spectrum analyzer.

Again thank you for putting the effort into this explanation.

[MrAl]

Hi again,

Oh ok no problem, so this more or less falls into the category of self study, which is always a good thing.  You can learn a lot like that. 

Thank you for response.

I love learning and do so every chance I get.  To me self-study has been fairly easy especially with the wealth of information out there nowadays.  I also know that things in the real world are different than textbooks and simplified equations.  So I also try to learn from others that already went through the growing pains on whatever particular subject.

The bottom line to your original question seems to me to go like this.
If you have meter A and you read 80v, and you have meter B and you read 90v, then the two meters are using a different method to measure the voltage, or there is something wrong with one of the meters.  You could try a third meter C to see if it matches with either one of the first two.

This is just common sense and this was my oversight.  I should have pulled out my other meters to double check.

The same would apply to different settings on a single meter.  If setting A produces 80v and setting B produces a reading of 90v, then the two settings are using a different technique to measure the voltage, or else the meter is not made quite right.  Again another meter might help to decide which reading is right, or at least close to right.

The conclusion that I came to based on someone else’s explanation and actually testing the meter is that the auto range function was not doing its job properly.  Because when I went up to a higher range it showed the correct voltage (I assume it is correct but I did verify it with 2 other meters).

You should be aware that a true RMS reading is done differently than a regular AC measurement on a run of the mill meter.  A true RMS method is done in a way that samples the wave in different places and builds up a reading.  A regular AC measurement just measures the peak or the average and uses a correction factor (sometimes called the 'crest factor') to display the reading.  The two would read vastly different values on a full wave rectified waveform.

I appreciate the detail.  I understand this. I try to buy only “true RMS” meters.

I am not sure what measurements would be useful here either, except the true RMS value or the average DC value.  Once you add a capacitor though, the average DC value will change.  The capacitor can store energy during the low voltage time periods and thus the average DC voltage becomes higher in value, effectively giving you more power output.

This part I already know from my studies.  It was suggested that I try a high value resistor by somebody else. 

The theoretical values of the full wave output is of interest though.  You can also do a spectrum analysis to see the levels of the different harmonics.

I would love to.  However I don’t really have access to a spectrum analyzer.

Again thank you for putting the effort into this explanation.

Hi,

Oh you are welcome and happy to hear you are coming to grips with this.

Remember that whatever resistor you use has to be rated for the power it will have to dissipate.  A good idea is to double the theoretical power to choose the resistor rating.

If you would like to see some spectrum analysis you can get a digital scope.  These things can do spectrum analysis and display the results so you can see the different harmonics present.
If this is of interest to you I can post some harmonic values.  It is interesting that the output is comprised of all even harmonics of the input fundamental.  That means that for a 50Hz input we will see 100Hz, 200Hz, 400Hz, 600Hz, etc., up to a very high even number which would be considered infinite in theory.  That's very theoretical though as the diodes are nonlinear and will introduce some other frequencies too especially under load.
It is also interesting that under load the diode current can be very very unusual, with very high peaks when there is only a capacitor filter.  The capacitor charges near the peaks of the wave and so can draw a very high current near the peaks.  Sometimes an inductor is used to smooth out the current wave, although I think this is done less today except maybe in high power devices with extremely high current peaks.

Now if you care to you can look into the solutions for the DC output with a capacitor filter as that is more common today.  In the older wall warts almost all of them used this.