Hi,
I have a problem with a transformer that is humming quite loud now and then. Out of curiosity I started logging the voltage with a multimeter and an oscilloscope. I logged the voltage for 48 hours. The problem is that the results from the meter and oscilloscope are quite different. The multimeter says that the DC voltage has been between -1,067 V and +1,176 V, the oscilloscope says the average voltage has been between -3,7258 V and -2,0095 V.
First, I am not quite sure how to measure the DC component with an oscilloscope. Most instructions on the internet for measuring DC component on the AC line are done with a multimeter. I did some basic test with a signal generator and the oscilloscope; I had a symmetrical sine wave where I adjusted the DC component and the AVG measurement at the oscilloscope gave me the correct DC voltage. I have assumed that is correct to use the AVG measurement. Unfortunately, my mains voltage isn’t very symmetrical, it is quite distorted.
So, my questions are basically:
Any other input/comments are also welcome.
I have a problem with a transformer that is humming quite loud now and then. Out of curiosity I started logging the voltage with a multimeter and an oscilloscope. I logged the voltage for 48 hours. The problem is that the results from the meter and oscilloscope are quite different. The multimeter says that the DC voltage has been between -1,067 V and +1,176 V, the oscilloscope says the average voltage has been between -3,7258 V and -2,0095 V.
First, I am not quite sure how to measure the DC component with an oscilloscope. Most instructions on the internet for measuring DC component on the AC line are done with a multimeter. I did some basic test with a signal generator and the oscilloscope; I had a symmetrical sine wave where I adjusted the DC component and the AVG measurement at the oscilloscope gave me the correct DC voltage. I have assumed that is correct to use the AVG measurement. Unfortunately, my mains voltage isn’t very symmetrical, it is quite distorted.
So, my questions are basically:
- Is it correct to use the oscilloscope’s AVG measurement to read the DC component?
- Are there other methods that are better than using the AVG measurement?
- Why are the results from the multimeter and oscilloscope (very) different?
Any other input/comments are also welcome.
Attachments
You could take another tack, such as measuring the magnetising current in the toroidal transformer primary winding (eg. with no loading on the transformer).
Measuring mains current is a lot easier and safer with a LEM style sensor than measuring voltage. Statistical measurements on the current sensor output could identify aspects such as crest factor or peak levels per polarity (or whatever your meter can present to you), and whether they are consistent or vary sporadically or have some repetition character, or relate to the humming episodes.
Measuring mains current is a lot easier and safer with a LEM style sensor than measuring voltage. Statistical measurements on the current sensor output could identify aspects such as crest factor or peak levels per polarity (or whatever your meter can present to you), and whether they are consistent or vary sporadically or have some repetition character, or relate to the humming episodes.
The oscilloscope is not needed/not suitable for measuring small DC on the mains.
A multimeter preceded by a LP filter is more accurate and much safer, example here:
How to measure DC offset on the mains voltage?
A multimeter preceded by a LP filter is more accurate and much safer, example here:
How to measure DC offset on the mains voltage?
If the transformer humming episodes are short lived, eg. changing in intensity over less than say 5-10 seconds, then using any kind of RC filtering (as per Elvee's network) may attenuate or disguise the character of the DC shift being experienced by the transformer.
If the humming is related to DC imbalance, then taking the effort to look at the transformer primary current waveform will likely confirm an asymmetry resulting from entering core saturation each mains cycle (eg. see Rod Elliott's waveform in Mains DC and Transformers). LEM LA and LTS series for example can provide a simple connection to a hardware or software scope to show mains current waveform.
If the humming is related to DC imbalance, then taking the effort to look at the transformer primary current waveform will likely confirm an asymmetry resulting from entering core saturation each mains cycle (eg. see Rod Elliott's waveform in Mains DC and Transformers). LEM LA and LTS series for example can provide a simple connection to a hardware or software scope to show mains current waveform.
Last edited:
For 240Vac users extracting an offset of 0,1Vdc is not easy, because of the 80dB ratio (680Vpk to pk to 0,1V) and the noisy environnement. The DC value will depend of the filter characteristics either passive with analog voltmeter, LP filter or digital computation. There is no time constant standard to compare figures, a 0,1Hz or 5Hz filter will give different DC offsets.
I use a N4L PPA5520 power analyzer, the DC offset will substantially depends of the settings, with the possibility to record samples to better evaluate the DC component of a mains.
Because DC does usually not "flow" trough transformers, the DC mains offset is likely a local issue, often very local.
regards,
Pierre
I use a N4L PPA5520 power analyzer, the DC offset will substantially depends of the settings, with the possibility to record samples to better evaluate the DC component of a mains.
Because DC does usually not "flow" trough transformers, the DC mains offset is likely a local issue, often very local.
regards,
Pierre
Last edited:
Toroidal transformers are more prone to buzz off mains dc offset.
A simple 2 cap and 4 diode circuit will block the dc.
Loads of circuits on Google.
If its not toroidal then its more likely loose laminations.
A simple 2 cap and 4 diode circuit will block the dc.
Loads of circuits on Google.
If its not toroidal then its more likely loose laminations.