Wierd one....

The LED drivers are using a switch mode power supply, and being low wattage probably use a design with a single power transistor rather than a pair in push pull. This makes the voltages on the transformer windings and the AC waveform prior to rectification  highly assymetric.

(  optional side track into cheap flyback converter design)

The meter is trying to respond to a waveform that is very far from the simple DC or 50Hz sinewaves forr which it is intended. There is probably a much higher peak voltage than your meter reports, but only for a very small fraction of the time. Also the frequency will be too high for the rectifier in your meter to actually rectify properly.

The meter being unable to respond to the fast pulses decides the signal is lower than it truly is (it will be more than the 20V the AC range reckons too), and switches its internal range scaling resistors incorrectly,, so the analogue to digital converter (ADC) clips on the peaks. Then, due to the adc internal details, clipping is not the same for positive overshoots as negative, as inside the chip is supplied by a battery in a 'negative earth' sort of way and the 'earth' is created from a voltage part way up the battery supply.  In your case it seems the ADC interprets the assymetrically clipped fast waveforms as an average value that is slightly negative - regardless of the input polarity - it is the asymmetry of the overload response inside the meter electronics that cause the 'DC' reading to always be the same way up.



Adding more converters will add more pulses that may reinforce or cancel the effect depending on phasing, so it is hard to predict what will happen.


High frequency "Conducted emissions" as the EMC folk call them are very hard to measure properly, and to do it justice you really need a measurement receiver.

However, I suspect these units either fail EMC standards, or only scrape a pass in some carefully contrived configuration.

Earthing the output may solve the tingle problem, but the switching waveform is clearly escaping as an equivalent voltage source between the mains wiring and the LED dive lines, presumably via the inter-winding capacitances.


To design these things properly requires inter-winding screens on the high frequency transformers, either earthed or connected to the mid point of L and N via a pair of capacitors, and properly chosen  L-R-C filtering on the input.

The pressures of value engineering mean that such things often get left off, and while external chokes and filters can be added, it is not usually possible to be as good as  if it was well designed from the beginning.

The LED drivers are using a switch mode power supply, and being low wattage probably use a design with a single power transistor rather than a pair in push pull. This makes the voltages on the transformer windings and the AC waveform prior to rectification  highly assymetric.

(  optional side track into cheap flyback converter design)

The meter is trying to respond to a waveform that is very far from the simple DC or 50Hz sinewaves forr which it is intended. There is probably a much higher peak voltage than your meter reports, but only for a very small fraction of the time. Also the frequency will be too high for the rectifier in your meter to actually rectify properly.

The meter being unable to respond to the fast pulses decides the signal is lower than it truly is (it will be more than the 20V the AC range reckons too), and switches its internal range scaling resistors incorrectly,, so the analogue to digital converter (ADC) clips on the peaks. Then, due to the adc internal details, clipping is not the same for positive overshoots as negative, as inside the chip is supplied by a battery in a 'negative earth' sort of way and the 'earth' is created from a voltage part way up the battery supply.  In your case it seems the ADC interprets the assymetrically clipped fast waveforms as an average value that is slightly negative - regardless of the input polarity - it is the asymmetry of the overload response inside the meter electronics that cause the 'DC' reading to always be the same way up.



Adding more converters will add more pulses that may reinforce or cancel the effect depending on phasing, so it is hard to predict what will happen.


High frequency "Conducted emissions" as the EMC folk call them are very hard to measure properly, and to do it justice you really need a measurement receiver.

However, I suspect these units either fail EMC standards, or only scrape a pass in some carefully contrived configuration.

Earthing the output may solve the tingle problem, but the switching waveform is clearly escaping as an equivalent voltage source between the mains wiring and the LED dive lines, presumably via the inter-winding capacitances.


To design these things properly requires inter-winding screens on the high frequency transformers, either earthed or connected to the mid point of L and N via a pair of capacitors, and properly chosen  L-R-C filtering on the input.

The pressures of value engineering mean that such things often get left off, and while external chokes and filters can be added, it is not usually possible to be as good as  if it was well designed from the beginning.