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DIY: Mains Line/Noise/EMI Filtering

restorer-john

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#21
but in several places adding a DC blocking filter did fix some transformer hum. I've forgotten the details, but IIRC the magnitude of common-mode offset needed to induce hum in a toroidal transformer was a lot less than I expected (led to me doing a little more research to find out why).
I've got an oldie but a goodie- 'squeaky clean' line filter on my bench feed to run all my test gear- in fact everything (32 outlets) runs through it and it has DC blocking in the form of diodes/caps along with a bunch of other filters all in a heavy steel case with potted filters etc. Pretty sure there are 2 banks of three series connected diodes IIRC when I pulled it apart to replace a few neon indicators many years ago. I think there was a 1.5v differential between input and output voltage. It's riveted together so not fun to dismantle.

Don't have a schematic, but it is very effective at killing toroid hum/buzz and even is effective on some marginally noisy EI transformer primaries.

Our mains waveform here is terrible looking on my DSO, makes me think we should all be locally regenerating our own.
 

DonH56

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#22
I've got an oldie but a goodie- 'squeaky clean' line filter on my bench feed to run all my test gear- in fact everything (32 outlets) runs through it and it has DC blocking in the form of diodes/caps along with a bunch of other filters all in a heavy steel case with potted filters etc. Pretty sure there are 2 banks of three series connected diodes IIRC when I pulled it apart to replace a few neon indicators many years ago. I think there was a 1.5v differential between input and output voltage. It's riveted together so not fun to dismantle.

Don't have a schematic, but it is very effective at killing toroid hum/buzz and even is effective on some marginally noisy EI transformer primaries.

Our mains waveform here is terrible looking on my DSO, makes me think we should all be locally regenerating our own.
I want one! :)

The usual scheme is caps to block DC with diodes across (shunting) them to keep the voltage across them small under transient conditions. That way, you can use large (high-value) caps with modest voltage ratings.

I think the problem with toroids is that it is very difficult to gap them in manufacturing like normal EI transformers making them extremely prone to saturation with small DC offsets applied. Toroids better confine the EM field to reduce noise in packed electronic components but they do have their drawbacks.
 

trl

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#23
"[...] A number of potential sources of DC current injection have been identified, particularly the use of high frequency ballasts within fluorescent lighting loads, and switch mode power supplies for PCs. Additionally there are no limits for DC harmonics defined under EN61000-3-2, and additionally items taking less than 75W do not have any harmonic limits defined at all (exceptions under clause 7). Hence for - some household equipment, such as mobile phone chargers, it is not known whether any DC current component is produced. It is therefore recommended that a selection of these devices are tested to measure and attempt to quantify any DC current components. [...]"
 
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