Anyone using commercial EMI-Filters in his PSU ?

[coolnose]

Hi Everybody,

I just stumbled across the following datasheet while doing a google search on filtering AC current and was wondering if anyone used commercial EMI filters in his PSU as mentionned in the header ?


Schaffner FN 343

Any hints on how useful this might be are appreciated.

Cheers

[zener_diode]

Commercial EMI filters are a cool idea, but I think they target really high
frequencies. I designed a power supply with the Schurter DD22, which is a
complete module with IEC connector, switch, fuse and EMI filter: http://www.schurterinc.com/pdf/english/typ_dd22.pdf

The problem is that the filter doesn't do anything for lower frequencies, such as
dimmer noise and other power line noise that comes from noisy sources. In my
case, a light fixture with a noisey supply. It was easy to test my supply filter by
just turning on the light fixture.

I settled on the Schurter DD21, with IEC connector, switch and fuse and made
my own filter with a 22mH common mode choke and a 1uF capacitor. It works
quite well.

[dmills]

I routinely use DD22 style modules[1] (mostly because it makes the metalwork easier) mainly to help keep AM radio out of the gear.

If dimmer noise is a problem then you either need to balance the audio lines, solve the pin one problems or get better dimmer racks, there is no excuse for it (At work we run ~200 * 15A dimmers spread across all three phases and the amp racks plugged in next to them are totally silent).

Regards, Dan.

[1] I favour the variants that only fuse the live.

[coolnose]

Thanks a lot for your answers.

I actually have a couple of EMI-filters available from a salvage and thought "why not use them" ...as I also have some already included in the IEC-socket, the link was just an example...

As far as I understand It doesn't hurt...

@dmills

Any reason you prefer fusing the live only ?

Afaik fusing both shouldn't change anything... plus giving an additional security in case the hot/cold wiring is inversed for any reason..

Cheers,

Max

[dmills]

Originally Posted by coolnose

Any reason you prefer fusing the live only ?

Basically, so that if the fuse blows the downstream components are not at a high voltage relative to the case.

Yes, I have an isolation transformer and don't work live unless I have to, but if the power on indicator is out then it is nice if that means the power is probably off!

Mainly paranoia, combined with the fact that UK practise has long standardised on just fusing the live (Our plugs have been polarised practically forever).

Regards, Dan.

[zigzagflux]

In the US, it is against Code to fuse the neutral. Should never be done.

In exotic systems, there is overcurrent neutral sensing that will simultaneously open all 'hot' phase conductors, but there is never a need to actually open the neutral.

Those IEC type input modules do have the capability for fusing both lines, but if you are using it in a 120V single phase application (hot, neutral, ground), you are not permitted to install both fuses. The neutral side of the input module should have a shorting bar installed. Since the module is rated for 240V operation, they provide the means for installing an additional fuse if both lines are hot.

[stratus46]

Originally Posted by coolnose

Hi Everybody,

I just stumbled across the following datasheet while doing a google search on filtering AC current and was wondering if anyone used commercial EMI filters in his PSU as mentionned in the header ?


Schaffner FN 343

Any hints on how useful this might be are appreciated.

Cheers

Seems like a good idea but here is the problem with it. It treats the AC power as if it were a balanced line. It's the 2 capacitors to the ground (earth for our European friends) that cause problems as the neutral to ground should have nearly 0 volts and nearly 0 current into ground while the hot has 120 (North America) relative to ground. This throws an 'accidental' imbalance and starts ground currents that shouldn't be there. I work in facilities with hundreds of units plugged in this way and it causes problems. In one place we had another transformer with the center tap connected to ground so it was 60VAC + 60VAC and the power was truly balanced. Hum was excellent but all the circuit breakers had to be duals with twin 'hot' leads to each load - a considerable extra cost. The 'hospital grade' versions leave out the capacitors to ground and introduce no offset currents so require no extra work. BTW those filters are in most computer equipment and introduces hum into analog audio interconnects. Who hasn't had that problem? Ground is _not_ a trivial issue.

G²

[dodo]

Netzfilter von THEL, Netzzentralen selber bauen

[Geek]

Or roll your own

Classic Valve Design - Original and Legacy Design PCB's
(docs have various configurations)

Cheers!

[star882]

Originally Posted by zigzagflux

In the US, it is against Code to fuse the neutral. Should never be done.

What about common wall warts that can plug in either way? When plugged in one way, only the neutral would be fused.