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
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
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.
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.
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.
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.
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
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
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.
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.
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.
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
Or roll your own 
Classic Valve Design - Original and Legacy Design PCB's
(docs have various configurations)
Cheers!
Classic Valve Design - Original and Legacy Design PCB's
(docs have various configurations)
Cheers!
What about common wall warts that can plug in either way? When plugged in one way, only the neutral would be fused.
I don't see why being in 240 V would mean 2 x hot .... single-phase is single phase... ie. phase (hot, live), neutral (cold) & earth (ground, etc. ..
Interesting link dodo, I already had a look at it.
Unfortunately in total contradiction to the posts in this thread. People interested should babelfish it as it's about building a "AC-Filter-Box" with multiple outlets using commercial EMI Filters...
Any thaughts ?
The wall wart does not have a specific neutral if the two prongs are physically sized the same, allowing you to plug it in either way. It's the wires and receptacle that have the neutral, and this neutral will never be fused (or have a breaker). The polarization of the wall wart has no effect on the power system installation and design.
Clarifying that I am only speaking of US code requirements, single phase 240V in residential applications are 120-0-120 with the center tap solidly grounded. Therefore there are two hots, and both of these must be fused (or breaker protected). There is, in industrial facilities, something called 240V grounded B, which is a three phase 3 wire 240V system with one phase solidly grounded. All devices (breakers, fused disconnects, etc) are to be two pole with the grounded phase solid and unprotected. Not saying this is a good system (it sucks), but it illustrates the simple rule that the grounded conductor is not to be fused.
Example: NEC 230.90 (B) Not in grounded conductor. No overcurrent device shall be inserted in a grounded conductor except a circuit breaker that simultaneously opens all conductors of the circuit.
That means you are permitted to use a 2 pole breaker to protect a single phase 120V hot-neutral circuit, but you cannot fuse it, as there is the potential to blow the neutral fuse but not the hot.
Zigaflux,
Indeed, per NEC in the States, one cannot fuse a neutral. I believe we (you or I) might be misinterpreting what is meant by 'fuse'. As in all things NEC, it stops at the wall. In other words, one cannot, per NEC, breaker/fuse a neutral at a point of service, breaker/fuse box. This is not applicable in an electrical 'appliance' external to mains service within a structure, which materially effects only the connected 'appliance', not other 'appliances' connected to that circuit. Beyond the wall, ETL (and appropriate ANSI) standards apply. The most conservative of the bodies in the States, Underwriters Laboratories [UL], has no problem giving their certification to 'appliance' located dual fused power entry or entry/conditioning assemblies [for either single ended or split phase service].
Regards,
Paul
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Paul:
I wouldn't exactly say the NEC stops at the wall. In fact, it covers all residential electrical equipment in a generic sense (Article 90-2 Scope (2) and (3)). It does grant exception to equipment that carries approval, identification, labeling, or listing by an outside lab, such as UL (Article 110, Requirements)
That being said, I have found two typical responses at DIYAudio:
1) "the Code doesn't cover anything beyond the outlet". This allows one to do whatever they want to do with their equipment. Unfortunately, this statement is not true, and the Code does have jurisdiction over what we build. Fortunately, the requirements are (a) simple (b) practical - they do not adversely affect noise free operation and audio quality and (c) provide for a safe installation. I have found the acceptance of item #1 often leads to a statement along the lines of #2
2) "I don't care what the Code says". Fair enough. No one can force you to obey every law we have, and maybe you take too many deductions on your taxes, too. I'm not a fan of big government, so do what you will in the privacy of your own home.
Although you can find equipment that has one or two fuses, 120 or 240 operation, what makes it unique is that it carries a certification or listing from an organization such as UL. It is checked for safe operation in the event of a blown fuse or reverse-wired receptacle, and may be double insulated, too. Our DIY equipment has none of this, and we should follow the very few simple rules.
I wouldn't exactly say the NEC stops at the wall. In fact, it covers all residential electrical equipment in a generic sense (Article 90-2 Scope (2) and (3)). It does grant exception to equipment that carries approval, identification, labeling, or listing by an outside lab, such as UL (Article 110, Requirements)
That being said, I have found two typical responses at DIYAudio:
1) "the Code doesn't cover anything beyond the outlet". This allows one to do whatever they want to do with their equipment. Unfortunately, this statement is not true, and the Code does have jurisdiction over what we build. Fortunately, the requirements are (a) simple (b) practical - they do not adversely affect noise free operation and audio quality and (c) provide for a safe installation. I have found the acceptance of item #1 often leads to a statement along the lines of #2
2) "I don't care what the Code says". Fair enough. No one can force you to obey every law we have, and maybe you take too many deductions on your taxes, too. I'm not a fan of big government, so do what you will in the privacy of your own home.
Although you can find equipment that has one or two fuses, 120 or 240 operation, what makes it unique is that it carries a certification or listing from an organization such as UL. It is checked for safe operation in the event of a blown fuse or reverse-wired receptacle, and may be double insulated, too. Our DIY equipment has none of this, and we should follow the very few simple rules.
I got some PSU cases from a recycling bin that I am going to use as a temporary prototype case for my next Class-T amp implementation - and they came with some of these EMI filters - look like they will be handy for cleaning the power before it goes into the transformer?
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