EMI ungrounded case

It will block the e-field almost entirely, and typically will have a modest effect on the h (magnetic) field, especially if it's a typical box folded (not welded) from mild steel. Depends on frequency too, with higher audio frequencies probably blocked more. Grounding won't matter to the h-field.

I tried a welded-seam steel Hoffmann box. It had less than 2:1 attenuation for a 60 Hz magnetic field.

But the definitive answer: Try it!

John

You cant legally use an ungrounded case !!! It is not safe unless you are using a mains socket based low voltage power supply.

The low frequencies from audio should be too much of a problem unless there is some HF generation in your cct.

I would have gone for an earthed case with ferrites on all the inputs and outputs if I didnt have EMI test gear to hand.

Hard to say. It depends a lot bit on exactly what audio frequency you are looking for and how far the air-cored coil is from the steel case. If the steel case looks like some kind of shorted turn to the coil (measure the Q of the coil with and without the steel case), you've got to figure that the magnetic field coming out of the coil is going to induce a current in the steel case that will oppose the magnetic field being generated at the case. You still end up with enough magnetic field at the case to generate enough volts to drive the counteracting current around the steel plate against the resistance of the steel plate,which may still be high enough for you to measure outside the case.

A slot in the case in the right place may help a lot, though for anything really dramatic you'd want a slot that is long enough to accomodate a quarter of the free-air wavelength of the radiation being emitted - this is roughly 250 feet at 1MHz, so entirely irrelevant at audio frequencies.

-- Bill Sloman, Nijmegen

I don't think this is true. For example my set-top box is not grounded and is mains powered. It's just that more stringent standards of insulation must be met, if the case is *not* grounded.

Or batteries.

He said he *wanted* emissions!

I agree with John Larkin, there will be almost no e-field but perhaps some magnetic field depending on the thickness of the metal.

I can assure you need to have a grounded chassis if it is conductive ! The good thing about a metal case is its screening properties.

You can use plastic cases but they need conductive film on the inside that is grounded. Unless of course your electronics dont radiate.

Just because you have equipment in your house not grounded doesnt mean it passes EMC checks. There is a get out clause where you have to simply show you tried to stop EMI.

I spent 6 months doing nothing else but EMC testing so know what passes and what doesnt.

Er - did you mean illegal because of electrical safety (electrocution) or EMC?

Either way I don't believe this is always true.

In other words it is perfectly *possible* to have electronics in plastic cases (or unearthed metal ones) provided they have been designed to suit.

These are units that have been made like this for 15+ years by major manufacturers. There are 10's of million of them in European countries (and everywhere else I expect). Are you really saying these are all illegal? It seems unlikely.

Really? I had not heard of this. So hard do you have to try? :)

I am sure you have seen plenty of devices fail and have accumulated a lot of experience about the failure reasons.

Nevertheless many devices *are* designed to pass with plastic (or unearthed metal) cases.

Just because people churn stuff out doesnt mean it was properly EMC tested !!!

I would never send out a metal chassis with mains in it without it being earthed. If you think that is fine then do it but get used the first time someone gets a shock !

I wouldn't either, actually, but I do not design products for mass-market.

Funnily enough, you *do* get a shock from these things. The switch mode power supplies allow enough leakage so that you can feel the current if you touch the case while earthing yourself! But evidently it is below the limit in the safety standard.

I agree this is a bad thing (and this has been discussed here before). But you made a blanket statement that it is flat-out

I simply would not leave myself open to being sued when someone gets a shock.