I was looking at an old PC power supply, and I notice in a couple of places slots have been cut into the PCB. I think it's done as part of the safe isolation between the mains and the low voltage outputs.
On Thu, 12 Dec 2013 10:53:21 +1100, Sylvia Else Gave us:
It does what you stated. A surface can propagate carbon trails. An air gap cannot. Examine the definition for the term "creepage". The electrical definition.
Usually seen on HV supplies, but smart folks making power supplies place them anywhere the voltage is getting up there.
On a related note, and referring to the document whose link I posted previously,
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It seems odd to me that the functional (not safety) clearance requirements at mains voltages (see table 2, and note that I'm in Australia with 240V AC mains) are higher than the distance between the pins for a typical mains voltage triac. For example,
I've seen the results when a numb-n*ts soldered a TO-220 triac with acid-core solder (not pretty, but quite fragrant).
I guess if you have to conform to that, you'll have to use something like a TO-218 with > 4mm creepage.
TO-220 will exceed the 0.6mm spacing for functional isolation < 150VAC RMS and the "F" class.
They could make triacs with an overmold on the middle lead like they used to do on horizontal output transistors, but I can't recall having seen such a thing.
Best regards, Spehro Pefhany
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"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
The creepage distance in a basic safety isolation barrier and a functional creepage distance are handled in pretty much the same way. If insufficient, the distance is shorted to see if an unsafe condition is created.
If it does not exhibit excessive operating leakage current, still passes safety hipot, has no open traces resulting and no flame hazard is created etc, then it's OK. This repeated for all single fault abnormals.
Obviously, any short of an isolation barrier runs the risk of creating excess leakage current or a hipot failure, even without the addition of tracking due to destroyed internal components that may result. So creepage shorctcomings in these areas are not acceptible.
Optocouplers can be used to couple across supplementary isolation - effectively 2xbasic isolation, only if internal and external physical dimensions and materials are controlled sufficiently to do so, by demonstration to the certifying agency.
Recent consolidation of safety standards has removed any advantage that clearance distances had over creapage distances, in safety isolation measurements, so slots are a waste of effort from a regulatory point of view. Practically, however their function remains unaltered.
Shorting seems an unduly optimistic test- a short will result in no power dissipation at the short. Arcing and tracking could result in significant heat, flames, toxic gases. But if that's how they do it.. I guess it would be best to make real sure about the flammability ratings of everything in the vicinity.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
As to my question about the triac pin separation, clearly, since any triac may fail shorted anyway, the circuitry has to handle that possibility, and will thus also handle a short across the pins.
Is there now a single place from which a definitive statement of the safety requirements can be obtained?
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Post your question there and you'll get definitive, knowledgeable answers specific to any country you choose within hours usually. Including the EXACT wordings of requirements and penalties involved for non-compliance. From memory, if your equipment has not been approved/certified and: if your equipment destroys a building in US; you're liable for the equipment cost. If your equipment destroys a building in Canada, you're liable for the equipment cost and the building cost and the cost of putting out the fire, or something along those lines. Not actually that different, but I was surprised to how much more Canadians hold people culpable compared to the US.
I remember those optocouplers. Didn't I feel like a bright spark including them in a new layout......and didn't their hollow tubular component turned out to be made of highly flammable plastic .... ...well, not self-extinguishing, anyways.
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