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Slots cut into power supply PCB.

S

Sylvia Else

Jan 1, 1970
0
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.

Anyone know the rationale for the slots?

Sylvia.
 
S

Sylvia Else

Jan 1, 1970
0
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.

Anyone know the rationale for the slots?

Sylvia.

Never mind, although my initial searches producing nothing, further
effort yielded this TI document.

http://www.ti.com/lit/ml/slup227/slup227.pdf

Sylvia.
 
M

Martin Riddle

Jan 1, 1970
0
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.

Anyone know the rationale for the slots?

Sylvia.

Creepage distance. A milled slot offers greater creepage distance vs
FR4.

Cheers
 
D

Don Y

Jan 1, 1970
0
Hi Sylvia,

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.

Anyone know the rationale for the slots?

Termite damage! ;)
 
S

Sylvia Else

Jan 1, 1970
0
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.

Anyone know the rationale for the slots?

Sylvia.

On a related note, and referring to the document whose link I posted
previously,

http://www.ti.com/lit/ml/slup227/slup227.pdf

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,

http://www.farnell.com/datasheets/1726515.pdf

Anyone have any thoughts on that?

Sylvia.
 
S

Spehro Pefhany

Jan 1, 1970
0
On a related note, and referring to the document whose link I posted
previously,

http://www.ti.com/lit/ml/slup227/slup227.pdf

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,

http://www.farnell.com/datasheets/1726515.pdf

Anyone have any thoughts on that?

Sylvia.

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
 
A

Adrian Tuddenham

Jan 1, 1970
0
[...]
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,

http://www.farnell.com/datasheets/1726515.pdf

Anyone have any thoughts on that?

I have seen the control board of a dehumidifier with tiny clearances and
no slots. When it was placed in a humid environment and switched on, it
blew up. :)
 
L

legg

Jan 1, 1970
0
On a related note, and referring to the document whose link I posted
previously,

http://www.ti.com/lit/ml/slup227/slup227.pdf

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,

http://www.farnell.com/datasheets/1726515.pdf

Anyone have any thoughts on that?

Sylvia.

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.

RL
 
S

Spehro Pefhany

Jan 1, 1970
0
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.

RL


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
 
W

whit3rd

Jan 1, 1970
0
It seems odd to me that the functional (not safety) clearance
requirements at mains voltages...are higher than the distance between the
pins for a typical mains voltage triac.

It's probably because the triac has normal conduction there (so the triad
pins becoming a short is protected by other components, i.e. a fuse).
Careful users of HV components don't always use the TO-220 center
pin, that can be cut off and the tab used for GT2 connection instead, which
adds to the printed wiring board standoff distance. Also common, bending
leads to make a triangle of the PWB pads.

Safety clearances are intended to keep ground wires from conducting even in
short multi-kilovolt transients, like lightning events. Unless your triac
has a ground (as opposed to neutral) wiring connection, it isn't in the path
of greatest concern.
 
S

Sylvia Else

Jan 1, 1970
0
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.

Thanks for the info.

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?

Sylvia.
 
R

RobertMacy

Jan 1, 1970
0
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 is my understanding that BEFORE AC mains protection - observe all
safety and creepage distances sometimes requiring slots in the PCB. [after
all we're talking about 10 to 20 Amps of current available here.] AFTER
some type of AC mains protection - be careful about spacing but don't need
slots anymore because you're circuitry is 'behind' the protection network.

Further, testing at NRTL's is done by simulating catastrophic events, if
that results in fire, you fail. That's why you send a 'sacrificial' unit
to the Safety NRTL's, because they cut it up, blast it, burn it [testing
for the generation of toxic fumes, etc. sometimes give them a couple. PS:
saves a bit *if* you can list every material as already allowed, provide
schematics showing you've done everything right, and give them
construction details you know they'll agree to [like appropriate 3 level
isolation-types of 3=5 mil kapton inside transformers, built-in fuse
links, those kinds of things]
 
R

RobertMacy

Jan 1, 1970
0
...snip...
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?

Sylvia.

join the IEEE discussion group [don't have to be a member to join the
forum] for Product Safety and EMC
Website: http://www.ieee-pses.org/
Instructions: http://listserv.ieee.org/request/user-guide.html
List rules: http://www.ieee-pses.org/listrules.html
If you have any problems/questions email me directly and I'll provide
email addresses for the group's moderators.

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.
 
L

legg

Jan 1, 1970
0
I can recall building a power supply to, IIRC, VDE specs... 3/4" long
opto-coupler straddling a slot in the PCB.

...Jim Thompson

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.

I'm sure that's all been fixed by now.

RL
 
R

Robert Baer

Jan 1, 1970
0
Yes, those slots are exactly to increase isolation and safety.
They seem to be more common in CFL circuits, especially at/near the
xfmr outputs..
 
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