C
Chris Carlen
- Jan 1, 1970
- 0
Hi:
Any tips I might want to consider for safety when building a gadget (my
Nixie clock) that will plug into 120-240VAC and use only a two-wire cord?
I have a 5x20mm 250VAC fuse in the hot before the transformer primary.
I even have pads on the PCB if I want to install a thermal fuse which
would cut the line voltage if the transformer core overheats due to an
overload that isn't enough to blow the line fuse, a failure mode which
could start a fire without the thermal fuse, but which is rarely
protected against in consumer electronics.
I am planning to put a 1Meg resistor from the signal GND to the neutral
of the power line. That's so any static discharges get dissipated and
the signal ground can't float with respect to earth, assuming of course
that the wall socket is wired with the correct polarity. But with 1Meg,
even if the polarity is backwards, it can't kill anybody.
But perhaps I should also put a cap from the signal GND to neutral?
Does it have to be a "safety" cap? What exactly is the point of safety
caps anyway? Are they just rated for high surge voltages and able to
tolerate continuous, albeit negligible, dissipation? The only thing I
can see it would do is in the event of a static discharge, it would
prevent a large peak voltage from developing on the secondary side.
I have plenty of creep distance, at least 10mm, from any line traces to
the low voltage traces.
Should I add any other surge protection or safety features?
Comments appreciated.
Good day!
Any tips I might want to consider for safety when building a gadget (my
Nixie clock) that will plug into 120-240VAC and use only a two-wire cord?
I have a 5x20mm 250VAC fuse in the hot before the transformer primary.
I even have pads on the PCB if I want to install a thermal fuse which
would cut the line voltage if the transformer core overheats due to an
overload that isn't enough to blow the line fuse, a failure mode which
could start a fire without the thermal fuse, but which is rarely
protected against in consumer electronics.
I am planning to put a 1Meg resistor from the signal GND to the neutral
of the power line. That's so any static discharges get dissipated and
the signal ground can't float with respect to earth, assuming of course
that the wall socket is wired with the correct polarity. But with 1Meg,
even if the polarity is backwards, it can't kill anybody.
But perhaps I should also put a cap from the signal GND to neutral?
Does it have to be a "safety" cap? What exactly is the point of safety
caps anyway? Are they just rated for high surge voltages and able to
tolerate continuous, albeit negligible, dissipation? The only thing I
can see it would do is in the event of a static discharge, it would
prevent a large peak voltage from developing on the secondary side.
I have plenty of creep distance, at least 10mm, from any line traces to
the low voltage traces.
Should I add any other surge protection or safety features?
Comments appreciated.
Good day!