3 ms·

[Disclaimer: I've put products successfully through UL] The OP is correct, for a non-isolated product (one with user exposed metal bits and connectors) you're

[Disclaimer: I've put products successfully through UL] The OP is correct, for a non-isolated product (one with user exposed metal bits and connectors) you're not paying enough attention to separation and protection. UL would probably require double insulated wires, wire guides, and moving your wire lug away from logic circuits, adding a fuse, along with all sorts of requirements on labeling, etc etc. Also, are you calculating RMS current or average current? Without a voltage tap, you also can't determine power due to power factor.

trafficlight 15y ago
Do you have any recommended documentation for doing these things properly? I'm pretty naive when it comes to this stuff, so any information about best practices would be appreciated.
- 0x12 15y ago
  I think the easiest way for you to significantly improve the circuit from a safety perspective is to modify the layout in such a way that the high voltage part of the circuit is on one end of it and the low voltage part on the other. A (very safe) alternative to using the hall sensor the way it is wired up right now: consider wrapping the current carrying wire around a ferrite core and putting a number of turns around the other side of the core to pick up the current (a very basic transformer). That way there is absolutely no need to go 'near' to the low voltage part of the circuit with the high voltage portion of the wiring. That's called a current sensing transformer. You could also place the hall sensor if you really want to use that in the gap of a slotted toroid around which the primary wire is wound a number of turns. Adding a fuse is a basic precaution, you should size it based on the wire thickness that you are using. You are doing pretty good but beware of knowing 'just enough to be dangerous'. Alternatively once you've finished prototyping the circuit you could hand off the process of packaging it in a way that complies with the various standards to an electrical engineering firm, from a liability perspective that might be a good idea anyway. hth
jrockway 15y ago
I don't think he cares about accuracy of the result; all he needs is a binary on off. So you measure the output of the circuit with the TV off, make a note, measure the output of the circuit with the TV on, make a note, and set the threshold (in software), somewhere in between. Then you know if the TV is off or not. (I would personally have pasted a photocell onto the power LED, if I really needed to know the TV's state, but that's just me. My TV doesn't have a button for "power on", but it responds to a remote code for one.)

trafficlight 15y ago

Do you have any recommended documentation for doing these things properly? I'm pretty naive when it comes to this stuff, so any information about best practices would be appreciated.

0x12 15y ago
I think the easiest way for you to significantly improve the circuit from a safety perspective is to modify the layout in such a way that the high voltage part of the circuit is on one end of it and the low voltage part on the other. A (very safe) alternative to using the hall sensor the way it is wired up right now: consider wrapping the current carrying wire around a ferrite core and putting a number of turns around the other side of the core to pick up the current (a very basic transformer). That way there is absolutely no need to go 'near' to the low voltage part of the circuit with the high voltage portion of the wiring. That's called a current sensing transformer. You could also place the hall sensor if you really want to use that in the gap of a slotted toroid around which the primary wire is wound a number of turns. Adding a fuse is a basic precaution, you should size it based on the wire thickness that you are using. You are doing pretty good but beware of knowing 'just enough to be dangerous'. Alternatively once you've finished prototyping the circuit you could hand off the process of packaging it in a way that complies with the various standards to an electrical engineering firm, from a liability perspective that might be a good idea anyway. hth

jrockway 15y ago

I don't think he cares about accuracy of the result; all he needs is a binary on off. So you measure the output of the circuit with the TV off, make a note, measure the output of the circuit with the TV on, make a note, and set the threshold (in software), somewhere in between. Then you know if the TV is off or not. (I would personally have pasted a photocell onto the power LED, if I really needed to know the TV's state, but that's just me. My TV doesn't have a button for "power on", but it responds to a remote code for one.)