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Thread: Possible danger when using low voltage signal wiring from 240vac black box?

  1. #1
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    Possible danger when using low voltage signal wiring from 240vac black box?

    I was wanting to control a cheap ($7) rgb floodlight that comes with IR remote with an arduino. We'll say teensy but really probably a pro mini or something of similar quality to the light.

    I have made a slight tweak to to the IRremote library to let me send the IR code directly to the floodlight without the need for a IR LED on my side or the IR receiver on the floodlights side.

    The question I have is I'm connecting my 5vdc external wiring to this lamps 5vdc wiring coming out of its resin sealed black box.

    What is an easy way to insure isolation between the floodlights data/gnd and the floodlights 240vac supply?

    Or put another way, prevent the possibility of a failure in the floodlights black box can't liven up my external arduino and wiring.

    If inside my floodlights black box is a voltage devider across the mains and rectified.

    Thanks.

    I'm an electrician, so I feel safe maintaining correct isolation when making connections inside floodlight.(I know some people worry when reading posts like this)

    On a side note. What is done in normal certified equipment like a computer power supply to prevent a component failure livening up the elv supplies? E.g. If an isolation transformer fuses together

    Not the lamp in question, but took this the other week. A 50W floodlight installed in our factory.
    Click image for larger version. 

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    Last edited by Gibbedy; 08-07-2017 at 08:05 AM.

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    Senior Member Epyon's Avatar
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    A crowbar circuit an/or some other form of voltage limiting like zeners or TVS should protect your power supply and/or data lines from spikes of overvoltages. If you really want to isolate the Teensy you can use an isolated DC-DC converter for power and optocouplers on the data lines. Not that expensive, just some more work.

    Higher-end power-supplies use isolated DC-DC converters. Because the isolation transformer is in the high-frequency switching part of the supply, it can be made very small and thus cheap.

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    Right answer is DC-DC converter actually rated for isolating 240V + opto isolation for data, but that's a lot of work for a one off project.

    Given the quality wiring on show with the earth wire left unconnected I'd take it as a given that the power module will not offer any true isolation. First check is to with power off check if either red or black wires have continuity to blue or brown, and then with some care measure the voltage between black and true earth with power on, both DC and AC. Quite possible you will find it's actually about ~100 volts when not actually earthed. Have a couple of USB chargers around the place that do that and while the current isn't normally enough to shock it'll often make connecting to another eletronic device exciting.

    For a home project and given your knowledge you can probably come up with a design where 0V is properly earthed and the 5V wire has a fuse and a husky 12V zenier sized big enough to blow said fuse when more than 12V shows up. Then just apply some sane design to your buttons etc to give yourself the equivalent of double isolation and call it done.

    A check of the 5V supply during power on may also be worth a look in case it's one of these ones that over shoots a bit.

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    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by Gibbedy View Post
    On a side note. What is done in normal certified equipment like a computer power supply to prevent a component failure livening up the elv supplies? E.g. If an isolation transformer fuses together
    Most AC powered equipment uses earth grounding, called Class I. Small devices are often Class II, using 2 insulation layers. Isolation transformers in Class II gear typically have special plastic bobbins and sometimes special wire with extra insulation.

    https://en.wikipedia.org/wiki/Appliance_classes

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    Epyon: ok. I will look into TVS. One for gnd and one for IR signal down to earth I imagine.
    Gremlin: sounds like the way to go. Tested today and it 5v side is isolated from 240 at least to start with.
    Paul: Thanks.

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    Senior Member Epyon's Avatar
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    Quote Originally Posted by GremlinWrangler View Post
    Given the quality wiring on show with the earth wire left unconnected I'd take it as a given that the power module will not offer any true isolation.
    It doesn't need to, it's a class II appliance (see the double square icon). They are all VDE/ISO safety icons. It is also only rated for indoor use (icon of the house).

    Quote Originally Posted by Gibbedy View Post
    Gremlin: sounds like the way to go. Tested today and it 5v side is isolated from 240 at least to start with.
    I don't know how you measured it, but you can't measure it reliably with just the continuity test of a multimeter because of the high inductance of the internal coils. To know if it's isolated, the 'galvanic isolation' symbol should be printed on the power supply. The power supply should have this symbol:

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    http://www.cycloflow.com/2014/11/mar...nsformers.html

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    I didn't know any of the symbols other than double insulation so thanks for that. Remember this isn't the one I was asking questions about, but the floodlight as a whole is class 1 (or at least has no double insulated symbol on the outside).

    I beleive because in this case because there is single insulation between live conductors and the metal enclosure the enclosure has to be earthed.

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    Senior Member Epyon's Avatar
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    I can only speak for European situations, but here all touchable metal parts of a system or construction must be grounded, unless they are double insulated. The PSU in your image doesn't need to be grounded because it is double insulated, but the enclosure isn't and indeed so needs to be earthed. This is the general rule with TT earthing systems like here, but in other parts of the world it will probably be comparable.

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    I am not an EE, but I've read up on these things for a long while. Take what I say with a grain of salt, and I'd love any corrections if I got something wrong!

    TLDR: Grounded metal case, Physical separation design of PCB and wiring and mounting, Isolation rated input circuitry.

    In general, AC / DC systems are designed with sufficient clearance between the two domains (some number of millimeters) and additionally either using double-insulation, or grounded-chassis, as described above.

    If you take apart devices that have both low-voltage and high-voltage domains, you will typically see that the PCB has significant spacing between the two copper areas, and often a silk screen line showing where the isolation domains separate. Making sure that no loose wiring can stray between the domains is 90% of the additional work needed. Safety grounding a metal enclosure is also quite helpful. Don't connect safety ground to DC/DC converter output 0V reference, though! That's super bad!

    When you need to hook into the system above, there are safety concerns (don't kill humans,) and there are integrity concerns (don't malfunction.)
    TVS diodes on the low-voltage control wiring is mostly about integrity. A TVS that can do high enough power to protect against an accidental short to mains (which should be physically impossible because of physical isolation distance) will have too high capacitance to transmit signals at more than a few kilohertz reliably.

    To physically isolate the device, you will want to use an opto isolator rated for sufficient electrical insulation. The isolator would expose to wires to be connected to the outside world; these would be isolated from the rest of the appliance. The receiving end of the isolator would be powered by whatever DC converter is on the inside of the enclosure. Make sure the connector for the opto isolator input is physically isolated from the other wiring, and you'll be OK. A small-power TVS across the isolator input will also help protect against possible inductive spikes / EMF on the input wiring. (Ideally, use wiring with chokes on it, too, but now we're purely into integrity territory, not safety territory.) Finally, a resistor on the input will give it some robustness to "wrong" voltage on the input port. (Can be very effective together with TVS.)

    There are tons of parts made to solve exactly this problem, as long as your physical layout/assembly is safe. For example, LTV-817: http://optoelectronics.liteon.com/up...0201610%20.pdf
    This is the cheapest single-quantity part available at Digi-Key -- fancier parts have higher current transfer ratio, faster response time, built-in robustness features, and so forth. There's even parts that use active on-chip magnetics (rather than opto) to signal at very high data rates while retaining high isolation ratings -- not as cheap though :-)

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    Thanks for that. Interesting.

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