Help thinking about Grounding/Safety for LED device - Page 1
Forum section: electronics / beginners

author: jai.whitey
author_rank: Newbie
timestamp: October 19, 2021, 04:25:58 pm
content: Hello - I'm creating an LED device and I'd like to have a good understanding of grounding/safety so I don't accidentally harm my users!I'm powering my device with an AC/DC wall mount adapter that outputs 20V which connects to a barrel jack on my PCB. I'm using an LED driver as well that outputs 700mA constant current. My LED itself has a forward voltage of 17.2V at 700mA at test temperature (85C).I connect to my LED with wires off-the-board. I'm using screw terminal blocks on the PCB itself as this seems to be the most secure connection. And I solder my output wire leads to the LED for a secure connection.However, I'm using aluminum for the light stand/enclosure itself and I'm worried one of the LED hot wires could potentially become disconnected from the LED and then make a connection to the aluminum enclosure, giving it a voltage potential. And then when a user touches the device, they could become shocked.How can I make sure nothing catastrophic can take place? My understanding is that to ground a device, a wire from the metal enclosure should be connected to the ground plug in the AC plug, so if the enclosure gains a voltage potential, it will be dissipated through the ground wire.However, in my case I'm using the AC/DC wall mount adapter so I'm only dealing with DC within my device.As you can see, I'm a novice so any tips or pointing me in the right direction would be much appreciated.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2021, 05:14:59 pm
content: 20V is not a shock hazard. You can touch 20V and won't feel a thing. (FYI, around 60V is where safety starts to become an issue.)So if there's no mains power inside, then grounding the enclosure is not necessary for safety.What I would do, of course, is make sure the wires are well secured, since you want the thing to be reliable. In particular, where stranded wire is soldered tends to be a weak spot, so that end needs to be supported well.

author: jai.whitey
author_rank: Newbie
timestamp: October 19, 2021, 06:29:00 pm
content: Thanks a lot. Can you explain this more? Even with DC power, say the positive wire to the LED becomes detached somehow, wouldn't this represent a hot voltage of 20V or whatever voltage my driver is outputting. If I touch it, wouldn't the electricity want to flow through me to the earth ground? As you say, 20V is not shock hazard but at higher voltages this would be dangerous?

author: Benta
author_rank: Super Contributor
timestamp: October 19, 2021, 06:29:46 pm
content: Agree with tooki, electric shock is not a hazard here.What you do need to look at is fire hazards, eg, what happens if something is shorted or broken or gets too hot somewhere; and that can be quite challenging to analyze.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2021, 07:11:32 pm
content: Quote from: jai.whitey on October 19, 2021, 06:29:00 pmTechnically speaking, at 20V a tiny bit of electricity could flow through you to ground. But it's such a minuscule amount of current that it simply does not matter. You wouldn't feel it and it wouldn't harm you.By the international IEC standards, DC under 120V and AC under 50V is consideredextra-low_voltageand poses low risk of electric shock. Your 20V DC isfarbelow that. (You'd feel 120V DC, but it wouldn't hurt you.)Honestly, the biggest thing you can do to ensure customer safety is to buy top-quality wall adapters. The only situation where I could see your product becoming hazardous would be if a) a wire shorted to the metal case, AND at the same time b) the wall adapter failed in an unsafe way that put mains on its output. You're entirely responsible for a), but it's easy to do. The power supply manufacturer is responsible for b), so your responsibility is simply to include a quality wall adapter. That means not buying no-name stuff from Alibaba. Even a "budget" name brand like Mean Well will be perfectly safe.

author: jai.whitey
author_rank: Newbie
timestamp: October 19, 2021, 07:18:07 pm
content: Thanks a lot.As far as ensuring A (wire shorted to metal case) does not happen, this means making sure my solder joints are all solid and secure? Anything I'm missing here?

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 19, 2021, 07:18:25 pm
content: Nothing to be concerned about based on your description, unless it's to be used in a saltwater puddle.If you manage to feel anything from that what you'll feel is the tingle of AC leakage through the safety capacitor in the AC adaptor, which is normal and kept at a harmless current if the adaptor is properly designed and built (avoid the cheap unbranded ones).

author: themadhippy
author_rank: Super Contributor
timestamp: October 19, 2021, 07:43:42 pm
content: Quoteonly if your using a non isolated supply.Adding an electrical earth to the secondary side of an isolated supply may actually increase the risk of a shock in certain fault conditions

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2021, 07:46:00 pm
content: Quote from: jai.whitey on October 19, 2021, 07:18:07 pmSecure wires against flexing, vibration and chafing. So don't have them flopping around inside, route them nicely, secured where possible. Smooth or cover any rough edges the wire could rub against. Provide strain relief.

author: jai.whitey
author_rank: Newbie
timestamp: October 19, 2021, 11:37:44 pm
content: Quote from: themadhippy on October 19, 2021, 07:43:42 pmNon isolated supply meaning somehow AC voltage seeps through to the secondary DC side? And this represents a shock hazard simply because of the high voltage and not anything special about AC voltage?Can you explain what an electrical earth on the secondary side would be? Would this be if I connect a wire from my metal enclosure to the ground pad on PCB and then connect this pad to the Earth somehow? Or simply just a connection to the ground pad on the PCB?

author: tooki
author_rank: Super Contributor
timestamp: October 20, 2021, 01:24:11 am
content: Quote from: jai.whitey on October 19, 2021, 11:37:44 pmIsolation, in the context of power supplies, means "galvanic isolation", meaning there is no electrical connection between the primary and secondary side — it all happens via some other method. In the case of power supplies, a transformer provides galvanic isolation, and thus the energy is transferred as a magnetic field. (Galvanic isolation of signals is often done with magnetic fields (signal transformer) or light (optocoupler).)If you try to measure continuity from the primary to secondary of an isolated power supply, it should read open circuit.Non-isolated AC-DC supplies do exist, but are rarely found outside of "white goods" appliances and cheap LED bulbs. In such a supply, there is a direct electrical connection between the secondary side and mains.Quote from: jai.whitey on October 19, 2021, 11:37:44 pmCorrect.Quote from: jai.whitey on October 19, 2021, 11:37:44 pmLike if you took the housing and connected it to the earth pin of an AC plug, and connecting it to the negative ("ground") of your circuit on the secondary side.A few key concepts to understand:- what a "floating" circuit is- what your "reference" is (i.e. where is the circuit's 0V point? most often, in DC circuits, the negative)- what it means to reference a circuit to groundRead this textbook chapter on electrical safetyhere.

author: jai.whitey
author_rank: Newbie
timestamp: October 20, 2021, 01:32:39 am
content: Quote from: tooki on October 20, 2021, 01:24:11 amThanks a lot again. The wall wart is a 2 pronged inlet plug so I don't believe it makes a connection with the earth pin of the AC wall socket. So, with my current choice of wall wart, I don't think I could make a true electrical earth grounding.But I can still make a wire connection between the metal enclosure and the negative/ground of my circuit on the PCB and use a fuse to detect if there is a short circuit. This would still be worthwhile correct as a safeguard against fire hazard?

author: tooki
author_rank: Super Contributor
timestamp: October 20, 2021, 02:21:37 am
content: A fuse is never a bad idea. I would use one.But absent a specific reason to tie the enclosure to 0V (the "negative" of your power supply), I'd just leave it floating. Then you'd have to haveboththe negative and positive accidentally short to the case at the same time. (In which case the short circuit would immediately blow the fuse.)

author: jai.whitey
author_rank: Newbie
timestamp: October 20, 2021, 03:56:34 am
content: I see, I think I understand now. Th 0V/negative on my board is a more powerful pulling force of electricity from the positive lead than the earth ground. For some reason I was equating the two as a same negative return path.

author: tooki
author_rank: Super Contributor
timestamp: October 20, 2021, 09:12:53 am
content: Quote from: jai.whitey on October 20, 2021, 03:56:34 amErm… huh? Voltage is the name of the force (voltage is actually formally known as electromotive force). We often talk about “potential” when referring to it. Voltage is always thedifferencein potential between two points, but for convenience, we always define a 0V reference point, and usually call it “ground” (which causes confusion when distinguishing between circuit “ground” and earth ground). That way, we can talk about a voltage at a given node without having to always give a second node; when we see a voltage given at a single point, we assume it to mean “with respect to circuit ground”.In mains circuits, circuit ground (0V) and earth ground are at the same potential. It is strictly for human safety that we have separate ground and “neutral” (0V) wires in AC connections; the load itself wouldn’t care at all if ground and neutral were merged.When a circuit has no point of it connected to earth ground (i.e. not referenced to earth ground), it’s a “floating” circuit. It is a closed circuit within itself, so if you measure it with a meter, there’s 0V between earth ground andanypoint in your circuit.Back to your statement in bold:Electric current must flow in a closed loop. (Indeed, electrocution dangers arise when your body inadvertently closes a circuit of sufficient voltage.) Your circuit (the secondary side of the power supply, plus the load) is a complete closed circuit. Earth ground isn’t a part of the circuit, so there’s no path for current to flow from the 20V side of your circuit.

author: jai.whitey
author_rank: Newbie
timestamp: October 20, 2021, 03:06:02 pm
content: Quote from: tooki on October 20, 2021, 09:12:53 amI see thanks, so it is not a complete circuit path, so any shock I do receive when I touch a high voltage wire in a floating circuit would be a sudden shock of static electricity but won't be a sustained flow of current.

author: tooki
author_rank: Super Contributor
timestamp: October 21, 2021, 10:59:48 pm
content: Quote from: jai.whitey on October 20, 2021, 03:06:02 pmIn theory there wouldn’t be any static, either. A floating circuit, in principle, is safe to touchat any single pointbecause no current flows.This is demonstrated in dramatic fashion by the linemen who work on live high-tension power lines by floating upliterallyin a helicopter, which is then held at line voltage, as is the crew which then tethers to the live wire. So they’re actually floating up at hundreds of kV, but are safe because they’re not closing a circuit. But if they were to come too close to the (grounded) towers or one of the other lines, it’d be instant death as the high voltage closed the circuit through them.What floating doesn’t protect against is if you touchtwopoints in the circuit. If there’s enough voltage, it will shock you. This is why mains isolation transformers only provide limited safety. If you touch the 0V terminal of a floating transformer secondary with one hand, and touch the 120V terminal of the secondary with your other hand, you’ll be in just as much trouble as if you’d touched ground and the 120V mains, since the current flows from one hand to the other via your chest, which is exactly where we don’t want any current because it can stop your heart. (That’s why you keep one hand in your pocket when working around live mains, especially with an isolation transformer.)With that said: Your light, being powered by a DC wall adapter (no mains inside the metal case), is perfectly safe for you to work on or sell.