Help thinking about Grounding/Safety for LED device

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
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 board 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 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.
 

Tonyr1084

Joined Sep 24, 2015
6,459
First, welcome to AAC.

Now, if I understand you correctly you have a power supply that converts 120VAC into 20VDC. I have yet to see a power supply, what we often call a wall wart or a power brick with a ground wire, though with the power bricks it is more common to see them grounded. As for the low DC voltage, there's not much risk of hurting anyone. Still, grounding isn't a bad idea. Probably not necessary, but I'm just guessing at this point. Take a picture with your cell phone, then send the picture to your email and drag it to the computer desk top. Then click "Attach files" and upload a picture of exactly what you're working with. I'm leaning on saying you don't need to ground the lamp. Especially if the power supply itself is grounded. But it never hurts to be sure of exactly what we're dealing with.
 

BobTPH

Joined Jun 5, 2013
4,046
20V DC is not a shock hazard. And if nothing is grounded, touching one conductor cannot shock you even with a higher voltage.

Bob
 

Tonyr1084

Joined Sep 24, 2015
6,459
20V DC is not a shock hazard. And if nothing is grounded, touching one conductor cannot shock you even with a higher voltage.

Bob
I generally agree with this statement. However, some cheap Chinese made power supplies can present a dangerous voltage. I haven't seen it, but I've heard of it. Whether true or not - I don't know, but I heard of a 5V power supply supplying a lethal voltage to a person in a tub. May be a myth, maybe not. But again, in general, I agree with Bob on this one, as I stated in my first post that I don't see there being a need for a ground.
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
20V DC is not a shock hazard. And if nothing is grounded, touching one conductor cannot shock you even with a higher voltage.

Bob
Thanks a lot for the reply. Can you explain this more? My understanding is: say the positive wire to the LED becomes disconnected. Doesn't that represent a positive potential voltage of 20V or whatever voltage my LED driver is supplying? If I somehow touch that wire, wouldn't the electricity want to flow through my body to the earth ground?

If the wire somehow makes connection with my outer aluminum enclosure/stand it and I touch the outer enclosure, the same logic would apply?

As you noted 20V is not shock hazard, but at higher voltages would this not be dangerous?
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
First, welcome to AAC.

Now, if I understand you correctly you have a power supply that converts 120VAC into 20VDC. I have yet to see a power supply, what we often call a wall wart or a power brick with a ground wire, though with the power bricks it is more common to see them grounded. As for the low DC voltage, there's not much risk of hurting anyone. Still, grounding isn't a bad idea. Probably not necessary, but I'm just guessing at this point. Take a picture with your cell phone, then send the picture to your email and drag it to the computer desk top. Then click "Attach files" and upload a picture of exactly what you're working with. I'm leaning on saying you don't need to ground the lamp. Especially if the power supply itself is grounded. But it never hurts to be sure of exactly what we're dealing with.
Thanks a lot for the reply. I'm curious as to what safety/grounding practices the common, low-voltage DC consumer electronics devices I interact with employ, if any.

Also, as you noted, I have a AC/DC wall wart that connects to my PCB and then I employ a constant current buck converter for my LED driver. I think this may be redundant as I've seen AC/DC led drives. But that's another issue.
 

Tonyr1084

Joined Sep 24, 2015
6,459
The following illustrations assume the occupant is grounded in some way, shape or form.

When dealing with line voltages there's the potential for a shock hazard and possible death. This illustration shows that touching the line can shock you (or worse) whereas touching the neutral line of a properly wired and functioning circuit presents no danger. On the "Low V" side, the secondary is isolated from ground. Touching any of those lines, even if they're not low voltages, there's virtually no danger; except for touching both lines of the secondary "Low V" side.
1634667430871.png
This next illustration shows what can happen if the wiring becomes disconnected on the "Neutral" side. Touching an open neutral line can result in nearly the same full voltage as the line voltage. Yet, even in this failure mode touching the Low V side does not result in any danger, except as noted, touching both wires of the Low V side. However, "Low Voltage" is generally understood to be 50 volts or less Click Here for a link.
1634668184665.png

This poor guy, grounded or not, is having a really bad day.
1634668007685.png
 

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Tonyr1084

Joined Sep 24, 2015
6,459
I'm curious as to what safety/grounding practices the common, low-voltage DC consumer electronics devices I interact with employ, if any.
Generally, any secondary voltage is going to be isolated from the mains and main ground. The mains come from a center tapped transformer (commonly in the United States and possibly other places). The center tapped transformer is considered the neutral line, which is also tied to earth ground. The two ends of the transformer are considered L1 and L2. L1 to neutral gives you 120VAC, and L2 to neutral also gives you 120VAC. However, L1 to L2 gives you 240VAC. The neutral line on the transformer is ALWAYS grounded at the pole and should be grounded at your service panel as well. A grounded plug should have its own ground wire path back to ground and not to neutral. A power supply plugged into an outlet, grounded or not, should be isolated from the low voltage user side of the supply. This is not always true, however. And there have been reported cases where one leg of the secondary low voltage side may be actually connected to line voltage. This is what I was referring to in post #4. Some cheap supplies may use a capacitor phase blocking system (I don't know the correct name for it) to produce the lower voltage. I've certainly seen this in some cheap LED House Light Bulbs. They are horrible methods of reducing the voltage and can still present full voltage to the end user; and thus, a severe shock hazard may exist.

IF you ground the housing of the lamp, any such circumstances will be more safely channeled away from an end user, but I can't guarantee that. Still, grounding is a choice you make on your own. Me? I probably would forego grounding the lamp unit. But that'd be MY choice; not necessarily yours.
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
Generally, any secondary voltage is going to be isolated from the mains and main ground. The mains come from a center tapped transformer (commonly in the United States and possibly other places). The center tapped transformer is considered the neutral line, which is also tied to earth ground. The two ends of the transformer are considered L1 and L2. L1 to neutral gives you 120VAC, and L2 to neutral also gives you 120VAC. However, L1 to L2 gives you 240VAC. The neutral line on the transformer is ALWAYS grounded at the pole and should be grounded at your service panel as well. A grounded plug should have its own ground wire path back to ground and not to neutral. A power supply plugged into an outlet, grounded or not, should be isolated from the low voltage user side of the supply. This is not always true, however. And there have been reported cases where one leg of the secondary low voltage side may be actually connected to line voltage. This is what I was referring to in post #4. Some cheap supplies may use a capacitor phase blocking system (I don't know the correct name for it) to produce the lower voltage. I've certainly seen this in some cheap LED House Light Bulbs. They are horrible methods of reducing the voltage and can still present full voltage to the end user; and thus, a severe shock hazard may exist.

IF you ground the housing of the lamp, any such circumstances will be more safely channeled away from an end user, but I can't guarantee that. Still, grounding is a choice you make on your own. Me? I probably would forego grounding the lamp unit. But that'd be MY choice; not necessarily yours.
Thanks.

It seems there may a risk of fire hazard, if something gets shorted and too hot somewhere. I'm thinking of safety measures I could implement in my circuit to detect a short and shut down power somehow.
 

DrBearEE

Joined Feb 3, 2020
3
I don't disagree with the technical part of the answers above. If you are making one or two of these and just want to be sure they are safe to use, the advice so far is enough, especially the "use the correct fuse for your wire size".

You can think of this from the point of view of a safety hazard analysis. What are the hazards? Are they mitigated? You've already considered the line voltage hazard, and purchased components (your mains to 20 V DC wall wart supply) that have adequate protection built in. Some safety engineer test, and you already paid for it when you bought the supply. The supply *is* CE marked, or UL listed, right?

Now, consider if there is a shock hazard at the low voltage side? 20 volts is low enought that the US NFPA "electrical code" classifies it as "Class 2", and European Directives classify it Extra-Low Voltage (ELV), both saying there is no plausible scenario where a 20 V supply can push enough current through your body to cause pain or damage. So far so good. All that said, there is no reason NOT to ground any metal parts, and that brings us to the other hazard.

Another hazard to consider is whether there might be a failure mode where the fault current makes something get hot, maybe even hot enough to catch fire. Here, you have to consider not just the voltage, but the energy (or available current). An example would be a stick welder, where voltage may be ELV, but its intended to source 100 A, and that can certainly make things hot enough to burn. So to mitigate that hazard, you want to know where a fault current might flow, and protect that. In this case, a chafed wire, for example, might short the 20 V supply to the case. If that happened, enough current might flow to cause the cord to get hot, and burn you, or start a fire. The easiest way to mitgate that hazard is to ground the metal case to the secondary supply and place a fuse in the circuit that will blow if the current is more than the lamp ever needs normally, then size your wire so that it won't overheat before the fuse blows. Or, choose a power supply that has electronic energy limiting, such as fold back current limiting.

Consider the failure scenarios and mitigate them.

If you intend to make a bunch of these and sell them, then you likely have regulatory reponsibilities, and you need someone that knows the rules in your jurisdiction. Where I am in the US, there are labs that will assess a product against the UL safety standard and you can even get a product affirmatively listed as safe, but the testing and listing fees are not cheap. I try to design everything to meet the UL and EU standards, and most other jurisdictions coordinate with essentially the same rules.
 

Reloadron

Joined Jan 15, 2015
6,214
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).
Welcome Aboard!
Starting there we generally refer to AC/DC wall mount adapters as a wall wart as was mentioned. Speaking for here in the US and only in the US most of these devices do not include a ground lug and the output DC is isolated from the AC side mains. There should exist no shock hazard. That said if you want to and it certainly won't hurt place a grounding lug on your aluminum fixture and arrange for it to be grounded. The trick then becomes finding a ground to connect to. This is likely best done at the power source. You can look at any codes depending on your product distribution scheme but as mentioned placing a little UL (Underwriters Laboratory) comes with a cost. A 20 VDC wall wart is considered low voltage and presents no shock hazard. I would look at any applicable codes and then decide where you want to or have to go.

Ron
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
Welcome Aboard!
Starting there we generally refer to AC/DC wall mount adapters as a wall wart as was mentioned. Speaking for here in the US and only in the US most of these devices do not include a ground lug and the output DC is isolated from the AC side mains. There should exist no shock hazard. That said if you want to and it certainly won't hurt place a grounding lug on your aluminum fixture and arrange for it to be grounded. The trick then becomes finding a ground to connect to. This is likely best done at the power source. You can look at any codes depending on your product distribution scheme but as mentioned placing a little UL (Underwriters Laboratory) comes with a cost. A 20 VDC wall wart is considered low voltage and presents no shock hazard. I would look at any applicable codes and then decide where you want to or have to go.

Ron
Thanks, if I understand you correctly regarding the grounding lug - I would connect a wire from the metal enclosure to the ground pad on my PCB (the ground negative pad for my DC barrel jack)?

Taking a look at UL, it seems I would want a "UL 1598, Luminaires" certification.
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
I don't disagree with the technical part of the answers above. If you are making one or two of these and just want to be sure they are safe to use, the advice so far is enough, especially the "use the correct fuse for your wire size".

You can think of this from the point of view of a safety hazard analysis. What are the hazards? Are they mitigated? You've already considered the line voltage hazard, and purchased components (your mains to 20 V DC wall wart supply) that have adequate protection built in. Some safety engineer test, and you already paid for it when you bought the supply. The supply *is* CE marked, or UL listed, right?

Now, consider if there is a shock hazard at the low voltage side? 20 volts is low enought that the US NFPA "electrical code" classifies it as "Class 2", and European Directives classify it Extra-Low Voltage (ELV), both saying there is no plausible scenario where a 20 V supply can push enough current through your body to cause pain or damage. So far so good. All that said, there is no reason NOT to ground any metal parts, and that brings us to the other hazard.

Another hazard to consider is whether there might be a failure mode where the fault current makes something get hot, maybe even hot enough to catch fire. Here, you have to consider not just the voltage, but the energy (or available current). An example would be a stick welder, where voltage may be ELV, but its intended to source 100 A, and that can certainly make things hot enough to burn. So to mitigate that hazard, you want to know where a fault current might flow, and protect that. In this case, a chafed wire, for example, might short the 20 V supply to the case. If that happened, enough current might flow to cause the cord to get hot, and burn you, or start a fire. The easiest way to mitgate that hazard is to ground the metal case to the secondary supply and place a fuse in the circuit that will blow if the current is more than the lamp ever needs normally, then size your wire so that it won't overheat before the fuse blows. Or, choose a power supply that has electronic energy limiting, such as fold back current limiting.

Consider the failure scenarios and mitigate them.

If you intend to make a bunch of these and sell them, then you likely have regulatory reponsibilities, and you need someone that knows the rules in your jurisdiction. Where I am in the US, there are labs that will assess a product against the UL safety standard and you can even get a product affirmatively listed as safe, but the testing and listing fees are not cheap. I try to design everything to meet the UL and EU standards, and most other jurisdictions coordinate with essentially the same rules.
Thanks a lot for the reply. Yes the wall wart is UL listed.

To clarify, when you say ground the metal case to the secondary supply, this would be connecting a wire from the metal enclosure/case to the ground pad on my PCB (for the DC barrel jack connector), correct?

I'm in the US as well. It seems I would want "UL 1598, Luminaires" certification. From what I'e read, it is not legally necessary to sell but typically required to obtain product liability insurance.
 

Tonyr1084

Joined Sep 24, 2015
6,459
if I understand you correctly regarding the grounding lug - I would connect a wire from the metal enclosure to the ground pad on my PCB (the ground negative pad for my DC barrel jack)?
Well, we don't know. Not without seeing a schematic representation of the power supply. Some supplies I've worked with have the negative lead grounded to the ground pin of the power cord. Others have not. So we can't say for sure if that's a safe scenario. It also depends on the type and method the power supply employs to produce a regulated, and hopefully isolated source of power. As mentioned before, there are circuits that clip the AC voltage using a capacitor, then use the lower RMS voltage as a part of the supply. There IS a scenario where that could fail and cause application of full line voltage to the low voltage circuitry. Let me see if I have a diagram of such a circuit handy. I think I do, so it won't be long for me to post it.
 

Tonyr1084

Joined Sep 24, 2015
6,459
That didn't take long at all. Here's a partial diagram of a night light I blew out and investigated. The 824J 250V cap reduces the voltage, then DB1 rectifies it to somewhere around 10 volts (if memory serves). This is the very sort of circuitry that can pose a hazard. But since this is in an entirely encapsulated lamp there is adequate prevention of potential shock hazard to a user. Also note that there is no ground pins.
1634690474457.png
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
Well, we don't know. Not without seeing a schematic representation of the power supply. Some supplies I've worked with have the negative lead grounded to the ground pin of the power cord. Others have not. So we can't say for sure if that's a safe scenario. It also depends on the type and method the power supply employs to produce a regulated, and hopefully isolated source of power. As mentioned before, there are circuits that clip the AC voltage using a capacitor, then use the lower RMS voltage as a part of the supply. There IS a scenario where that could fail and cause application of full line voltage to the low voltage circuitry. Let me see if I have a diagram of such a circuit handy. I think I do, so it won't be long for me to post it.
I see. Here are the datasheets for the wall wart as well as the barrel jack connector. https://media.digikey.com/pdf/Data Sheets/Tri-Mag PDFs/L6R12_Series_2020-12.pdf
https://www.cuidevices.com/product/resource/pj-036ah-smt-tr.pdf

The wall wart is only a 2-prong plug so I don't believe it makes a connection with the earth ground pin in the AC wall socket. So with my current choice of wall wart, I don't think it would be possible to make a true earth grounding. But I can still make a wire connection between my metal enclosure and the negative/ground pad on my PCB, with a fuse to detect short circuiting as a safeguard against fire hazard.
 
Last edited:

BobTPH

Joined Jun 5, 2013
4,046
When you have a grounded supply, and you are grounded you can be shocked by touching just the ungrounded side if the supply. The circuit is completed through ground.

When neither side if the supply is grounded, you would have to touch both sides if the supply to to complete a circuit. No current can flow through ground since the supply has no connection to ground.

But none of this matters with a 20V supply, because that is not enough voltage to shock you unless you have both sides if the supply penetrate your skin. You can touch both sides of the supply and you will not feel anything, because your skin resistance prevents a large enough current to shock you.

Bob
 

Thread Starter

jai.whitey

Joined Oct 19, 2021
9
I see now that it may be better to leave the enclosure floating rather than ground it, so that both the positive and negative would have to short the enclosure at the same time.
 

Tonyr1084

Joined Sep 24, 2015
6,459
I believe Bob has explained it well enough. You don't need a ground, nor do you need to be concerned about it. 20 volts isn't going to hurt anyone.
 
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