Electrical safety for boosted voltage - Page 1
Forum section: electronics / projects

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 12:21:17 pm
content: I've designed a LED driver that can drive LED strings of up to 75V but it suddenly occurred to me that this might not be a safe voltage. I know about grounding for mains powered circuits but this is powered from 12V or 24V. How should one deal with electrical safety in such a case? It is going to be used in a box that people are going to stand on with lighting coming from below shining through metal grating.

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 13, 2011, 12:39:38 pm
content: Depending on where you live, there are probably definitions for the parameters of what's considered a safe low voltage circuit in your electrical code.In the US it will be called Low Voltage while in IEC countries it will be Extra Low Voltage.http://en.wikipedia.org/wiki/Low_voltagehttp://en.wikipedia.org/wiki/Extra_low_voltageEither way, 75V AC or DC will be out of the allowed range and you will need to either treat your system as mains voltage or reduce the available voltage to qualify.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 01:43:43 pm
content: The point of using a high voltage like this is to be able to drive two RGB LEDs (Vf ~ 30V) from three drivers instead of six so I would very much like to keep doing that. All of this is driven from one single 24V source which is a generic external power supply.When you say I should treat the system as mains voltage, what does that mean more specifically? My knowledge of electrical safety tells me that mains powered devices need either very thorough isolation or safety grounding of all exposed metal parts. In my case, there isn't any dedicated safety ground since the system is only indirectly powered from the mains but I could go ahead and ground the metal grating the system ground so as to provide a low resistance path to ground if any higher voltage conductor should come into contact with it.

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 13, 2011, 01:52:58 pm
content: This kind of engineering is a little out of my league, especially not knowing what your project is. It may be that you'll be fine just bonding any exposed metal parts to the system earth or it may be that you need to add fusing to your secondary circuits as well. There are a lot of variables that come into play like whether the light source is separate from the box that it will be installed into, what kind of transformer you're using, will energized parts be accessible by some curious kid etc.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 02:33:30 pm
content: The plan was to have the boards mounted in a stack on a platform a decimeter under the grating along with the LEDs. So in use, you won't be able to reach the electronics.

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2011, 04:07:50 pm
content: The basic safety principle is insulation and isolation so that people cannot deliberately or accidentally come into contact with high voltage parts.Earthing or grounding is irrelevant unless you are dealing with the mains. It has no effect on the safety of isolated battery powered circuits whatsoever.Your basic safety principle is to put the energized electrical parts inside a box which is closed with screws or other tamper-proof fastenings and to make sure there are no holes or openings that fingers can get through and touch bits inside. When thinking of safety, also consider hot parts as well as high voltage parts.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 04:39:55 pm
content: Quote from: IanB on October 13, 2011, 04:07:50 pmThanks, a bit of thinking made me realize this as well. I could very well mount the electronics in a box but the problem is that I cannot put the LED's themselves in a box but I guess I could make sure that they are not reachable. Would that be enough?

author: bilko
author_rank: Frequent Contributor
timestamp: October 13, 2011, 05:05:33 pm
content: If they are at ground level / below ground, wouldn't they have to be waterproof ?Have you thought of encasing the LEDS in epoxy ?

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2011, 05:14:55 pm
content: Quote from: shadewind on October 13, 2011, 04:39:55 pmYou will have to make sure your LEDs are safely behind a screen or grille or mesh of some kind to protect them, especially if you don't control access to the display area. Otherwise someone will fiddle and your installation will be broken before you can blink. You have to protect people from your installation, but you also have to protect your installation from people.You should of course make sure that wiring is all covered with insulation and uses insulated connectors. If there are any cable runs between electronics and lights, make sure it is mains rated cable so that it has sufficient mechanical robustness and insulation, and make sure it has appropriate stress relief so that it cannot be pulled if someone trips over it.Mainly, apply common sense and you should be fine.

author: Zero999
author_rank: Super Contributor
timestamp: October 13, 2011, 05:21:28 pm
content: The regulations depend on the jurisdiction but anyone who thinks 75VDC as dangerous as the mains is completely mistaken. Lots of people see a voltage of above 48VDC and think oh shit I'm going to die if I touch it!In reality 75VDC is much safer than 120VAC or 240VAC 50 to 60Hz. AC is generally more dangerous than the equivalent DC voltage because it's more prone to cause ventricular fibrillation. Another obvious reason is that an RMS AC voltage has a peak voltage of 1.414 times the RMS voltage so 120VAC is really 170V peak.In reality you may not even feel anything if you come into contact with 75VDC. You may get a bit of a belt if your skin is wet but it's much less likely to kill than the mainsIs the 24VDC source earth bonded? Is the negative of the 24VDC supply connected to a vehicle chassis?If not then it's safer under normal conditions because you'll need to touch two wires to get a shock. A word of caution here: if the 24VDC isn't earth bonded and the 75VDC is shorted to earth, the 24VDC line could float at a higher voltage.Is the 75VDC output galvanically isolated from the 24VDC input? If so it's even safer as it eliminates the risk of the 24VDC side floating at a higher voltage.What's the current limit? If it's under 75mADC it's very unlikely to cause death regardless of the voltage as most people can let go below this current level.Here in the UK 75VDC is treated as ELV however it does need to be insulated, any voltage above 60VDC or 24VAC at least requires basic insulation. It's just that appliances run from voltages below 60VDC don't need to be earthed. Appliances running off voltages above 120VDC or 50VDC require earthing unless they're double insulated.In general voltages of this order of magnitude don't need to be treated with the same level of caution as the mains but some precautions do need to be taken. In short, you should be fine as long as you ensure that the connections to the LEDs and on the secondary side of the DC-DC converter are insulated from the user. Again check the regulations in your country.

author: Neilm
author_rank: Super Contributor
timestamp: October 13, 2011, 06:34:19 pm
content: The safe voltage definition is also dependent on where the circuit will be used. For instance, if your appication could be used outside (for instance in a field) then the safe voltage could be as low as 25V-35V.Neil

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 06:54:16 pm
content: Quote from: Hero999 on October 13, 2011, 05:21:28 pmOkay, a little more information:The 24V come from a generic external AC/DC adapter with a regular circular plug. I haven't got the specific adapter in my hand but I believe these usually have an IEC mains connector. I don't really know how these are constructed so I cannot answer on earth bonding and things like that but perhaps somebody here knows how these work?This goes to a regular boost converter which drives the LEDs with constant current at 350 mA at up to 75V. This voltage limit is imposed using a Zener diode which inhibits switching if the voltage reaches levels above this (usually because of open circuit). Since this is a regular boost converter, it is not galvanically isolated from the 24V supply. When in use, the voltage is more likely to be 2x19V for the red channel, and 2x28V for the green and blue channels.It is not going to be used in an installation of any sort. It's going to be used by me and my band on stage. Something similar to this:http://www.egorizerstagelighting.com/

author: Zero999
author_rank: Super Contributor
timestamp: October 13, 2011, 08:18:29 pm
content: Quote from: Neilm on October 13, 2011, 06:34:19 pmYes, that's true, the UK regulations only state 25VAC or 60V ripple free DC not requiring insulation in dry areas, in "other areas" it's halved to 30VDC and 12VAC and in really wet areas, all electrical connections need to be insulated.Quote from: shadewind on October 13, 2011, 06:54:16 pmThe output is normally isolated from the earth.QuoteDon't forget that it's possible for components to work loose.What's the typical maximum voltage output should the voltage limiting fail?

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 09:22:51 pm
content: Quote from: Hero999 on October 13, 2011, 08:18:29 pmThis is a boost constant current driver where the feed back is the voltage across a current sense resistor so if there's an open circuit where no current flows and there is no limiting, there's theoretically no limit to the voltage. I haven't checked to see what the practical limit is since the MOSFET and diode will probably break as well.I'll add another question, what's the minimum clearance on a PCB with solder mask for these kinds of voltages?

author: Zero999
author_rank: Super Contributor
timestamp: October 13, 2011, 09:41:21 pm
content: Quote from: shadewind on October 13, 2011, 09:22:51 pmFor functional insulation (i.e. just so it works, no safety requirements), there's no minimum clearance other than what's practical with most PCB production methods. 75V is far too lower voltage to arc through air at atmospheric pressure unless it's a truly microscopic gap. If it's safety critical just give a couple of mm for good measure. It's not mains so you don't have to go too over the top.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 09:57:21 pm
content: Quote from: Hero999 on October 13, 2011, 09:41:21 pmOkay, but right now I only have 10 mil since I was never thinking I'm working with higher-than-usual voltage. I was just thinking "not mains". Will that be enough? I've obviously not seen any arcing yet or otherwise I would have gone back to remaking the board immediatelyRegarding the maximum voltage if the Zener breaks loose, I don't really see how there's anything else I can do about that?

author: vk6zgo
author_rank: Super Contributor
timestamp: October 13, 2011, 11:13:42 pm
content: I agree with IanB. Use mains rated cabling-it is rugged,& readily available.75 volts isn't a lot,but nobody ever got sued for over-insulating wiring!If your gig is out in the middle of a paddock somewhere,what kind of wiring do the audio & lighting guys use?OB crews drag mains power all over the place with out problems,so you need to make the stuff as rugged as the gear they use.VK6ZGOP.S.I think the Ego-rizer guys guys use open mesh covers because they are using incandescents & have to get rid of a lot of heat.With LEDs you can probably use glass or clear plastic (probably mechanically safer) under the metal grating & totally isolate the LEDs & wiring inside the boxes.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 13, 2011, 11:32:06 pm
content: The reason we're using grating/mesh instead of glass/plastic is because of cost reasons mainly.Still a bit worried about the PCB clearance though but I'm growing a bit tired of revising this board right now. The IPC standard calls for 0.6 mm but I only have 0.254 mm. On the other hand, I've seen those recommendations criticized a fair bit.

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2011, 11:49:22 pm
content: If you are building a one-off item for your own use your constraints are quite different from something that will go into production.If your tracks are too close together on a circuit board there is a danger of solder bridging causing a failure. Obviously if you build one board and examine it carefully before testing it, this will not be a problem for you.Furthermore, if you have a commercial low voltage "power brick" adapter as your power supply, you have basically isolated yourself from dangerous mains voltages. The fact you are boosting this voltage a bit to drive LEDs is not a big deal as long as you have robust construction and good insulation.Low voltage electricity does not leap out of wires and attack people. I hate to say this, but the dangers of electricity are greatly exaggerated. There is a far greater set of dangers around, such as falling off a stage, or having a badly attached light fall on your head from above.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 14, 2011, 06:14:54 am
content: Quote from: IanB on October 13, 2011, 11:49:22 pmSo what determines in commercial products that the clearance is enough? Testing? If I have the boards right here working, is there any reason to worry? My worry in this case is more reliability, not so much safety.Reading the IPC standard about clearance, I'm a bit unclear about which column I should go by as I don't have any particular coating but it's not open air either.I'm very pedantic when it comes to projects which bites me sometimes as I feel I have to go back and fix boards until I cannot think anything to improve.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 14, 2011, 01:25:07 pm
content: It seems like I need to order some boards for another project so I might as well change the design of this one as well.I added a design rule to maintain a minimum of 0.6 mm clearance between the switching island (the heatsink area in the top of the board), the LED+ output net (the C7/C8 net) and any other tracks. I am wondering a bit about the trace from J2 to D2 (the protection Zener diode) which passes under R7. Is that a really a good idea?