Is plywood box safe for mains powered PSU? - Page 1
Forum section: electronics / projects

author: electr_peter
author_rank: Unknown
timestamp: December 23, 2014, 10:16:14 pm
content: I am designing DC PSU for my needs. PSU will be mains powered, grounded, with floating output, ~100W power. I could make metal, plastic or plywood case for power supply.Plywood would be 3-5mm thick, three layers. Material is structurally strong for small-medium PSU.However, plywood may be not good enough from electrical safety standpoint (as mains is involved). I wonder about these issues:resistance of wood/plywoodresistance of common glue typespotential for arcing and carbonisation (also, pyrophoric-carbonisation)is some finishing (impregnation/lacquer) required to make it safe/usable?humidity effects on material (I will use it inside, but spills may happen)other issues I missed?Any experts on materials? Could I use plywood or should I throw it out and use metal/plastic instead?

author: radioFlash
author_rank: Regular Contributor
timestamp: December 23, 2014, 10:21:30 pm
content: I would avoid wood:* Fire hazard should something go really wrong* When dry, wood should have a pretty high resistance, but if it gets wet, it will soak in the moisture and resistance would drop* Grounding-a metal enclosure will make it easier to ensure that all exposed metallic components (bolts, etc.) are properly grounded

author: Dave Turner
author_rank: Frequent Contributor
timestamp: December 23, 2014, 10:28:55 pm
content: Wood enclosures were used for years without apparent problems.However nowadays for legal, and probably to satisfy elf & safety demons, a metal lining at least should be considered.

author: Towger
author_rank: Super Contributor
timestamp: December 23, 2014, 10:42:48 pm
content: Good enough for old valve TVs and radios for many years.

author: ajb
author_rank: Super Contributor
timestamp: December 23, 2014, 11:31:01 pm
content: I never really expected I'd have an opportunity to cite this on the EEVBlog forum, but here you go:TheWood Handbook: Wood as an Engineering Material(Full 15.1MB PDF).Full of all the things you didn't know you didn't know about wood. Chapter 4 has electrical properties for many common species at different moisture levels. Chapter 18 covers fire safety.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: December 23, 2014, 11:44:04 pm
content: https://www.eevblog.com/forum/chat/hammond-got-wood/msg112538/#msg112538

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2014, 11:57:20 pm
content: Providing the wood is dry, it shouldn't be a problem.As far as fire risk is concerned. Wood is no worse or better than many plastics which are flammable..If you don't have any metal parts, then there's no need for an earth connection.

author: ovnr
author_rank: Frequent Contributor
timestamp: December 24, 2014, 12:57:49 am
content: Quote from: Hero999 on December 23, 2014, 11:57:20 pmWell if he doesn't have any metal parts, it's bound to be a pretty bad PSU.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2014, 01:22:30 am
content: You certainly don't want to be, like, building tag strips or PCB from wood. Soldering on it, or putting power resistors and heatsinks against it, etc. But as a mechanical container, it's fine. No idea what UL would say about it these days -- it's obviously not flame-retardant (though it can be treated to help), but you can at least give it a good try by putting a metal chassis inside to give some thermal and short circuit protection.So, if that answers your question in the negative (you should still have grounded metal inside), I guess that covers it.I doubt vintage electronics would meet UL today (for a variety of reasons), but they had some good ideas anyway. The basic construction is a three or five sided box, with holes in the top for mounting tubes, transformers and other bulky or hot components, and terminal strips for mounting smaller components and wiring. The metal box provides good structure for mechanical stability, and safety, while the open bottom gives a compromise for easy access. Of course, a fully open bottom would be absurd, so these would be mounted on a wood shelf inside a cabinet, or covered with a metal plate.Tim

author: Red Squirrel
author_rank: Super Contributor
timestamp: December 24, 2014, 02:57:33 am
content: I got yelled at for having fused #2 12VDC wiring fastened to wood, so general consensus is probably no, but really, if you don't actually have anything live touching it I'd say it's fine. Maybe it would not hurt to line the bottom with some metal sheet of sorts just to play things safe though. Just think of how hard it is to set a piece of wood on fire with just a lighter though. While wood is combustible a big flat piece still requires decent effort and energy to actually get it going. Try starting a camp fire without kindling or paper.Chances are it would not pass any kind of safety stuff though. But this is a hobby forum, I doubt anything people do here would pass unless you get it UL certified and all that jazz.

author: compet17
author_rank: Regular Contributor
timestamp: December 24, 2014, 10:25:56 am
content: I already made some wood housings for 230V mains stuff. There is no reason why it should not be used. If you're concerned about the fire risks, Why not line the box out with adhesive aluminum tape? That layer should probable be earthed as well as any screws that become accessible from outside.One thing is to consider: wood is a good thermal insulator, the components inside could get hotter because heat is trapped inside. A few holes can help.

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2014, 11:11:58 am
content: Quote from: ovnr on December 24, 2014, 12:57:49 amOf course. Don't be pedantic. I meant if the box isn't metal duh.The extra low voltage side of the PSU should have double insulation, otherwise it needs to be earthed but that's fairly straightforward to do.You could also have a metal box and not earth it, providing it's double insulated from the mains.

author: xibalban
author_rank: Regular Contributor
timestamp: December 24, 2014, 11:41:20 am
content: I'd encourage wood to plastic or metal any day, provided it comes directly from a lumberjack. Metal mining and plastic manufacture do no good to the environment compared to felling a tree. Wood is bio-degradable anyway, so!

author: electr_peter
author_rank: Unknown
timestamp: December 24, 2014, 12:39:58 pm
content: Thanks for all replies.It seems that wood for a case is fine, as long as humidity is not too high. Also, suggestions about internal metal case/plating are worth considering.Let me clarify few PSU details - I will use toroidal transformer, PCB/copper clad for circuitry, nylon/metal stand-offs, plastic input/output jacks. PS output will be insulated with separate mains earth banana jack. Mains earth will be primarily used for noise suppression on mains input and transformer shield earthing (if I can get toroidal with a shield).Possible wood to metal connections could be earthed/floating screws, output LVDC jacks, internal AC wiring fault.I am a bit worried about internal heat from PS components, so I will test the temperature of PS without a case. Some vent holes/slots on the sides of a case will be included, be it metal, plastic or plywood box.Personally I prefer wood/plastic rather than metal because it is easier to work with thicker materials than thin piece of sharp metal. Also, I am for double insulation or insulated metal case rather than mains earth connected metal case.Thus, I will consider plywood as a possible material of choice. Final case design will depend of 3D placement of parts, heat generated and total size of the PS.

author: richard.cs
author_rank: Super Contributor
timestamp: December 25, 2014, 12:02:09 am
content: If you can't get a torroid with a screen between the windings that you can earth then you will need to use an EI transfomer with either a screen or a split bobbin if you want to avoid earthing the output. Otherwise nothing protects you from mains appearing on the output in the event of a transformer failure.If you earth the output you can float it to small signals by earthing via some diodes. I normally use a 35A bridge for this, it has redundant paths and can cope with a few hundred amps for long enough to blow a fuse.

author: timb
author_rank: Super Contributor
timestamp: December 25, 2014, 12:25:18 am
content: I wouldn't worry too much about heat. I was in the pallet business for many years and we had some specialty pallets that we would heat treat after building. Basically we'd stick 200 pallets into a giant kiln of sorts that was internally heated to between 80 and 100c for 24 to 72 hours, until the wood's core temperature got up to at least 65c. Most hardwoods have a very high auto ignition point. To illustrate my point, take a piece of plywood and hold a lighter under it. It'll scorch but won't catch fire.An interesting aspect of the PSU heating the wood in your enclosure is that it will dry the wood out, offsetting any humidity issues.Obviously you'll want to add some vent holes and maybe a 40 or 60mm fan; though that's for protection of the power ICs, not the box.Sent from my Tablet

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2014, 06:01:42 am
content: For what purpose are you interesting in low noise / shielding?Tim

author: electr_peter
author_rank: Unknown
timestamp: December 25, 2014, 10:49:13 am
content: Quote from: richard.cs on December 25, 2014, 12:02:09 amDo you mean loop breaker circuit/zero volt line (like in the picture, but without capacitor and resistor, with diodes only)?Sourcehttp://sound.westhost.com/earthing.htmAbove scheme with diodes does not look too terrific as output is always referenced to mains earth +/- diode drop. So such PSU is not floating and could not be connected in series safely.Quote from: T3sl4co1l on December 25, 2014, 06:01:42 amDC PSU will be semi precision and I want to insure maximum isolation from mains and mains earth on the output (for floating output. Separate earth jack will be included). Shielded transformer is a slightly better option due to construction.

author: Leuams
author_rank: Contributor
timestamp: December 25, 2014, 12:01:40 pm
content: For anyone interested in using wood as a engineering medium check outhttp://woodgears.ca. He makes some amazing machinery using mostly hardwoods and plywood.

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2014, 12:43:17 pm
content: Quote from: richard.cs on December 25, 2014, 12:02:09 amA toroidal transformer doesn't need to to have a screen. Just pick a transformer with class 2 insulation between the primary and secondary windings. In fact most modern toroidal transformers are designed so the secondary doesn't need to be earthed.Choose a transformer from a reputable distributor with a datasheet which describes the safety standards.http://www.rapidonline.com/pdf/82719.pdfhttp://www.rapidonline.com/pdf/525390.pdf

author: richard.cs
author_rank: Super Contributor
timestamp: December 25, 2014, 03:06:15 pm
content: I hadn't realised that torroids with class 2 insulation were common, that would be a good alternative.That is the type of ground breaking I was meaning and you're right, it doesn't allow series connection of power supplies, the output is only "floating" to signals of under a volt. This is useful for things like preventing your d.c. return from the circuit under test flowing partly via scope probes and mains earth (as this is all about the few tens of mV drop in the cables), a rather different problem.To have a true floating output you must have a transformer suitable for class 2. If torroids with suitable insulation are available then use them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2014, 06:53:04 pm
content: Don't go for a toroid, go for a split bobbin type transformer. The capacitance is probably 10 or more times lower. You'll get more isolation and lower noise that way, than trying to shield something so large as a toroid.Tim

author: mrpackethead
author_rank: Super Contributor
timestamp: December 27, 2014, 08:36:49 am
content: Maybe the box would be best made out of tissue paper, bubble gum and sellotape.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2014, 03:05:44 pm
content: Quote from: T3sl4co1l on December 25, 2014, 06:53:04 pmIt depends on the application.A torrid may have a higher capacitance than a split bobbin but it has less leakage flux.If you want isolation don't go for an SMPS which will have much more capacitance than any transformer.

author: edavid
author_rank: Super Contributor
timestamp: December 27, 2014, 04:35:29 pm
content: Wood is fine for low power projects, but it's a really bad idea to build a PSU inside a thermally insulating box. It will make cooling very difficult. Use a metal box.

author: electr_peter
author_rank: Unknown
timestamp: December 27, 2014, 06:01:17 pm
content: Local electronic shops had selection of torroids marked for halogen lamp use(?) and various non-torroid types. I chose split bobbin type 2*12V ~1.5A for main transformer. Two secondaries will be connected in series to boost available current. Other circuit components will be powered by low power 9-0-9V transformer.Quote from: edavid on December 27, 2014, 04:35:29 pmWill wooden box with vent holes be good enough at maximum total power of PSU at 40-50W?Link to thread with initial PSU schematic:DC PS with strict current limiting on short circuit

author: edavid
author_rank: Super Contributor
timestamp: December 27, 2014, 07:05:27 pm
content: Quote from: electr_peter on December 27, 2014, 06:01:17 pmNo, you would need a fan.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2014, 07:33:08 pm
content: Think about the Watts of heat produced and the physical size of the unit rather than the output power of the PSU. If you have an enormous heat sink, then a wooden box with adequate ventilation will be fine without any fans.Wood does look nice but one of the good things about metal is you can use it has a heat sink. I've designed a small LM317 bench power supply with the metal enclosure used as a heat sink and it worked perfectly.

author: Red Squirrel
author_rank: Super Contributor
timestamp: December 28, 2014, 12:01:30 am
content: If your PSU is efficient enough, it should not be outputting that much heat anyway.We have about 1600 amps @ 54vdc being pulled from our rectifiers at work and I can stand behind that rack without sweating...much. Mind you the room is AC cooled... it would get hot in there without the AC, but probably not anywhere as bad as any other equipment room.That said *DO* put fans and a thermal sensor so it can shut down and/or output an alarm if it overheats. If you want to play it very safe you throw in a smoke detector that also shuts down and outputs an alarm if there's smoke.

author: electr_peter
author_rank: Unknown
timestamp: January 15, 2015, 07:13:29 pm
content: I calculate worst case dissipation for any single device as 12W(3A*4V voltage drop). Quite manageable with heatsinks and holes in the case.I decided to use plywood box and cover all inside surfaces with copper or aluminium foil. Thermal conductivity is great, it forms a good shield as well. Electrical conductivity of foil glue is in question, but there are ways around it.