Is this power supply regarded safe? - Page 1
Forum section: electronics / projects

author: CuriousAbe
author_rank: Newbie
timestamp: March 19, 2019, 10:15:29 pm
content: So I came across this DIY drum machine project on a different forum I frequent. Was about to order one but something struck me about the power supply. I dont know if it's just me but would this psu be deemed safe or not in terms of clearance between live and neutral etc?I mean I wouldn't want somebody poking about all willy nilly since its on the same board as the drum machine itself and really close to the rest of the components.Asked a member who built one last week for some photos and additional information:

author: magic
author_rank: Super Contributor
timestamp: March 19, 2019, 10:30:37 pm
content: I have no idea what are recommended or legally required clearances between line and neutral. But what really makes me nervous here is the proximity of mains wiring to what appears to be low voltage ground and signal traces, probably directly or indirectly connected to some I/O jacks and UI controls the user will touchI would bypass that PCB and run mains cable far away from it straight to the fuse and transformer.

author: CuriousAbe
author_rank: Newbie
timestamp: March 19, 2019, 10:47:46 pm
content: Quote from: magic on March 19, 2019, 10:30:37 pmGood point! Looking at the pictures again the PSU part of the pcb (top section to the red) only seems to serve for mains input voltage selection

author: TimNJ
author_rank: Super Contributor
timestamp: March 19, 2019, 10:55:54 pm
content: In my experience, typical clearance between line and neutralbefore the fuseshould be 4mm or greater. Clearance after the fuse should be 2.5mm or greater. Just rule of thumb though. Depends on contamination level, etc. For a sealed unit, you can get away with less. If there's dust/dirt or if the board isn't de-fluxed properly, you should have more clearance.

author: bd139
author_rank: Super Contributor
timestamp: March 19, 2019, 11:02:14 pm
content: I've seen worse on old HP/Harrison power supplies !To note I probably wouldn't use that section of the board and just wire the transformer to the correct voltage permanently.

author: james_s
author_rank: Super Contributor
timestamp: March 19, 2019, 11:33:08 pm
content: I would certainly want a fuse in the line ahead of the board, but you should have that anyway. I also agree that the clearance between the mains side and low voltage is uncomfortably close. Also I cringe just looking at that board layout, the oddly angled traces just scream amateur.

author: wraper
author_rank: Unknown
timestamp: March 19, 2019, 11:39:49 pm
content: Distance between "Neutral" (which can be Line just as well when connected to mains) and low voltage GND seem to be less than 1mm. It's nuts. That part of board should never be used. Not to say it's not a good idea to put high voltage on such board to begin with.

author: floobydust
author_rank: Super Contributor
timestamp: March 19, 2019, 11:48:33 pm
content: You want me to tell you the bridge will not collapse based on a picture- strictly speaking I can't do that.There are several spacings violations and maybe a missing fuse. I've highlighted some in yellow. Just a few observations:PCB creepage/clearance minimum is 1.5mm for 300VAC Cat. II and if the drum machine is not protective earth-grounded, then you need double-insulation for 3mm minimum.So- between line-neutral 1.5mm spacings, and between line or neutral to chassis ground/secondary 3mm minimum - if the drum machine is not earth grounded, otherwise it's all 1.5mm min.The jumpers do not have 240VAC across them, as you have drawn.If the power transformer is classified as inherently short-circuit protected/energy limited, then no primary fuse is required. It would also need to be rated for double-insulation if the drum machine is not grounded.The PCB mounting spacer could be too close to hazardous live on the underside, and I really don't like the transformer primary (neutral) being so close to the ground pour. Line and Neutral easily get reversed in North America, so they are treated the same.You would not have mains wiring or nodes on a PCB next to VCO op-amps, for hum and noise. I would not have mains on this PCB, with so much analog circuitry.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 20, 2019, 12:47:19 pm
content: As others have said, I'm puzzled why they even put the A.C. lines on the PC board. If you still wanted to use the runs as intended I'd use a Dremel tool or Exacto knife to remove some of the pad like this modified photo. Note there are other pads that are also close that need to be modified, including on the front side of the board.

author: wraper
author_rank: Unknown
timestamp: March 20, 2019, 01:18:31 pm
content: Quote from: ArthurDent on March 20, 2019, 12:47:19 pmThat's the smallest problem and actually a non-issue. Look on the pad on bottom left corner.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 20, 2019, 01:57:48 pm
content: QuoteThat pad is also very close to the ground plane on the front side of the board. That is another place that I would modify, as well as the small pads on the back side for the voltage select jumpers that are very close to the trace that goes between them.I assume they rationalized that the pad you mentioned was zero volts so it was o.k.to be that close to the ground plane. I'd eliminate the whole (or is it hole?) problem by not using that section of the PC board at all. You could make a small perf board version of that section with proper spacing and eliminate the entire mess.

author: CuriousAbe
author_rank: Newbie
timestamp: March 20, 2019, 03:05:10 pm
content: Thanks for the replies everyone, my hunch about this was correct. I've forwareded the thread to my friend who sent me the photos(who I think is also a member here).

author: Dr. Photon
author_rank: Contributor
timestamp: March 20, 2019, 03:55:14 pm
content: You could always buy the kit but just do the mains wiring off-board?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 20, 2019, 04:34:36 pm
content: As some others have pointed out, you have missed the bigger problem completely.Live - neutral clerance is, if not safe and up to regulations, at least... quite OKish, when compared to the massively horrible and immediately lethally dangerous lack of clearance between primary and secondary (through the secondary side main copper pour). (I'm assuming that the low voltage side ends up being touchable by the end user, through connectors or otherwise. If the complete product is fully physically isolated, please ignore this post.)Exact requirements vary per legislation, but primary-secondary creepage and clearance requirements are always bigger than the live-neutral limits, and for a good reason: arcing between L and N only tends to cause increased risk of fire, whereas between the live (FYI: neutral wire is always treated as live as well) and the protected side, the result is not only a risk of fire, but an imminent, invisible electrocution death trap as well.Eyeballing from the image, primary-secondary clearance seems way below 1mm if not 0.5mm - reason for immediate product recall due to being a massive death trap.You tend to see beginner issues like this on Western products designed by amateurs, often accompanied with a marketing assumption of high quality. Chinese power supplies get a lot of attention for doing this sort of thing, but theytypicallyhave better idea about what they are doing.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: March 22, 2019, 12:28:53 pm
content: Well, I cannot regard it as safe, as others said. There is not enough insulation clearance, and I see jumpers that pass 240V flying above the board, unprotected. I don't know what was the idea of the designer, but if I was on his shoes, I would move the jumper/240V section out of the board. Only SELV would go into that board, by my standards.If you need to use high voltage on your board, because you are using one of those encapsulated transformers, you should:1- Leave enough clearance;2- Apply a guard ring connected to protective earth, around the tracks you wish to isolate from the secondary, provided that point 1 also applies to the guard ring.See the attached files to see what I mean. However, I should note that this design is far from ideal, just because the mains terminal block doesn't have enough clearance between the pins. Despite that, the mains are adequately isolated from the ground plane. The board layout that I've attached would be 100% correct if a different terminal block was used at that point.Kind regards, Samuel Lourenço

author: floobydust
author_rank: Super Contributor
timestamp: March 22, 2019, 07:07:25 pm
content: In North America, you are forbidden from putting a fuse or switch in Neutral, in a product. It can only go to the Line or hot side.I'm not sure about Portugal's electrical codes, but your schematic seems odd to have double fusing and double switches for mains.

author: magic
author_rank: Super Contributor
timestamp: March 22, 2019, 07:22:20 pm
content: In continental Europe most plugs are reversible and sockets are wired randomly so both conductors are considered equally "hot". If something needs to be earthed for safety it uses PE, which bypasses power switches. Switches are often double pole to completely and certainly cut power from the device.

author: wraper
author_rank: Unknown
timestamp: March 22, 2019, 08:14:02 pm
content: Quote from: floobydust on March 22, 2019, 07:07:25 pmPlease explain how it works with devices which don't have ground in the plug?

author: bloguetronica
author_rank: Frequent Contributor
timestamp: March 22, 2019, 08:19:30 pm
content: Quote from: floobydust on March 22, 2019, 07:07:25 pmThe electrical code in Portugal is a completely different animal. It is good practice, even recommended, to break both live and neutral. The same with fuses. This is because the Schuko type plugs are reversible. One can never be too safe.Why breaking neutral is forbidden in US, I don't know. It doesn't make sense for a single phase distribution. In a three phase distribution, yes, you can't cut the neutral, as that would unbalance the phases and damage the load. In a single phase device or distribution, only earth should be unbroken.Mind that the neutral is not guaranteed to have no voltage. In fact, the main OC breaker at the entry point of my house breaks both live and neutral. The same happens with the main RCD breaker.Having said that, the US and the Portuguese electrical codes are incompatible, and not comparable in the slightest. For example, we don't have pole pigs, we have different color codes, we only have single 230V~ instead of dual 120V~ for a house. And it doesn't end here.Kind regards, Samuel Lourenço