Connecting Parallel Transformer Primaries with Faston Connectors - Page 1
Forum section: electronics / projects

author: grbk
author_rank: Contributor
timestamp: March 09, 2019, 04:56:34 pm
content: I'm wiring up a mains transformer inside a metal chassis and want to make sure I'm following best practices for safety & reliability. This is for a personal project, nothing commercial, and low power, but I do want to know what the "best" way to do this is. Ideally any solution would pass a UL certification, etc.The transformer has two primaries, and I'm on 120V mains, so I'm connecting them in parallel. I'm using crimped Faston/quick connect terminals to make the connections. I have line/neutral from the IEC inlet to a front panel power DPST switch, with faston connectors on the inlet and the switch. But now I have two transformer primary wires to connect to each tab on the switch.How do I connect two transformer primaries to a single tab, preferably with crimped faston connectors?To be clear, I understand in theory how to connect the primaries in parallel (I know which wires need to be connected together). My question is how to make the physical connections. I see several possibilities, but I don't love any of them and I'm hoping I'm missing an obvious solution:Ditch the crimped terminals and solder the wires directly. I worry about the reliability of the soldered connections to freehanging wires, but mostly this is not an option because I specifically want to be able to swap out the transformer.Crimp both primary leads into a single faston connector -- this seems dubious.Crimp both primary leads into one side of a butt splice. Crimp the other side of the splice onto a single wire, which then gets crimped into a faston connector, which connects to the switch. For some reason this seems slightly better than 2) above, but it's basically the same in that I'm still crimping two wires into a single terminal.There are all sorts of freehanging 2-to-1 wire splice options out there. For some reason these still seem a bit amateur/how-ya-doin', but maybe I'm being unreasonable?make the connections with a chassis mounted terminal block and spade terminalsmake the connections with a PCB -- overkill to design a PCB and get it fabbed, plus now I have to think about PCB creepage distances.Edit: I suppose I could use a 220V/120V selector switch. That seems superfluous for a device that will only be used by me."Piggyback"andtwo-to-onefaston terminals exist, but most of them are uninsulated. I'm using an earthed metal chassis, so uninsulated is not the end of the world, but I'd still prefer an insulated solution. Still, this might be my best option.Are there any better solutions that I am missing?Are there any resources (books, articles, etc.) that are helpful for this kind of mains chassis wiring that anyone could point me to? I've found some information in Art of Electronics and in Doug Self's Power Amplifier book, but neither are as comprehensive as I'd like.One further question: I've seen several recommendations to put an X2 capacitor across the power switch. Is this best practice? How would one make the connections? Soldering it would be easy, but that seems incompatible with using Faston terminals.Thanks for any advice.

author: Gregg
author_rank: Super Contributor
timestamp: March 09, 2019, 05:32:40 pm
content: There are plenty of DIN rail mount terminal blocks that would suit your purposes. That is if you have room in your enclosure.

author: grbk
author_rank: Contributor
timestamp: March 09, 2019, 06:42:48 pm
content: Quote from: Gregg on March 09, 2019, 05:32:40 pmThanks for the suggestion. I knew very little about these so I just did some reading. They seem very useful. I think they will be too big for this project, and perhaps overkill for the limited number of connections I'm making, but I'll keep them in mind for future projects.

author: helius
author_rank: Super Contributor
timestamp: March 09, 2019, 06:59:04 pm
content: You could also use Wago 221 or 222 lever-lock connectors, either free-hanging or mounted to a rail using Wago system carriers.

author: Benta
author_rank: Super Contributor
timestamp: March 09, 2019, 08:48:16 pm
content: The "piggyback" solution is the right one, with heat-shrink insulation if needed.

author: 2N3055
author_rank: Super Contributor
timestamp: March 09, 2019, 09:04:07 pm
content: I second the Wago...

author: grbk
author_rank: Contributor
timestamp: March 09, 2019, 09:18:57 pm
content: Quote from: helius on March 09, 2019, 06:59:04 pmThese look pretty nice; I'll keep them in mind. Not available at Digikey/Mouser which is a bit of a drag, but Newark does carry them. Thanks for the suggestion.Quote from: Benta on March 09, 2019, 08:48:16 pmThis is what I'm leaning towards, plus I foundsome from Molexthat are fully insulated. Thanks for the confirmation that I'm on the right trackDoes anyone have suggestions on reading material or other resources for this sort of chassis wiring?

author: rdl
author_rank: Super Contributor
timestamp: March 09, 2019, 10:04:56 pm
content: Since this is a one-off project, I vote for #2. Twist and crimp the two wires together in the same connector.As a matter of fact, I believe I did nearly the same thing for a power supply I built over 10 years ago. I'd have to open it up and look to be sure though.

author: David Hess
author_rank: Super Contributor
timestamp: March 10, 2019, 04:57:36 pm
content: If you do not want to solder, then I would use #2 and crimp two leads into a single connector maintaining an insulated connection. I do not understand why you think this is dubious.

author: Gregg
author_rank: Super Contributor
timestamp: March 10, 2019, 06:05:17 pm
content: Another solution might be to use ring terminals with a small machine screw and cover the junction with heat shrink. Then the transformer can be removed at a later date and still have usable terminals attached.Ring terminals back to back nest quite nicely especially uninsulated ones; you often can remove the insulation from the ones with the colored plastic sleeves.For solid wire like transformer magnet wire, the ring terminal can be lightly crimped to keep it in place and soldered.I have often put two stranded wires into a single crimp by stripping long, fanning out the strands, stacking the fanned ends on top of each other and twisting them together to form a well-mixed array of strands then cutting the bundle to length before stuffing it into the crimp sleeve.

author: grbk
author_rank: Contributor
timestamp: March 11, 2019, 12:18:28 am
content: Quote from: rdl on March 09, 2019, 10:04:56 pmQuote from: David Hess on March 10, 2019, 04:57:36 pmI appreciate the input; this is probably the easiest option and I guess it isn't as sketchy as I think. My reluctance comes from a lot of time spent debugging poorly constructed race car wiring harnesses, and after that experience I get nervous with anything other than a single-conductor-per-crimp (soldered connections and substandard tooling give me nightmares!).But after receiving these replies, I spent some time re-reading NASA workmanship standards (the gold standard as far as I'm concerned) and crimping multiple wires into a single terminal is explicitly allowed (my memory was the opposite). Given that my application is vastly less demanding than aerospace or even motorsport, I agree my concern's probably misplaced.Quote from: Gregg on March 10, 2019, 06:05:17 pmI've done this before and it worked quite well.