Help me get 35 Amps onto my PCB! - Page 1
Forum section: electronics / general

author: frogblender
author_rank: Regular Contributor
timestamp: October 05, 2021, 04:39:57 pm
content: Howdy... I need to get 35 amps onto my PCB. Attached pic shows my first stab at it. The other end of the busbar is attached to a another PCB in the same manner.The busbar is huge and can easily handle 35amps.The 7mmDia PCB pad is the part I am worried about... It'll have local topside flood, and a bunch of vias (both in the pad, and in the topside flood (not shown) surrounding the 7mm pad.Anyone:a) have any better ideas?b) knowledge on whether the 7mm pad can handle 35amps?

author: Wolfram
author_rank: Frequent Contributor
timestamp: October 05, 2021, 04:58:25 pm
content: That busbar looks awfully big for 35 A. Having rigid busbars connect to PCBs can lead to a lot of potential problems. Differential thermal expansion between the copper and the PCB can cause a lot of stress on the interconnection, which can lead to failure over time. I use flexible wires for up to around 70 A, and even going beyond this is easily done by using multiple wires in parallel. Wire with high temperature insulation can help buy some margin here, 125 or 155 degree rated XLPE is a good match for the temperature handling of normal PCB materials. Busbars are viable for higher currents, but need to be designed with flexible sections to limit strain in every plane.The connection between the insert nut and the board itself can easily be designed to handle 35 A. Make sure the whole current path is as wide and short as possible, and increase the copper thickness in case you need more margin in your design. Skip the thermal reliefs if practical, that way your wire can help heatsink the interconnection and plane.Edit: I just noticed that the PCB itself will be compressed by the screw and the nut. PCBs will creep at higher temperature under compression, and once you lose compression of the joint then you also lose the integrity of the electrical connection. Consider either pressfit terminals where the PCB isn't compressed by the screw, or soldered threaded terminals.

author: frogblender
author_rank: Regular Contributor
timestamp: October 05, 2021, 05:17:37 pm
content: Quote from: Wolfram on October 05, 2021, 04:58:25 pmThanks Wolfram. I've heard that belleville washers (cup-shaped, such that as you tighten them down, the cup shape starts to flatten out, and it acts like a spring) are often used with busbars, presumably to maintain force even if things creep over time. Ever hear of this?

author: m98
author_rank: Frequent Contributor
timestamp: October 05, 2021, 05:18:30 pm
content: There are dedicated Connectors like Molex PowerPlane or Würth PowerOne for such applications.

author: Wolfram
author_rank: Frequent Contributor
timestamp: October 05, 2021, 05:28:59 pm
content: Quote from: frogblender on October 05, 2021, 05:17:37 pmOf course I've heard of belleville washers. The problem is that the PCB will continue creeping as long as there is an applied force, and quantifying how much mechanical compliance you need to always maintain an acceptable lifetime, while ensuing that the pads don't disconnect from inner planes during compression, sounds a lot more difficult than using a proper interconnection in the first place. If you don't care about reliability then anything goes of course. Belleville washers are useful when making interconnections between materials that don't flow under compression, like connecting solid metal pieces together.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2021, 05:44:40 pm
content: 35A is not much. Is that busbar a requirement, or is this negotiable, i.e., can you choose something else to carry the 35A?The challenge isn't current, the challenge is reliable mechanical mounting. I'd suggest simple soldered THT screw terminals, those with a threaded hole on a block, the block having a leg in each corner so the block is soldered to the PCB. Wurth Electric at least carry those, they can easily carry some 100A.

author: frogblender
author_rank: Regular Contributor
timestamp: October 05, 2021, 05:50:42 pm
content: Quote from: Siwastaja on October 05, 2021, 05:44:40 pmNo, busbars are not a requirement. It can be anything, except for pinthru solder (this board has lots of pressfit, so a pressfit solution is good; but wavesolder is not allowed).

author: wraper
author_rank: Unknown
timestamp: October 05, 2021, 05:58:48 pm
content: It can be simply a few connectors in parallel or a single multi pin connector where several wires share the current. For example in computers you don't see any beefy wires because multiple wires pass the current into a single multi pin connector. Or use as screw terminal as already suggested.

author: daqq
author_rank: Super Contributor
timestamp: October 05, 2021, 06:06:42 pm
content: Take a look at what Wuerth is offering:https://www.we-online.com/catalog/en/em/redcube_terminals

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2021, 06:50:16 pm
content: I'm fond of these:https://www.digikey.com/en/products/detail/keystone-electronics/8197/316833Basically same as the above, just the slimmer sheetmetal version, by a different brand.The machined block ones obviously take quite a lot of heat to solder, keep that in mind. An average 70W iron should be fine either way, just be patient; and if you have big copper pours hanging off it, consider preheating the board. Or for production, make sure the wave soldering is done at the right rate.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2021, 07:03:57 pm
content: AWG8 or 8...10mm^2 wire instead of busbars, and classical THT PCB mount screw connector is an option at 35A. Even a single beefy connector, or as wraper suggests, paralleled connections.Teslacoil linked roughly what I was talking about earlier. Solid metal block versions can support even higher currents but at 35A that one is definitely OK. Can use either busbars, or crimp a simple round lug on a wire.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2021, 11:51:20 pm
content: Yeah, 35A isn't all that much, it's not far above plain old 1/4" quick connects; but those run out around 20-30A so would be a bit marginal here. A bolted connection is probably a good idea but it doesn't need to be anything particularly fancy.Tim

author: james_s
author_rank: Super Contributor
timestamp: October 06, 2021, 12:17:10 am
content: I've used XT60 connectors with success. Those screw terminals Tim posted look like a good option too. In a pinch you can also use multiple pins in any random connector you want to use, that is commonly done with various Molex/Amp types, just use as many wires and pins connected together as you need to carry the load.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 06, 2021, 12:38:25 am
content: Mind that connectors won't generally share current perfectly -- so, in addition to the usual heating effect*, allow some extra derating -- but as you can easily pack three times as many connections in a header, this often ends up in your favor.*Mind that most connectors are rated for only one circuit active. No heat from neighboring pins screwing things up. When they show derating for more (up to full header active), take note of how much the capacity goes down by -- it can be half (per pin) or thereabouts!Also, ultimately it's a tradeoff for resistance, temp rise and reliability; if you really don't care, don't have any standards to meet or anything, you can run connectors way over their ratings. Just be careful that they can still carry fault current so as to clear fuses when a fault does occur!Tim