High Current PCB Trace - Page 1
Forum section: electronics / projects

author: awef
author_rank: Contributor
timestamp: June 30, 2017, 02:59:40 pm
content: So I used some online calculator to come up with a PCB trace that's let's just say 1 inch wide for 30A current.The idea is to have an Arduino control a relay.Now the typical Relay "pin" is very narrow even if it is rated for 16A or 30A or whatever. Well... at least it looks that way since the power cable from the wall/circuit breaker looks much thicker.So let's say I put 220V in at one end of the 1 inch PCB trace a relay in the middle, and the output on another fat trace.I have this picture in my head that all the current is going to just run in the middle of the trace (assuming input and relay pins are centered) and completely ignore the rest of the trace. (at least relatively speaking).Also let's say for the sake of argument that current on the PCB goes from point A to point B. The power input is at A, relay pin at B. Let's say point B is surrounded on all sides by 0.5 inch of trace (since it's centered).As current runs from point A to point B, is it all going to "traffic jam" on the "A-side" of point B and ignore the rest of the fat trace on the "far side" of point B?I acknowledge there are many variables, but I figured I'd check in on this "fundamental" item.

author: georges80
author_rank: Frequent Contributor
timestamp: June 30, 2017, 03:27:11 pm
content: Current flows through the entire trace cross-section/width (not just down the middle), certainly for 50/60Hz or DC.The online calculators allow you to specify temperature rise you are will deal with. You are likely using conservative defaults...You can specify high oz copper for a high current board, not unusual to go to 4oz copper...You can run traces on both the top and bottom layers (parallel) if needed.It's also not uncommon to expose part of the copper trace (remove the soldermask) so that during the soldering process, solder is deposited onto the exposed copper trace to 'thicken' the trace, not as good as copper for conductivity, but it does help.cheers,george.

author: DBecker
author_rank: Frequent Contributor
timestamp: June 30, 2017, 05:24:52 pm
content: With circuit board traces you should think at least as much in terms of thermal performance as conductivity.The reason thin copper foil can support high currents, despite a small cross-section, is that it has a large surface for cooling. Doubling the copper thickness won't double the current supported because you can't get rid of double the heat. But doubling the surface area will improve the thermal performance, even if it doesn't decrease the point-to-point resistance that much.Solder isn't that great of a conductor, so solder reinforcement doesn't improve the thermal or electrical performance as much as it appears. But it's so much less expensive than increasing the copper thickness that it's quite common.

author: sheitbrink
author_rank: Newbie
timestamp: June 30, 2017, 06:16:02 pm
content: I think what you are getting is the fact that when we get to this level of detail you have to need to stop treating traces as a lumped resistances, capacitance, and inductors and start to treat them as continuous distributed systems. From what I understand trace current ratings are really generated by looking at the resistance that is a function of the cross sectional area of your copper, assuming you have a constant current density throughout the copper, and comparing the amount of temperature rise you are willing to take.What you are describing is the fact that you aren't going to have current have a current density that is the same thru your out your system, so the easy plug and chug way of getting trace resistance, and current ratings doesn't quite fit. If you are really interested in this you could look at the following:http://www.ultracad.com/articles/fusingr.pdfPCB Trace and Via Currents and Temperatures: The Complete Analysis, 2nd Edition

author: DBecker
author_rank: Frequent Contributor
timestamp: June 30, 2017, 06:27:57 pm
content: Moving away from the circuit board design details: have you considered a relay with quick-connect tabs on top?They solve several problems at once. They avoid high current running through the circuit board, provide a low cost robust wire-to-board connector, and remove heat from the relay though the connecting wires instead of the circuit board.If you expect to support 30A at a high duty cycle, you should consider an off-board relay (for low voltage DC) or contactor.

author: awef
author_rank: Contributor
timestamp: June 30, 2017, 08:40:19 pm
content: Quote from: DBecker on June 30, 2017, 06:27:57 pmI had no idea there was such a thing. Thanks for mentioning it!

author: awef
author_rank: Contributor
timestamp: June 30, 2017, 08:41:59 pm
content: Quote from: DBecker on June 30, 2017, 05:24:52 pmAh! Good point. I almost missed that.

author: georges80
author_rank: Frequent Contributor
timestamp: June 30, 2017, 10:41:10 pm
content: Quote from: awef on June 30, 2017, 08:41:59 pmBut doubling the copper thickness halves the resistance of the trace so the I^2 x R heat loss is also halved... After all that heat is due to trace resistance which is due to copper width x height (area of the trace) and then length of trace. increase the copper area reduces resistance (and thermal losses) as does reducing the trace length.I've designed power distribution boards that handle 12A (DC loads) via PCB thruhole pinned relays and it is quite feasible to do with 4oz copper and 100 to 120 mil traces that are kept as short as possible. These are production boards used for motorbike lighting - so not the most benign environment, i.e. potentially high ambient temperatures etc.cheers,george.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2017, 10:46:48 pm
content: Another example of binary versus continuous thinking:"Current follows the path of least resistance"So, like,thepath, and none others?No. Definitely not!Such statements are found all over the place. They perhaps appeal to our baser instinct to categorize, and hence survive despite being on-the-face complete bollocks.Indeed,almost everywhere you can look in reality, if you are told there are categories, or steps or levels, or absolutes, almost everywhere, those statements are strictly wrong.The real world is continuous, so you shouldalwaysexpect to see a range of outcomes. Likewise, you should always expect to re-interpret peoples' statements: when they say some absolute, some categorization, it's actually that a continuous phenomenon has been binned. "A short circuit has zero resistance." "Current follows the path of least resistance." "Minimize loop inductance."How can we reinterpret this statement, then?Suppose it were true. Maybe the first tiniest bit of current follows the shortest path; but now, there's a tiny voltage drop along that path, so the next little bit of current has to follow a slightly longer path. Once the tiny currents reach the edge, they start building up again in the center. But now the center has two tiny voltage drops worth, and so on. So, if we conceptually divide "a current" into a bunch of tiny currents that find a path independently, they will build up a density of paths, which looks exactly like the current density in a real conductor. And, by the way, in the process of dividing a current into an infinite number of tiny parts, we've assembled a continuous whole!So, maybe that helps a little. A better statement would be, "current flows proportional to the voltage dropped across the conductor". Which is just a crude way of saying Ohm's law: I = V/R. There isn't really anything more you can say about a conductor, without knowing how the conductor is shaped. Then it's just an electric field problem.Tim

author: Nusa
author_rank: Super Contributor
timestamp: June 30, 2017, 11:17:29 pm
content: Another option would be to solder bare copper wire to the high-current traces during assembly. I've also heard of people using solderwick braid instead of wire. Leave the solder mask off the traces, of course.Or skip the long traces and use through-hole jumper wires on the board. Especially useful if you're short on component space.

author: digsys
author_rank: Unknown
timestamp: July 01, 2017, 12:59:49 am
content: Completely concur with georges80. We make several 20-30A P/Supplies, and I use 4oz, 2 layers, 80% mask removed tracks and according to FLIR, no discernible temp rise.Even tested to extra 10A+. If there are even higher currents expected, I provide pads, down the middle of the track, to solder solid wire links. (broken into ~1cm lengths)

author: DBecker
author_rank: Frequent Contributor
timestamp: July 01, 2017, 05:09:43 am
content: For small-scale production 4oz copper boards are cost prohibitive. It's often less expensive to hand solder wire reinforcement or bus bars, or redesign the board to use a different approach.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2017, 12:58:21 pm
content: Quote from: digsys on July 01, 2017, 12:59:49 amYeah, what trace width?Tim

author: digsys
author_rank: Unknown
timestamp: July 01, 2017, 01:17:01 pm
content: Pretty wide !! :-)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2017, 04:43:39 pm
content: (Yeech, the JPEG, it burns)Yeah fine, but......What trace width?Around 200 mils I'd eyeball, but come on...Tim

author: rdl
author_rank: Super Contributor
timestamp: July 02, 2017, 01:54:52 pm
content: I've never seen two fuses in parallel.

author: digsys
author_rank: Unknown
timestamp: July 02, 2017, 01:59:48 pm
content: Quote from: rdlPolyfuses. They share nicely. Don't even have to be of the same value.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2017, 02:04:12 pm
content: Fuses can be connected in parallel, but it's not recommended. Expect one fuse to blow before the other, i.e., the actual rating* is somewhere between min(fuse1, fuse2) and (fuse1 + fuse2).*The rating is nominal maximum, not the melt value. Actual failure is 20-100% (within a duration of ~1 min) above the rating. Keep this in mind as well.Tim

author: fcb
author_rank: Super Contributor
timestamp: July 02, 2017, 02:51:23 pm
content: Quote from: DBecker on July 01, 2017, 05:09:43 amI use 4oz routinely on a particular clients projects - 32A with 9mm wide traces on both sides (18mm effective width). You can barely measure the temperature increase.The problem is that there aren't many companies happy to do 4oz, my favourite supplier starts with 3oz and plates up to 4oz. It has some bearing on cost but not huge (perhaps 1.5x), certainly much cheaper than the alternatives. My CEM had to frig around with wave-solder speeds though to get a nice joint.

author: Daque
author_rank: Contributor
timestamp: July 02, 2017, 06:39:23 pm
content: Quote from: DBecker on June 30, 2017, 05:24:52 pmI think I see where your coming from 1/2 the resistance is only 41% more current because current's effect is the square of the change in current.I^2*R=P sqrt(2)^2*(R/2)=pRight?

author: fcb
author_rank: Super Contributor
timestamp: July 02, 2017, 06:45:29 pm
content: Quote from: Daque on July 02, 2017, 06:39:23 pmI would have thought that the rough rule of thumb would be: Double the Cu thickness, halve the resistance, halve the losses?This is a really useful tool BTW:http://www.saturnpcb.com/pcb_toolkit.htm

author: digsys
author_rank: Unknown
timestamp: July 02, 2017, 11:16:02 pm
content: Quote from: fcbYup, I'm having trouble finding someone who does in-house 4oz. No problems with 1-3oz. It adds days, as they send it off to some unknown 3rd party. So far all ok.Who are you using?

author: fcb
author_rank: Super Contributor
timestamp: July 02, 2017, 11:24:41 pm
content: For short run (100-400pcs) we use a company in the UK.

author: digsys
author_rank: Unknown
timestamp: July 02, 2017, 11:31:54 pm
content: Quote from: T3sl4co1lYes, what you say is relevant. In my case, I do carry out extensive testing to get the profile I want. Sometimes, I'll even heatshrink the 2(+) together to get a slightlyfaster trigger and slower release. In the case of the bridge-caps, the caps take care of fast transients (shorts etc), and the polys are a failsafe. If required, I have other"primary" protection. Parallel polys also help with reducing dropout (small as it may be) in my ULDO units. After 25+ yrs, so far, never had a failure. ymmv

author: digsys
author_rank: Unknown
timestamp: July 02, 2017, 11:38:50 pm
content: Quote from: fcbAnother trick I use, when I really want to be SURE te batch of PCBs is made correctly - is to add a few "test strips" to the design (or on smaller boards, snap-off along anedge). eg I'd do say 5-20mmW, 50-100mmL, with + without exposed mask. Then run test currents through these strips to see if they behave as expected.Formulas are fine, doesn't help if someone screws up on the batch :-)

author: Neilm
author_rank: Super Contributor
timestamp: July 04, 2017, 06:50:49 pm
content: Quote from: digsys on July 02, 2017, 11:38:50 pmI would want to see something like that if I was getting boards made in volume from China. The way to get cheap boards is to reduce the amount of copper, especially on the inner layers. The prototype boards work fine but as soon as they go offshore it all goes to pot.

author: DBecker
author_rank: Frequent Contributor
timestamp: July 05, 2017, 02:16:22 pm
content: Quote from: Daque on July 02, 2017, 06:39:23 pmYes, that's the theoretical formula that describes the effect and lets you estimate.Of course in real life everything is slightly different.. as the joke goes, "first, assume a spherical cow".