PCB Trace Width and Current - Page 1
Forum section: electronics / beginners

author: JLynch
author_rank: Contributor
timestamp: August 13, 2021, 11:58:14 pm
content: Is there a rule of thumb for how wide traces should be in relation to current?My board in standby / idle mode draws around 14mA. When driving the geared micro-motor it draws (normally) somewhere in the range of 70-165mA. If the motor stalls current is limited through the DRV3388 chip to 200mA.The micro motor runs infrequently and only for short durations, max of about 800ms at a time.So 200mA is the max current the board should ever see and that is limited to a duty cycle of less then a second.Any suggestions of how wide the trace widths should be for the motor would be appreciated.Thanks.

author: bob91343
author_rank: Super Contributor
timestamp: August 14, 2021, 12:11:47 am
content: You didn't state the copper thickness.Actually there are two major considerations. First, the current needs to be so low that negligible heating occurs. Second, the resistance needs to be so low that negligible voltage drop occurs.So the answer to your question is the annoying 'it depends'.There is a major, but often overlooked, situation that can occur. The current, as you might expect, doesn't spread uniformly over the copper, but is controlled somewhat by the terminations. If you put a hole in the board and solder a wire in, the current density near that point will be much higher than elsewhere.This is exemplified by some tube type audio amplifiers where the tube heaters are connected by circuit traces. The edges where the current enters and leaves have very high current density and there is local heating which, in several cases I have seen, is sufficient to melt solder and char boards and, finally, cause an open - with the result being that the amplifier stops working. Or catches fire.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2021, 12:20:05 am
content: Some numbers may be helpful. With 1/2 oz copper, 20 mil wide traces are fine up to an ampere or two. Width depends on copper resistance, thickness and desired temp rise; there are many calculators online.Tim

author: JustMeHere
author_rank: Frequent Contributor
timestamp: August 14, 2021, 12:28:04 am
content: I use Electrodroid on Android.

author: bob91343
author_rank: Super Contributor
timestamp: August 14, 2021, 03:45:51 am
content: As mentioned above, be wary of current distribution. Width alone isn't enough.

author: JLynch
author_rank: Contributor
timestamp: August 17, 2021, 08:30:27 pm
content: Thanks for the help with trace widths. I found several calculators online and while getting a few varying results I was surprised to learn how narrow the trace recommendations were.If I’m using the calculator correctly a trace width of 0.03mm is sufficient for 200mA which seems really small.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2021, 11:32:28 pm
content: You'll get different results because they'll be based on IPC-2221B or IPC-2152; the latter is more accurate, and also more general (includes the heat-spreading effect of inner layers, if applicable).Also, note that these figures are valid under the test conditions they were derived from, including traces being very long so that heat sinking through the ends is negligible (temp rise highest in the middle of the route).If you have a bunch of short, fat traces, it matters a whole lot less. For example, if you need a nominal 25mm wide trace to connect some transistors switching 20 or 30A, but the transistors are adjacent (<50mm apart), the current flow isn't going to be uniform, at all, it's going to be concentrated around the two (or however many) pins and the space between them. You still use a wide pour, because the bulk serves to sink heat away from the pins.Another way you can use these calculators, is to set the current, and adjust temp rise until you see the trace width you're using. I'm not aware offhand of one that does this automatically..? Probably would be easy enough to write, but it's a one-to-one function, easy enough to solve in this way by hand. Anyway, that also gives you some idea about your safety factor.Tim