Hi - I'm working on a board that will be switching a fairly high current (I think maybe 5A) 120V AC line. I'm trying to design the board to handle as much current as possible. I'm making all of the traces carrying this signal as wide as possible. I was struck with an idea though - would it be OK to mirror all these traces? In other words - have a duplicate of each trace directly below each trace on the bottom of the board? Being that they're the same signal I don't think there will be any issues - but I thought it'd be best for me to check first. Thanks,
Hi, Mike. There's no potential between the traces if you mirror them so that should be safe. Try less expensive solutions first, though. Specify 2 oz. copper and solder coat. These are a lot cheaper than getting a double sided board, and will boost your safe current rating.
Obviously you're increasing the trace heat sink surface area, so you'll be increasing power dissipation for a given trace temperature. However, if you've got significant heating with both traces, you'll tend to hotbox the circuit board between, causing discoloration and tending to delaminate the traces from the board. That might be a bit of a problem.
I would tend to have some doubts about your solution being that much better, but it should help some. I would guess the positives would somewhat outweigh the negatives, especially if there's a fan or it's not buried in still air. Mounting the board vertically rather than horizontally will also help if there's no air circulation.
Sorry that doesn't give you a good answer, but if you give more information, you might get a better answer than yes and no. What's your trace width, and how long does it run? Are you expecting elevated environmental temperatures, or is this running in free air at room temperature?
Don't forget to worry about how you get the current into and out of the trace. For thin copper, the current density right at the connection pin can get high enough to cause long-term heat problems. Might need to think about multiple input points on the trace. I see a lot of power supplies and TVs where the solder joint cracked over time. mike
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John Larkin wrote in news: snipped-for-privacy@4ax.com:
Hi - the traces will be 35um copper (1 oz) - I think about 3/8" of an inch wide. Is there a good way to calculate how much current that can safely take, and if I need a dual trace on the bottom of the board? Thanks,
mike wrote in news: snipped-for-privacy@netscape.net:
The relay that I'm using has two pins for each contact, so that should help somewhat with this. I also thought I'd make the pads very large and then use a much larger than normal amount of solder, giving more surface area to the connection.
"Chris" wrote in news: snipped-for-privacy@g14g2000cwa.googlegroups.com:
Unfortunately I don't have control over the copper thickness - it will have to be 1 oz. For other reasons (primaryily due to a RF receiver needing a groundplane) the board has to have two sides anyways, so making it two sided is a non-issue.
It will be mounted vertically
I believe trace width will be about 3/8". I'm making the final layout later today so I'll know then. It will be run inside a home, so normal environmental temperatures for the most part. It will not have much air circulating over it though, and a fan is not an option, unfortunately.
Hi, Mike. There are trace width calculators all over the net. Here's one I've used before:
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It says that you should be OK with single side, 1 oz. copper at over .108" outer trace width (10 degree C rise in temp over ambient being the unofficial standard for the start of deep doodoo). As long as you don't have an internal trace, that is.
Take these results with a grain of salt -- your results may vary. However, I can unequivocally guarantee you have nothing to worry about conducting 5 amps with a 3/8" wide 1 oz. single copper trace. You won't need to double up, for sure.
It's great you're being cautious about these things -- that's a good sign, and you're headed in the right direction.
I would be surprised if you run into problems with 5 amps at 120v, i worked on smpsu designs ages ago wich handled 15 amps on single sided pcbs, although in such cases its not too clever if you have long runs as cross regulation becomes an issue, i measured the voltage acsross a wire link once at 50 mv.
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