In a current project, I needed a contractor that will reliably run a 1HP 120VAC motor, or 2 HP 240V, and continue to do so for a long time, (I calculated that I needed 20,000 on off cycles over the product life). After messing with several relays that theoretically "should" work, but probably wont have the longevity I wanted, I had a good long consult with one of the friendly Engineers at Potter and Brumfield, and decide to use their T92 series relay. Here's the data sheet.
This is a very robust relay with two 40 amp contacts, rated for switching the loads I need 100,000 cycles!. OK, it will cost a little more, but the product will be reliable.
Unfortunately, good things often DON'T come in small packages when it comes to power. My only hope of making this relay fit in my enclosure is to use their "Mounting Code #1" version, which is a PCB mount.
My question is about figuring out the most economical way to make PCB traces that will safely carry the current, without heating up much. I DID say this relay was overkill, as I'll never need to carry much more than 13 Amps continuous, based on the the name plate data of the motors I'll be controlling. So I thought I would just run the biggest traces I could (I did copper pours within drawn shapes actually), which came out to about 0.350 inch wide, and ran them to a hefty 30 amp PCB mount barrier strip. Then I duplicated those main traces on both sides of the PCB, and I figured I'd also go for 2, 3, or 4 oz copper.
Well apparently the China board house I usually use ( PCBWAY.COM ) is very good about doing 1 oz copper boards at super low prices (like 10 for $5 plus shipping). But as soon as you go to 2 oz copper, the same number of boards would be $36, and 3oz jumps to a whopping $140!!! (they don't even offer 4 oz).
I don't know why the higher density copper costs rise exponentially like this, but I'm still prototyping, and would rather avoid the expense until its proven to be necessary. So I'm trying to figure out whether I need that extra copper thickness. Based on what I explained already, can anyone offer some advice as to whether I could get away with plain 1oz copper? This image probably won't show up in a browser to scale, but my board looks like the below. Remember the wide traces are about 0.350 wide, are duplicated on side 2, and will never have to carry more than 15 amps continuous.
If 1 oz copper is definitely NOT up to the task, a couple of other things crossed my mind...
- I could wad up a lot of extra solder on the bottom side of the board to increase its overall metal thickness
- Knowing the Relay pins are about 0.156 long, I could stack two boards (0.062) together to simulate a single higher density copper board
- Someone could tell me my board house is being ridiculous, and tell me about another "budget board" company that doesn't treat copper like gold