Hello,
I'm redesigning a motor control system. Currently it is a discrete design with din relays. I would like to go to a integrated PCB design with onboard relays; mainly to save on manufacturing costs. The motor that I am controlling draws a pulse of around 25 amps @ 24 VDC for approx 100ms then goes to a steady state of 2.5 amps at the same voltage. With that turn on pulse, what would be the appropriate sizing for the PCB trace on the board. AND why, I have looked for information that would relate to this and have came up dry. I would most likely be using 2.5 oz trace weight. Thanks for the help.

 

Here is a link to a trace width calculator, you may have seen similar ones elsewhere.
http://www.4pcb.com/index.php?load=content&page_id=95

According to the calculator, a 25 amp load on a 2.5g copper and a 20 degree temperature rise requires a 0.263 inch wide trace.  This can handle 25 amps indefinitely and should be a safe value.  You could probably use a smaller trace, but keep a good safety margin.  It is all about heat dissipation.  Fusing will play an important factor here, make sure the fuse will blow if the motor pulls 25 amps for too long.  If you can use a 5 amp slow blow fuse, you may be able to size the trace in the 5 to 10 amp range.  Be conservative as possible and keep the trace wide, if you can.  If you plan for a 0.263 inch or wider trace, you have one less worry on your mind.

Bill Sturm

Abbeytronics LLC

 

Thinking about this a little more, it might be better to size the trace for 25 amps and use a higher temperature rise to account for the short duration.  25 amps with 2.5 oz copper and a 50 degree C heat rise calls for a .151" wide trace.  You might take a look at your boards temperature ratings.  The board temperature will be ambient plus the heat rise.

Bill Sturm

Abbeytronics LLC