# Weird value of track width with calculator

I am currently working on a design where the power input will be connected to battery of car. I have a series of LDo and DC-Dc but the entry point is 9-36v and I am unsure what the current can be . I read here and there on the internet that car batteries can deliver until 40A. Considering this I have checked with track width calculator but the result looks conpletely wrong (I find 111mm around). What's wrong there ? What I am missing ? What should the be track width for car battery ?

• You never want to try to put that much current through a trace. – Ignacio Vazquez-Abrams Jun 9 '16 at 7:47
• Design should be driven not by the battery but by an awareness of the power your circuit is intended to draw under normal conditions, and the power it could draw under fault conditions until some protection device trips (using PCB traces as fuses is usually a design flaw rather than an intentional feature) – Chris Stratton Jun 9 '16 at 7:54
• We have a protection device in the circuit, we use LM74610QDGKRQ1, but before this we still need to have some track between connector and this device. – chris Jun 9 '16 at 7:56
• If the "protection device" opens, how much current will flow through the the power input connector? Trying to design a circuit board without a better understanding of circuitry is a bit like trying to run before learning to walk. If your assignment is only to physically implement a circuit electrically designed by someone else and to do so without you first fully understanding it, ask them to calculate the current. And make sure they review and approve your work before it goes to the fab. – Chris Stratton Jun 9 '16 at 8:00
• The maximum current totalalling all the devices will be around 3A – chris Jun 9 '16 at 8:21

First of all as already commented to your question, you don't design a circuit according to what the power supply can deliver, but to what the circuit in design will draw as an absolute maximum (plus some safety margin).

What I usually do, is to write a comment to each section of the schematic with the estimated current consumption. Add them up and you know your complete current consumption. Even if you are for some parts of your circuit not completely aware about the current requirements you can usually find some information in the datahsheet, estimate or simulated (by something like LTSPice for example). On top of that you want to add some safety and you have your maximum current your complete circuit will draw.

With this information you can either use a lookup table or one of the plenty trace with calculators around (e.g. http://www.circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/) and you get your minimum trace with.

If space is not absolutely critical usually your PCB design software uses some default trace widths, select the next higher one to the calculated one an you are good to go.

Besides that various methods exist to increas the current capabilities of tracks, such as

• using thicker copper (2oz instead of 1oz thickness for example)