Sep 292018
 

So, I was busy routing a board having come up with a basic schematic.  I wasn’t going to order the board yet, I wanted to just play around with the design, see how compact I could make this.

One thing that was niggling in the back of my mind, was how the traces would cope with the current.  I use 6AWG cable from the solar panels, and 8AWG from the batteries.  How wide should I make the traces? One calculator reckoned I should make them about 7cm wide!  Another option was to use heavier gauge traces, maybe 3oz copper.  A 5cm×5cm 2-layer board would cost a staggering AU$263 for just the PCB!

Okay, so I can work around this by fiddling the solder mask in Kicad and just solder some copper wire along the trace.  Not a show stopper.  I’ll just make wide traces so I know where to lay the wire and have plenty of area to solder it.

I was making the traces as wide as Kicad would let me, but something didn’t seem right.  The inductor, just seemed so, small…

When I did the search on Mouser, their interface allows you to pick a value, then hit the ≥ button to select everything “greater than”.  What I missed, is the option right down the bottom:

The “-” option, better known as “we couldn’t be stuffed looking up what the real value is”, is seen as “greater than everything else”.

A check of the datasheet itself, revealed the truth.

In short, there is no way that little tiddler is going to manage the current I was contemplating throwing at it!

What’s the biggest I can get that will handle that current?  Well if I take the “-” option out of the equation, they suggest this monster .  It’s 10uH instead of 33, so my ripple voltage will increase.  At $837.84, it is also a rare exception to the free shipping over $60 offer.

I might need to go play with some numbers to see what I can get away with.  The good news is that discontinuous output is not a show stopper for a battery charger.  I might have to make do with nanohenries of inductance instead of microhenries.