I was fiddling with one of my robot boards the other day — popping some passives on and off and checking out subs and alternate values. I was doing this on a couple of boards at the same time. Everything was going along fine until I started to do a power-on test. The first board was fine. The second one would briefly light the power indicator LED. It would start a full brightness and then fairly quickly fade out.
My first thought was that I had been too aggressive with my soldering iron and had burned something out. (Who has already guessed what really happened?) Turns out, that wasn’t the case. I put it aside and came back to it a few days later. This time, I gave it the finger test and discovered that my regulator was hot. Darn. Next, I found a hot tantalum cap. Nothing looked out of the ordinary/ I stared at it for a while. The + side was on the left in both parts and… The plus side was on the left in both parts. One was supposed to be on the right. Oops. The cap had a high enough voltage rating that it didn’t blow up. It just pulled down the supply until the over-current protection in the regulator shut it down.
I’ve heard a number of folks recommend that you keep all your polarized parts facing the same way. It’s not always possible, but it can certainly reduce opportunities for errors like I made here.
Duane Benson
Left, right. Left, right. Left, right. Left, left. Left, right …