Wednesday, 27 November 2013

Dangerous Assumptions

When You Assume....

Back in the start of my Alarm interface project, I ran into problems with my current limiting circuit.  My little 3A FET would go up in smoke with limited to 500mA and I just put this down to a need to heatsink the FET.  The thinking here is that passing 500mA when switched on was too much for the little SOT23 package.

I was wrong!

But before I knew that, I decided to beef up my circuit, switching the PMV48XP for a TD2955PT4G which comes in a D-PAK package, and rated to run with a Heavy Duty 12A steady state current.  500mA should be nothing for this beast, and the bigger case might compensate for the lack of heatsink (I was trying to get a physically small solution).

However, the only thing I knew was that I was going on guesswork here.  I didn't have any hard maths that proved what I was thinking was correct, and rather than commit to winging it on my next run of Seeed studio PCB's I built a test circuit to check it out.

Rather glad I did that now!

High Side Limit Schematic

High Side Limit PCB