Value Engineering

A project log for Magnetic Switches for RC Aircraft

Four versions of a simple on/off power switch activated by a magnet.

bud-bennettBud Bennett 03/29/2018 at 00:140 Comments

I did not build any of the 2nd pass low current mag switch prototypes. They just didn't measure up against the Zepsus product (and I was too cheap when designing them). My decision to use three AO3400 FETs instead of a more capable power FET seemed wrongheaded. Spending $0.50 more for a better power FET would  add capability and robustness. So why not?

I found a relatively cheap ($0.61/each in QTY=10) FET in a 3.3mm x 3.3mm DFL-8 package -- TSM038N03PQ33. Its RDSon = 4mΩ typ. @ VDS=4.5V, which is 3x lower than the combined AO3400 FETs, with the same 30V VDS max rating. But it cost $0.61 (Digikey) vs. $0.026 (eBay) for the AO3400.

I bit the bullet and scrapped the second pass low current switch boards. The single big FET will increase the current handling to at least 7A, while allowing a short term burst current capability of 10A. Another consideration in making the switch is that these DFL packages, while seemingly difficult to solder no-lead packages, were surprisingly easy to solder (if you know the trick) with consistent results.

The latest schematic for the low current version is posted in the details section. I've ordered 9 PCBs from OSH Park in 2 oz. copper. They should arrive in a few weeks.