Part of my latest project is a custom a DCC decoder that needs to fit into frame of a dumpcar. This means it needs to be much smaller than a standard DCC decoder, with required width of mere 6 mm.

For this, I performed a search of the smallest electronic components possible, that would still do the job, with the requirement that they are also hand-solderable, i.e. have legs and are not too small. And they need to be available at LCSC. Being available for JLCPCB PCB assembly is a bonus.

Comparison of different small footprints (and corresponding devices)

Linear regulators

Usecase: drop track voltage (12-25V) to power MCU and other logic, i.e. 3.3V or 5V. For small footprints are are limited to low currect output (<50mA), but this should be fine for the usecase.

Suggested package: SC-70-5 aka SOT-353 or SOT323-5. It’s smaller than SOT23 while still being legged and solderable.

  • ST’s ST715C50R - 5V output, 24V, 85mA

  • ST715C33R is a 3.3V option. It is comparatively expensive and doesn’t have a lot of availability (<100 in stock at the moment)

  • TechPublic TPS71550DCKR-TP - 5V output, 24V input, 50mA current capacity.

  • TPS71533DCKR-TP - 3.3V variant, 24V, 50mA. Judging by specs and product number, these are a relatives to ST’s part. They are noticable cheaper and are easily available.

In my decoder I used TechPublic ones.

Dual transistors

Usecasae: control several high-power outputs with an MCU. For example, to have standatized decoder outputs for lamps, couplers, smoke machines etc.

Package: SOT-363 aka SC-70-6. Same size as SOT-353 before, but with 6 legs.

  • AO7800. 900mA, Vds=20V. These are values from Alpha&Omega, the device is available from many manufacturers.

  • NX3008NBKS-TP. 800mA, Vds=30V, works on 3.3V.

  • NTJD4001 from VBSemi. 2.6A Vds=20V. Same device from other manufacturers offer much smaller currents.

  • BJTs are also possible, as seen in this project. It uses MMDT3904 in SOT-363 (200mA), available from a lot of manufacturers. MMDT2222 should also work and is seemingly better suited due to higher current capacity.

Diodes

Usecase: rectify DCC track voltage. Secondary usecase: to feed DCC signal to MCU (as part of a voltage conversion circuit).

Package: SOD-523 (SOD stands for Small Outline Diode). It is comparable to 0603 is size. There are 0603 (and 0805) diodes, but they have solder pads only at bottom (as compared to resistors and capacitors that have all sides as leads), so not solderable with an iron.

To build a rectifier, 4 of these diodes use less space than a single-chip rectifier. Smallest full recifier I found is in MBS package (~5x8mm footprint)

  • B5819WT Shottky diode, 40V, 1A, 610mV@1A. Many options are available by this name, with different characteristics.

Next diode size is SOD-323. It’s slightly larger.