Archive for September, 2019

Silly Subproject: Switching 7805 / 7833 TO-220 replacement

Thanks to an oopsie on a larger project that involved not doing the math before dropping a small 3.3V linear regulator into a >20V input circuit design, I had a need to swap it for something that generated less finger-burning, board-cooking, smoky-smelling heat. I’m sure everyone and their dog has done one of these already, but I was putting in for boards anyway and had the PCB editor already open, so: A tiny, low cost switching regulator board that’s a drop-in replacement for a through-hole TO-220 linear regulator such as a 7805 (5V) or LM1117/LM3940/etc. (3.3V). You can also wire it up in place of a SMT regulator of course; I just liked the reusability of the iconic 7805-style pinout and form factor.

Download Gerbers, EAGLE (7.x) files and BoM below.

You can order the bare board itself from OSH Park for a couple bucks. Note, this is a surface mount design with 0603 size components, although there are only 6 components in total.

The heart of this minimal circuit is an AP632xx buck converter from Diodes Inc., with a maximum input voltage of 32V and claimed output current of 2A, with typical efficiencies in the 80-90% range (depending on input voltage, output voltage and current). The output voltage can be switched by stuffing either the AP63203 (3.3V) or AP63205 (5V). I’d be a little cautious of trying for the claimed 2A on this board, just because it’s so tiny (0.4 x 0.635 inches) and heat dissipation from the chip itself is, at least according to the datasheet recommendations, mainly by proximity (through an air gap!) of the plastic package to the groundplane beneath. On the other hand, in the circuit I made this for (~20V input, 3.3V out at ~300mA) the chip itself is barely detectable as warm to the touch. If in doubt, adding some thermal goop on the underside of the package might help.

Regulator in action on a silly “IoT” project that I’ll never have time to complete. The soldering and lack of cleaning are atrocious, but this is prototyping, don’t judge!

Not Dead

I’ve just been busy with a couple of little things.

Now that the littlest one is figuring out this whole sleep thing, I might have time for projects again :-)