USB-C Breakout and 3.3 V Regulator Building Block Board — Part 3

USB-C Breakout and 3.3 V Regulator Building Block Board — Part 3#

Today I received the 5 W resistors I needed to load test the USB-C and 3.3 V regulator building block board.

I also tested the short-circuit behaviour of the board, which required a little bit of debugging before I was happy with the result.

The first test was the short-circuit behaviour on the 5 V rail, followed by the 3.3 V rail. Initially, I saw a slightly confusing result when I shorted the 5 V output: the board appeared to turn off immediately.

After a moment of head scratching, I realised what was actually happening. My bench power supply was doing exactly what it was supposed to do and entering current limiting.

For a more realistic USB-powered test, I switched to a basic USB wall adaptor and used a small USB-C voltage/current meter to observe the current being drawn by the board.

When I shorted the 5 V rail to ground, the 1206L050/15YR PPTC responded by limiting the current. I then shorted the 3.3 V rail to ground and observed the AP2114D-3.3 regulator’s protection behaviour kicking in, again limiting the current drawn from the USB-C input.

This does not make the board indestructible, but it does mean the design has some basic protection against dead shorts on the exposed 5 V and 3.3 V rails. That should help protect both the power source and the regulator board itself in the event of a direct short.

It is worth being clear, though: this protection is mainly there for fault conditions such as dead shorts. It should not be treated as a guarantee that every connected device will survive every wiring mistake or overload condition.

After the short-circuit checks, I moved on to load testing the 3.3 V rail.

I tested the board at three load levels:

• 100 mA for 1 hour
• 200 mA for 1 hour
• 500 mA for 6 hours

For the 500 mA test, I used two 3.3 Ω 5 W resistors connected in series, creating a load close to the intended upper design limit of the board.

The results were encouraging:

My temperature measurement method was not exactly laboratory-grade — it was mostly the fingertip test — but as an initial practical soak test, the result was good.

In all cases, the output voltage remained steady at approximately 3.3 V, with no obvious voltage sag at the output pins.

I did not test beyond 500 mA. The PPTC is rated for 500 mA hold current and 1 A trip current, and the board was not designed to be used above that level. For that reason, I would describe this as a 500 mA-or-below board.

Taking it beyond that may work in some situations, but it is outside the intended operating range and not something I would recommend.

For my current projects, the board has now passed the basic checks I care about:

• correct 5 V USB-C input behaviour
• stable 3.3 V output
• short-circuit behaviour on the exposed rails
• sustained 500 mA load testing
• no concerning heat build-up during the test

At this point, I am happy to consider this first revision usable for my own prototyping work.

Next, I plan to publish the schematic, PCB layout, and BOM for this board. My current intention is to release those assets under the CERN-OHL-W licence and keep a small stock of the boards available for anyone in the UK who wants to use them in their own projects.

That should make this board both a useful module for my own projects and a small reusable reference design for others working on similar embedded prototypes.

If you are looking for reusable open hardware modules to speed up your bench testing, or need support designing custom breakout circuits, check out our catalog at Protoworks (NI).