Individually switchable powerstrips are a great start for building a smart Home Assistant-compatible powerstrip that can be remotely controlled via an ESPHome microcontroller.
Some Initial Remarks…
When building a DIY smart powerstrip, never forget that you are dealing with dangerous AC voltages and potentially high currents.
Save Work And Add Safety
Building such a powerstrip completely from scratch in a safe manner can be a challenge, and costly, too:
- Housing: you need to construct and 3D print a suitable housing. If you don’t invest good time, it will most likely look ugly and may not have the stability required.
- Sockets: you need to purchase sockets that can latch into your housing. Such sockets are quite costly.
- Accessoires: don’t forget that you’ll need a power cord and wires of sufficient diameter.
By purchasing a commercial powerstrip with individual mechanical switches per socket, you get great material to start with: you get a ready-to-use powerstrip with all wiring, typically in a nice-looking housing. All that’s left for you to do is to replace the mechanical switches with relays, and fit your microcontroller and a 5V power supply into the housing.
If the powerstrip you purchased has a USB power supply built-in (like the one pictured above), you can even save the power supply for your microcontroller.
Where To Get?
Powerstrips with individually switchable sockets aren’t as available as regular powerstrips but you can get them at places like AliExpress. Quite a lot of different designs are available.
Ordering powerstrips from untrusted overseas sources may not be recommended if you planned to use the powerstrip as-is - quality and safety of such components doesn’t always meet local standards. However, since you are going to open and repurpose the powerstrip anyway, you will have plenty of chances to review the build quality and make improvements where necessary.
The risk is low: powerstrips like the one pictured above was available for less than €6. Another advantage of powerstrips ordered from China is that they can be easily opened. Regular philips screws hold them together (whereas Western manufacturers typically make it very hard to open up their housings by either using specialized screws, or by molding the parts together in the first place).
Opening And Exploring
Most powerstrips from China use very basic components and can be easily opened using a philips screw driver. The one I used here is held together by six screws that are accessible from the backside.
Understanding Design
Once you removed the screws, you can examine its internal design:
Good Component Quality
This design is actually very straight-forward and perfect for adding smart switching capabilities:
- Neutral: the neutral AC line (brown) is connected to one continuous metal strip that supplies voltage to one side of each socket.
- Live: the live AC line (blue) is connected to a mechanical switch, and from there it is distributed to all other switches. The other side of each switch is connected to the remaining side of the respective socket. So each switch just needs to be replaced by a relais.
- Ground: the ground line (yellow-green) is connected to all ground contacts of all sockets.
- Signal LED: there is a red LED for each socket, located next to each switch. The LED is simply wired to both poles of each socket, using a simple current limiting resistor. You can keep this LED (if you want to continue to have a simple indicator LED), or replace it with a more sophisticated indicator LED (i.e. a bi-polar bi-color LED).
- USB Power Supply: the powerstrip I picked came with a USB power supply. It is connected to AC using two thin blue wires and cannot be switched off. You can either keep it connected as-is, make it switchable as well, or repurpose it to power your microcontroller - in which case you might want to remove access from outside to not overload it.
Bad Assembly Quality
When you look at the assembly quality of some of the AC cables, it becomes clear why it is generally not recommended to use cheap Chinese powerstrips out of the box for an end user:
Some of the solder joints are not directly connected to the metal and just bridged by solder. Keep in mind that this powerstrip is rated at 2400W. If you ever were to use this claimed maximum, and connected some beefy heaters to this powerstrip, most likely such joints would just heat up and melt away.
Fortunately, for DIY projects, the component quality is all that matters, and bad solder joints can be fixed.
Removing Unwanted Parts
Since the sockets are going to be controlled by a microcontroller, the bulky mechanical switches are not needed and can be removed, providing extra room for the new components.
De-solder the wires that are connected to the switches, and unscrew the buttons.
Examining Inlays And Openings
The particular powerstrip I used has neon-green inlays that hold the buttons. These inlays can simply be taken out:
This design is great because it makes it very easy to 3D design similar inlays as part of a structure that can be plugged into the openings. It turned out that each inlay has a diameter of 24mm and uses two M3 screws with a distance of 30mm.
It shouldn’t be hard to 3D design a suitable replacement, and the fact that simple M3 screws can be used to secure it is a welcome benefit.
Stripping Unwanted Parts
The built-in signal LEDs might be useful: they light up when a socket is powered. Connecting LEDs directly to AC isn’t the most recommended design, but it works and is simple.
Why connecting LEDs directly to AC shpuldn’t work (and still does)
Connecting a LED to 220V AC is not the recommended way, however it can work, and it is cheap.
There are two problems:
- Reverse Breakdown Voltage: LEDs have a very low reverse breakdown voltage of around 5-30V. Any voltage exceeding this that is “going the wrong direction” will destroy the LED. Obviously, 220V is exceeding this limit, and AC is sending voltage the “wrong way” every half wave.
- Heat: Since the LED requires only a fraction of the energy available, a current limiting resistor would need to convert the rest to heat. A lot of heat.
The simple signal LED works fine nevertheless because a very high current limiting resistor is used, i.e. 200kOhm. This limits the current to roughly 1.5mA. Many LEDs can tolerate such low currents when applied “in the wrong direction”, and they still emit enough light when applied “in the right direction”. The 50Hz flicker isn’t visible to the human eye.
If you decide to implement a safer or more sophisticated concept for your signal LEDs, pull out the LEDs and de-solder their wires.
USB Power Supply
The powerstrip comes with a built-in USB Power Supply that is connected to AC via two thin blue wires.
Here are a few options:
- Remove: unscrew and remove the board to get more space. That’s especially useful if you need to use larger relays.
- Keep: leave as-is, and use a constantly powered USB supply
- Make Switchable: add a relay to one of the blue power cables, and make the USB supply remotely switchable. This can in fact be a great idea if you would like to remotely control USB devices.
- Combined Use: share the USB power supply with the end users and your microcontroller, and run your microcontroller off it while keeping the ability to also connect external USB devices.
- Repurpose: Remove the ability for end-users to plug in USB devices, and use the 5V from this board exclusively as a dedicated power supply for your microcontroller.
Power Microcontroller And Relais
It would be absolutely convenient to power the microcontroller and relais off the USB power supply found in the powerstrip - saving a dedicated AC/5V converter plus the space it requires.
To check whether this USB power supply is at all capable of running your components, you need to know what the maximum power consumption of your smart electronics is that you want to fit into the powerstrip.
What matters is the absolute maximum peak current that your components may draw in the worst circumstance. If the maximum current that the power supply can deliver is exceeded only for a very short time, i.e. because of a WiFi burst the microcontroller sends out while enabling all four relais, then this would result in an immediate voltage drop which in turn would brown-out your microcontroller.
Microcontroller
Here are peak currents for some popular microcontrollers:
| Microcontroller | Peak Currents (WiFi Transmit, mA) |
|---|---|
| ESP32S | 300 |
| ESP32-C3 | 200 |
| ESP32-S2 | 290 |
| ESP8266 | 300 |
The old single-core ESP8266 requires roughly as much energy as the more modern dual core ESP32S. The even more modern single core ESP32-C3 however is much more efficient.
Relais
The type of relais affects power consumption considerably:
- A solid state relay requires around 15mA, and with four sockets, this adds up to 60mA.
- A mechanical relay requires hefty 100mA, summing up to 400mA for four sockets.
Overall Peak Consumption
You have considerable options to fine tune energy consumption. Depending on the microcontroller type and the type of relays you pick, overall power consumption for a four-socket powerstrip varies between 300mA and 800mA (including 15% safety margin):
- Efficient (300mA): ESP32-C3 and solid state relays
- Inefficient (800mA): ESP8266 and mechanical relays
Available USB Power
For the powerstrip used, its USB power supply is rated at 2.1A at 5V. This would leave 1.3A for end users, and you could be intreagued to run your microcontroller/relais off it and also enable the end user to charge devices.
Here are two caveats to consider:
- Overload: if you keep public access to the USB power supply, then you always run the risk that end users overload it, cause a shortcut, or whatnot, effectively browning out or shutting down your microcontroller. So if you want public access to the USB power supply, use a different and dedicated power supply for your microcontroller/relays.
- False Specs: if you plan to use the internal USB power supply to power your microcontroller/relays, do not trust the vendor specifications. They are often wrong or imprecise (maybe you can draw up to 2.1A, but for how long?). Always take a look at the hardware, test it, and judge for yourself. Else, use a dedicated AC/5V power supply from trusted sources to power your mission critical components.
Examining USB Power Hardware
This is the USB power supply board found in the powerstrip:
Circuit Analysis
It comes with four USB-A and one USB-C port. All five USB port are connected to each other, so there is just one power supply for all ports.
There are two chips identifyable on the board with hardly readable markings. One is a MT6704 with a logo:
The other one is a PL3369:
Both are unknown to me, so from looking at the components, it seems impossible to verify the vendors’ claims of 2.1A at 5V.
Testing True Peak Current
This leaves testing as only remaining option. For this, an electronic load is needed to simulate varying loads:
- Connect the electronic load to the USB output. The USB-C connector has two easily accessible solder pads, or you simply use a USB cable, plug it in, and connect its other end to the electronic load. Just make sure you use a cable capable of handling the test currents, and you connect the USB output in correct polarity: some electronic loads cannot handle reverse polarity.
- Next, connect the USB power supply board to AC power using the two blue wires. Obviously, take great care. AC power is dangerous and can kill.
- The electronic load now shows the output voltage of 5V.
- Switch the load to constant current (CC) mode, set an initial current of 200mA, and enable the load. The electronic load now acts as a load, drawing exactly the specified current.
- Now increase the constant current gradually up to the claimed maximum of 2.1A. Should the voltage break in (drop considerably below 5V), or if the power supply shuts off unexpectedly, then the true maximum current was exceeded.
Testing True Sustainable Current
Once you know the true maximum peak current (that the supply can handle for a short period of time), you need to determine the sustainable maximum load that can be drawn for a long period of time. For this, you need to define a maximum temperature you want the board to not exceed. This temperature is somewhat arbitrary. It is the highest temperature that you want to allow inside the powerstrip.
It is not unusual for power supplies to reach temperatures up to 100C and more. This does not mean that such temperatures are your safe limit. I wouldn’t want the internals of my powerstrip to ever get hotter than 70C.
Keep drawing the peak maximum current, and closely monitor the PCB and its components with an infrared camera. It tells you the temperatures and hot spots.
When temperatures rise above your threshold temperature, lower the current until the temperature starts to drop. Adjust the constant current to the sweet spot where the component temperature stabilizes at your temperature threshold. Continue to draw this current for an hour, and verify that the temperature is stable.
This is the maximum sustainable current the board can deliver.
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(content created Aug 29, 2024)