Recently, we purchased a Timer Smart plug for our office use as we wanted to test one equipment that needs to be tested for a particular time period and then should be put off. There were many other timer switch available in the market but this was more suitable as we could directly connect the equipment’s plug into it without needing any extra wiring.
As I am always curious to know how someone has built a product, I opened it to void warranty instantly (penalty) and the reward was I could see how they designed the product.
I am sharing this so that you can also learn how such devices I built.
In order to open you just need to open three screws and the plastic cover will come out.
The top cover has a cutout for the 7-segment display and 3 switches, with the help of which the user can start/stop the timer. The two switches are for increasing or decreasing the time. They have given 15 minutes as the resolution, So you can program it from 9 hrs 45 Min to 15 min for operation.
I think they could have given a resolution of 1 minute for better flexibility. I’m not sure why they did not provide that, but anyway, it’s not a big issue.
On the top side of the PCB, we can see the ST mark, which shows that it was probably manufactured by Shogni Technology, a well-known PCB manufacturer in Pune.
Looking at the PCB design, components it looks like designed and manufactured in India only which is really great.
If you look at the PCB, it is clear that they have used a Capacitor based power supply which converts 230V AC to 24V DC, why 24V DC because the relay they have used is of 24V.
Have a closer look at the power supply section:
Is it a good idea to use capacitive power supply for such devices? well capacitor based power supply basically uses a series capacitor to drop the voltage and then a bridge rectifier is used to convert low voltage AC to low voltage DC, in this case 24V DC. This type of power supply only works when you have low current requirement say 25-35mA or may be 50mA max. These type of power supplies are cheap that why they are popular in ultra low cost devices. You will find them in domestic electricity meters as well.
While looking at PCB, one more thing I saw, there is no LDO on the PCB but a zener diode with 5.1V( by looking at the marking on the PCB). 5V is required for MCU and other circuity to function.
Why they used a Zener, I would have used 3.3V or 5V LDO instead of a Zener.
The MCU used is Nuvoton MS51DB9AE which is an 8 bit MCU based on 8051-core and has 18KB Flash more than enough for such a simple device.
They have used 105 degree C rated capacitors’ which is good choice for any power electronic product or anything which has AC interface. Goof for reliability.
Relay used is HF3FF 024-1ZST, which is 24DC coil and 250VAC/10A contact rating relay. The relay is UL rated, Made in China
From electrical wiring point of view. The Earth connection is direct from input port to output port, PCB is not involved in this.
Neutral and Phase are going to the board for power supply and Phase is coming via Relay contact to the output plug.
I am wondering why they used a thin wire in the neutral connection and quite thick wire for Phase connections. Ideally Neutral connection should also be with a thick wire.
One good feature I saw was that if the input voltage is lower than 150V AC which is min operating voltage, it will show Lo on 7 segment display and timer will not work, which is a good feature. I was able to test it and it works 🙂
I hope you found this teardown useful and have learned something new. I will publish a new blog soon. Have a great learning!
I am running an Embedded Design House, CAPUF Embedded Pvt. Ltd, located in Bangalore, India. At CAPUF, we help companies build embedded products with our hardware and firmware design services.
We also help in design optimizations for power consumption, cost, mass manufacturing, and performance.
Additionally, we develop PCB testing jigs and provide cloud-based device / production monitoring solutions.
Yes same question where Do I get casing like this enclosure with socket and 3 pin
Hi pallav,
Where do i get the casing like this?