Building a Smart Master/Slave Switch:
An affordable and straightforward solution is available for creating a master/slave switch that can be seamlessly integrated into power strips. This innovative switch enables automatic control of the “slave” devices based on the status of the “master.” Equipped with a built-in sensor, it constantly monitors the master unit’s current consumption. Once the current surpasses a predefined threshold, the slave outputs are promptly activated. Originally designed for desktop PCs, this device simplifies the experience by automatically powering on all associated peripherals, such as the monitor screen, printer, scanner, multimedia speakers, and more, whenever the desktop PC is turned on.
The circuit centers around two essential components: a current transformer and a compact SMPS module. Specifically, the current transformer (CT) used is a 5-A/1,000:1 model equipped with a built-in “burden resistor,” while the SMPS module chosen is a 5-V/3-W variant known as HLK-PM01. The current transformer serves a dedicated purpose in monitoring current, allowing you to wrap several turns of mains-insulated wire around its core to obtain a usable output from the secondary side. When the current transformer detects a substantial load current from the master unit, it triggers an electromagnetic relay (RL1) to power all connected devices on the slave side. Conversely, RL1 deactivates when the master unit is switched off, ensuring energy efficiency.
As depicted, the circuit is primarily designed to accommodate 10-A relays, which typically possess a coil resistance of 400 Ω or higher. However, it remains adaptable to driving relays with lower coil resistance values. The recommended minimum coil resistance is 200 Ω.
Effectively sensing the load current from the master unit can be somewhat challenging, but the inclusion of a current transformer lends versatility to the process. The 5-A/1,000:1 ratio (5 A to 5 mA) CT incorporates a 200-Ω burden/load resistor across its output. This setup allows for the calculation of AC current by measuring the voltage drop across the resistor. In other words, when subjected to a 5-A load current (primary current divided by the turns ratio and multiplied by the burden resistor’s value), the output registers at 1 V.
It’s important to note that the specific number of primary turns (wire loops) required when using this CT depends on both the CT type and the current drawn by the master unit. For the mentioned transformer, commencing with one to three turns is a suitable starting point, with the option to adjust the number of turns as needed to accommodate varying load currents. Additionally, you have the flexibility to replace the onboard 200-Ω burden resistor of the CT with a resistor of higher value or even a trimpot. This adjustment is made without the need to overly concern yourself with the inherent saturation and frequency response issues associated with the current transformer.
*By strategically passing the wire under monitoring through the current transformer two or three times, we effectively create an illusion of a greater current flow than what truly exists. As a consequence, the current transformer perceives a doubled or tripled current value. However, it’s important to note that the current transformer employed in this design possesses a theoretical maximum current sensing capacity of 5 A. Attempting to sense currents beyond this limit has two noteworthy consequences.
First, there is the potential for the output voltage to increase. Second, surpassing the 5-A limit pushes the transformer into saturation, leading to a deterioration in its linearity. In applications where precise measurement of current values is crucial, these effects would be of concern. However, in our case, our sole concern is whether the transformer is active or inactive.
The circuit is intentionally designed to incorporate cost-effective components, with several essential parts sourced from eBay merchants. Many of these components are not highly sensitive or demanding. Nevertheless, unlike commercial counterparts, this master/slave switch does necessitate an initial adjustment of the load threshold, which may be considered somewhat cumbersome.
As previously discussed, one option for achieving a more flexible range of load threshold adjustments involves replacing the burden resistor with a trimpot, such as a 1K resistor or a similar component, connected across the output of the current transformer (CT).
For practical applications, it’s advisable to create a customized printed circuit board (PCB) to ensure a safer and more organized setup. Using a typical veroboard can be highly hazardous when dealing with mains voltages. Once the design is finalized, it should be enclosed within an appropriate insulated enclosure.
To optimize performance, consider positioning the current transformer in close proximity to the master plug under surveillance. It’s crucial to emphasize that this component is responsible for sensing currents at potentially lethal mains voltages. Therefore, utmost care must be taken to ensure that all aspects of the mains-side wiring and safety adhere to established standards and remain isolated from other components.
A word of caution: A minor error in this setup could have dire consequences for you or others, so diligence is paramount.
Within numerous electronics labs, an array of instruments and power tools are frequently employed in tandem. For instance, a dust extractor may be paired with a rotary tool or a jigsaw. This circuit can find valuable application in such environments by automating the operation of all the slave devices in synchronization with the master unit.
CT Test in Lab
I ran some random tests on the current transformer module with the on board 200-Ω burden resistor. At full current (5,000 mA), I got 1,000 mV across the burden resistor — exactly as expected — with a single turn.
Furthermore, when utilizing five turns on the primary winding, the recorded output across the burden resistor registered approximately 5,000 mV with a primary current of 5,000 mA. It’s worth noting that the burden resistor is situated in parallel with the secondary winding, making it more practical to monitor voltage across it as opposed to tracking the current flowing through it. This choice is based on the convenience of working with an output voltage rather than an output current.