We offer for repetition an inexpensive and easy-to-assemble Master / Slave switch for automatically switching the driven load from turning on the master. A built-in sensor monitors the current drawn by the master unit, and when the current exceeds a predetermined level, the slave outputs automatically turn on. In fact, this device is intended for use in desktop PCs. When the PC is turned on, all peripheral devices (such as a monitor, printer, scanner, speakers , and so on) also turn on automatically.
DESCRIPTION OF SWITCH CIRCUIT
The circuit consists of two key blocks: a current transformer (CT) and a compact SMPS module. The current transformer (CT) is a 5-A / 1000:1 with built-in “terminating resistor” and the SMPS module is a 5-V / 3-W type (HLK-PM01).
The current transformer is specially designed to control the current in the line, it is possible to wind several turns of a wire insulated from the mains through its core to obtain an output signal from the secondary winding. When the current transformer detects a high load current from the master unit, the electromagnetic relay (RL1) turns on to power whatever is connected to the slave, and turns off again when the master is turned off.
The circuit is designed for 10A relays with coil resistance of 400 ohms or more, although it can also drive lower resistance coils. The recommended minimum value would be a coil resistance of 200 ohms.
In general, determining the load current of the lead unit can be a little tricky, but using a current transformer makes it easy. Since a current transformer with a ratio of 5A/1000:1 (5A to 5mA) has a 200 ohm load resistor at the output, AC current can be calculated by measuring the voltage drop across the resistor; that is, we get 1 V at the output at a load current of 5 A (primary current divided by the transformation ratio and multiplied by the value of the load resistor).
When using a CT, the number of turns of the primary winding (loop) required depends on the type of CT itself and the current drawn by the lead unit. With the transformer mentioned here, just start with one to three turns and try increasing/decreasing the number of turns for lower/higher load currents.
Because the CT used in this design has a theoretical maximum current sensing capability of 5A, attempting to measure more current will have two effects. First, the output voltage can rise, and another effect of exceeding the 5-A limit causes the transformer to saturate and degrade its linearity. For projects that accurately measure current, this will matter, but all we’re interested in here is whether current flows or not.
SCHEMATIC ASSEMBLY TIPS
The circuit is designed using inexpensive components, you can buy the main parts on Aliexpress. The finished structure can be placed in a suitable insulated case (after all, 220 V). The current transformer can be placed next to the monitored main line.
Be aware that it measures currents at life-threatening mains voltage, so care must be taken to ensure that everything connected to the mains side is done in accordance with proper electrical safety standards!
Several tests were carried out on a current transformer module with a built-in 200 ohm terminating resistor. At full current (5000 mA), we got 1 V across the load resistor – as expected – in one turn.
And with five turns on the primary, the observed output across the load resistor was about 5V at a primary of 5000mA. Since the load resistance is placed in parallel with the secondary winding, the voltage across it was controlled, and not the current through it, because it is easier to take the output voltage to work than the output cu
