LED light-emitting diode

Many people are familiar with electric heating rods, but how much do you really know about their structure, working principle, and safety features? In this article, I’ll break down the inner workings of a typical hot water heater and explain some of the key concepts behind its operation. Understanding these elements not only helps you appreciate the technology better but also allows you to use it more safely and efficiently.

As shown in the diagram, SB is the start button switch. When using the device, the water level detection electrodes A and B are placed together with the heating element inside a thermos filled with water. When you press SB, the transformer T steps down the voltage from the main power supply, converting the alternating current into a lower voltage. This results in approximately 12V AC on the secondary side, which is then rectified by VD (a diode) and used to detect the water level via the resistance between electrodes M and N. This process creates a DC voltage of around 10V across capacitor C, which powers the relay K.

The relay K is designed to operate at a low voltage, typically 8V. Once activated, the relay closes its contacts, allowing the LED to light up and the heating element RL to be energized. Even after releasing the SB button, the relay remains in a self-holding state, keeping the circuit closed and maintaining the heating function. However, once the water in the bottle starts boiling, the formation of bubbles due to intense cavitation causes the electrodes M and N to momentarily disconnect. This interrupts the circuit, causing the relay K to de-energize and open its contacts, thus stopping the heating process.

If the water level in the bottle is too low, the resistance between M and N becomes very high or even open. In such cases, even if you press the SB button, the relay won't activate because it doesn’t receive enough operating voltage. This is a built-in safety feature that prevents the heating element from running dry, which could lead to overheating or damage.