LED light-emitting diode

Many people use electric heating rods for warming water, 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 these devices and explain how they function. Understanding their design can help you assess their safety and use them more efficiently and responsibly.

As shown in the diagram, SB is the start button. When used, the water level detection electrodes A and B are placed along with the heating element inside a thermos filled with water. Pressing SB activates the transformer T, which steps down the voltage from the main power supply. The secondary coil outputs approximately 12V AC, which is then rectified by VD and passed through the resistance between electrodes M and N, creating a DC voltage of around 10V on capacitor C. This voltage powers the relay K, which operates at a low voltage of 8V. Once activated, the relay turns on the LED, indicating that the device is active.

When the relay K is engaged, it closes its contact, allowing current to flow through the heating element RL, which begins to heat up. Even after releasing the SB button, the relay remains in a self-latching state, keeping the contact closed and maintaining the heating process. However, when the water in the bottle reaches boiling point, vapor bubbles form due to intense cavitation, temporarily disconnecting the M and N electrodes. This causes the relay to lose power, opening the contact and stopping the heating process.

If the water level is too low, the resistance between M and N becomes very high or even open. In such cases, pressing SB won't activate the relay because it doesn't receive enough operating voltage. This feature ensures that the heating element only works when there's sufficient water, enhancing safety and preventing dry burning.