Application Note | Position and Level Sensing in Refrigerators and Freezers

Home appliances like refrigerators, freezers and wine coolers usually consume a lot of energy and are therefore not very environmentally friendly.

Like many modern home appliances, microprocessor-based controllers for refrigerators and freezers allow for the use of a variety of sensors to ensure proper, safe, and energy-efficient operation. Littelfuse magnetic sensor products help make these appliances as reliable as ever.

Both reed and Hall effect sensors are very reliable and have millions of operating cycles and long term integrity when used with microprocessor electrical loads. Both reed and Hall effect sensors operate on a magnetic field. Reed switches and sensors consume no power, while Hall effect devices are semiconductors that consume a small amount of current in both the activated and deactivated states.

Figure 1: Components of a refrigerator

Refrigerators equipped with microprocessor control units are compatible with the use of non-contact sensing technologies such as reed switches/sensors and Hall effect sensors. These technologies are highly compatible with the low DC voltage and current requirements of the microcontroller.

The traditional mechanical and plunger switches that were previously used to directly switch 120-volt incandescent lamps are no longer needed. Modern appliances using microcontrollers have switched to internal LED lighting. This is a huge energy saving advantage.

When the door of a refrigerator, freezer or wine cooler is opened, the lighting is turned on, illuminating the interior compartment. To achieve this, proximity sensors are used to detect whether the appliance door is open or closed.

A reed sensor or a Hall effect sensor can be used to detect the position of the door. For example, in a refrigerator, the reed sensor is mounted on the refrigerator frame and a permanent magnetic actuator is mounted on the door. Each reed switch or Hall effect sensor and magnetic actuator are mounted in such a way that when the door is closed, the reed switch or Hall effect sensor is activated and the lighting is turned off. When the door is opened, the magnet on the door is outside the sensor activation range, the sensor is disabled, and the lighting turns on in this mode. The controller uses inverse logic to control the lighting relative to the sensor activation.

The sensor units are invisible as they are mounted behind a panel in the frame or door, greatly improving the external aesthetics of the appliance. Unlike mechanical or plunger switches which have a visible handle or button, these sensors are 'contactless' and operate without physical contact.

Figure 2: Sensor location in refrigerator

Littelfuse recommends the following products as door position sensors:

Table 1. Recommended products for door position sensors

The ice dispenser on the front of the French door refrigerator currently does not have a sensor within the mechanism to detect the position of the ice dispenser switch. Adding a sensor will allow the microcontroller to close the loop of this unit.

There are a few options available, digital signal or rotary proportional output, or even linear proportional output.

The digital version of the solution is simply to mount a reed or hall effect sensor on the fixed frame and a magnet on the moving rod. Using this system, the water or ice will maintain a certain fill or flow rate; that is, it is either on or off.

When using a ratiometric output Hall effect sensor, an instantaneous feedback of the exact position of the joystick/magnet is obtained over a linear response range. The microcontroller detects the exact position and the system can release water at different flow rates. It is best to use a non-contact sensing solution to improve long-term reliability. Programmable rotary Hall effect sensors are well suited for this application.

For example, the voltage output of the Hall Effect sensor can be programmed to 0.5 Vdc when the joystick is in the normal open position, while the maximum swing voltage of the joystick movement will be programmed to 4.5 Vdc. Between these values, the water flow can be controlled relative to the instantaneous position of the joystick.

Figure 3: Sensor and magnet locations on a cold drink dispenser

Littelfuse recommends the following products as dispenser lever position sensors:

Table 2. Recommended products for use as water dispenser lever position sensors

There are many potential sensing applications around the various compartments within refrigerators and freezers. The door of a butter compartment or a drawer containing vegetables, meats and deli items could also be equipped with sensors to detect when it is open or closed.

All of these sensors can be either digital reed type or Hall effect output type. The magnet will be mounted on the moving tray or drawer and the sensor will be mounted in a fixed position on the frame. If it fails to close correctly, the lighting in the unit will activate, allowing the user to adjust correctly.

Littelfuse recommends the following products as pallet and compartment cover position sensors:

Table 3. Recommended products for use as tray and compartment cover position sensors

Refrigerators/freezers with internal ice makers and ice buckets usually use a robotic arm on the ice bucket to detect when the bucket is full. When the bucket is full, the arm turns off the ice maker.

Currently mechanical switches and relative valves are used to control this. In home appliances containing microcontrollers, reed sensors can be mounted on the frame and magnets can be attached to the mechanical arms to achieve a more reliable sensing mechanism than current mechanical devices.

Littelfuse recommends the following products as ice bucket fill level sensors:

Table 4. Recommended products for use as ice bucket fill position sensors

Both freezer and refrigerator compartments have rotary temperature adjustment knobs. These are usually either resistive film or potentiometer type regulators.

In systems equipped with a microcontroller, a contactless solution would be preferred to improve long-term reliability. For this application with an output voltage of 0.5V to 4.5V and a rotation of more than 360 degrees, a programmable rotary Hall effect sensor is an ideal choice.

Figure 4: Temperature control knob mechanism using a Hall effect sensor

Littelfuse recommends the following products as temperature control knob position sensors:

Table 5. Recommended products for use as temperature control knob position sensors

There is a shallow drain pan at the bottom of most refrigerators to collect water from leaking refrigerator or freezer compartments. Since this pan is not visible, it is difficult to detect a leak unless you see water on the ground. A reed level sensor would be of great benefit for this application.

A digital output sensor can detect when the water level in the drain pan is too high, and the controller can then activate an alarm or lighting.

Littelfuse recommends the following products as level sensors for drain pans:

Table 6. Recommended products for drain pan level sensors

The reed sensor is a magnetically operated switch that is normally open in most situations when there is no magnetic field. When a properly aligned and strong magnetic field is present, the contacts of the reed switch will close and form a circuit.

Reed switches and Hall Effect sensors are magnetic switching devices, sometimes referred to as “contactless” devices; this is because the actuator/magnet does not physically contact the switch in order to change state, unlike a microswitch or mechanical switch.

Reed switches are the perfect technology for switching microprocessor electrical loads with 1-12V DC voltages and small milliamp currents. Reed switches use precious metal contacts sealed in a nitrogen-filled glass capsule. Due to the inherent reed switch design, the switching cycle can last for millions of operations without contact wear.

The digital Hall effect sensor is a semiconductor with an unlimited life if the proper voltage and current are applied. Reed and Hall effect devices are durable and very reliable in applications with very high humidity and temperature variations.

In addition to sensor products, Littelfuse offers the world's broadest and deepest portfolio of circuit protection and power control solutions.

In these applications, Littelfuse recommends the following products:

Table 7. Circuit protection and power control solutions