Resettable Circuit Protection for Robot Vacuums


In recent years the development of information technology, artificial intelligence, sensor techniques and mobile robot technology has led to intelligent systems being applied to smaller household appliances. One of the most representative products of this trend is the robot vacuum cleaner. Since the launch of the first commercial robot vacuums in the U.S. in the early 2000’s, these appliances have become increasingly popular around the world.

Because a robot vacuum operates independently, it may be used when no one is there to supervise it. In the event of mechanical or electrical failure, the components must be protected by a timely shutdown to avoid possible damage. Consequently, for robot vacuum manufacturers, ensuring high reliability is a primary concern. Moreover, they must comply with safety standards, including requirements specified in the IEC/EN60950, UL1017 and EN 60335-1.

TE Connectivity (TE) Circuit Protection’s PolySwitch PPTC (polymeric positive temperature coefficient) devices can help provide overcurrent protection in robot vacuums. As resettable devices, PolySwitch PPTC devices are well-suited to help protect the product’s motors, batteries and charging ports from damage caused by stalled motors, overcharging and other failures that can be encountered during normal operation. PolySwitch PPTC devices feature low resistance, rapid response time, small size and reset functions – all of which help circuit designers develop safe and reliable products.

Motor Protection
While floor cleaning is in progress, the motorized parts of a robot vacuum may become entangled with debris, causing the motors to stall. When this happens, voltage is still being supplied to the motor even though current can no longer flow, which causes the temperature of the motor to rise rapidly. When the temperature exceeds the rating of the motor coil, the coil can burn out and the motor can fail. This not only inconveniences the consumer, it can also add to the cost of warranty repairs incurred by the manufacturer.

PolySwitch resettable devices are well-suited for protecting motors during these overcurrent events. Like traditional one-use fuses, PolySwitch PPTC devices limit the flow of dangerously high current during fault conditions. Unlike fuses, however, PolySwitch devices reset after the fault is cleared and power to the circuit is cycled. And compared to many dual-metal breaker products, PolySwitch PPTC devices enable greater design flexibility, longer service life and reduced electromagnetic interference (EMI).

Figure 1a illustrates the real-time response of a PolySwitch miniSMDC050F surface mount PPTC device being used to protect a stalled wheel motor. As shown in the graph, the protection response time is 3.0 seconds, which is far less than the 10 seconds generally required in these applications. Figure 1b shows a PolySwitch radialleaded PPTC device (RUEF400) being used in a dust-removal motor stall. The real-time response time in this graph is 6.0 seconds; again, much lower than the 10-second response time requirement.

TE-TECP_Fig1aFigure 1a. Real-time response time of a surface mount PolySwitch PPTC device protecting a stalled wheel motor.
TE-TECP_Fig1bFigure 1b. Real-time response time of a radial-leaded PolySwitch PPTC device protecting a stalled dust-removal motor.


Protecting Power-Charging Ports and Rechargeable Batteries
Since a robot vacuum operates autonomously, it uses a rechargeable battery for its power supply. This allows for unmanned control and helps improves the product’s functionality and flexibility since, when fully charged, a robot vacuum can operate nonstop for several hours. Although convenient for the user, having the charging ports exposed means that designers must provide circuit protection to help protect against external short circuits and other failures.

Resettable PolySwitch devices help provide an effective port protection solution in robot vacuum applications. Figure 2 shows the charging circuit in the robot vacuum. Since the vacuum’s rechargeable battery will not exceed 30V, PolySwitch devices with voltage ratings equal to or greater than 30 VDC should be used. These low voltage rated PolySwitch devices can be placed on the AC mains charger for secondary side protection. Based on the working current levels and ambient operating temperatures for robot vacuums, the PolySwitch RUEF400 device generally can meet the relevant requirements.

Figure 2. Circuit diagram showing a PolySwitch PPTC device in a charging port of a robot vacuum.


Various types of batteries can be used in robot vacuum battery chargers, including nickel-metal hydride, lithium ion and lithium polymer – all of which require protection against electrical surges or failures. PolySwitch PPTC devices are widely used as secondary protection devices in these applications to help provide both overcurrent and over-charging protection. The overcurrent function of the PolySwitch device helps protect against abnormally high-charging or discharging currents to prevent damage and ensure battery safety. The over-charging protection function of the device senses the battery’s core temperature and helps prevent over-charging by limiting the current if such a condition occurs.