Safe Torque Off (STO), motor temperature and mechanical brake wiring

Himalaya Evaluation Board includes some safety systems that are controlled externally. The safety systems are:

  • Safe Torque Off (STO)
  • Motor temperature sensor
  • Electromechanical brake

Safe Torque Off (STO)

The STO is a safety system that prevents motor torque in an emergency event while Himalaya remains connected to the power supply. When STO is activated, the power stage is disabled automatically (no mater what control or firmware does), and the motor shaft will slow down until it stops under its own inertia and frictional forces.

The Himalaya STO works with negative logic, deactivating the power stage by default. In order to activate the power stage, and therefore allow the motor operation, two differential inputs must energized. These inputs activate two optocouplers that enable the power stage operation. On the contrary, if the STO inputs are not energized, the transistors of the power stage are turned off and a STO fault is notified. During this state, no torque will be applied to the motor no matter configuration, or state of a command source. This input should not be confused with a digital input configured as enable input, because enable input is firmware controlled and does not guarantee intrinsic safety as it can be reconfigured by a user.

To simplify product development, two STO override switches are available to emulate STO performance. 

The following diagram shows an schematic of the STO input circuit.


STO inputs have an input voltage range from +4.5 V to +24 V. In order to simplify the wiring, the STO connector includes a 5V and GND pins. The following table summarizes some specifications:



Type of input

Optocoupled, dual channel

ESD protected

High level input voltage2 V < Vin < 24V
Low level input voltage0 < Vin < 0.8 V

Maximum voltage

24 V

Motor temperature sensor

The Himalaya Evaluation Board provides a connection for an external motor temperature sensor based on changes of resistance (PTC thermistor, bimetal, NTC, PT100, silicon temperature sensors). This analog input includes a pull-up for directly connecting a PTC thermistor and a low-pass filter for reducing noise,  as shown in the following figure:


In order to correctly dimension the overtemperature levels it is suggested to use this calculator tool:

Suggested PTC

The suggested PTC thermistor value is a 1 kΩ nominal resistance (@ 25 ªC) as Vishay PTC (TFPTL10L1001FL2B).

Main specifications of the external temperature sensor input are shown in the next table:



Type of input

Single ended analog filtered
1 kΩ pull-up

1st order filter cutting frequency1.6 kHz
Pull-up resistor value1 kΩ

Electro-mechanical brake

Electro-mechanical brakes are needed in critical applications where the disconnection of the motor or a lack of electric braking could be dangerous or harmful (i.e. falling suspended loads). Himalaya Evaluation Board includes a brake output in the Brake connector to that end.

The brake output consists on an open drain MOSFET (1 A full temperature range) and an two supply terminals, one connected at Logic Supply, and another connected to Power Supply. This allows to easily connect high-voltage or low-voltage brakes. Next figure shows the brake schematic:


Brake connection using the main power supply

The on board supplies are useful when:

  • The brake nominal voltage is the same than the power supply voltage.
  • The brake nominal voltage is the same than the logic supply voltage.
  • The brake nominal voltage is lower than the power or logic supply voltages. In this case, the brake output voltage can be easily configured via the PWM duty cycle.

Although effective brake voltage will be reduced by the PWM, ensure the brake withstands the peak voltage.

Brake operation details

The brake output is an N-Channel MOSFET internally connected to PWM_BRAKEIts operation is usually configured for normally locked electro-mechanical brakes; that is, brakes that by default block the movement of the motor shaft. For this reason, the switch is controlled with inverted logic, being activated to allow the rotation of the shaft. This kind of brakes increase the safety of the application, because in a drive power failure, the switch would be opened therefore the brake activated.

Main ratings of the brake switch are detailed in the next table:



Type of output

N-Channel MOSFET

Maximum voltage

250 V

Maximum current

1 A (full temperature range)