I am looking for a way to add a relay-based safety feature to my BLDC controller.
In case of an emergency the user must be able to push an e-stop (probably consisting of an NC contacts) completely disconnecting the power to the motor.
After the E-stop is pushed, the relay(s) may not close until the E-stop is reset and the RESET button is pressed.
Basically I want to mimic the workings of a safety relay with an E-stop and reset. What type of IC and/or circuit can I use here?

  • \$\begingroup\$ Do you want to have a reset button and an e-stop, or just the e-stop? What purpose does the second button serve. \$\endgroup\$ Commented Sep 26, 2018 at 13:17
  • \$\begingroup\$ @SomeoneSomewhere to make sure the motor is always in a known state and won't start turning the moment you release the E-stop. \$\endgroup\$
    – Alex Er
    Commented Sep 26, 2018 at 13:19
  • \$\begingroup\$ There is a reason relays with 'force guided contacts' are a thing, but you will basically end up building a safety relay, they are designed for exactly this situation. Don't go for silicon for this sort of thing, go for relays with guided contacts so you can set up a system which will not allow reset if there is a stuck relay in the control path (This is what safety relays do). \$\endgroup\$
    – Dan Mills
    Commented Sep 26, 2018 at 14:28

1 Answer 1


As you think through the logic of how this would work do consider the possibility that the power can go off and put things into a state that needs to be safe. One way to deal with this is to allow the system to disarm itself when the power goes off.

I could envision a system where the E-Stop switch cuts off power completely when the switch is in its pushed in state. When the E-Stop is twisted so it pops back up it would reconnect the main power connections. It is a good practice to use a double pole switch for the E-Stop so that both sides of the AC mains are cut off by the E-Stop switch.

An arming button (or reset button as you call it) would be a momentary switch that activates the coil to an arming relay that is wired up in a way to latch the relay on through a set of NO contacts. This relay's latched behavior ceases when the E-Stop switch cuts off the power or there has been a power loss. This arming relay coil is best if powered from the mains power as supplied through the E-Stop switch. Some applications like to use a power supply to run the arming relay from a lower voltage but the holdup time of the power supply can cause this relay to not unlatch when there is a momentary AC mains power loss.

The arming relay in turn activates your main power relay to allow power to go to your motors. A pair of NO contacts on the arming relay are used for this purpose and the main power relay coil may powered from either AC mains or some other power supply derived voltage source. The NO contacts on this main power relay can be used to switch power into the motor controller or could be used to switch the motor controller output leads to the motor itself. This choice of how to wire this part is dependent on the types of motors involved and how the motor is being controlled.

There are additional considerations that should be taken into consideration in designing a safety interlock system. Sometimes redundant relays are used in series if there is a possibility that relay contacts can weld together thus preventing circuit disconnect.

The idea presented here is intended to use a relatively low power relay for the arming relay and using that to create the latching action. Lower current switching can make this more reliable as opposed to trying to create the latching function on the main power relay itself.

  • \$\begingroup\$ have you used such setups before? Any pros and cons of cutting the input to the controller vs cutting the output from the controller? \$\endgroup\$
    – Zy Gan
    Commented Sep 26, 2018 at 13:53
  • \$\begingroup\$ I have worked on systems that had similar interlock and arming mechanisms. In some cases the arming latch was performed in MCU software but there are safety critical environments where software interplay is considered less than optimal. I cannot comment about the choice of method for cutting actual power to the motor because, as I said in my answer, it is dependent upon the system design and components used. \$\endgroup\$ Commented Sep 26, 2018 at 14:01
  • \$\begingroup\$ Ok, i also need to design similar system but couldn't get much details on the standard practice for such system. For simplicity's sake i just decided to cut off the power input to the controller, but the drawback is that my main PC couldn't detect the controller when the emergency button is pushed. \$\endgroup\$
    – Zy Gan
    Commented Sep 26, 2018 at 14:07

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