I want to do a relays output with enhanced safety level (redundancy and EDM).
For that, I plan to use two force-guided relays driven each by N-channel MOSFETs. However, I'm wondering if I can use only one MCU outputs to drive these two MOSFETs or if I should use one MCU output by MOSFET.
Can I assume the MCU transistor output will never be shorted when used to drive MOSFET ? Or should I use two MCU outputs (one per MOSFET).
Here is my schematics :
- Relay1 is the MCU output
- Relay_1_EDM is connected to MCU input and is used for External Device Monitoring
- START_STOP_COM and START_STOP are the relay output with enhanced safety level
This device is intended to be used on a crane.
Currently, this crane works with a button box without any safety at all (except limit switches). It has a power line contactor driven by a start button with self-maintain and a stop button Each motor has its contactor. There is any E-stop, external monitoring on contactor or safety device.
My work is to replace the button box by a radio remote.
In order to have a safe communication between RF remote and station, the station MCU will send a heartbeat with nonce (autoincrement byte) and the remote MCU should reply to this heartbeat with the right nonce value within a timeout delay.
In case of problem, the station MCU will stop the crane by acting on the power line contactor (On the schematic above,
START_STOP will be connected to terminal 3 and
START_STOP_COM to terminal 1 which is connected to
P power line contactor coil).
This is why I need this output with enhanced safety level.
For now, I think the redundant force-guided relays output with EDM and, as proposed by Dan, driven with two charge pump (one for each relay) and two MCU outputs would be a good solution. I would also include the crane power line contactor monitoring into the EDM.