We have designed and supplied a disability control system for motorised beds. The system enables voice control of the postion of the bed and is used by people with spinal cord injuries. This system was installed in December 2023. We have recently heard from the customer saying several of the beds (the total order was 18 units) are jamming on and running the motor to the end of travel. The entire project has been recalled due to safety concerns, and we are now analysing the components.
At least 3 units activate one relay as soon as power is applied, even if the controlling microprocessor is disconnected. It is obvious the MAX4820 is driving the relay with no data and the SET input high. The RESET would also be high as it is driven by by a watchdog timer.
The logic is powered from 3.3V. The relays are powered from a regulated 5V rail, and are activated when the MAX4820 open collector input conducts to GND. The relays provide isolation from the bed motor control and switch logic level current <20 mA.
There is a pattern developing:
Failed units with the relay locked on measure between 2.04 and 2.24V between the output pin and GND. It should be 5V when the relay is not activated. The Voltage drops to approx 60mV when the output is active.
One unit has a 2.3V measurement, dropping or 66mV when activated. This unit triggers the relay when activated, but the relay does not release. I assume the 2.3V is just not enough to pull the relay in, but is enought to keep it pulled in.
The relay used is QYZK - QY23 signal relay with 5V coil. (123 Ohms, 40mA @ 5V)
There appears to be a failure in the output driver or flyback protection in the MAX4820 which is showing up 7 months after commissioning in multiple units.
We are now retrofitting external 1N4007 SMD diodes across the relay coils and replacing any MAX4820 ICs that show any voltage drop on the inputs when they shouldn't.
I'd be grateful for any comments or suggestions, especially strategies to give us confidence that the products are suitably remediated and ok to return to the customer.
This has also been submitted as a support ticket, and still awaiting a response.