Automation pHat relay problems


#1

I am trying to use an automation pHat to activate a garage door opener. I’ve simply connected the two wires that run from the opener mechanism to the local “press for open/close” switch across the NO and COM contacts of the pHat relay. However, this mechanism only works for a while before the darlington driver (pins 1 -> 16 of the ULN2003A) driving the relay stops working. I’ve connected another darlington driver( pins 2->15 ) in place of the failed one and the same thing happens - after working for a few switches, the driver fails.
I’ve checked and everything on the relay side of the driver seems well within spec, but I am not sure what the transistor/regulator/? that is connected to the COM (pin 9) of the ULN2003A is supposed to do and whether it is working correctly. Any help would be appreciated.


#2

Are you just closing the relay on the automation pHat momentarily? Like pushing, then releasing a button?


#3

The “transistor/regulator” is a 33v zener diode used to sink the high voltage spike that occurs from the relay coil’s collapsing magnetic field. This diode being faulty could expose the ULN2003A to high voltage spikes which would, indeed, kill it.

You should drop us an email at support@pimoroni.com referencing this thread and request a replacement.


#4

Thank you both.
Alphanumeric, I found that the relay needs to be energised for between 0.5 and 1.0 secs. I guess that is because of some debounce logic in the door controller.
gadgetoid, OK - I had noticed there could be a flyback problem but was unable to determine whether the component in question would dealt to it. I’ll send an email.