I am building a project that will lock several cabinet doors using ULN2003AGP chips and security electro magnets similar to this:
This magnet has a max current draw of 150ma, which is well below the 500ma that each ULN2003 channel limit. My plan was to attach six magnets to a single chip, which should draw a max of 900ma, which is also well below the max current limit for the entire chip. (note, my power supply is a box-plug 12v 3A supply.) My design is shown below. (showing 1 of the 6 connected magnets)
These magnets will typically stay on for hours, then turn of for some amount time between 1 and 30 minutes. All of them will spend at least an hour on, all at the same time.
My original plan was to power 9 magnets using 2 ULN2003 chips. However, after connecting one magnet to one chip and two magnets to another chip, I returned a week later to find that the current flowing through each circuit had fallen significantly (the magnets no longer functioned.) Initial investigation (without a multi-meter) showed that the transistor array was not functioning correctly (though in hindsight, I did not rule out the optocoupler.) Do I need to remove the 100nf caps, add diodes, or modify the way the optocoupler is connected? Is there another, better way to connect electromagnets to a ULN2003 chip?