I own some animatronic robots that use solenoid valves to operate their mechanical movements. Recently I created a pcb that uses three 8 channel Darlington Arrays ULN2803A to control the individual solenoids and make the robot move. Each Solenoid is 6.7 watts (or about 300milliamps) and 24 volts. The power supply I use is 4.16 amps and 24v. I use an arduino to control the darlington arrays. When I first tested the darlington arrays, the robot moved and it's solenoids were triggered by the darlington arrays, but upon inspection of the board, the darlington arrays began to smoke. I replaced them, and tested them again. I tested them multiple times having similar results, only twice did they work without failing. When they didn't fail, I noticed the chips get extremely hot. This has concerned me, because I know someone who also uses them and has had no heating or failure issues with them. I did not use diodes for the outputs as it says they have built in output clamps diodes. I don't understand how I could possibly be overloading them because they can handle up to 50 volts and 500 milliamps. Is there any output protection I should have used? perhaps addition of some resistors? Thanks
With Vce-sat of maybe 1.6 V (at 500 mA), each darlington will be heated by ~0.8 W when run at maximum current.
If all 8 darlingtons are active at once that's 6.4 W.
Junction-to-ambient thermal resistance is 66 C/W (for some assumed copper pattern on the board).
So with with all 8 darlingtons active at 500 mA, you can expect a temperature rise of 422 degrees.
What this means in practice is: Don't operate more than one or two of the darlingtons at full current at the same time.