I am aware of three situations that will cause the ULN2xxx chips to fail: 1) over current. Even a momentary short will destroy one or more channels. 2) over voltage. Exceeding 50V causes damage or destroys the=20 channel. Easiest thing is to use the built-in clamp diodes (tie pin=20 10 to the relay supply + line) 3) excessive input voltage to the channel. This failure is usually=20 pretty rare. Question: did the chip fail spontaneously or was the system being=20 worked on when it failed? dwayne At 02:31 PM 12/11/2013, Carl Denk wrote: >My house heating zoning system board has a ULN2803A to complete the >circuits controlling 4 solenoid valves, each with a 1500 ohm coil @24 >VDC =3D 16 ma current by completing the ground side of the circuit. To 2 >of the outputs, I add a relay to each output with a 17 ma coil and a >suppression diode. Supposedly the ULN2803A has an allowable current of >500 ma. There has been 2 failures of the chip drivers, one totally open, >the other shows 5VDC, when should be open. I'm assuming the 5 VDC >control from the CPU. I am thinking of adding say 100ma fast acting >fuses to my connections as protection if the vendor complains. > >Comments?? --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .